energetics questions

1. Which of these diagrams correctly represents an endothermic reaction?

(Total 1 mark)

2. This question is about methanol and the energy changes that accompany some of its reactions.

(a) Complete the diagram (using dots and crosses) to show the bonding in methanol, CH3OH.You should show outer electrons only.

(2)

Sri Lankan School 1

(b) The Hess cycle below can be used to calculate the standard enthalpy change of combustion of methanol, using standard enthalpy changes of formation.

(i) Complete the cycle by filling in the empty box.

(2)

(ii) Define the term standard enthalpy change of formation of a compound, making clear the meaning of standard in this context.

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(3)

(iii) Use your cycle and the data below to calculate the standard enthalpy change of

combustion of methanol, ΔHcӨ

ΔHfӨ /kJ mol–1

CO2(g) –393.5

H2O(l) –285.8

CH3OH(l) –239.1

(2)

Sri Lankan School 2

(c) An experiment was carried out, using the apparatus below, to estimate the standard enthalpy change of combustion of methanol.

Sri Lankan School 3

After burning the methanol for a few minutes, the temperature of water in the beaker had risen by 20.7 °C and the mass of methanol burnt was 0.848 g.

(i) Calculate the amount of energy transferred to the water.

Energy transferred (J) = mass of water × 4.18 × temperature change

(1)

(ii) Calculate the number of moles of methanol, CH3OH, burnt during the experiment.

(1)

(iii) Use your answers to (c)(i) and (ii) to calculate the experimental value for the standard enthalpy change of combustion. Include a sign and units in your answer, which should be given to three significant figures.

(1)

(iv) Compare your answers to (b)(iii) and (c)(iii) and give TWO reasons to explain any differences.

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(2)(Total 14 marks)

3. Which of the following is true for the exothermic reaction shown below?

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

A ΔH positive

B ΔSsurroundings positive

C ΔSsystem negative

D ΔStotal negative

(Total 1 mark)

Sri Lankan School 4

Sri Lankan School 5

4. (a) Crystals of hydrated cobalt(II) chloride, CoCl2.6H2O, lose water when they are heated, forming anhydrous cobalt(II) chloride, CoCl2.

CoCl2.6H2O(s) → CoCl2(s) + 6H2O(l)

(i) Calculate the entropy change of the system, ΔSӨsystem, at 298 K. Include a sign and

units in your answer. You will need to refer to your data booklet.

(2)

(ii) Explain whether the sign of your answer to (a)(i) is as expected from the equation for the reaction.

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(1)

(iii) The standard enthalpy change for the reaction, ΔHӨ, is +88.1kJ mol–1. Calculate

the entropy change in the surroundings, ΔSӨsurroundings, at 298 K for this reaction.

Include a sign and units in your answer.

(2)

(iv) Calculate the total entropy change, ΔSӨtotal, at 298 K for the reaction.

(1)

(v) Does your answer to (a)(iv) indicate whether hydrated cobalt(II) chloride can be stored at 298 K without decomposition? Explain your answer.

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(1)

Sri Lankan School 6

(b) A student attempted to measure the enthalpy change of solution of anhydrous cobalt(II)

chloride by adding 2.00 g of cobalt(II) chloride to 50.0 cm3 of water in a well-insulated container. A temperature rise of 1.5 °C was recorded.

The student used a balance which reads to 0.01g, a 50.0 cm3 pipette, and a thermometer which can be read to 0.25 °C.

(i) Which measuring instrument should be changed to give a result which is closer to the accepted value? Justify your answer.

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(2)

(ii) Suggest ONE other change the student could make to give a result which is closer to the accepted value. Justify your suggestion.

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(2)

Sri Lankan School 7

(c) The lattice energies of magnesium chloride, MgCl2, calcium chloride, CaCl2, and strontium chloride, SrCl2 are shown in the table below.

Chloride Lattice energy/kJ mol–1

MgCl2 –2526

CaCl2 –2258

SrCl2 –2156

(i) Use data on ionic radii, from your data booklet, to explain the trend in these values.Estimate a value for the lattice energy of cobalt(II) chloride, giving ONE piece of data to justify your estimate.

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(4)

Sri Lankan School 8

(ii) Explain how lattice energy values, together with other data, can be used to predict the solubility of ionic compounds.

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(3)

(d) Cobalt forms another chloride, CoCl3, but scientists predict that MgCl3 cannot be made. Suggest a reason for this.

You should consider the enthalpy changes in the Born-Haber cycle, which provide evidence about why cobalt(III) chloride is known but magnesium(III) chloride is not.

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(2)(Total 20 marks)

Sri Lankan School 9

5. Which equation represents the reaction for which the enthalpy change is the standard enthalpy

change of formation, ΔHfө, of sodium nitrate, NaNO3?

A 2Na(s) + N2(g) + 3O2(g) → 2NaNO3(s)

B Na(s) + ½N2(g) + 1½O2(g) → NaNO3(s)

C Na(s) + N(g) + 3O(g) → NaNO3(s)

D Na(g) + ½N2(g) + 1½O2(g) → NaNO3(g)

(Total 1 mark)

6. Which equation represents the reaction for which the enthalpy change, ΔH, is the mean bond enthalpy of the C–H bond?

A ¼CH4(g) → ¼C(g) + H(g)

B CH4(g) → C(s) + 2H2(g)

C CH4(g) → C(g) + 4H(g)

D CH4(g) → C(g) + 2H2(g)

(Total 1 mark)

7. Magnesium chloride can be made by reacting solid magnesium carbonate, MgCO3, with dilute hydrochloric acid.

(a) Write an equation for the reaction, including state symbols.

(2)

(b) Give TWO observations you would make when the reaction is taking place.

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(2)

Sri Lankan School 10

(c) In an experiment to make crystals of hydrated magnesium chloride, MgCl2.6H2O,

magnesium carbonate was added to 25 cm3 of hydrochloric acid with concentration 2.0

mol dm–3. The molar mass of magnesium carbonate is 84.3 g mol–1.

(i) How many moles of acid are used in the reaction?

(1)

(ii) What mass of magnesium carbonate, in grams, reacts with this amount of acid?

(1)

(iii) Suggest why slightly more than this mass of magnesium carbonate is used in practice.

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(1)

(iv) How would you separate the magnesium chloride solution from the reaction mixture in (iii)?

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(1)

(v) The magnesium chloride solution was left to crystallise. The crystals were separated and dried carefully. A sample of 3.75g of hydrated crystals, MgCl2.6H2O,

which have molar mass 203.3 g mol–1, was obtained. Calculate the percentage yield of this reaction.

(2)

(vi) Give ONE reason why the yield of crystals is less than 100%, even when pure compounds are used in the preparation.

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(1)


(d) Lattice energies can be measured using the Born-Haber cycle, or calculated from electrostatic theory. Lattice energies of magnesium chloride and magnesium iodide are shown below

Salt

Lattice energy fromBorn-Haber cycle using

experimental data

/ kJ mol–1

Lattice energy fromelectrostatic theory

/ kJ mol–1

MgCl2 –2526 –2326

MgI2 –2327 –1944

(i) What does this data indicate about the bonding in magnesium chloride?

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(1)

(ii) Explain why there is a greater difference between the experimental (Born-Haber) and theoretical lattice energies for magnesium iodide, MgI2, compared with magnesium chloride.

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(2)


(e) Blood plasma typically contains 20 parts per million (ppm) of magnesium, by mass.

(i) Calculate the mass of magnesium, in grams, present in 100 g of plasma.

(1)

(ii) Magnesium chloride can be used as a supplement in the diet to treat patients with low amounts of magnesium in the blood. Suggest ONE property which makes it more suitable for this purpose than magnesium carbonate.

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(1)(Total 16 marks)

8. This question is about hexane, C6H14, and hex-1-ene, C6H12.

(a) What test would you use to distinguish between hexane and hex-1-ene? Give the results of the test for each substance.

Test: ............................................................................................................................

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Result with hexane: ....................................................................................................

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Result with hex-1-ene: ................................................................................................

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(2)


(b) Hex-1-ene has a number of isomers, including two stereoisomers of hex-2-ene.

(i) Complete the formula to show the structure of E-hex-2-ene.

(1)

(ii) Explain why stereoisomerism can occur in alkenes, and why hex-2-ene has stereoisomers but hex-1-ene does not.

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(2)

(c) The enthalpy change of combustion of hexane was measured using a spirit burner to heat a known mass of water in a calorimeter. The temperature rise of the water was measured. The results of the experiment are shown below.

Mass of hexane burnt 0.32 g

Mass of water in calorimeter 50 g

Initial temperature of water 22 °C

Final temperature of water 68 °C

The specific heat capacity of water is 4.18 J g–1°C–1.

(i) Calculate the energy in joules produced by burning the hexane. Use theexpression

energy transferred = mass × specific heat capacity × temperature change.

(1)

(ii) Calculate the enthalpy change of combustion of hexane. The mass of 1 mole of hexane is 86 g.

Give your answer to TWO significant figures. Include a sign and units in your answer.

(3)


(iii) The value for the enthalpy change of combustion in this experiment is different from the value given in data books. Suggest TWO reasons for this difference.

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(2)

(iv) A student suggested that the results would be more accurate if a thermometer which read to 0.1°C was used. Explain why this would not improve the accuracy of the result. A calculation is not required.

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(1)

(d) Hex-1-ene can be converted to hexane in the following reaction.

C6H12(l) + H2(g) → C6H14(l)

(i) What catalyst is used in this reaction?

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(1)


(ii) The enthalpy change of this reaction ΔHreaction can be calculated from the followingenthalpy changes of combustion.

SubstanceEnthalpy change of combustion

/kJ mol–1

Hex-1-ene, C6H12 –4003

Hydrogen, H2 –286

Hexane, C6H14 –4163

Complete the Hess cycle by adding labelled arrows. Use your cycle to calculate theenthalpy change ΔHreaction.

(3)


(iii) The enthalpy change for the reaction of some other alkenes with hydrogen is shown below.

ReactionStandard enthalpy change

/ kJ mol–1

C3H6 + H2 → C3H8 –4003

C4H8 + H2 → C4H10 –286

C5H10 + H2 → C5H12 –4163

Explain why the values are so similar.

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(1)(Total 17 marks)

9. At 100 °C, pure water has a pH of 6, whereas at 25 °C it has a pH of 7. This is because

A the dissociation of water is endothermic, so the concentration of hydrogen ions is lower at100 °C than it is at 25 °C.

B the dissociation of water is exothermic, so the concentration of hydrogen ions is lower at 100 °C than it is at 25 °C.

C the dissociation of water is endothermic, so the concentration of hydrogen ions is higher at 100 °C than it is at 25 °C.

D at 100 °C, water has a higher concentration of hydrogen ions than of hydroxide ions.

(Total 1 mark)


10. Some mean bond enthalpy values are given in the table below.

Bond Mean bond enthalpy / kJ mol–1

H―H +436

I―I +151

H―I +299

What is the enthalpy change for the reaction shown below in kJ mol–1?

H2(g) + I2(g) → 2HI(g)

A +436 + 151 – 299 = +288

B –436 – 151 + 299 = –288

C +436 +151 – (2 × 299) = –11

D –436 – 151 + (2 × 299) = +11

(Total 1 mark)

11. Which of the following covalent bonds is the shortest?

A H―F

B H―Cl

C H―Br

D H―I

(Total 1 mark)


12. The Born-Haber cycle for the formation of sodium chloride from sodium and chlorine may be represented by a series of steps labelled A to F as shown.


(a) (i) Complete the table below by adding the letters A to F next to the corresponding energy changes.

Energy change Letter ΔH

/kJ mol–1

Lattice energy for sodium chloride –775

Enthalpy change of atomization of sodium +109

Enthalpy change of atomization of chlorine +121

First ionization energy of sodium +494

First electron affinity of chlorine

Enthalpy change of formation of sodium chloride –411(3)

(ii) Calculate the first electron affinity of chlorine, in kJ mol–1, from the data given.

(2)


(b) Lattice energies can be calculated from electrostatic theory (theoretical values) as well as by Born-Haber cycles (experimental values).

Compound Experimental lattice energy

/ kJ mol–1Theoretical lattice energy

/ kJ mol–1

NaCl –770 –766

Agl –889 –778

(i) Comment on the fact that there is close agreement between the values for sodium chloride, NaCl.

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(1)

(ii) Explain, in terms of chemical bonding, why the experimental value for silver iodide, AgI, is more exothermic than the value calculated theoretically for the samecompound.

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(2)


(c) Suggest why the first ionization energies of the Group 1 elements decrease down the group.

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(2)(Total 10 marks)


13. Propanone, C3H6O, undergoes complete combustion to form carbon dioxide and water.

C3H6O(l) + 4O2(g) → 3CO2(g) + 3H2O(l)

(a) In an experiment to calculate the enthalpy change of combustion for propanone, 2.90 g ofpropanone was burned completely in oxygen.

The heat energy from this combustion raised the temperature of 200 g of water from 20.2 °C to 78.4 °C.

The specific heat capacity of water is 4.18 J g–1°C–1.

(i) Calculate the number of moles of propanone present in 2.90 g.

[The molar mass of propanone is 58 g mol–1.]

(1)

(ii) Use the expression

energy transferred (J) = mass × capacity

heatspecific

× change

etemperatur

to calculate the heat energy transferred to raise the temperature of 200 g of water from 20.2 °C to 78.4 °C.

(2)

(iii) Use your answers to (a)(i) and (ii) to calculate a value for the enthalpy change of combustion of propanone. Give your answer to three significant figures and include a sign and units.

(3)


(b) In another experiment, the enthalpy change of combustion for butanone, C4H8O, was

found to be –1300 kJ mol–1.

A Data Book value for the standard enthalpy change of combustion for butanone is –2440

kJ mol–1.

(i) Suggest a reason why the value obtained in the experiment is so different from the Data Book value.

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(1)

(ii) This Data Book value (–2440 kJ mol–1) refers to the following equation.

C4H8O(l) + 211

O2(g) → 4CO2(g) + 4H2O(l)

How would the value be different if it referred to the formation of water in the gaseous state? Justify your answer.

Difference.........................................................................................................

Justification.......................................................................................................

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(2)

(c) Standard enthalpy changes of combustion can be used to calculate the standard enthalpy change of formation of a compound.

(i) Define the term standard enthalpy change of formation, making clear the meaning of standard in this context.

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(3)


(ii) Use the standard enthalpy changes of combustion, ΔHcӨ, given in the table below

to find the standard enthalpy change of formation for ethanoic acid, CH3COOH, in

kJ mol–1.

Substance ΔHcӨ

/ kJ mol–1

C(s, graphite) –394

H2(g) –286

CH3COOH(l) –870

2C(s, graphite) + 2H2(g) + O2(g) → CH3COOH(l)

(3)(Total 15 marks)

14. In the reaction profile below, which energy change would alter if a catalyst was added to the reaction?


A

B

C

D

(Total 1 mark)

15. Which equation represents the reaction for which the enthalpy change is the lattice energy of sodium fluoride, NaF?

A Na(s) + ½F2(g) → NaF(s)

B Na(g) + F(g) → NaF(s)

C Na+(g) + F–(g) → NaF(s)

D Na(g) + ½F2(g) → NaF(s)

(Total 1 mark)

16. Theoretical lattice energies can be calculated from electrostatic theory. Which of the following

affects the magnitude of the theoretical lattice energy of an alkali metal halide, M+X–?

A The first electron affinity of X.

B The first ionization energy of M.

C The enthalpy of atomization of M.

D The radius of the X– ion.

(Total 1 mark)


17. This question is about some standard enthalpy changes, ΔHӨ

A enthalpy of reaction

B enthalpy of combustion

C mean bond enthalpy

D bond enthalpy

(a) Which enthalpy change is represented by p?

CH4 (g) → CH3(g) + H(g) ΔHӨ= p

A

B

C

D

(1)

(b) Which enthalpy change is represented by q?

CH4 (g) → C(g) + 4H(g) ΔHӨ= 4q

A

B

C

D

(1)


(c) Which enthalpy change is represented by r?

H2C=CH2(g) + ½O2(g) → ΔHӨ= r

A

B

C

D

(1)(Total 3 marks)

18. Which of the equations shown below represents the reaction for which DH is the standard

enthalpy change of formation, DHοf 298, for ethanol, C2H5OH. Ethanol melts at 156 K and boils

at 352 K.

A 2C(g) + 6H(g) + O(g) C2H5OH(g)

B 2C(s) + 3H2(g) + O2(g) C2H5OH(l)

C 2C(s) + 3H2(g) + O(g) C2H5OH(g)

D 2C(s) + 3H2(g) + ½O2(g) C2H5OH(l)

(Total 1 mark)

19. Airbags, used as safety features in cars, contain sodium azide, NaN3. An airbag requires a large volume of gas to be produced in a few milliseconds. The gas is produced in this reaction:

2NaN3(s) 2Na(s) + 3N2(g) DH is positive

When the airbag is fully inflated, 50 dm3 of nitrogen gas is produced.


(a) Calculate the number of molecules in 50 dm3 of nitrogen gas under these conditions.

[The Avogadro constant = 6.02 × 1023 mol–1. The molar volume of nitrogen gas under the

conditions in the airbag is 24 dm3 mol–1].

(2)

(b) Calculate the mass of sodium azide, NaN3, that would produce 50 dm3 of nitrogen gas.

(3)

(c) What will happen to the temperature in the airbag when the reaction occurs?

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(1)


(d) The airbag must be strong enough not to burst in an accident. An airbag which has burst in an accident is hazardous if the sodium azide in it has decomposed.

Explain why this is so.

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(2)(Total 8 marks)

20. A student investigated a reaction which could be used to warm up coffee in self-heating cans.

Mg(s) + Cu(NO3)2(aq) Mg(NO3)2(aq) + Cu(s)

In the self-heating cans, the bottom has a compartment containing copper(II) nitrate solution. When a button on the bottom of the can is pressed, the magnesium powder is released into the compartment where it reacts with the copper(II) nitrate solution.

(a) A student investigated the enthalpy change for this reaction by measuring

50.0 cm3 of 0.300 mol dm–3 copper(II) nitrate solution into a 100 cm3 beaker and adding 1g (an excess) of magnesium powder.

The results are shown below.

Temperature of copper(II) nitrate solution at start = 22 °CTemperature of mixture after reaction = 43 °C

(i) Calculate the energy change which took place. The specific heat capacity of the

solution is 4.20 J g–1 K–1.

Which is the correct value for the energy change in joules?

(1)


(ii) How many moles of copper(II) nitrate were used in the experiment?

(1)

(iii) Calculate the enthalpy change for the reaction. You should include a sign and units in your answer.

(2)

(iv) Suggest two changes you would make to the equipment used in order to improve the accuracy of the result.

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(2)


(b) The ionic equation for the reaction is shown below:

Mg(s) + Cu2+(aq) Mg2+(aq) + Cu(s) DH= –532 kJ mol–1

Would the following affect the value of the experimental result?

Explain your answer, stating the effect, if any, on the value of the enthalpy change obtained.

(i) The student used 2 g rather than 1g of magnesium.

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(2)

(ii) The heat losses that occurred from the student’s beaker.

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(2)

(c) The temperature in the self-heating can needs to increase by 60 °C to produce a hot drink.

Suggest a change you could make to the mixture in the experiment in (a) to produce a greater temperature rise. You are not expected to do a calculation.

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(1)(Total 11 marks)


21. The following data can be used in a Born-Haber cycle for copper(II) bromide, CuBr2.

Enthalpy change of atomisation of bromine DHοat[½Br2(l)] +111.9 kJ mol–1

Enthalpy change of atomisation of copper, DHοat[Cu(s)] +338.3 kJ mol–1

First ionisation energy of copper, Em1[Cu(g)] +746.0 kJ mol–1

Second ionisation energy of copper, Em2 [Cu(g)] +1958.0 kJ mol–1

Electron affinity of bromine, Eaff[Br(g)] –342.6 kJ mol–1

Enthalpy change of formation of CuBr2(s), DHοf [CuBr2(s)] –141.8 kJ mol–1

(a) On the following outline of a Born-Haber cycle complete the boxes A, B, and C by putting in the formula and state symbol for the appropriate species and writing the name of the enthalpy change D.

C u ( g ) C u B r2 +2 ( s )2 B r ( g )–

C u ( s ) + B r 2 ( l )

B

A

C D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

+

(3)

(b) Use the data to calculate a value for the lattice energy of copper(II) bromide.

Give a sign and units in your answer.

(3)


(c) When the lattice energy of copper(II) bromide is calculated from ionic radii and charges, the result is a value numerically about 10% less than the one obtained from the Born-Haber cycle.

(i) What does this suggest about the nature of the bonding in copper(II) bromide?

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(1)

(ii) Draw a diagram to show how the smaller copper ion alters the shape of the larger bromide ion.

(1)(Total 8 marks)


22. An experiment was carried out to measure the enthalpy change for the reaction of zinc with aqueous copper(II) sulphate.

The equation for the reaction is

Zn + CuSO4 → ZnSO4 + Cu

• A measuring cylinder was used to transfer separate 50 cm3 samples of 1.25 mol dm–3 copper(II) sulphate solution into polystyrene cups.

• Weighed amounts of zinc powder were added to each sample in turn.

• Each mixture was stirred thoroughly and the temperature rise noted with a thermometer accurate to 0.5 °C.

The results of this experiment are summarised on the graph below.

0 . 0 0 1 . 0 0 2 . 0 0 3 . 0 0 4 . 0 0 5 . 0 0 6 . 0 0 7 . 0 0

7 0 . 0

6 0 . 0

5 0 . 0

4 0 . 0

3 0 . 0

2 0 . 0

1 0 . 0

0 . 0 ×

×

× ×

×

×

×

×

×

×

×

× ×

M a s s o f z i n c / g

T e m p e r a t u r ec h a n g e / º C

(a) Explain why the graph initially shows a rise in temperature and then levels off.

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(2)


(b) (i) Suggest why the mass of metal is not used in the calculation of the heat change.

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(1)

(ii) The graph shows that the maximum temperature change is 63.5 °C. Use this value to calculate the maximum heat change, in joules, in this reaction.

You should assume that the density of the solution is 1.00 g cm–3 and its heat

capacity is the same as water, 4.18 J g–1 °C–1.

(1)

(iii) From the heat change calculated in (b)(ii) calculate the enthalpy change, in kJ mol–

1, for the reaction. Include the appropriate sign and give your answer to three significant figures.

(4)


(c) (i) It is suggested that the precision of the experiment would be improved by using a thermometer accurate to 0.1 °C.

Explain why this suggestion is incorrect.

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(1)

(ii) Suggest a simple practical change to the method that would make the experiment more accurate.

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(1)(Total 10 marks)

23. (a) The following data were collected to use in a Born-Haber cycle for silver fluoride, AgF.

Value

/kJ mol–1

enthalpy of atomisation of silver +285

first ionisation energy of silver +731

enthalpy of atomisation of fluorine +79

enthalpy of formation of silver fluoride –205

lattice energy of silver fluoride –958


On the following outline of a Born-Haber cycle, complete boxes A and B by adding the formula and state symbol for the appropriate species. Write the name of the enthalpy change at C.

A g ( g )+

A g ( s ) + ½ F 2 ( g )

F ( g )– A g F ( s )+

B o x B

B o x A

C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(3)

(b) ΔHlatt (theoretical) is the lattice energy calculated assuming the crystal lattice is completely ionic.ΔHlatt (experimental) is the lattice energy determined experimentally using the Born-Haber cycle.

Values for the silver halides are listed below.

Formula of halide ΔHlatt

(theoretical)

/ kJ mol–1

ΔHlatt

(experimental)

/ kJ mol–1

ΔHlatt (theoretical)minus

ΔHlatt (experimental)

/ kJ mol–1

AgF –920 –958 38

AgCl –833 –905 72

AgBr –816 –891 75

AgI –778 –889 111


(i) Explain why the theoretical lattice energies become less exothermic from AgF to AgI.

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(3)

(ii) Explain why the values of the theoretical and experimental lattice energies are different.

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(2)

(iii) Explain why the difference between the theoretical and experimental lattice energies increases from AgF to AgI.

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(2)(Total 10 marks)


24. (i) Use the data below to calculate a value for the enthalpy change of solution, ΔHsolution, for silver fluoride.

Value

/kJ mol–1

lattice energy of AgF (s) –958

enthalpy of hydration of Ag+ (g) –464

enthalpy of hydration of F– (g) –506

(2)

(ii) Use your answer to part (c)(i) to suggest whether you would expect silver fluoride, AgF, to be soluble or insoluble in water at room temperature.

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(2)(Total 4 marks)


25. The enthalpy change for some reactions cannot be determined directly.

One such reaction is the thermal decomposition of potassium hydrogencarbonate, which in a closed system at 200 °C is an equilibrium reaction.

2KHCO3 K2CO3 + H2O + CO2

However, by determining the enthalpy change for the neutralisation of the two potassium salts with hydrochloric acid, ΔH for the reaction above can be found. The equations for the neutralisation reactions are:

K2CO3 + 2HCl → 2KCl + H2O + CO2 ΔH1

KHCO3 + HCl → KCl + H2O + CO2 ΔH2

ΔH1 and ΔH2 for the neutralisation reactions were determined as follows:

• 30 cm3 of 2 mol dm–3 hydrochloric acid (an excess) was placed in a polystyrene cup, and its temperature measured to the nearest 0.1°C.

• A weighed quantity of the potassium salt (either the carbonate or the hydrogencarbonate) was added to the acid with rapid stirring, and the temperature measured again when the reaction was complete.

For the neutralisation using potassium carbonate, the results were as follows:

Amount of potassium carbonate used = 0.0187 molInitial temperature = 23.7 °CFinal temperature = 30.1 °C

(a) State Hess’s Law.

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(1)


(b) Use the data for the neutralisation of potassium carbonate to calculate the value of ΔH1 totwo significant figures. Remember to include a sign and units in your answer.

[Assume that the heat capacity of the solution is 4.18 J g–1 °C–1, and that it has a mass of 30 g.]

(3)

(c) (i) Show how the two equations for the neutralisation reactions and their ΔH values can be combined to find a value of ΔH for the thermal decomposition of potassium hydrogencarbonate.

Calculate this enthalpy change using your value for ΔH1 from part (b), given that

ΔH2 = + 29.3 kJ mol–1.

(3)


(ii) Explain why you would need to include the enthalpy of vaporisation of H2O(l), in order to obtain an accurate value of the enthalpy of decomposition of potassium hydrogencarbonate.

2KHCO3(s) K2CO3(s) + H2O(g) + CO2(g)

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(1)

(d) State and explain the effect of a decrease in temperature on the value of the equilibrium constant for the decomposition reaction and hence on the composition of the equilibrium mixture.

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(2)(Total 10 marks)

26. The Hess cycle below can be used to find the enthalpy change, ∆Hr, for the reaction between hydrogen sulphide and sulphur dioxide, using standard enthalpy changes of formation.

S O 2 ( g ) + 2 H 2 S ( g ) 3 S ( s ) + 2 H 2 O ( l )

H r

H 2 H 1


(i) Complete the cycle by filling in the empty box.

(2)

(ii) What is meant by the standard enthalpy change of formation, ∆Hfο, of a compound?

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(3)

(iii) Use the cycle and the data below to calculate the enthalpy change of the reaction, ∆Hr.

∆Hfο / kJ mol–1

SO2 (g) –296.8

H2S (g) –20.6

H2O (l) –285.8

(2)(Total 7 marks)

27. This question is about ammonia, NH3, which is produced as shown in the following equation.

N2(g) + 3H2(g) 2NH3(g)

(a) Use oxidation numbers to explain why this is a redox reaction.

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(2)


(b) (i) Use the average (mean) bond enthalpy data to calculate a value for the enthalpy change for this reaction. You are reminded to show all your working.

BondAverage bond enthalpy

/ kJ mol–1

N≡N 944

H—H 436

N—H 388

(3)

(ii) The actual standard enthalpy change for this reaction is –92 kJ mol–1. Explain why the value you calculated in (b)(i) is not the same as this.

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(1)


(iii) At room temperature, a mixture of nitrogen and hydrogen is thermodynamically unstable with respect to ammonia, but is kinetically stable.

Use the data in (b)(i) and (ii) to help you explain why this mixture is

thermodynamically unstable

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kinetically stable

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(3)


(c) The manufacturer of ammonia would like to achieve a high rate of reaction and a high equilibrium yield of product.

(i) State and explain, in terms of collision theory, TWO ways to increase the rate of the reaction. An increase in pressure does not alter the rate in this process.

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(6)


(ii) State and explain TWO ways to increase the equilibrium yield of ammonia.

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(4)(Total 19 marks)

28. The enthalpy change for the reaction between aqueous sodium hydroxide solution and aqueous hydrochloric acid was determined by the following method:

• Aqueous hydrochloric acid was titrated against 25.0 cm3 of 1.50 mol dm–3 aqueous sodium hydroxide solution using a suitable indicator. The mean (or average) titre was

22.75 cm3.

• 25.0 cm3 of the sodium hydroxide solution was carefully measured into a polystyrene cup

and 22.75 cm3 of the hydrochloric acid was transferred to a clean dry beaker.Both solutions were allowed to stand for five minutes before their temperatures were noted.

• The hydrochloric acid was then added to the sodium hydroxide solution, the mixture stirred thoroughly and the highest temperature noted.

• The experiment was repeated three times giving an average temperature change of +10.5°C.


(a) (i) Calculate the heat produced in the reaction, in joules.

Use the approximations that the density of the final solution is 1.00 g cm–3 and its

specific heat capacity is 4.18 J g–1 K–1.

(2)

(ii) Calculate the enthalpy change for the reaction, in kJ mol–1.

(3)

(b) State ONE assumption made when calculating this enthalpy change, other than those stated in (a)(i).

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(1)(Total 6 marks)


29. The Born-Haber cycle below represents the enthalpy changes when calcium hydride, CaH2, is formed from its elements.

H 5

C a H ( s )2

C a 2 + ( g ) + 2 e – + 2 H ( g )

C a 2 + ( g ) + 2 e – + H 2 ( g )

C a ( g ) + H 2 ( g )

C a ( s ) + H 2 ( g )

C a 2 + ( g ) + 2 H – ( g )

H 1

H 2

H 3

H 4

Δ H 6

(a) Write down in terms of one of the symbols ΔH1 to ΔH6

(i) the lattice energy of calcium hydride ................................................................

(1)

(ii) the first electron affinity of hydrogen ................................................................

(1)


(b) Use the data below to calculate the standard enthalpy of formation of calcium hydride, CaH2(s).

value

/kJ mol–1

enthalpy of atomisation of calcium +178

first plus second ionisation energies of calcium +1735

enthalpy of atomisation of hydrogen +218

first electron affinity of hydrogen –73

lattice energy of calcium hydride –2389

Calculation:

(2)

(c) Explain why the lattice energy of magnesium hydride, MgH2(s), is more exothermic than the lattice energy of calcium hydride, CaH2(s).

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(3)(Total 7 marks)


30. Two experiments were carried out in order to calculate the enthalpy change of formation of magnesium carbonate, MgCO3.

A Hess cycle for these reactions is shown below.

H 3

E x p e r i m e n t 1

M g + C + O 2 M g C O ( s )3

M g C l 2 ( a q ) + H 2 ( g ) + C + O 2

H f

H 1

H 2– 1= – 6 8 0 k J m o l

+ 2 H C l ( a q )

+ 2 H C l ( a q ) E x p e r i m e n t 2

M g C l 2 ( a q ) + H 2 O ( l ) + C O 2 ( g )

(a) Complete the Hess cycle above for the formation of magnesium carbonate from its elements by balancing the equations and adding state symbols.

(2)

(b) In Experiment 1 the temperature of 100 cm3 of hydrochloric acid was measured.After one minute, 0.100 g of magnesium was added to the excess acid and the temperature measured every minute. The following results were obtained:

Time / min 0 1 2 3 4 5 6

Temp / °C 21.0 21.0 25.3 25.1 24.9 24.8 24.7

(i) How many moles of magnesium were used in this experiment?

Use the Periodic Table as a source of data.

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(1)


(ii) The initial concentration of the hydrochloric acid was 2.00 mol dm–3.

Calculate the number of moles of hydrochloric acid at the start and hence the number remaining at the end of the experiment.

(3)

(iii) Plot the graph of temperature against time.

(2)


(iv) Calculate the energy change in this experiment assuming the temperature rise is 4.5 °C. Use the expression

Energy change (J) = 4.2 × mass of solution × temperature change

[Assume that 1 cm3 of solution has a mass of 1 g]

(1)

(v) Use your answer to (iv) to calculate ∆H1 for one mole of magnesium reacting with hydrochloric acid. Include a sign and units in your answer.

(2)

(vi) Suggest why a temperature rise of 4.5 °C was used in the calculation in (iv).

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(1)


(c) 2.2 g of magnesium carbonate was added to 100 cm3 of the same acid in Experiment 2.

The temperature changed from 21.0 °C to 23.5 °C resulting in an energy change of 1.05 kJ.

(i) Calculate the mass of one mole of magnesium carbonate, MgCO3 and hence the number of moles of magnesium carbonate used in this experiment.Use the Periodic Table as a source of data.

(2)

(ii) Using the method in part (b)(v), calculate ∆H3.

(1)


(d) Using your answers to (b)(v) and (c)(ii), calculate the enthalpy change of formation, ∆Hf, of magnesium carbonate, MgCO3.Include a sign and units in your answer.

(2)

(e) Why is it impossible to measure ∆Hf of MgCO3(s) directly?

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(1)(Total 18 marks)

31. Calculate the standard enthalpy change of formation of gaseous silicon tetrachloride,

ΔHοf [SiCl4(g)].

Your answer should include a sign and units.

Use the Hess cycle below and the following data at 298 K.

oatH [Si(s)] = +455.6 kJ mol–1

oatH [½Cl2] = +121.7 kJ mol–1

Bond energy, E (Si-Cl) = +407.4 kJ mol–1

H

S i C l 4 ( g ) S i ( g ) + 4 C l ( g )

H f [ S i C l 4 ( g ) ]

S i ( s ) + 2 C l 2 ( g )

(Total 3 marks)


32. (a) When excess chlorine and methane are mixed at room temperature and pressure no reaction takes place but when ultraviolet light is shone into the mixture an explosion occurs, producing carbon and hydrogen chloride.

CH4(g) + 2Cl2(g) uv C(s) + 4HCl(g) ∆H = –219 kJ mol–1

Calculate the mass of methane needed to produce 1000 kJ of energy.

(2)


(b) Draw a labelled reaction profile for the reaction between methane and chlorine and use it to explain why the reaction does not take place unless ultraviolet light is present.

P r o g r e s s o f r e a c t i o n

E n t h a l p y

Explanation

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(5)(Total 7 marks)

33. (a) Define the term standard enthalpy of combustion.

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(3)


(b) The following standard enthalpies of combustion are needed to calculate the standard enthalpy of formation of ethanol, C2H5OH.

SubstanceStandard enthalpy of

combustion /kJ mol–1

carbon, C (s, graphite) –394

hydrogen, H2(g) –286

ethanol, C2H5OH (l) –1371

(i) Complete the Hess’s Law cycle by filling in the box and labelling the arrows with the enthalpy changes.

H

2 C ( s ) + 3 H 2 ( g ) + 3 ½ O 2 ( g ) C 2 5H O H ( l ) + 3 O 2 ( g )

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . + . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

f

(3)

(ii) Use your Hess’s Law cycle to calculate the standard enthalpy of formation of ethanol.

(2)(Total 8 marks)


34. The apparatus used and the recordings made by a student, carrying out an experiment to determine the enthalpy of combustion of methanol, are shown below.

Diagram

t h e r m o m e t e r

b e a k e r

w a t e r

m e t h a n o l

s p i r i t l a m p

Results

Molar mass (methanol) = 32 g mol–1

Volume of water in beaker = 50 cm3

Mass of water in beaker = 50 g

WeighingsSpirit lamp + methanol before combustion = 163.78 gSpirit lamp + methanol after combustion = 163.44 g

TemperaturesWater before heating = 22.0 °CWater after heating = 43.5 °C

Specific heat capacity of water = 4.18 J g–1 °C–1

Observations

• When the spirit lamp was being weighed its mass was continually falling.

• A black substance formed on the bottom of the beaker as the methanol burned.


(a) (i) Calculate the amount (moles) of methanol, CH3OH, burned.

(2)

(ii) Calculate the heat gained by the water. Give your answer in kJ.

(2)

(iii) Use your values from (i) and (ii) to calculate the enthalpy of combustion of

methanol in kJ mol–1. Include a sign with your answer.

∆H = ........................................ kJ mol–1

(2)

(b) (i) The thermometer used in the experiment can be read to an accuracy of ±0.5 °C.Calculate the percentage error in the temperature change.

(1)



(ii) Calculate the maximum temperature change that could have occurred during the experiment.

(1)

(c) (i) Give a reason why the mass of the spirit lamp fell as it was being weighed.

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(1)

(ii) Suggest the identity of the black substance that forms on the beaker. State the effecton the value of the enthalpy of combustion obtained.

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(2)(Total 11 marks)


35. (a) An incomplete Born-Haber cycle for the formation of magnesium oxide, MgO, from its

constituent elements is shown below. All numerical values are in kJ mol–1.

l a t t i c e e n e r g y o fm a g n e s i u m o x i d e

f i r s t p l u s s e c o n d i o n i s a t i o n e n e r g y o f m a g n e s i u m = + 2 1 8 6

e n t h a l p y o f a t o m i s a t i o n o f o x y g e n = + 2 4 9

M g 2 + ( g ) + O ( g ) + 2 e –

2 +M g ( g ) + 12 O 2 ( g ) + 2 e –

H = – 6 0 2

H = + 1 5 0

M g ( g ) + 12 O ( g )2

M g ( s ) + 12 O ( g )2

M g O ( s )

H = + 6 5 73

1

2

(i) Complete the empty box on the cycle by writing in the formulae of the missing species. State symbols are required.

(1)

(ii) Identify each of the following enthalpy changes by name:

ΔH1 ....................................................................................................................

ΔH2 ....................................................................................................................

ΔH3 ....................................................................................................................

(3)


(iii) Use the Born-Haber cycle to calculate the lattice energy of magnesium oxide.

(2)

(b) Magnesium iodide is another compound of magnesium. The radius of the magnesium ion is 0.072 nm, whereas the radius of the iodide ion is much larger and is 0.215 nm.

(i) Describe the effect that the magnesium ion has on an iodide ion next to it in the magnesium iodide lattice.

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(1)

(ii) What TWO quantities must be known about the ions in a compound in order to calculate a theoretical lattice energy?

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(2)


(iii) Suggest how the value of the theoretical lattice energy would compare with the experimental value from a Born-Haber Cycle for magnesium iodide.

Give a reason for your answer.

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(2)(Total 11 marks)

36. (i) Define the term enthalpy of hydration, ΔHhyd, of an ion.

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(2)


(ii) The table below gives some information about the sulphates of the Group 2 elements magnesium and barium.

sulphate lattice energy

/ kJ mol–1

hydrationenthalpy of cation

/ kJ mol–1

solubility

/ mol dm–3

MgSO4 –2874 –1920 1.83

BaSO4 –2374 –1360 9.43 × 10–6

Use the lattice energy and hydration enthalpy values to explain the difference in the solubility of the two salts.

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(4)(Total 6 marks)


37. Ammonia can be oxidised with oxygen in two ways.

Reaction I: 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g),

Reaction II: 4NH3(g) + 3O2(g) → 2N2(g) + 6H2O(g)

The first reaction is used in the conversion of ammonia to nitric acid.

(a) Define the term enthalpy of formation.

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(2)

(b) The following enthalpies of formation are needed for this question.

ΔHf / kJ mol–1

NH3(g) – 46.1

NO(g) + 90.2

H2O(g) – 242

(i) Calculate the enthalpy change for Reaction I.

(2)


(ii) Calculate the enthalpy change for Reaction II.

(1)

(c) Whenever ammonia is oxidised, the two reactions occur at the same time: they are competing reactions.

(i) Suggest, with a reason, which reaction you would expect to be more likely thermodynamically.

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(1)

(ii) Explain how the use of a catalyst can favour Reaction I over Reaction II.

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(2)

(d) In the nitration of benzene, a mixture of concentrated nitric and sulphuric acids is used at a temperature not exceeding 55 °C.

(i) What is the reason for using sulphuric as well as nitric acid? Support your answer with an equation.

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(2)



(ii) On the basis of your answer to (d)(i), which of nitric and sulphuric acid is the stronger acid? Give a reason.

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(1)

(iii) Give the mechanism for the nitration of benzene.

(3)

(iv) If the temperature of the reaction mixture rises much above 55 °C, other compounds are formed in addition to nitrobenzene.

Suggest the structural formulae of ONE of these compounds.

(1)

(v) Explain, in terms of structure and energetics, why benzene usually reacts via substitution rather than addition.

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(3)(Total 18 marks)



38. Calcium hydroxide decomposes on strong heating to form calcium oxide and water.

Ca(OH)2(s) → CaO(s) + H2O(l)

Two samples of calcium hydroxide were taken, each weighing exactly 1.00 g.

The first sample was cautiously added to 25.0 cm3 of dilute hydrochloric acid contained in a glass beaker. The temperature rise was measured and found to be 16.5 °C.

The other sample was heated for some time. It was then allowed to cool and then added to another 25.0 cm3 portion of hydrochloric acid as before. In this case the temperature rose by 25.5 °C.

In both cases, the acid used was an excess.

(a) (i) Calculate the energy produced by the reaction of each solid with the acid.

Use the relationship

Energy produced = mass of solution × 4.2 × temperature rise

/ J / g / J °C–1 g–1

You may assume that 1.0 cm3 of solution has a mass of 1.0 g. Ignore the mass of the solid.

For the solid calcium hydroxide

For the solid calcium oxide

(1)


(ii) How many moles of calcium hydroxide were used in each experiment?

[Molar mass of Ca(OH)2 = 74.0 g mol–1]

(1)

(iii) Using your answers to (a)(i) and (ii), calculate the enthalpy changes for each reaction.

Give your answers to two significant figures. Include the sign and units for each answer.

For the solid calcium hydroxide, ΔH1

For the solid calcium oxide, ΔH2

(2)

(b) A Hess cycle for all these reactions is shown below.

C a ( O H ) 2 ( s ) C a O ( s ) + H 2 O ( l )

C a C l 2 ( a q ) + 2 H 2 O ( l )

H r e a c t i o n

H 1 H 2

2 H C l ( a q ) 2 H C l ( a q )


(i) Use this Hess cycle and your answers in (a)(iii) to calculate ΔHreaction. Include a sign and units.

(2)

(ii) Apart from the approximations involved in using the equation given in (a)(i), give TWO other potential sources of error which are likely to affect the accuracy of the results.

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(2)

(iii) Suggest why ΔHreaction is difficult to determine directly by experiment.

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(1)(Total 9 marks)


39. (a) Define the term standard enthalpy of formation.

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(3)

(b) In the Haber process, ammonia is manufactured from nitrogen and hydrogen as shown in the equation.

N2(g) + 3H2(g) 2NH3(g)

(i) Use the bond enthalpies below to calculate the standard enthalpy of formation of ammonia.

Bond Bond enthalpy / kJ mol–1

N≡N in N2 +945

H–H in H2 +436

N–H in NH3 +391

(4)


(ii) Draw a labelled enthalpy level diagram for the formation of ammonia in the Haber process.

E n t h a l p y

(2)

(iii) State the temperature used in the Haber process and explain in terms of the rate of reaction and position of equilibrium, why this temperature is chosen.

Temperature .......................

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(3)


(iv) Identify the catalyst used in the Haber process and state what effect, if any, it has on the equilibrium yield of ammonia.

Catalyst ................................................................

Effect on yield ...................................................................................................

(2)

(v) Explain why it is necessary to use a catalyst in this process.

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(1)

(c) The pressure used in the Haber process is 250 atmospheres.

(i) State and explain an advantage of increasing the pressure to 1000 atmospheres.

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(2)

(ii) Suggest a disadvantage of using a pressure of 1000 atmospheres.

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...........................................................................................................................

...........................................................................................................................

(1)(Total 18 marks)



40. The enthalpy change for the reaction of anhydrous aluminium chloride, AlCl3, with water can befound as follows:

• Add about 100 cm3 of distilled water to a weighed polystyrene cup.

• Measure the steady temperature of the water.

• Add anhydrous aluminium chloride to the polystyrene cup, with stirring.

• Measure the highest temperature reached.

• Re-weigh the polystyrene cup and contents

DataMass of anhydrous aluminium chloride = 4.00 gMass of solution = 104 g

Initial temperature = 17.5 °CHighest temperature reached = 43.5 °C

Specific heat capacity of the solution = 4.09 J g–1 °C–1

(i) Calculate the heat change in this experiment.

(2)

(ii) Assuming that 100 cm3 of water is a large excess, calculate the enthalpy change, in

kJ mol–1, when one mole of aluminium chloride reacts. Include a sign and unit in your answer.

(3)(Total 5 marks)



41. (a) (i) Write the equation which represents the change occurring when the standard enthalpy of atomisation of bromine is measured.

...........................................................................................................................

(2)

(ii) Define lattice energy.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(3)

(b) A Born-Haber cycle for the formation of magnesium chloride is shown below.

M g 2 + ( g ) + 2 C l ( g ) + 2 e –

M g 2 + ( g ) + C l 2 ( g ) + 2 e – 2 +M g ( g ) + 2 C l – ( g )

M g + ( g ) + C l 2 ( g ) + e –

M g ( g ) + C l 2 ( g )

M g ( s ) + C l 2 ( g )

M g C l ( s )2

Value / kJ mol–1

Enthalpy of atomisation of magnesium +150

1st ionisation energy of magnesium +736

2nd ionisation energy of magnesium +1450

Enthalpy of atomisation of chlorine +122

Enthalpy of formation of magnesium chloride –642


Lattice energy of magnesium chloride –2526


The theoretically calculated value for the lattice energy of magnesium chloride is

–2326 kJ mol–1.

Explain, in terms of the bonding in magnesium chloride, why the experimentally

determined value of –2526 kJ mol–1 is significantly different from the theoretical value.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(2)

(c) The table shows values for the lattice energies of the metal chlorides of some Group 2 metals.

Group 2 metalchloride

MgCl2 CaCl2 SrCl2 BaCl2

Lattice energy/

kJ mol–1 –2526 –2237 –2112 –2018

Explain why these lattice energies become less exothermic from MgCl2 to BaCl2.

(3)(Total 10 marks)


42. An experiment was carried out to find the enthalpy change for the reaction of zinc powder with copper(II) sulphate solution.

Zn(s) + CuSO4(aq) ZnSO4(aq) + Cu(s)

50cm3 of copper(II) sulphate solution, of concentration 1.0 mol dm–3, was put into a polystyrenecup and the temperature of the solution measured. After one minute, 5.0 g of zinc powder was added, the mixture stirred with a thermometer and the temperature measured every 30 s.

(a) (i) What is meant by a spectator ion?

...........................................................................................................................

...........................................................................................................................

(1)

(ii) Give the formula of the spectator ion in this reaction.

...........................................................................................................................

(1)

(iii) Write the equation for this reaction, omitting the spectator ion.

(1)

(b) How would you measure the 50 cm3 of copper(II) sulphate solution?

.....................................................................................................................................

.....................................................................................................................................

(1)


(c) Give TWO reasons why it is better to use a polystyrene cup, rather than a metal container,to obtain more accurate results.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(2)

(d) Calculate the number of moles of each of the reactants and hence deduce which reactant is completely used up.Use the Periodic Table as a source of data.

Moles of zinc powder

Moles of copper(II) sulphate

Reactant used up .................................................................

(3)

(e) The following results were obtained.

Time /s 0 60 90 120 150 180 210

Temperature /°C 22 22 60 65 63 61 59


(i) On the graph paper below, plot the results of this experiment.

7 0

6 0

5 0

4 0

3 0

2 0

1 0

0

T e m p e r a t u r e/ º C

0 4 0 8 0 1 2 0 1 6 0 2 0 0 2 4 0

T i m e / s

(2)

(ii) Explain the shape of your graph

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)



(iii) The maximum recorded temperature in this experiment was 65°C. Use your graph to estimate a more accurate maximum temperature.

...........................................................................................................................

(1)

(f) (i) Calculate the energy change in this experiment using your answer to (e)(iii) and therelationship

energy change = mass of × specific heat capacity × temperature risesolution of solution

/J /g /J °C–1 g–1 /°C

You may assume that

• 1.0 cm3 of solution has a mass of 1.0 g

• The specific heat capacity of the solution is 4.2 J °C–1 g–1

(1)

(ii) Use your answers to (d) and (f)(i) to calculate ∆H for this reaction. Include a sign and units in your answer.

(3)(Total 18 marks)


43. This question is about the chemistry of propane, C3H8.

Propane is sold for use as a fuel for camping stoves. On complete combustion it forms carbon dioxide and water.

(a) The enthalpy change of combustion of propane, ΔHc, can be measured by burning a known mass of propane below a container of water and measuring the temperature rise ofthe water.

The heat capacity of the apparatus (the energy required to raise the temperature of the apparatus by 1 °C) is found by calibrating it with a fuel with known enthalpy change of combustion.

The results of an experiment are shown below.

Mass of propane burned 0.500 gTemperature of water at start 21.0 °CFinal temperature of water 39.0 °C

Heat capacity of apparatus 1.35 kJ °C–1

(i) Calculate the number of kilojoules of energy transferred when the 0.500 g sample of propane burns in this experiment.

(1)


(ii) Use your answer to (i) to calculate ΔHc for propane in kJ mol–1. Give your answer to three significant figures.


(2)

(iii) The Book of data gives the value of ΔHc for propane as –2220 kJ mol–1.

Calibrating the apparatus means that the answer you calculated in (ii) allows for errors due to heat loss.

Suggest the other main source of error which makes the experimental result different from the data book value.

...........................................................................................................................

...........................................................................................................................

(1)


(b) A value of ΔHc for propane can be calculated using mean bond energies and the Hess cycle below.

(i) Complete the Hess cycle, and use the mean bond energies to calculate ΔH1.Hence calculate ΔHc.

Mean bond energies

/kJ mol–1

C=O 805

H–O 464

H c

C H3 8 ( g ) + . . . . . . . . . . O 2 ( g ) 3 C O 2 ( g ) + 4 H 2 O ( g )

+ 6 4 9 0 k J m o l – 1 H 1 = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 C ( g ) + 8 H ( g ) + . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ΔHc = ................................................................................................................

(3)

(ii) Give ONE reason why the value you calculated in (b)(i) also differs from the value for the heat of combustion of propane in the Book of data.

...........................................................................................................................

...........................................................................................................................

(1)

(c) When propane reacts with chlorine in the presence of ultraviolet light one of the products is 2-chloropropane.

(i) Name the mechanism and type of this reaction.

Mechanism .......................................................................................................

Type ..................................................................................................................

(2)



(ii) In this reaction a small quantity of an alkane, C6H14, is produced.

Explain how this occurs. Include an equation in your answer.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)

(d) 2-Chloropropane and 2-iodopropane are both colourless liquids at room temperature.They can be distinguished by their reactions with aqueous silver nitrate.

(i) What would you see when the reaction is carried out with each halogenoalkane?

2-chloropropane ...............................................................................................

2-iodopropane ..................................................................................................

(2)

(ii) Write an ionic equation showing how silver ions react in the mixture made from 2-iodopropane and aqueous silver nitrate. Include state symbols in your answer.

(2)

(iii) Both 2-chloropropane and 2-iodopropane form the same organic product in the reaction with aqueous silver nitrate.

Name, or give the structural formula of, this organic product.

...........................................................................................................................

(1)(Total 17 marks)


44. This question is about some of the chemicals used in car engines and their reactions.

(a) Compound X, shown below, is one component of petrol.

H — C — — C — — C — — C — — C — H

H C H H H H

H C H H HC H

3

3 3

(i) Name X.

...........................................................................................................................

(1)

(ii) Give the empirical formula of X.

...........................................................................................................................

(1)

(iii) X can be made by cracking decane, C10H22.

Assuming only one other product forms in a cracking reaction, deduce the molecular formula of this other product.

(1)

(iv) What is the sign of the enthalpy change for the reaction in which decane is cracked? Give a reason for your answer.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(1)


(v) If the air supply in a car engine is poor, there is not enough air for carbon dioxide to be produced.

Use this information to suggest ONE possible equation for the combustion of X in this engine. Use the molecular formula of X in your equation.

(2)

(b) When air enters a car engine, as well as the fuel burning, nitrogen and oxygen can react toform nitrogen(II) oxide.

N2(g) + O2(g) 2NO(g) ΔH = + 180 kJ mol–1

(i) What, if any, is the effect on the percentage of nitrogen(II) oxide in an equilibrium mixture of these three gases if the pressure and temperature are increased?Explain your answers.

Increase in pressure

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

Increase in temperature

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)

(ii) In a car exhaust pipe, nitrogen(II) oxide passes over a catalytic converter.The following reaction occurs.

2NO(g) + 2CO(g) → N2(g) + 2CO2(g) ΔH = –746 kJ mol–1

Explain why this reaction speeds up when the car engine has been running for a few minutes.

...........................................................................................................................

...........................................................................................................................

(1)



(iii) A textbook says “The catalytic converter converts the gases coming out of the engine into less harmful ones”.

State, with a reason, which of the four gases in the equation in (ii) you consider to be least harmful.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(1)

(iv) The diagram below shows the reaction profile for the change which occurs in the catalytic converter.

E n e r g y

2 N O ( g ) + 2 C O ( g )

N ( g ) + 2 C O ( g )2 2

P r o g r e s s o f r e a c t i o n

On the diagram, show the activation energy, EA.

Add a line showing the reaction profile if no catalyst is present.

(2)(Total 12 marks)



.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(3)

(b) The dissociation of phosphorus pentachloride is a reversible reaction.

PCl5(g) PCl3(g) + Cl2(g)

(i) Use the values of enthalpy of formation given to calculate ∆H for the forward reaction.

∆Hf /

kJ mol–1

PCl5(g) – 399

PCl3(g) – 306

(1)

(ii) Explain, with reasons, the effect that raising the temperature would have on the composition of the equilibrium mixture.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)


(iii) Other than by changing the temperature, suggest how the amount of PCl5 present atequilibrium could be increased. Give a reason for your answer.

...........................................................................................................................

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(2)(Total 8 marks)

46. The following data relate to the thermodynamic changes which occur when Group 2 hydroxidesdissolve in water.

Enthalpy of hydration of Mg2+ –1890 kJ mol–1

Enthalpy of hydration of Ba2+ –1275 kJ mol–1

Enthalpy of hydration of OH– –550 kJ mol–1

Lattice energy of Mg(OH)2 –2995 kJ mol–1

Lattice energy of Ba(OH)2 –2320 kJ mol–1

(a) (i) Define the term enthalpy of hydration.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)


(ii) Explain why this enthalpy change is always exothermic.

...........................................................................................................................

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...........................................................................................................................

(2)

(b) Why is the lattice energy of magnesium hydroxide more exothermic than that of barium hydroxide?

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(3)


(c) (i) Draw a labelled Hess’s law cycle to show how the lattice energy and the enthalpies of hydration are related to the enthalpy of solution of magnesium hydroxide, Mg(OH)2(s).

(3)

(ii) Use your cycle and the data to calculate the enthalpy of solution of magnesium hydroxide. Include a sign and units with your answer.

(2)


(d) Use the data to explain how the solubility of barium hydroxide compares with that of magnesium hydroxide.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

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(3)(Total 15 marks)

47. Iron is produced from the ore haematite, which contains iron(III) oxide.

Some standard enthalpies of formation are given below.

Compound ∆Hfο / kJ mol–1

Fe2O3 (s) –822

CO (g) –110

CO2 (g) –394


(a) (i) Write the equation for the complete reduction of iron(III) oxide by carbon monoxide.

Calculate the enthalpy change for this reaction.

(3)

(ii) Iron(III) oxide can also be reduced by carbon.

Fe2O3 + 3C → 2Fe + 3CO ∆Hο = +492 kJ mol–1

Explain whether the reduction process in (i) or (ii) is more likely to occur.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(1)


(b) On heating, the following exothermic reaction occurs

3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g)

At 600 °C, a mixture of iron and steam is allowed to reach equilibrium. The equilibrium partial pressures of hydrogen and steam are 1.6 atm and 1.2 atm respectively.

(i) Write the expression for the equilibrium constant, Kp, for the reaction. Calculate its value and state the units.

(2)

(ii) State the effect, if any, on the value of Kp when the temperature is increased. Justify your answer.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)


(c) When an iron nail is left in a test tube of water containing a little phenolphthalein, it starts

to react and region X becomes pink, due to the formation of OH– ions.

X

Y

Z

Write the half-equations for the reactions taking place in regions X and Y.

Name the green precipitate that forms in region Z.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

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.....................................................................................................................................

(3)


(d) Anhydrous iron(III) chloride is made by passing dry chlorine gas over heated iron. It is formed as a dark red covalent gas with formula Fe2Cl6 and has a similar structure and reactions to aluminium chloride.

Draw a diagram to show the structure of the Fe2Cl6 molecule. Label the types of bonding present.

State the shape around each iron atom.

(3)

(e) Hydrated iron(III) chloride is ionic and soluble in water.

(i) Describe a test for aqueous Fe3+ ions.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)

(ii) Explain why an aqueous solution of hydrated iron(III) chloride is acidic.

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(3)(Total 19 marks)



48. The enthalpy change for the thermal decomposition of calcium carbonate cannot be measured directly, but can be found by carrying out two reactions as shown in the Hess cycle below.

C a C O ( s ) C a O ( s ) + C O ( g ) H r e a c t i o n

3 2

H 3 H 4

E l e m e n t s i n t h e i r s t a n d a r d s t a t e s

(a) Suggest ONE reason why it is difficult to measure Hreaction directly by experiment.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(1)

(b) In an experiment to find H1 a student added 2.00 g of finely powdered calcium

carbonate to 20.0 cm3 of 2.50 mol dm–3 hydrochloric acid solution (an excess) in a polystyrene container. The temperature rose from 20.5 °C to 23.0 °C.

(i) Why is the calcium carbonate used in this experiment finely powdered, rather than in lumps? Explain why this is important for an accurate result.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)


(ii) Calculate the energy change using the relationship below.

Energy change = 4.2 × mass of solution × temperature change

/J /J g–1 K–1 /g /K

Assume that the mass of the solution is 20 g.

(1)

(iii) Calculate the enthalpy change, DH1. Include a sign and units in your answer.

[The molar mass of CaCO3 is 100 g mol–1]

(3)


(iv) In another experiment, the value of H2 was found to be –181 kJ mol–1.

Use this result and your answer to (iii) to calculate the value of Hreaction.

(2)

(c) The student checked the experimental results using information from the Book of data in another Hess cycle.

C a C O ( s ) C a C O ( s ) + C O ( g ) H r e a c t i o n

3 2

H 3 H 4


Name the enthalpy change represented by DH3.

.....................................................................................................................................

(1)(Total 10 marks)


49. The Hess cycle below can be used to estimate the enthalpy change of formation, DHf, of the unstable gaseous compound with the formula HOCl(g).

H O C l ( g )

H f


+ 5 8 9 k J m o l – 1

+ 6 6 7 k J m o l – 1

G a s e o u s a t o m s o f t h e e l e m e n t s

(a) (i) Insert formulae, with state symbols, into the appropriate boxes, to show the correct quantities of each element.

(1)

(ii) Use the cycle to calculate a value for the enthalpy change of formation, fH

[HOCl(g)].

(1)


(iii) Assuming that the H—O bond energy is +464 kJ mol–1, calculate a value for the O—Cl bond energy.

(1)


(b) (i) Draw a ‘dot and cross’ diagram for the HOCl molecule showing outer electrons only.

(2)

(ii) Predict the HOCl bond angle. Justify your answer.

Angle ................................................................................................................

Justification ......................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)


(c) HOCl(g) can be made from chlorine(I) oxide by the reversible reaction

Cl2O(g) + H2O(g) 2HOCl(g)

What effect, if any, would an increase in pressure have on the proportion of HOCl(g) at equilibrium? Justify your answer.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(2)(Total 9 marks)

50. One stage in the manufacture of sulphuric acid is the exothermic reaction

2SO2(g) + O2(g) 2SO3(g)

(a) In a closed container this mixture of gases would be in dynamic equilibrium.State the meaning of the words dynamic and equilibrium in this context.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(2)

(b) (i) State the conditions of temperature and pressure used industrially for the manufacture of SO3.

...........................................................................................................................

...........................................................................................................................

(2)


(ii) Justify the choice of temperature for this reaction in terms of yield and rate.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(3)

(iii) The yield of products would be greater if a higher pressure were to be used for the reaction.

Suggest a reason why a higher pressure than you have given in (i) is not used.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(1)

(c) (i) Calculate ΔH for the forward reaction, given the enthalpies of formation below.

ΔHf / kJ mol–1

SO2(g) –297

SO3(g) –395

O2(g) 0

(2)


(ii) State why the enthalpy of formation of oxygen, O2(g), is zero.

...........................................................................................................................

...........................................................................................................................

(1)

(d) (i) State the formula of the catalyst used in the industrial process.

...............................................................

(1)

(ii) Draw an enthalpy level diagram to show the reaction profiles of the uncatalysed and catalysed reactions.

(3)

(iii) Explain how the catalyst increases the reaction rate.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)



(e) Suggest why the sulphur trioxide produced is passed into concentrated sulphuric acid rather than water to form sulphuric acid at the end of the process.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(1)(Total 18 marks)

51. (a) State Hess’s Law.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(2)

(b) Methane burns in oxygen.

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

(i) Calculate the enthalpy change for this reaction, using the bond enthalpies given below.

Bond enthalpy

/ kJ mol–1

C – H +435

O = O +498

C = O +805

H – O +464

(3)

(ii) State the name of this enthalpy change.

...........................................................................................................................

(1)


(iii) The value of this enthalpy change, under standard conditions, is –890 kJ mol–1. State the meaning of standard conditions.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)

(iv) Suggest, with a reason, why the enthalpy change calculated in (i) is different from the standard value quoted in (iii).

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)

(c) Although the reaction between methane and oxygen is exothermic, it does not occur unless the mixture is ignited.

Use these facts to explain the difference between thermodynamic and kinetic stability.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

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(4)(Total 14 marks)



52. A student carried out an experiment to find the concentration of a solution of nitric acid and alsoits enthalpy of neutralisation.

• The solutions of nitric acid and sodium hydroxide were allowed to reach the same temperature.

• 50.0 cm3 of the nitric acid was pipetted into a polystyrene cup.

• A burette was filled with a solution of 2.0 mol dm3 sodium hydroxide, NaOH.

• The initial temperature of the acid was recorded.

• The sodium hydroxide was added to the acid in 5.0 cm3 portions.

• After each addition, the mixture was stirred and the maximum temperature recorded.

• This was repeated until 45 cm3 of the sodium hydroxide solution had been added.


The student plotted the results, as shown below.

3 0

2 8

2 6

2 4

2 2

2 0

1 8

1 60 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 4 5 5 0

T e m p e r a t u r e/ ° C

V o l u m e o f 2 . 0 m o l d m s o d i u m h y d r o x i d e s o l u t i o n a d d e d / c m– 3 3

(a) Complete the graph by drawing two intersecting straight lines of best fit.

(1)


(b) The point where the lines cross represents the neutralisation of the nitric acid by the sodium hydroxide solution.

Use the graph to find:

(i) the volume of 2.0 mol dm3 sodium hydroxide solution, NaOH, that reacts exactly

with the 50 cm3 of the nitric acid.

...........................................................................................................................

(1)

(ii) the maximum temperature change, DT, in the reaction.

...........................................................................................................................

(1)

(c) The information in (b)(i) and the equation below can be used to calculate the concentration of the nitric acid.

NaOH(aq) + HNO3(aq) NaNO3(aq) + H2O(l)

(i) Calculate the amount (moles) of sodium hydroxide used to neutralise the 50 cm3 ofnitric acid.

(1)

(ii) Write the amount (moles) of nitric acid in 50.0 cm3 of the solution.

...........................................................................................................................

(1)

(iii) Hence calculate the concentration of nitric acid, HNO3, in mol dm3.


(2)


(d) (i) Use the data from (b) to calculate the heat change for this reaction.

The density of the mixture produced at neutralisation is 1.0g cm–3 and the specific

heat capacity of the mixture is 4.2 J g–1 °C–1.

Heat change = mass × specific heat capacity × DT

(2)

(ii) Use your answer from (d)(i) and (c)(iii) to calculate the enthalpy of neutralisation per mole of nitric acid, HNO3. Include a sign and units with your answer.

(3)

(e) The enthalpy of neutralisation found by this method may be less exothermic than the data book value because of heat loss.

Suggest ONE way to reduce the error due to heat loss.

.....................................................................................................................................

.....................................................................................................................................

(1)(Total 13 marks)


53. (a) Calculate the number of atoms in 3.50 g of lithium.


[The Avogadro constant, L = 6.02 × 1023 mol–1]

(2)

(b) The equation for the reaction of lithium with hydrochloric acid is shown below.

2Li(s) + 2HCl(aq) 2LiCl(aq) + H2(g)

(i) Rewrite this equation as an ionic equation, omitting the spectator ions.

(1)


(ii) Draw a ‘dot and cross’ diagram of lithium chloride showing all the electrons. Indicate charges clearly on your diagram.

(2)

(iii) The value of the standard enthalpy change for the reaction, DHο, is –557 kJ mol–1. State TWO of the reaction conditions necessary for this enthalpy change to be standard.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)(Total 7 marks)


54. This question is about the arene, naphthalene. The structure of naphthalene can be shown as

(a) What is the molecular formula of naphthalene?

.....................................................................................................................................

(1)

(b) The enthalpy change, ΔH, for the addition of hydrogen to cyclohexene to form

cyclohexane is –120 kJ mol–1.

+ H = – 1 2 0 k J m o l H – 1 2

(i) Calculate the enthalpy change of the hydrogenation reaction shown below.

+ 5 H 2

ΔH = ...................... kJ mol–1

(1)

(ii) Experimental work shows that ΔH for the hydrogenation of naphthalene is actually

–333 kJ mol–1. What does this suggest about the stability and structure of naphthalene?

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(2)


(iii) Would you expect naphthalene to decolorise bromine solution? Justify your answer.

...........................................................................................................................

...........................................................................................................................

...........................................................................................................................

(1)

(c) The Friedel-Crafts reaction enables an alkyl group to be attached to an arene ring.

(i) Suggest the reagent and catalyst you would need to makeC H ( C H )3 2

from naphthalene.

Reagent ..............................................................................

Catalyst ..............................................................................

(2)

(ii) Name the type of reaction and its mechanism.

...........................................................................................................................

(2)(Total 9 marks)

55. (a) Enthalpy changes can be calculated using average bond enthalpy data.

(i) The enthalpy change to convert methane into gaseous atoms is shown below.

CH4(g) → C(g) + 4H(g) ∆H = +1664 kJ mol–1

Calculate the average bond enthalpy of a C—H bond in methane.

(1)


(ii) Use the data in the table below and your answer to (a)(i) to calculate the enthalpy change for

2C(g) + 2H2(g) + Br2(g) → CH2BrCH2Br(g)

Bond

Average bond

enthalpy / kJ mol–1 Bond

Average bond

enthalpy / kJ mol–1

C—C +348 H—H +436

Br—Br +193 C—Br +276

(3)

(b) The standard enthalpy of formation of 1,2-dibromoethane, CH2BrCH2Br, is

–37.8 kJ mol–1.

Suggest the main reason for the difference between this value and your calculated value in (a)(ii).

................................................................................................................................

................................................................................................................................

................................................................................................................................

(1)(Total 5 marks)


56. (a) 2,2,4-trimethylpentane, C8H18, is one of the hydrocarbons present in petrol.

(i) Draw the structural formula of 2,2,4-trimethylpentane.

(1)

(ii) To which homologous series does 2,2,4-trimethylpentane belong?

......................................................................................................................

(1)

(b) (i) Define the term standard enthalpy of combustion.

......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

(3)

(ii) Write the equation to represent the complete combustion of butane, C4H10.

......................................................................................................................

(2)


(c) The enthalpies of combustion of some compounds in kJ g–1 and kJ cm–3 are given below.

Compound ∆Hc /kJ g–1 ∆Hc /kJ cm–3

Butane, C4H10(g) –0.12

Ethanol, C2H5OH(l) –30 –21

2,2,4-trimethylpentane, C8H18(l) –48 –33

(i) The standard enthalpy of combustion of butane is –2877 kJ mol–1.

Calculate the enthalpy of combustion of butane in kJ g–1.

(2)

(ii) Use the information in the table to compare the advantages and disadvantages of these three compounds as fuels for a motor vehicle.

......................................................................................................................

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(4)(Total 13 marks)


57. In an experiment to find the enthalpy change for the reaction

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

a student was given the following list of instructions:

weigh out 5.0 g of zinc powder into a weighing bottle

use a measuring cylinder to transfer 50 cm3 of 1.0 mol dm–3 aqueous copper(II) sulphate

into a polystyrene cup, firmly held in a 250 cm3 beaker

stir the solution with the thermometer and record the temperature to the nearest 0.5 °C

continue to stir the solution, recording its temperature every minute

at exactly 3.5 minutes, add the zinc powder to the aqueous copper(II) sulphate, stirring continuously

record the temperature of the solution every minute from 4.0 to 9.0 minutes.

The temperature readings obtained are shown in the table below.

Time/min 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

Temperature/°C 20.0 20.0 20.0 20.0 63.0 60.5 59.0 57.0 55.5 53.0


(a) (i) Plot a graph of temperature against time on the grid below.

T e m p e r a t u r e/ º C

T i m e / m i n u t e s

0 1 . 0 2 . 0 3 . 0 4 . 0 5 . 0 6 . 0 7 . 0 8 . 0 9 . 0 1 0 . 0

7 0 . 0

6 0 . 0

5 0 . 0

4 0 . 0

3 0 . 0

2 0 . 0

1 0 . 0

0

(2)

(ii) Use the graph to calculate the maximum temperature change, DT. Show clearly on the graph how you obtained your answer.

DT = ................................................°C

(2)


(iii) Give ONE reason why a series of temperature readings is obtained instead of just the starting and maximum temperatures.

......................................................................................................................

......................................................................................................................

(1)

(b) (i) Calculate the heat change, in joules.

The specific heat capacity of the solution is 4.18 J g–1 °C–1.

(1)

(ii) What assumption have you made about the solution in your calculation in (i)?

......................................................................................................................

......................................................................................................................

(1)

(iii) Calculate the amount (moles) of copper (II) sulphate, CuSO4, in 50 cm3 of a

1.0 mol dm–3 solution.

(1)


(iv) Calculate the enthalpy change for this reaction in kJ mol–1.

(2)

(c) Suggest TWO improvements that could be made to the experimental procedure.

Give a reason for each.

Improvement 1 ............................................................................................................

................................................................................................................................

................................................................................................................................

Reason .........................................................................................................................

................................................................................................................................

................................................................................................................................

Improvement 2 ............................................................................................................

................................................................................................................................

................................................................................................................................

Reason ..........................................................................................................................

................................................................................................................................

................................................................................................................................

(4)(Total 14 marks)


58. The values of the lattice energies of potassium iodide and calcium iodide experimentally determined from Born-Haber cycles and theoretically calculated from an ionic model are shownbelow.

Experimental latticeenergy

/kJ mol–1

Theoreticallattice energy

/kJ mol–1

Potassium iodide, KI(s) – 651 – 636

Calcium iodide, CaI2(s) –2074 –1905

(i) Explain why the experimental lattice energy of potassium iodide is less exothermic than the experimental lattice energy of calcium iodide.

...............................................................................................................................

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...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

(3)


(ii) Explain why the experimental and theoretical values of the lattice energy are almost the same for potassium iodide, but are significantly different for calcium iodide.

...............................................................................................................................

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(3)(Total 6 marks)

59. (a) (i) Draw a labelled Hess’s Law cycle for the dissolving of solid calcium hydroxide in water, and use it and the data below to calculate the lattice energy of calcium hydroxide.

∆H/ kJ mol–1

Enthalpy of hydration of Ca2+(g) –1650

Enthalpy of hydration of OH–(g) –460

Enthalpy of solution of Ca(OH)2(s) –16.2

(4)



(ii) State and explain the trend in solubility in water of the Group 2 hydroxides.

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(4)

(b) The dissolving of sparingly soluble calcium hydroxide in water can be shown by the equilibrium reaction

Ca(OH)2(s) + (aq) Ca2+(aq) + 2OH–(aq) ∆Hο = –16.2 kJ mol–1

State and explain the effect on the solubility of calcium hydroxide of

(i) increasing the temperature

......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

(3)


(ii) adding sodium hydroxide solution.

......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

(3)

(c) When concentrated sulphuric acid is added to solid calcium chloride, one acidic gas is given off.

When concentrated sulphuric acid is added to solid calcium bromide, three gases are given off.

Identify the three gases given off in the reaction with calcium bromide. Explain why only one gas is given off in the reaction with calcium chloride.

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

(4)(Total 18 marks)

60. When solutions of potassium carbonate and calcium chloride are mixed together, the following reaction takes place

CaCl2(aq) + K2CO3(aq) → CaCO3(s) + 2KCl(aq)

(a) Re-write the above equation as an ionic equation. Include state symbols, but omit any spectator ions.

(2)


(b) An experiment was carried out to measure the enthalpy change for this reaction. 50 cm3

of a 1.00 mol dm–3 solution of potassium carbonate was added to 50 cm3 of a

1.00 mol dm–3 solution of calcium chloride. The temperature fell by 1.5 °C.

(i) Calculate the energy taken in from the surroundings using the relationship

energy = mass of × specific heat capacity × temperaturesolution of solution change

/J /g /J g–1 °C–1 /°C

You may assume that

1.0 cm3 of solution has a mass of 1.0 g.

The specific heat capacity of the solution is 4.2 J g–1 °C–1.

Energy taken in = .................... J

(1)

(ii) How many moles of calcium chloride are used in this experiment?

(1)

(iii) Calculate the enthalpy change for the reaction, giving your answer to two significant figures. Include a sign and units in your answer.

(2)



(iv) Which measurement is likely to have caused the major source of error in this experiment? Explain your answer.

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

(1)

(v) What apparatus should be used to contain the reaction mixture during this experiment?

................................................................................................................................

(1)

(c) If the experiment in (b) was repeated, but using only 25 cm3 of each solution, predict what the fall in temperature would be.

.......................................................................................................................................

(1)(Total 9 marks)

61. This question is about the chemistry of methanol, CH3OH.

(a) (i) Draw a ‘dot and cross’ diagram for methanol, showing outer shell electrons only.

(1)


(ii) Textbooks show the displayed formula of methanol as follows

However, this is not a true representation of the shape of the molecule.Explain why the shape of methanol is not as shown above.Label the correct value of ONE bond angle on the displayed formula.

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

(2)

(b) When methanol burns in a poor supply of air, one of the products is carbon monoxide.A ‘dot and cross’ diagram of carbon monoxide is shown below.

(i) Draw the displayed formula for carbon monoxide. Show the TWO types of bond which are present.

(1)

(ii) The length of the bond between carbon and oxygen in methanol is 0.143 nm. Would you expect the length of the bond between carbon and oxygen in carbon monoxide to be longer, the same or shorter than this? Explain your answer.

................................................................................................................................

................................................................................................................................

................................................................................................................................

(2)



(c) The energy of the bond between carbon and oxygen in methanol (the C—O bond) can be calculated from data on enthalpy changes of atomisation.

(i) Write an equation, including state symbols, for the atomisation of one mole of methanol vapour.

(1)

(ii) Use the data below to calculate the energy of the C—O bond in methanol.

standard enthalpy change of atomisation of methanol vapour, ∆Hοat = +2039 kJ mol–1

energy of C—H bond, E(C—H) = +413 kJ mol–1

energy of O—H bond, E(O—H) = +464 kJ mol–1

(2)


(iii) Complete a balanced Hess cycle which you can use to calculate the standard

enthalpy change of formation of methanol vapour, DHοf.

You should use the value of the standard enthalpy change of atomisation of

methanol vapour, DHοat, given in (ii) and the data on enthalpy changes given

below. Write the correct numerical data beside the arrows in the cycle.

Equation Enthalpy change of atomization

/kJ mol–1

C(graphite) C(g) +716.7½ O2(g) O(g) +249.2½ H2(g) H(g) +218.0

Use your cycle to calculate the value of DHοf for methanol vapour.

(3)


(iv) Methanol is a liquid at room temperature. Would you expect the standard enthalpy change of formation of liquid methanol to be more or less negative than the value you calculated in (iii)? Justify your answer.

................................................................................................................................

................................................................................................................................

(1)

(v) Methanol is a liquid at room temperature although alkanes with similar molecular mass are gases.

Draw a diagram to show a bond between two methanol molecules that causes it to be a liquid at room temperature.

Give the value of this bond angle on your diagram.

(2)

(d) Methanol can be manufactured in the following reaction.

CO(g) + 2H2(g) CH3OH(g) ∆Hο = –93.3 kJ mol–1

Decide whether a high or low temperature and a high or low pressure would give the greater proportion of methanol at equilibrium. Justify your choice in each case.

Temperature ..................................................................................................................

.......................................................................................................................................

.......................................................................................................................................

Pressure .........................................................................................................................

.......................................................................................................................................

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(2)(Total 17 marks)



62. (a) The distribution of the energy of particles in a gas at temperature T1 is shown below.

(i) On the diagram above, draw the distribution of energy of particles at a lower temperature, T2.

(2)

(ii) Use the diagram to explain why the rate of a reaction increases with an increase in temperature.

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...............................................................................................................................

...............................................................................................................................

(3)


(iii) Explain fully why a catalyst increases the rate of a reaction.

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................................................................................................................................

(2)


(b) The fermentation of glucose is an exothermic reaction and is catalysed by enzymes in yeast.

C6H12O6(aq) → 2C2Η5ΟΗ(aq) + 2CO2(g)

The reaction is slow at room temperature.

(i) Describe, with the aid of a diagram, an experiment you could do to follow the progress of this reaction at different temperatures.

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

(4)


(ii) Would you expect ∆Ssystem to be positive or negative for this reaction? Justify your answer with TWO pieces of evidence.

................................................................................................................................

...............................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

(2)

(iii) Deduce the sign of ∆Ssurroundings. Show your reasoning.

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(2)(Total 15 marks)

63. Methane, CH4, is used as a domestic and industrial fuel and as a reagent in the petrochemical industry.

(a) Define the term standard enthalpy of combustion.

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

(3)


(b) Methane burns in oxygen according to the equation:

H C H ( g ) + 2 O O ( g ) O C O ( g ) + 2 H O H ( g )

H

H

Use the average bond enthalpy data shown below to calculate the enthalpy change of this reaction.

Bond Bond enthalpy/kJ mol–1

C H +435

O==O +498

C==O +805

H O +464

(3)


(e) Methane is the feedstock in the manufacture of hydrogen according to the equation:

CH4(g) + 2H2O(g) CO2(g) + 4H2(g)

Given the enthalpy of formation data below, draw a labelled Hess’s law cycle and use it to calculate the enthalpy change of this reaction.

Substance Enthalpy of formation/kJ mol–1

CH4(g) –75

CO2(g) –394

H2O(g) –242

(4)(Total 10 marks)

64. In an experiment to find the enthalpy of neutralisation of a monobasic acid, HX, with an alkali, the following procedure was followed:

Step 1 25.0 cm3 of 1.00 mol dm–3 dilute aqueous acid, HX, was measured into a polystyrene cup.

Step II A 0-100 °C thermometer was placed in the acid. The temperature of the acid was immediately read and recorded.

Step III 5.00 cm3 portions of aqueous sodium hydroxide were added to the acid from a burette. After each addition, the temperature of the solution was read and recorded. The thermometer was removed and rinsed with water between each addition. A

total of 50.0 cm3 of aqueous sodium hydroxide was added.


(a) Suggest ONE change that could be made at Step II and ONE change that could be made at Step III to improve the accuracy of the experiment.

Step II ...................................................................................................................

...............................................................................................................................

Step III ..................................................................................................................

...............................................................................................................................

(2)

(b) The readings of temperature and volume are plotted on the grid. Draw two separate straight lines of best fit, extending the two lines so that they intersect.

T e m p e r a t u r e/ ° C

2 5

2 0

1 50 1 0 2 0 3 0 4 0 5 0

V o l u m e o f s o d i u m h y d r o x i d e a d d e d / c m 3

×

×

×

×

××

×× × ×

×

(2)


(c) From the graph, read off the maximum temperature rise, DT, and the volume of aqueous sodium hydroxide added at neutralisation, VN.

DT = ..................................... C VN = ................................ cm3

(2)

(d) (i) Use the formula below to calculate the heat evolved in the neutralisation.

Heat evolved =

1000

18.4T25VN

kJ

(1)

(ii) Given that the amount (moles) of acid neutralised was 0.025 mol, calculate the

enthalpy of neutralisation, DHneut, in units of kJ mol–1.

DHneut = ............................... kJ mol–1

(2)(Total 9 marks)


65. (a) Define the term lattice energy.

...............................................................................................................................

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...............................................................................................................................

...............................................................................................................................

(3)

(b) A Born–Haber cycle for the formation of lithium chloride is shown below.

L i ( g ) + e + C l ( g )

L i ( g ) + e + C l ( g )

L i ( g ) + C l ( g )

L i ( s ) + C l ( g ) H

H

L i C l ( s )

L i ( g ) + C l ( g )

l a t t

a

+

+

+

–

–

– 1

1

1

2

2

2

2

2

2

Enthalpy change

/kJ mol–1

Enthalpy of formation of lithium chloride(s) –409

Enthalpy of atomisation of lithium +161


1st ionisation energy of lithium +519

Electron affinity of chlorine –349


(i) Calculate the lattice energy, DHlatt, of lithium chloride.

(2)

(ii) The enthalpy change DHa is the enthalpy of atomisation of lithium metal.

Suggest, and explain, whether you would expect the value of DHa for potassium to be more or less endothermic than that for lithium.

......................................................................................................................

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......................................................................................................................

......................................................................................................................

(2)

(c) (i) State TWO properties of ions that affect the value for the lattice energy of ionic compounds.

......................................................................................................................

......................................................................................................................

(2)


(ii) The calculated value for the lattice energy of potassium chloride is almost exactly the same value as that found experimentally.

Why is the theoretical value for the lattice energy of silver chloride, –833 kJ mol–1,

so different from the experimentally determined value of –905 kJ mol–l?

......................................................................................................................

......................................................................................................................

......................................................................................................................

(2)

(d) (i) Draw a Hess’s law cycle to show how the lattice energy and the enthalpy of hydration are related to the enthalpy of solution of an ionic compound, MX(s).

(2)

(ii) How are the enthalpy values used to suggest whether MX(s) is soluble in water.

......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

(2)


(iii) The table below gives some information about the sulphates of two of the elements in Group 2.

formula of sulphate lattice energy

/kJ mol–1

hydration enthalpy ofthe cation

/kJ mol–1

solubility

/mol dm–3

CaSO4 –2480 –1650 4.7 × 10–2

BaSO4 –2374 –1360 9.4 × 10–6

Comment on the trend in the solubility of these sulphates in relation to the lattice energies and hydration enthalpies as shown in the table.

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(2)(Total 17 marks)

66. A reaction of ammonium dichromate(VI) is shown by the following equation.

(NH4)2Cr2O7(s) heat N2(g) + 4H2O(g) + Cr2O3(s)

(a) What type of reaction is this?

......................................................................................................................................

(1)


(b) The enthalpy change for this reaction can be calculated from standard enthalpy changes of formation.

(i) State fully what is meant by the standard enthalpy change of formation, ΔHfο,

of a compound.

...............................................................................................................................

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...............................................................................................................................

...............................................................................................................................

(3)

(ii) Complete the Hess cycle for the reaction so that you can calculate the enthalpy change of the reaction from standard enthalpy changes of formation.

(3)

(iii) What is the value of ΔHfο[N2(g)]? ......................................................................

(1)


(iv) Calculate ΔHοr for the reaction using the following data. Remember to include a

sign and units in your answer.

ΔHfο[(NH4)2Cr2O7(s)] = –1810 kJ mol–1

ΔHfο[H2O(g)] = –242 kJ mol–1

ΔHfο[Cr2O3(s)] = –1140 kJ mol–1

(3)

(c) In this reaction, water vapour is formed which condenses to liquid water on cooling.Is this reaction H2O(g) H2O(l) exothermic or endothermic?

Justify your answer.

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(2)(Total 13 marks)


67. Hydrochloric acid, formed when hydrogen chloride is dissolved in water, can be converted to chlorine using an aqueous solution of hydrogen peroxide:

2HCl(aq) + H2O2(aq) Cl2(g) + 2H2O(l)

(i) Give the oxidation numbers of

chlorine in HCl .......... chlorine in Cl2 ..........

oxygen in H2O2 .......... oxygen in H2O ..........

(2)

(ii) Name the reducing agent in this reaction.

.....................................................................................................................................

(1)

(iii) Explain why the oxidation numbers you have given in (i) are consistent with the fact that two moles of hydrochloric acid react with one mole of hydrogen peroxide.

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(1)(Total 4 marks)


68. Chlorine can be converted to the gas chlorine(I) oxide, Cl2O.

The standard molar enthalpy change of formation of chlorine(I) oxide and the standard molar enthalpy changes of atomisation of chlorine and oxygen are given below:

ΔHfο [Cl2O(g)] = + 80.3 kJ mol–1

ΔHatο [½Cl2(g)] = +121.7 kJ mol–1

ΔHatο [½O2(g)] = +249.2 kJ mol–1

A partially completed Hess cycle involving chlorine(I) oxide is shown below:

(i) Insert the appropriate formulae, showing the correct quantities of each element, into the box above. Include state symbols in your answer.

(1)

(ii) Insert arrows between the boxes, writing the correct numerical data alongside the appropriate arrows.

(1)


(iii) Use the cycle to calculate ΔHatο [Cl2O(g)].

(1)

(iv) Calculate the Cl—O bond energy in chlorine(I) oxide.

(1)(Total 14 marks)


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...............................................................................................................................

...............................................................................................................................

(3)

(b) The complete combustion of the fuel methane is shown in the following equation.

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)


(i)

Formula ofSubstance

Standard enthalpy of formation

/kJ mol–1

CH4(g) –75

O2(g) 0

CO2(g) –394

H2O(l) –286

Use the data in the table above to calculate the standard enthalpy change for the complete combustion of methane.

(3)

(ii) Explain the significance of the sign of the value of this enthalpy change.

......................................................................................................................

......................................................................................................................

(1)

(c) Ethanol, C2H5OH, is also a fuel. Write the equation for the complete combustion of ethanol.

...............................................................................................................................

(2)


(d) The table shows some properties of ethanol and methane.

Ethanol Methane

State at room temperature Liquid Gas

Cost per tonne/£ 500 50

Enthalpy released per gram/kJ 30 56

Use these data to suggest advantages and disadvantages of ethanol and methane as motor car fuels.

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(3)

(e) Ethanol can be oxidised by potassium dichromate(VI) mixed with sulphuric acid, to produce either ethanal or ethanoic acid. Write an equation in each case to show these reactions. You may use [O] to represent the oxidising agent.

Production of ethanal

...............................................................................................................................

Production of ethanoic acid

...............................................................................................................................

(2)


(f) Ethanol can be dehydrated to produce ethene. State the reagent and conditions required for this conversion.

Reagent ...................................................................................................................

Conditions ...............................................................................................................

(2)(Total 16 marks)

70. In two similar, separate experiments the enthalpy changes for the reactions of sodium hydrogencarbonate and sodium carbonate with excess dilute hydrochloric acid were determined.

(a) The first experiment was to find the enthalpy change, DH1, for the reaction

NaHCO3(s) + HCl(aq) NaCl(aq) + CO2(g) + H2O(l)

Measurement Reading

Mass of solid sodium hydrogencarbonate added tohydrochloric acid.

5.00 g

Volume of hydrochloric acid 50.0 cm3

Temperature of hydrochloric acid before additionof solid sodium hydrogencarbonate

22.0 C

Final temperature of solution 15.5 C

Molar mass of sodium hydrogencarbonate 84.0 g mol–1

Specific heat capacity of solution 4.18 J g–1 C–1


(i) Calculate the amount (moles) of sodium hydrogencarbonate used.

(1)

(ii) Calculate the heat absorbed in the reaction in kJ.

[Assume that 1 cm3 of solution has a mass of 1 g]

(2)


(iii) Calculate the value of DH1 in kJ mol–1. Include a sign in your answer expressing it to a number of significant figures suggested by the data in the table.

(2)

(b) In the second experiment the enthalpy change for the reaction between sodium carbonate and dilute hydrochloric acid was measured.

Na2CO3(s) + 2HCl (aq) 2NaCI(aq) + CO2(g) + H2O(l)

The molar enthalpy change, DH2, was calculated to be –35.6 kJ mol–1

(i) Give TWO ways in which the temperature change differs when equal molar amounts of sodium hydrogencarbonate and sodium carbonate react separately with the same volume of hydrochloric acid.

......................................................................................................................

......................................................................................................................

......................................................................................................................

(2)

(ii) Give ONE assumption that has been made in calculating the values of DH1, and DH2 from experimental results.

......................................................................................................................

......................................................................................................................

(1)(Total 8 marks)


71. (a) The enthalpy of hydrogenation of cyclohexene is –120 kJ mol–1.

+ H 2

(i) Predict the value of DH for the reaction:

+ 2 H 2

......................................................................................................................

(1)

(ii) Suggest the value of DH for the hydrogenation of the hypothetical molecule1,3,5-cyclohexatriene:

+ 3 H 2

......................................................................................................................

(1)

(iii) The enthalpy of hydrogenation of benzene is –208 kJ mol–1. Explain in terms of thestructure and bonding in benzene why this value is different from your answer to (a)(ii).

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......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

(3)


(b) Benzene, C6H6, reacts with bromine to form bromobenzene, C6H5Br, and hydrogen bromide.

(i) Give the formula of the catalyst that is needed for the reaction.

......................................................................................................................

(1)

(ii) Give the mechanism for the reaction, making clear the role of the catalyst.

(4)

(iii) State the type of mechanism that is commonly found with reactions of benzene andits derivatives.

......................................................................................................................

(1)(Total 11 marks)


72. In the manufacture of beer, brewers often add small amounts of salts of Group 2 elements to the water used. These salts influence the chemical reactions during the brewing process.Two such salts are calcium sulphate and magnesium sulphate.

(a) A flame test can be used to confirm that a sample of a salt contains calcium ions.

(i) Describe how you would carry out a flame test.

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

(3)

(ii) A positive test results in a brick-red flame colour. Describe the changes that occur in calcium ions to produce a colour.

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

(2)

(iii) Impurities in the salt may lead to other colours being observed in the flame.What metal ion is likely to be present if a yellow flame is seen?

............................................................................................................................

(1)


(b) Magnesium sulphate can be used in its anhydrous form, MgSO4(s), or in its hydrated form, MgSO4.7H2O(s).

An experiment was carried out to find the enthalpy change when hydrated magnesium sulphate dissolved completely in water.

MgSO4.7H2O(s) waterexcess MgSO4(aq) + 7H2O(l)

12.3 g of hydrated magnesium sulphate was added to 100 g of water in a simple calorimeter and the temperature was found to fall by 1.1 °C.

(i) Calculate the energy change, in joules, that occurred in the experiment, using the relationship

Energy change (J) = 4.18 × mass of water × temperature change

(2)

(ii) Calculate the number of moles of hydrated magnesium sulphate used in the experiment. Use the Periodic Table as a source of data.

(2)

(iii) Use your answers to (i) and (ii) to calculate the enthalpy change for the reaction.Include a sign and units in your final answer, which should be given to 2 significant figures.

(2)



(c) The enthalpy change as hydrated magnesium sulphate is converted to anhydrous magnesium sulphate is very difficult to measure. The Hess Cycle below can be used to find this enthalpy change, ΔHr.

(i) Use the cycle to write an expression for ΔHr using ΔH1 and ΔH2.

(1)

(ii) Use your expression in (c)(i) and your answer from (b)(iii) to calculate ΔHr.

Include a sign and units in your final answer, which should be given to 2 significant figures.

(2)(Total 15 marks)

73. Phosphine, PH3, is a hydride of the Group 5 element, phosphorus.

(a) (i) Draw a ‘dot-and-cross’ diagram of a phosphine molecule. You should include only outer shell electrons.

(1)



(ii) Draw the shape you would expect for the phosphine molecule, suggesting a value for the HPH bond angle.

HPH bond angle .......................................................................................................

(2)

(iii) Explain the shape of the phosphine molecule you have given in your answer in (ii).

Justify your value for the HPH bond angle.

.............................................................................................................................

.............................................................................................................................

.............................................................................................................................

.............................................................................................................................

(2)

(b) (i) Write a balanced equation, including state symbols, for the atomisation of phosphine gas.

.............................................................................................................................

(1)


(ii) Use your answer to (i) and the data below to calculate the standard enthalpy changeof atomisation of phosphine at 298 K. Include a sign and units in your answer.

ΔHοf[PH3(g)] = + 5.4 kJ mol-1

ΔHοat[½H2(g)] = + 218.0 kJ mol-1

ΔHοat[P(s)] = + 314.6 kJ mol-1

(3)

(iii) Calculate a value for the bond energy of the bond between phosphorus and hydrogen, using your answer to (ii).

(1)(Total 10 marks)

74. The equation below shows a possible reaction for producing methanol.

CO(g) + 2H2(g) CH3OH(l) ΔHο = -129 kJ mol–1


(a) The entropy of one mole of each substance in the equation, measured at 298 K, is shown below.

Substance

Sο

/J mol-1 K-1

CO(g) 197.6

H2(g) 130.6

CH3OH(l) 239.7

(i) Suggest why methanol has the highest entropy value of the three substances.

.............................................................................................................................

.............................................................................................................................

(1)

(ii) Calculate the entropy change of the system, ΔSοsystem, for this reaction.

(2)

(iii) Is the sign of ΔSοsystem as expected? Give a reason for your answer.

.............................................................................................................................

.............................................................................................................................

.............................................................................................................................

(1)


(iv) Calculate the entropy change of the surroundings ΔSοsurroundings, at 298 K.

(2)

(v) Show, by calculation, whether it is possible for this reaction to occur spontaneouslyat 298 K.

(2)

(b) When methanol is produced in industry, this reaction is carried out at 400 ºC and 200 atmospheres pressure, in the presence of a catalyst of chromium oxide mixed with zinc oxide. Under these conditions methanol vapour forms and the reaction reaches equilibrium. Assume that the reaction is still exothermic under these conditions.

CO(g) + 2H2(g) CH3OH(g)

(i) Suggest reasons for the choice of temperature and pressure.

Temperature ........................................................................................................

.............................................................................................................................

.............................................................................................................................

.............................................................................................................................

Pressure ...............................................................................................................

.............................................................................................................................

.............................................................................................................................

(3)


(ii) The catalyst used in this reaction is heterogeneous. Explain this term.

.............................................................................................................................

.............................................................................................................................

(1)

(iii) Write an expression for the equilibrium constant in terms of pressure, Kp, for this reaction.

CO(g) + 2H2(g) CH3OH(g)

(1)

(iv) In the equilibrium mixture at 200 atmospheres pressure, the partial pressure of carbon monoxide is 55 atmospheres and the partial pressure of hydrogen is 20 atmospheres.

Calculate the partial pressure of methanol in the mixture and hence the value of theequilibrium constant, Kp. Include a unit in your answer.

(2)


(c) The diagram below shows the distribution of energy in a sample of gas molecules in a reaction when no catalyst is present. The activation energy for the reaction is EA.

(i) What does the shaded area on the graph represent?

.............................................................................................................................

(1)

(ii) Draw a line on the graph, labelled EC, to show the activation energy of the catalysed reaction.

(1)(Total 17 marks)

75. This question is about the chemical reaction between two gases, oxygen, O2, and nitrogen monoxide, NO.

2NO(g) + O2(g) 2NO2(g)

(a) (i) Define the term standard enthalpy of formation, DHοf.

......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

(3)


(ii) Use the data below to calculate the enthalpy change for the reaction between oxygen and nitrogen monoxide.

DHοf (NO) = + 90 kJ mol–1

DHοf (NO2) = + 34 kJ mol–1

(2)

(iii) Construct a reaction profile to represent the reaction in (ii).

(3)


(iv) On the basis of your calculation in (ii), state why you would expect the reaction to take place.

..........................................................................................................................

......................................................................................................................

(1)

(v) Explain why a reaction, which is expected to take place, may actually proceed so slowly as to appear never to happen.

......................................................................................................................

......................................................................................................................

(1)

(b) The reaction is reversible and produces a dynamic equilibrium.

2NO(g) + O2(g) 2NO2(g)

(i) Explain what is meant by the term dynamic equilibrium.

......................................................................................................................

......................................................................................................................

......................................................................................................................

(2)

(ii) State, with a reason, the effect on the position of equilibrium of increasing the total pressure of the system at constant temperature.

......................................................................................................................

......................................................................................................................

......................................................................................................................

(2)


(iii) Although the reaction is an equilibrium reaction, industrially this and other similar reactions do not usually achieve equilibrium. Suggest why this is so.

......................................................................................................................

......................................................................................................................

(1)(Total 15 marks)

76. The apparatus shown in the diagram below may be used to find the enthalpy of combustion of alcohols.

w a t e r

s p i r i t l a m p +e t h a n o l

Using the apparatus, a student recorded the results included in the table below.

Alcohol = ethanol, C2H5OH

Molar Mass (C2H5OH) = 46.0 g mol–1

Volume of water in beaker = 200 cm3

\ mass of water in beaker = 200 g

WeighingsSpirit lamp + ethanol before combustion = 198.76 gSpirit lamp + ethanol after combustion = 197.68 g

TemperaturesWater before heating = 19.5 CWater after heating = 38.1 C

Specific heat capacity of water = 4.18 J g–1 C–1


(a) What assumption is the student making about water to be able to state that its mass is numerically equal to its volume?

...............................................................................................................................

(1)

(b) Calculate the heat gained by the water. Give your answer in kJ.

(2)

(c) Calculate the amount (number of moles) of ethanol used.

(2)


(d) Using your values from (b) and (c), calculate the enthalpy of combustion of ethanol. Giveyour answer to a number of significant figures consistent with the readings in the table. Include a sign and units in your answer.

(3)

(e) The student’s evaluation of the experiment is given below.

My calculated value of the enthalpy of combustion wasnumerically much less than the data book value. Thereasons for my low value include:

1 heat losses to the surrounding air;

2 when I re-checked the mass of the spirit lamp andethanol after combustion, I noticed that it had lostmass even when it was not being used;

3 a black solid which formed on the base of the beaker.

(i) Explain why the spirit lamp and ethanol lost mass even when not in use.

......................................................................................................................

......................................................................................................................

(1)


(ii) Suggest the identity of the black solid. Explain why its formation will lead to a lowvalue for the enthalpy of combustion.

Identity ..........................................................................................................

Explanation ...................................................................................................

......................................................................................................................

(2)(Total 11 marks)

77. This question is about a self-heating can of coffee.

The bottom of the can has a compartment containing copper(II) nitrate solution. When a button on the bottom of the can is pressed, magnesium powder is released into the compartment where it reacts with the copper(II) nitrate solution.

(a) (i) Write an ionic equation for the reaction between magnesium powder and copper(II)ions. Include state symbols, but omit any spectator ions.

(2)

(ii) Show how the standard enthalpy change for this reaction could be calculated from the standard enthalpies of formation of copper(II) ions and magnesium ions. You should include a Hess cycle in your answer.

(3)


(b) The can contains 150 g of a solution of coffee in water.

The temperature of the solution needs to increase by 60 °C to produce a hot drink.

(i) Calculate the energy change needed to produce a temperature increase of 60 °C in the coffee, using the relationship

Energy change = 4.2 × mass of solution × temperature change.

Remember to include a unit in your answer.

(2)

(ii) The standard enthalpy change for this reaction is –530 kJ mol–1.

Calculate the number of moles of reactants needed to produce the energy change in(i).

(1)

(iii) A solution of copper(II) nitrate of concentration 8.0 mol dm–3 is used.

Use your answer to (ii) to calculate the volume, in cm3, of copper(II) nitrate solution needed.

Your answer should be given to two significant figures.

(1)


(c) Suggest TWO reasons why the temperature of the coffee may not increase by as much as 60 °C.

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

(2)(Total 11 marks)

78. The reaction between chlorine and methane, in the presence of ultraviolet light, involves the formation of free radicals and includes the following steps:

A Cl2 2Cl• ΔΗο = +242 kJ mol–1

B CH4 + Cl• HCl + CH3• ΔΗο = +4 kJ mol–1

C Cl2 + CH3• CH3Cl + Cl• ΔΗο = –97 kJ mol–1

D Cl• + Cl• Cl2

E CH3• + CH3

• CH3CH3

F Cl• + CH3• CH3Cl ΔΗο = –339 kJ mol–1

(a) (i) What is meant by a free radical? ....................................................................

...........................................................................................................................

...........................................................................................................................

(1)


(ii) Draw a ‘dot-and-cross’ diagram, showing outer shell electrons only, for a chlorine free radical.

(1)

(iii) What type of bond breaking occurs in step A?

...........................................................................................................................

(1)

(b) Which of the steps, A to F, are chain propagation steps?

...........................................................................................................................

(1)

(c) (i) Write the equation for the overall reaction between one mole of chlorine and one mole of methane molecules.

(1)

(ii) Calculate the standard enthalpy change, ΔΗο, for this reaction.

(2)


(d) (i) What is the value of ΔΗο for step D? ................................................................

(1)

(ii) Would you expect step E to be exothermic or endothermic? Justify your answer.

..........................................................................................................................

..........................................................................................................................

..........................................................................................................................

(1)

(e) The overall reaction was repeated using bromine gas instead of chlorine gas.

Would you expect step A for bromine to be more or less endothermic than step A for chlorine? Justify your answer.

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

(2)(Total 11 marks)

79. Urea, which is used as a fertillser in much of mainland Europe, Asia and Africa, is manufacturedby the reaction of ammonia and carbon dioxide.

2NH3(g) + CO2(g) NH2CONH2(s) + H2O(l)

(a) Define the term standard enthalpy of formation, DHf , of urea.

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

(3)


(b) Calculate the enthalpy change, DH for the reaction above, given the following standard enthalpies of formation.

Substance DHf / kJ mol–1

NH3(g) –46.2

CO2(g) –393.5

NH2CONH2(s) –632.2

H2O(l) –285.8

(3)(Total 6 marks)

80. (a) Halogenoalkanes react with many nucleophiles.

Define the term nucleophile.

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

(2)

(b) (i) Identify the reagent and conditions necessary for the conversion of iodoethane to ethylamine, C2H5NH2.

Reagent:.........................................................................................................

Conditions:.....................................................................................................

......................................................................................................................

(3)



(ii) State why the rate of reaction would be slower if bromobutane were used in place of iodoethane, with all other conditions remaining the same.

......................................................................................................................

......................................................................................................................

(1)

(c) Iodoethane reacts with water to form ethanol and hydrogen iodide.

C2H5I + H2O C2H5OH + HI DHf = +36 kJ mol–1

Use some or all of the data below to calculate the C I bond enthalpy.

Bond Bond enthalpy

/ kJ mol–1Bond Bond enthalpy

/ kJ mol–1

C H 413 H I 298

C C 347 C O 358

H O 464

(3)


(d) Ethanol was heated under reflux with an excess of a mixture of potassium dichromate(VI)and dilute sulphuric acid. Draw the full structural formnula of the organic product.

(1)(Total 10 marks)


81. (a) Ammonia is manufactured by the Haber process

N2(g) + 3H2(g) 2NH3(g) DH = –92.4 kJ mol–1

The usual conditions for this process are a catalyst of iron, a temperature of 400 °C and a pressure of 200 atmospheres.

Draw, on the axes below, an energy profile diagram for the uncatalysed reaction. Mark on your diagram the activation energy and the enthalpy change.

E n t h a l p y

E x t e n t o f r e a c t i o n ( r e a c t i o n c o - o r d i n a t e )

(4)

(b) (i) Draw, on the axis below, the Maxwell-Boltzmann distribution that could apply at 400°C and mark on your diagram the activation energies for the catalysed and the uncatalysed reaction.

F r a c t i o n o fm o l e c u l e s o fe n e r g y E

E n e r g y

(3)



(ii) Use your diagram to explain why the reaction is faster in the presence of the iron catalyst.

......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

......................................................................................................................

(3)

(c) A mixture of nitrogen and hvdro2en is kinetically stable at 25°C but kinetically unstable at 400°C. Explain why this is so.

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

...............................................................................................................................

(3)(Total 13 marks)

82. (a) Consider the following equilibrium, which illustrates one industrial method used to produce hydrogen:

CH4(g) + 2H2O(g) CO2(g) + 4H2(g)

In a certain experiment, 10 g of methane, CH4, and 54 g of water, H2O, were heated in a

container of volume 4 dm3. At equilibrium, 2.0 moles of hydrogen, H2, had formed. Writean expression for the equilibrium constant, Kc, for the system, and use the data to calculate a value for Kc, with units.

(8)


(b) The following table shows some data for enthalpies of formation, DHf.

Substance DH f /kJ mol–1

CH4(g) –76

H2O(g) –242

CO2(g) –394

Use these data to calculate the enthalpy change for the reaction in (a).

(3)

(c) In practice, the industrial production of hydrogen by this method is conducted at the moderately high pressure of 30 atm, and the high temperature of 750 °C, in the presence of a nickel catalyst. Suggest why these conditions are used, considering the factors of rateand yield.

(7)(Total 18 marks)

83. (a) The equation below shows the reaction which occurs when ammonia is dissolved in water.

NH3(g) + H2O(1) NH4 (aq) + OH–(aq)

(i) Explain why water is classified as an acid in this reaction.

..........................................................................................................................

..........................................................................................................................

(1)

(ii) The ammonia is acting as a weak base in this reaction.

What is the difference between a weak base and a strong base?

..........................................................................................................................

..........................................................................................................................

..........................................................................................................................

(1)


(b) Ammonia reacts with oxygen to form the gases nitrogen(II) oxide and steam.

(i) Complete the Hess cycle below so that ΔHreaction can be calculated using standard enthalpy changes of formation. Include state symbols.

(2)

(ii) Calculate ΔHreaction for this reaction using the following data.

ΔHf [NH3(g)] = – 46.1 kJ mol–1

ΔHf [NO(g)] = + 90.2 kJ mol–1

ΔHf [H2O(g)] = – 241.8 kJ mol–1

Include a sign and units in your answer and give your answer to three significant figures.

(3)(Total 7 marks)


84. This question is concerned with hydrogen cyanide, HCN.

(a) (i) Draw a ‘dot and cross’ diagram for a molecule of hydrogen cyanide, showing outershell electrons only.

(1)

(ii) Predict the numerical value of the bond angle in hydrogen cyanide. Justify your prediction.

Bond angle ........................................................................................................

Justification .......................................................................................................

..........................................................................................................................

..........................................................................................................................

(2)

(b) Write a balanced equation for the combustion of hydrogen cyanide in oxygen, assuming that the products are water, carbon dioxide and nitrogen.

(1)


(c) Hydrogen cyanide is an extremely toxic, volatile liquid that is used to make useful compounds, such as ‘Perspex’. Do you think it is acceptable for it to be used in this way?

Justify your answer.

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

(1)

(d) The standard enthalpy change of formation, ΔHfο, for gaseous hydrogen cyanide,

HCN(g), is +110 kJ mol–1.

The standard molar enthalpy changes of atomisation of hydrogen, carbon and nitrogen are

given below, in kJ mol–1.

ΔHοat/kJ mol–1

½H2(g) H(g) + 218C(s, graphite) C(g) + 717

21

N2(g) N(g) + 473

The C–H bond energy in hydrogen cyanide is + 413 kJ mol–1.

This information can be represented on a Hess cycle in the following way, and then used to calculate a bond energy.


(i) Insert formulae, showing the correct quantities of each element, into the appropriate boxes.

(2)

(ii) Insert arrows between the boxes and write the correct numerical data alongside the appropriate arrows.

(2)

(iii) Use the cycle to calculate ΔHοat[HCN(g)] and then the carbon to nitrogen bond

energy in hydrogen cyanide.

(1)(Total 10 marks)

85. (a) Define the term standard enthalpy of combustion, making clear the meaning of standard in this context.

………….…………………………………………………………………………..

………….…………………………………………………………………………..

………….…………………………………………………………………………..

………….…………………………………………………………………………..

………….…………………………………………………………………………..

(3)


(b) Use the enthalpies of combustion given below to find the enthalpy change for the reaction:

2C(graphite) + 2H2(g) + O2(g) CH3COOH(l)

DHcombustion/kJ mol–1

C(graphite) –394

H2(g) –286

CH3COOH(l) –874

(3)

(c) With reference to ethanoic acid, CH3COOH, what is the enthalpy change obtained in (b) called?

………….…………………………………………………………………………..

(1)


(d) Draw an enthalpy level diagram to represent the enthalpy change for the combustion of graphite. Show both the enthalpy levels of the reactants and products and an energy profile which represents the activation energy for the reaction.

(3)(Total 10 marks)

86. (a) This question is about finding the formula of copper hydroxide. The method is as follows:


20.0 cm3 of an aqueous solution of a copper salt of concentration 1.00 mol dm–3 was placed in a polystyrene cup and its temperature measured using a thermometer graduated in 0.1 °C intervals.

A burette was filled with aqueous sodium hydroxide, of concentration 2.00 mol dm–3.

2.00 cm3 of sodium hydroxide solution was run into the solution of the copper salt and the temperature was measured immediately.

As soon as possible a further 2.00 cm3 of sodium hydroxide solution was run in and the temperature measured again.

This process of adding 2.00 cm3 portions of sodium hydroxide solution and measuring

the temperature was continued until a total of 36.0 cm3 of the sodium hydroxide solution had been added.

The temperature readings are shown in the graph below.

3 0

2 9

2 8

2 7

2 6

2 5

2 4

2 3

2 2

2 1

2 00 4 8 1 2 1 6 2 0 2 4 2 8 3 2 3 6 4 0

V o l u m e o f N a O H ( a q ) / c m

T e m p e r a t u r e / º C

– 3


(i) Explain why the temperature reaches a maximum and then falls slightly on additionof further sodium hydroxide solution.

………….……………….……………………………………………………..

………….……………….……………………………………………………..

………….……………….……………………………………………………..

(2)

(ii) From the graph, what volume of the aqueous sodium hydroxide was required for complete reaction?

………….……………….……………………………………………………..

(1)

(iii) Calculate the amount (number of moles) of sodium hydroxide in this volume of solution.

(1)

(iv) Calculate the amount (number of moles) of copper ions that have reacted.

(1)

(v) Write the ratio of moles of copper ions to hydroxide ions reacting.

(1)

(vi) Write the formula of the copper hydroxide that is produced.


(1)


(b) The data call be used to find the enthalpy change for the reaction between sodium hydroxide and the copper salt.

(i) Use the graph to find the temperature rise that occurs for complete reaction.

………….……………….……………………………………………………..

(1)

(ii) Find the heat change, q, that occurs in the polystyrene cup for complete reaction.Use the formula

q = 168 × DT joules

(1)

(iii) Use your results from (a)(iv) and (b)(ii) above, to find the molar enthalpy change, DH, for the reaction. Give the correct sign and units to the answer.

(3)

(c) Identify one potential source of error in this experiment, and say what you would do to reduce its effect.

………………...……………….……………………………………………………..

………………...……………….……………………………………………………..

………………...……………….……………………………………………………..

………………...……………….……………………………………………………..

………………...……………….……………………………………………………..

(2)(Total 14 marks)



87. An excess of zinc powder was added to 20.0 cm3 of a solution of copper(II) sulphate of

concentration 0.500 mol dm–3. The temperature increased by 26.3 °C.

(a) How many moles of copper(II) sulphate were used in this experiment?

(1)

(b) Calculate the enthalpy change, ΔH, in kJ mol–1 for this reaction given that:

energy change = specificheat capacity

mass ofsolution

temperaturechange

/J /J g-1 K-1 /g /K

Assume that the mass of solution is 20.0 g and the specific heat capacity of the solution

is 4.18 J g–1K–1.

(2)(Total 5 marks)

88. (a) A mixture of hydrogen iodide, hydrogen and iodine (all in the gaseous state) establishes dynamic equilibrium if a constant temperature is maintained.

2HI (g) H2 (g) + I2 (g) ΔH = +9.6 kJ mol–1

(i) Explain the meaning of the term dynamic equilibrium.

............................................................................................................................

............................................................................................................................

............................................................................................................................

(2)



(ii) How, if at all, would the proportion of hydrogen iodide present at equilibrium change if the temperature were to be increased? Justify your answer.

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

(2)

(iii) The reaction is catalysed by metals such as gold and platinum. How, if at all, wouldthe proportion of hydrogen iodide present at equilibrium change if the reaction were to be catalysed? Justify your answer.

............................................................................................................................

............................................................................................................................

(1)


(b) Part of an energy profile for this reaction is shown below. It is not intended to be to scale.

Complete the profile showing:

the products;

the progress of both uncatalysed and catalysed reactions;

labelled arrows to indicate the activation energies of both the uncatalysed and catalysed reactions.

(4)(Total 9 marks)

89. Hydrogen peroxide decomposes to form water and oxygen.

H2O2(aq) H2O(l) + 21

O2(g)

(a) Suggest a method for following the rate of this reaction.

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

.....................................................................................................................................

(2)


(b) The reaction is first order with respect to hydrogen peroxide.

(i) Explain what is meant by the term first order.

............................................................................................................................

............................................................................................................................

(1)

(ii) The overall order of the reaction is one. Give the rate equation for the reaction.

Rate =

(1)

(iii) How would you use a graph of hydrogen peroxide concentration against time to show that the reaction is first order?

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

(2)

(c) The data in the table show the effect of temperature on the rate of this reaction.

T/K

Rate

/ mol dm–3 s–11/T

/K–1 ln(rate)

293 1.6 ×10–6 3.41 ×10–3 –13.3

302 4.2 ×10–6 3.31 ×10–3 –12.4

314 14.4 ×10–6 3.19 ×10–3 –11.1

323 33.8 ×10–6 3.10 ×10–3 –10.3


(i) On the axes below, sketch graphs for two temperatures, T1 and T2, where T2 is greater than T1, and use them to explain why increasing temperature has a dramaticeffect on the rate of this reaction.

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

(4)


(ii) Plot a graph of ln(rate), on the vertical axis, against 1/temperature, on the horizontal axis, on the grid below.

(3)


(iii) Use your graph and the equation below to calculate the activation energy, EA, for this reaction.

ln(rate) = constant RAE

(1/T) where R = 8.31 J K–1 mol–1

You should include the sign and units with your answer which should be given to two significant figures.

(3)(Total 16 marks)

90. The reaction of an acid with a base to give a salt is an exothermic reaction. In an experiment to

determine the enthalpy of neutralisation of hydrochloric acid with sodium hydroxide, 50.0cm3

of 1.00 mol dm–3 HCl was mixed with 50.0 cm3 of 1.10 mol dm–3 NaOH. The temperature rise obtained was 6.90 °C.

(a) Define the term enthalpy of neutralisation.

....................................................................................................................................

....................................................................................................................................

(1)

(b) Assuming that the density of the final solution is 1.00 g cm–3 and that its heat capacity is

4.18 J K–1 g–1, calculate the heat evolved during the reaction.

(3)



(c) 0.0500 mol of acid was neutralised in this reaction; calculate DHneutralisation in

kJ mol–1.

(2)

(d) Suggest why sodium hydroxide is used in slight excess in the experiment.

....................................................................................................................................

....................................................................................................................................

(1)(Total 7 marks)

91. (a) (i) Define the term standard enthalpy of combustion.

............................................................................................................................

............................................................................................................................

............................................................................................................................

............................................................................................................................

(3)

(ii) The values for the standard enthalpy of combustion of graphite and carbon monoxide are given below:

DHc /kJ mol–1

C (graphite) –394

CO(g) –283


Use these data to find the standard enthalpy change of formation of carbon monoxide using a Hess’s law cycle.

C(graphite) + 2

1

O2(g) CO(g)

(3)

(iii) Suggest why it is not possible to find the enthalpy of formation of carbon monoxide directly.

............................................................................................................................

............................................................................................................................

(1)

(iv) Draw an enthalpy level diagram below for the formation of carbon monoxide from graphite.

(1)


(b) Natural gas consists of methane, CH4. When methane burns completely in oxygen the reaction occurs as shown in the equation

CH4(g) + 2O2(g) CO2(g) + 2H2O(l) DHc = –890 kJ mol–1

Methane does not burn unless lit.

Use this information to explain the difference between thermodynamic and kinetic stability.

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

(4)(Total 12 marks)

92. A student was required to determine the enthalpy change for the reaction between iron and copper sulphate solution.

The student produced the following account of their experiment.

A p i e c e o f i r o n , m a s s a b o u t 3 g , w a s p l a c e d i n a g l a s s b e a k e r . 5 0 c m o f0 . 5 m o l d m a q u e o u s c o p p e r s u l p h a t e s o l u t i o n w a s m e a s u r e d u s i n g am e a s u r i n g c y l i n d e r a n d a d d e d t o t h e b e a k e r . T h e t e m p e r a t u r e o f t h em i x t u r e w a s m e a s u r e d i m m e d i a t e l y b e f o r e t h e a d d i t i o n a n d e v e r y m i n u t ea f t e r w a r d s u n t i l n o f u r t h e r c h a n g e t o o k p l a c e .

F e + C u S O F e S O + C u4 4

– 3

3

T i m i n g b e f o r ea d d i t i o n

1 m i n 2 m i n s 3 m i n s 4 m i n s 5 m i n s

T e m p e r a t u r e / ° C 2 2 2 7 2 9 2 6 2 4 2 2


(a) Suggest two improvements you would make to this experiment. Give a reason for each ofthe improvements suggested.

Improvement 1 ...........................................................................................................

....................................................................................................................................

Reason 1 ....................................................................................................................

....................................................................................................................................

Improvement 2 ...........................................................................................................

....................................................................................................................................

Reason 2 ....................................................................................................................

....................................................................................................................................

(4)

(b) In an improved version of the same experiment a maximum temperature rise of

15.2 °C occurred when reacting excess iron with 50.0 cm3 of 0.500 mol dm–3 aqueous copper sulphate solution.

(i) Using this data and taking the specific heat capacity of all aqueous solutions as

4.18 Jg–1 deg–1 calculate the heat change.

(1)

(ii) Calculate the number of moles of copper sulphate used.

(1)


(iii) Calculate the enthalpy change of this reaction in kJ mol–1.

(2)(Total 8 marks)

93. (a) The bombardier beetle Metrius contractus persuades potential predators to disappear by firing a boiling mixture of irritants at them. The reaction producing this ammunition is a redox reaction, H2O2 being the oxidising agent.

The two half-reactions involved are:

O H O

O H O

+ 2 H + 2 e + 0 . 7 0+ –

+ –

E / V

+ 1 . 7 7H O + 2 H + 2 e 2 H O2 2 2

(i) Write the overall equation for the reaction and show that the reaction is feasible.

(3)

(ii) The beetle makes use of an enzyme catalyst in the reaction. Explain in general terms how catalysts increase the rate of a chemical reaction using a graph of the Maxwell-Boltzmann distribution of molecular energies.

(5)


(iii) The reaction is highly exothermic; in principle its enthalpy of reaction could be found by using average bond enthalpies. By a consideration of the structure and bonding in the compounds involved, suggest why the use of the average bond enthalpies for C==O, C C, C==C and O H would give a highly inaccurate answer for the enthalpy of reaction.

(2)

(b) On heating hydrogen peroxide decomposes according to the equation

2H2O2 2H2O + O2

Hydrogen peroxide is marketed as an aqueous solution of a given ‘volume strength’. The

common 20-volume solution gives 20 dm3 of oxygen from 1 dm3 of solution. What is the

concentration in g dm–3 of such a solution? (Molar volume of any gas at the temperature

and pressure of the experiment is 24 dm3.)

(3)

(c) Hydrogen peroxide, H2O2, can also act as a reducing agent.

The rapid oxidation of hydrogen peroxide was used in World War II to generate steam to launch the V1 ‘flying bomb’. H2O2 (100 volume) was reacted with acidified potassium manganate(VII) solution.

(i) Write the half-equation for the oxidation of hydrogen peroxide to oxygen, O2.

(1)

(ii) The MnO–4 ions are reduced to Mn2+ during the reaction. Derive the overall

equation for the reaction between H2O2 and acidified KMnO4.

(2)

(iii) Suggest in terms of the collision theory of chemical kinetics why 100-volume

hydrogen peroxide (this gives l00 dm3 of oxygen from 1 dm3 of hydrogen peroxidewhen it decomposes to water and oxygen) was used rather than the more common 20-volume solution.

(2)(Total 18 marks)



94. (a) (i) Define the term standard enthalpy of formation, DHf .

..........................................................................................................................

..........................................................................................................................

..........................................................................................................................

..........................................................................................................................

(3)

(ii) The following table shows some values of standard enthalpy of formation.

Name Formula DHf /kJ mol–1

ethene C2H4(g) +52.3

hydrogen bromide HBr(g) –36.2

bromoethane C2H5Br(g) –60.4

Use the data in the table above to calculate the standard enthalpy change for the following reaction.

C2H4(g) + HBr(g) C2H5Br(g)

(2)

(iii) State the significance of the sign of the value obtained in part (a)(ii) above.

..........................................................................................................................

..........................................................................................................................

(1)



(b) Enthalpy changes can also be calculated using average bond enthalpy data.

Bond Average bond enthalpy/kJ mol–1

C == C +612

C C +348

C H +412

C Br +276

H Br +366

Use the data in the table above to recalculate the enthalpy change for the reaction in part (a)(ii).

C2H4(g) + HBr(g) C2H5Br(g)

(3)

(c) Suggest why the value obtained in part (b) above is likely to be less accurate than that obtained in part (a)(ii).

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

(2)(Total 11 marks)


95. The formation of magnesium chloride from magnesium and chlorine may be represented by the following Born-Haber cycle:

M g ( g ) + 2 C l ( g ) + 2 e

M g C l ( s )

M g ( g ) + 2 C l ( g )

2 +

2 +

M g ( g ) + C l ( g )2

M g ( s ) + C l ( g )2

2

–

M g ( g ) + C l ( g ) + 2 e2 +2

– –

(a) Define the terms:

Lattice enthalpy.

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

(3)

Enthalpy of atomisation.

....................................................................................................................................

....................................................................................................................................

....................................................................................................................................

(2)

(b) (i) Identify on the diagram the chance representing the enthalpy of atomisation of magnesium.


(1)


(ii) Use the data below to calculate the first electron affinity of chlorine.

Enthalpy changeValue of the enthalpy

change / kJ mol–1

Enthalpy of atomisation of magnesium

1st Ionisation energy of magnesium

2nd Ionisation energy of magnesium

Enthalpy of formation of magnesium chloride

Enthalpy of atomisation of chlorine

Lattice enthalpy of magnesium chloride

+150

+736

+1450

–642

+121

–2493

(2)

(c) Hydrogen gas reacts with sodium metal to form an ionic solid, NaH, which contains sodium cations.

Draw a Born-Haber cycle which could be used to determine the electron affinity of hydrogen.

(3)(Total 11 marks)


96. (a) The Born-Haber cycle for the formation of sodium chloride is shown below.

N a ( g ) + C l ( g ) + e

N a ( g ) + C l ( g ) + e

N a ( g ) + C l ( g )

N a ( s ) + C l ( g )

N a ( g ) + C l ( g )

N a C l ( s )

–

–

–

–

+

+

+

+

1

1

1

2

2

2

2

2

2

Use the data below to calculate the lattice enthalpy of sodium chloride.

Enthalpy changeValue of the

enthalpychange

/kJ mol–1

Enthalpy of atomisation of sodium +109

1st ionisation energy of sodium +494

Enthalpy of formation of sodium chloride –411


Electron affinity of chlorine –364

(2)


(b) Sodium chloride and magnesium oxide have very similar crystal lattices. Suggest why the lattice enthalpy of magnesium oxide is very much larger than that of sodium chloride.

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

(2)

(c) The lattice enthalpy of silver iodide can be calculated but the experimental value does notmatch the calculated value as well as those for sodium chloride match each other.

Explain why the calculated and experimental values for silver iodide are different.

................................................................................................................................

................................................................................................................................

................................................................................................................................

................................................................................................................................

(2)(Total 6 marks)

97. (a) The covalent compound urea, (NH2)2C==O, is commonly used as a fertiliser in most of the European Union whereas in the UK the most popular fertiliser is ionic ammonium nitrate, NH4NO3.

(i) Calculate the percentage of available nitrogen in urea.

(2)

(ii) Apart from the nitrogen content, suggest two advantages of using urea as a fertiliser compared with using ammonium nitrate.

(2)

(b) Some organic nitrogen compounds are used to manufacture polyamides by condensation polymerisation.

With the aid of diagrams, define the terms condensation polymerisation and polyamide.


(4)


(c) The ammonium ion in water has an acid dissociation constant,

Ka = 5.62 × 10–10 mol dm–3. The conjugate acid of urea has Ka = 0.66 mol dm–3. Use this data to explain which of ammonia or urea is the stronger base.

(2)

(d) Ethanamide, CH3CONH2, can be converted into methylamine, CH3NH2.

(i) State the reagents and conditions for carrying out the conversion.

(3)

(ii) Suggest the formula of the likely product if urea were used instead of ethanamide in this conversion.

(1)

(e) Ammonium nitrate can explode when heated strongly.

NH4NO3(l) N2O(g) + 2H2O(g) DH= –23 kJ mol–1

With moderate heating the ammonium nitrate volatilises reversibly

NH4NO3(s) NH3(g) + HNO3(g) DH = + 171 kJ mol–1

(i) State why the expression for Kp for the reversible change does not include ammonium nitrate.

(1)

(ii) 6.00 g of ammonium nitrate was gently heated in a sealed vessel until equilibrium

was reached. The equilibrium constant was found to be 15.7 atm2 under these conditions. Calculate the partial pressure of ammonia present at equilibrium and, hence, the percentage of the ammonium nitrate which has dissociated.

(One mole of gas under these conditions exerts a pressure of 50 atm.)

(5)

(iii) Explain the concepts of thermodynamic and kinetic stability with reference to thesetwo reactions.

(5)(Total 25 marks)



98. (a) State Hess’s Law.

..................................................................................................................................

..................................................................................................................................

..................................................................................................................................

(2)

(b) Define the term standard enthalpy change of combustion.

..................................................................................................................................

..................................................................................................................................

..................................................................................................................................

(3)

(c) The equation for the combustion of ethanol in air is

C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l)

and the structural representation of this is:

H C C O H + 3 O O 2 O C O + 3 H O H

H

H

H

H

(i) Calculate the enthalpy change for this reaction using the average bond enthalpy values given below.

Bond Average bondenthalpy/

kJ mol–1

Bond Average bondenthalpy/k

J mol–1

C H +412 C C +348

C O +360 O H +463

O O +496 C O +743

(3)


(ii) Draw and label an enthalpy level diagram to represent this reaction.

e n t h a l p y

(2)(Total 10 marks)

99. The enthalpy change of combustion of two fuels is listed below.

fuel enthalpy of combustion/kJ mol–1

hydrogen, H2 –280

octane, C8H18 –5510

(a) Calculate the enthalpy change per unit mass for each of the fuels.

Hydrogen:

Octane:

(3)


(b) Suggest, giving two reasons, which substance is the more useful as a fuel for motor cars and give your reasoning.

Reason 1.................................................................................................................

...............................................................................................................................

Reason 1.................................................................................................................

...............................................................................................................................

(2)

(c) Suggest one disadvantage of using the fuel chosen in (b)

...............................................................................................................................

(1)(Total 6 marks)

100. In an experiment to find the enthalpy change when copper is displaced from a solution of

copper ions excess zinc was added to 50.0 cm3 of 1.00 mol dm–3 aqueous copper(ii) sulphate in a plastic cup.

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

The temperature of the solution in the cup was measured every minute for 10 minutes with the zinc being added after 3.5 minutes.


The temperature readings are shown on the graph below.

7 0

6 0

5 0

4 0

3 0

2 0

1 00 1 2 3 4 5 6 7 8 9 1 0

T i m e / m i n u t e s

Tem

pera

ture

/°C

× × ×

××

××

××

×

(a) Suggest two reasons why a series of temperature readings is taken rather than simply initial and final readings.

First reason..............................................................................................................

................................................................................................................................

Second reason..........................................................................................................

................................................................................................................................

(2)


(b) Use the graph to calculate the maximum temperature change, DT, as the reaction takes place.

DT = ...........................................°C

(2)

(c) Calculate the enthalpy change for the reaction using the formula below, giving your answer to an appropriate number of significant figures.

DH = –4.18 × DT kJ mol–1

(2)(Total 6 marks)

101. (a) Define:

(i) the standard enthalpy of formation of benzene, C6H6(l);

......................................................................................................................

......................................................................................................................

......................................................................................................................

(2)

(ii) the standard enthalpy of combustion of benzene, C6H6(l).

......................................................................................................................

......................................................................................................................

......................................................................................................................


(2)


(b) Calculate the standard enthalpy of formation of benzene, C6H6(l), using the following enthalpy of combustion data:

Substance DHc /kJ mol–1

C6H6(l) –3273

H2(g) –286

C(s) –394

(3)

(c) If the standard enthalpy of formation is calculated from average bond enthalpy data assuming that benzene has three C==C and three C––C bonds, its value is found to be

+215 kJ mol–1.Explain, with reference to the structure and stability of benzene, why this value differs from that calculated in (b). Use an enthalpy level diagram to illustrate your answer.

................................................................................................................................

................................................................................................................................

................................................................................................................................

(4)


(d) Benzene reacts with bromine when gently warmed in the presence of a catalyst of anhydrous iron(III) bromide.

(i) The reaction is first order with respect to benzene and first order with respect to bromine. Write the rate equation for the reaction.

......................................................................................................................

(1)

(ii) The mechanism of this reaction involves an attack by Br+ followed by loss of H+.

+

+

+

+

B r

H

H

B r

B r

B r+ H

S t e p 1 .

S t e p 2 .

Deuterium, symbol D, is an isotope of hydrogen, and the C––D bond is slightly stronger than the C––H bond. If step 2 were the rate-determining (slower) step, suggest how the rate of this reaction would alter if deuterated benzene, C6D6, were used instead of ordinary benzene, C6H6, and explain your answer.

......................................................................................................................

......................................................................................................................

......................................................................................................................

(2)(Total 14 marks)

102. (a) Phosgene, COCl2, was used in the First World War as a poison gas. It can be prepared by reacting carbon monoxide with chlorine.

CO(g) + Cl2(g) COCl2(g) DH = – 112 kJ mol–1

1.0 mol of carbon monoxide and 1.0 mol of chlorine were placed in a vessel and heated to200 ºC. When equilibrium had been reached, it was found that the total pressure was 1.3 atm and that 85% of the carbon monoxide had reacted.


(i) Write the expression for Kp.

(1)

(ii) Calculate the value of Kp, stating its units.

(5)

(iii) State and explain the effect that an increase in temperature would have on the valueof the equilibrium constant.

......................................................................................................................

......................................................................................................................

......................................................................................................................

(2)

(b) The bond enthalpy of the bonds in carbon monoxide is +1065 kJ mol–1. The average C–

O bond enthalpy is +360 kJ mol–1 and that of C==O is +743 kJ mol–1. Use these data to suggest the number of bonds in carbon monoxide.

(1)


(c) Both phosgene and ethanoyl chloride, CH3COCl, contain a C==O group. Suggest an equation for the reaction of phosgene with excess ammonia.

................................................................................................................................

(2)

(d) The following reactions are those of compounds containing the C==O group. Draw the structural formulae of the organic products of the reactions between:

(i) ethanamide and bromine followed by the addition of sodium hydroxide solution,

(1)

(ii) ethanamide and phosphorus(V) oxide, P4O10,

(1)

(iii) propanal and hydrogen cyanide in the presence of a trace of alkali.

(1)


(e) Draw the stereoisomers of the product in (d)(iii).

(1)(Total 15 marks)

103. (a) (i) Draw ‘dot and cross’ diagrams to show the electronic structure of the ammonia andof the boron trifluoride molecules. Hence deduce their shapes and suggest values for the HNH and FBF bond angles in these molecules.

(5)

(ii) Explain, in terms of the intermolecular forces involved, the variation of the boiling temperatures of the Group 5 hydrides listed below.

Hydride Boiling Temperature/ K

Ammonia, NH3 240

Phosphine, PH3 183

Arsine, AsH3 218

Stibine, SbH3 256

(5)

(b) When ammonia and boron trifluoride are mixed, an addition compound, H3NBF3, is formed.

(i) Suggest how the nitrogen-boron bond forms between the two molecules in the addition compound.

(1)

(ii) Suggest how the HNH and the FBF bond angles would change when the addition compound forms.

(2)


(c) One of the early rocket fuels was hydrazine. It burns in oxygen as follows:

H2N––NH2(g) + O2(g) N2(g) + 2H2O(g)

When 1.00 kg of hydrazine is burnt in excess oxygen 1.83 × 104 kJ of heat energy is released. Use this and the average bond enthalpies below to calculate the N––N bond enthalpy.

BondBond

enthalpy/kJ

mol–1

Bond

Bondenthalp

y/kJ

mol–1

N––H +388 O==O +496

NN +944 H––O +463

(4)(Total 17 marks)

104. (a) Benzocaine, C9H11O2N, is an aromatic compound which is used commercially in creams to alleviate sunburn.

Benzocaine reacts with dilute acids to form the ion C9H12O2N+ and with ethanoyl chloride to form C11H13O3N.

When benzocaine is heated under reflux with aqueous sodium hydroxide and the solution obtained is neutralised, two compounds X and Y are formed.

X has a formula of C7H7O2N and is a solid with a melting temperature of 190 ºC. It issoluble in water.

Y is a volatile liquid with a formula C2H6O which gives steamy fumes with phosphorus pentachloride.

X reacts with sodium hydrogencarbonate solution to give a gas which turns lime watermilky. It also reacts with a solution of sodium nitrite and hydrochloric acid between 0 ºC and 5 ºC to produce a substance which reacts with phenol to give an orange precipitate, Z.


These reactions are summarised as follows.

C H O N

C H O NX

C H OY

C H O NC H O NB e n z o c a i n e

9 92

2 2 6

67 2

77

33

2+

+

1 2 1 1 1 31 1H ( a q ) C H C O C l

1 . N a O H ( a q ) h e a t / r e f l u x2 . H C l ( a q ) u n t i l n e u t r a l

+

21 . H N O b e t w e e n 0 º C a n d 5 º C2 . p h e n o l

3N a H C O ( a q )

o r a n g e p p t . ZC H O N N a

(i) Deduce a structural formula for benzocaine and explain its three reactions shown above. You may either describe the types of reaction or write the equations for the reactions.

(6)

(ii) Write equations for the two reactions of X. Include in your answer the structural formula of Z.

(3)

(iii) Explain why substance X has a fairly high melting temperature and why it is soluble in water.

(3)

(b) Substance X is a weak monobasic acid and for the purpose of the remainder of this question you may write its formula as HA.X has a relative molecular mass of 137, with a pKa value of 4.92 at 25 ºC.

(i) Calculate the pH of a solution containing 21.37g of X per dm3 at a temperature of 25 ºC.

(4)

(ii) 50.0 cm3 of this solution was mixed with 50.0 cm3 of a 0.100 mol dm–3 solution of sodium hydroxide. Calculate the concentration of the salt of X produced, and the concentration of the acid X left unreacted.Hence calculate the pH of the mixed solution.


(4)


(c) The standard enthalpy change at 25 ºC for the neutralisation of a strong acid by a strong

base is –57.2 kJ mol–1.

The standard enthalpy change for the ionisation of the weak acid HA in water is

+8.3 kJ mol–1.

(i) Write the ionic equation for the neutralisation of a strong acid by a strong base and hence calculate the standard enthalpy of neutralisation of the acid HA.

(3)

(ii) State and explain how the value of Ka of the acid X and hence the pH of the solution in (b)(i) would change if the temperature of the solution were increased.

(2)(Total 25 marks)

105. (a) Some standard enthalpy change of combustion values are listed below:

Substance DH /kJ

mol–1

C (graphite) –393.5

H2(g) –285.8

ethane C2H6(g) –1560.0

(i) Define standard enthalpy change of combustion

............................................................................................................................

............................................................................................................................

(2)

(ii) Complete the following thermochemical equation for the standard enthalpy change of combustion of ethane.

..C2H6(..)+..O2(..) ..CO2(..)+..H2O(..) DH =............................................

(3)


(iii) Use the data to calculate the standard enthalpy change of formation of ethane. Draw a Hess’s Law cycle as part of your answer.

(3)

(b) Consider the reaction

C2H6(g) + Cl2(g) C2H5Cl(g) + HCl(g) DH = 112.6 kJ mol–1

This reaction does not proceed at room temperature in the absence of light, but reacts rapidly when exposed to a bright light. Use these facts to illustrate the concept of thermodynamic and kinetic stability.

Thermodynamic stability ...........................................................................................

....................................................................................................................................

Kinetic stability .........................................................................................................

....................................................................................................................................

(2)

(c) Ethane and chlorine react in a homolytic free radical substitution reaction when exposed to light. The two propagation steps are

step 1:

C H + C l C H C + H C l

H H

H H

C H 3 3

step 2

C + + C l C l C l C H C C l

H H

H H

C H 3 3

(i) Explain the meaning of the term free radical.

............................................................................................................................

(1)


(ii) Explain the meaning of the term substitution reaction.

............................................................................................................................

(1)

(iii) Explain the movement of the C-H bond electron pair in step 1.

.........................................................................................................................

.........................................................................................................................

(2)

(iv) Give an example of a possible termination step in this reaction.

.........................................................................................................................

(1)

(d) The reaction of ethane with bromine proceeds in a similar way. Given the following

average bond enthalpies in kJ mol–1

C–H + 412 H–Br + 366

calculate the enthalpy change for step 1 of the reaction involving bromine.

(2)


(e) The product of bromination of ethane is bromoethane. This reacts with potassium cyanide in a solution of ethanol and water. The rate of this reaction was studied and the results are given below.

Experiment [CN–]/mol dm–3 [C2H5Br]/mol

dm–3Initial rate/mol dm–

3s–1

1 0.060 0.020 1.0 × 10–5

2 0.060 0.040 2.0 × 10–5

3 0.120 0.020 2.0 × 10–5

Deduce, showing your reasoning, the rate equation.

(3)

(f) Two routes can be suggested for the reaction in (e).

Route 1

C N + C H C H B r N C C B r C H C H C N + B r

H

H

C H

3 3

3

2 2. . . . . . . . . . . . . .

Route II

C H C H B r C H C H + B r3 32 2s l o w + –

then C H C H + C N C H C H C N3 32 2f a s t+ –

(i) Explain which route is consistent with the rate equation in (e).

...........................................................................................................................

...........................................................................................................................

(1)


(ii) This exothermic reaction is catalysed by iodide ions. Draw the enthalpy level diagram for both the uncatalysed reaction, labelling each clearly.

e n t h a l p y

(3)(Total 24 marks)

106. The Born-Haber cycle for the formation of sodium chloride from sodium and chlorine may be represented by a series of stages labelled A to F as shown.

N a ( g ) + C l ( g ) + e

N a ( g ) + C l ( g ) + e

N a ( g ) + C l ( g )

N a ( s ) + C l ( g )

+

+

+ –2

2

212

12

12

A

B

C

D

F

E

N a ( g ) + C l ( g )

N a C l ( s )

–


(a) (i) Write the letters A to F next to the corresponding definition in the table below

definition letter DH/kJm

ol–

1

1st ionisation energy of sodium +494

1st electron affinity of chlorine –364

the enthalpy of atomisation of sodium +109

the enthalpy of atomisation of chlorine +121

the lattice enthalpy of sodium chloride –770

the enthalpy of formation of sodiumchloride

(3)

(ii) Calculate the enthalpy of formation of sodium chloride from the data given.

(2)

(b) The lattice enthalpies can be calculated from theory as well as determined experimentally.

ExperimentalDH/kJ

mol–1

Theoretical

DH/kJ

mol–

1

Sodium chloride –770 –766

Silver iodide –889 –778


Why is the experimental value of the lattice enthalpy of silver iodide (–889kJmol–1) so different from the value calculated theoretically?

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(2)

(c) Explain the trend in first ionisation energies of the elements of Group 1 in the Periodic Table.

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(3)(Total 10 marks)


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