Mark Scheme Jan 1997 Unit-6

8/11/2019 Mark Scheme Jan 1997 Unit-6

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Winter 1997 CH6 Mark Scheme

1

1. Carvone

C

C

C

O

CH

CH

CH3

CH2 CH2

CH2H C3

O

CH3

CH2H C3

which may alsobe shown as

is the main flavouring material in spearmint oil.

(a) Draw the structure of the product obtained when carvone reacts with

CH

(2)

3

3

NNH 2

2

NO

2NO

any NN link at carbonyl (1)

CH BrBr

Br

Br

CH

O

(2) (1)

(2)

for each double bond that has been reactedif use bromine water if get addition to give ignore relative positions of these

(i)

(ii)

Br OH

(4)

(b) Explain why carvone does not react with ammoniacal silver nitrate.ketone so not easily oxidised (and therefore no reaction)/

no reducing group present / no aldehyde group present (1)

(1)

(c) Carvone is chiral and shows optical activity.

(i) Explain the meaning of chiral.mirror-image and original non superimposable /two non-superimposable mirror images /molecule can be left handed or right handed /molecule with a carbon atom attached to 4 different groups (1)

(1)

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(ii) Given that Kafor this reaction is 1.35 105mol dm3at 298 K, calculate the pH of

a 0.100 mol dm3solution of propanoic acid at this temperature.

[ ] . . ( )H K ca+

= = 135 10 01 15

= 0.00116 mol dm3(1)marked consequentially on first step

pH = 2.93 (1)allow 2.9/2.9( ) marked consequentially on above answermust be 2 or 3 significant figures if not (1)if correct answer and no working (1)only

(3)

(iii) If the solution referred to in (ii) were to be diluted tenfold, it might be expected thatthe pH would rise by one unit. In fact it rises by less than this. Explain.concentration of H+should decrease tenfold (1)but this will move equilibrium to the right / increase degree of dissociation (1)so reduction in [H+is less than tenfold / expected (1)if show statement to be true by calculation of new pH with no reason whychange occurs (max 2)

(3)

(b) During a titration NaOH solution of concentration 0.100 mol dm3was added to 30.0 cm3of 0.100 mol dm3propanoic acid.

(i) What property is shown by the reaction mixture when 15.0cm3of NaOH has beenadded to the propanoic acid?buffer (1)(OWTTE)

(1)

(ii) Calculate the pH of the reaction mixture at this stage. Propanoic acid has Ka= 1.35

103mol dm3,pKa= 4.87.

pH pK gsalt

acidK ressiona= + 1 12

[ ]

[ ]/ exp ( )

at stated point in titration [salt] = [acid] (1)pH = pKa= 4.87/4.9 (1)

(3)(iii) What criteria must b considered in the selection of an indicator for this titration?

pHlnof the indicator / indicator with sharp colour change at this value /

the range of pH over which the indicator changes colour (1)

(2)

(c) When hydrochloric or nitric acid is reacted with sodium hydroxide solution, the enthalpy

change is 57.1 kJ mol1. If propanoic acid is used instead of other acids, the anthalpy

change is less negative.

(i) Suggest why this is so.propanoic acid weak/partially ionised (1)Hdissociationof the weak acid is endothermic (1)

(2)

(ii) What would be the effect on the value of H for the reaction of using ammonia

solution instead of sodium hydroxide with propanoic acid?less negative / more positive / less exothermic (1)not lower/smaller/less

(1)(Total 16 marks)



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3. Propanoic acid dissociates in aqueous solution according to

CH3CH2CO2H(aq) + H2O(1) CH3CH2(aq) + H3O+(aq)

(a) Write the expression for Kafor propanoic acid.

KCH CH C H

CH CH COOHa =

+[ ][

[ ]( )

]

3 2

3 2

001

if water on the bottom line then (0)(1)

(b) Given that Kafor this reaction is 1.35 105mol dm3at 298 K, calculate the pH of a

0.100 mol dm3solution of propanoic acid at this temperature.

[ ] . . ( )H K ca+

= = 135 10 01 15

= 0.00116 mol dm3(1)marked consequentially on first step

pH = 2.93 (1)allow 2.9/2.9( ) marked consequentially on above answermust be 2 or 3 significant figures if not (1)if correct answer and no working (1)only

(3)

(c) If the solution referred to in (b) were to be diluted tenfold, it might be expected that the

pH would rise by one unit. In fact it rises by less than this. Explain.concentration of H+should decrease tenfold (1)but this will move equilibrium to the right / increase degree of dissociation (1)so reduction in [H+is less than tenfold / expected (1)if show statement to be true by calculation of new pH with no reason whychange occurs (max 2)

(3)(Total 7 marks)

4. During a titration NaOH solution of concentration 0.100 mol dm3was added to 30.0 cm3of0.100 mol dm3propanoic acid.

(a) What property is shown by the reaction mixture when 15.0cm3of NaOH has been addedto the propanoic acid?

buffer (1)(OWTTE)

(1)

(b) Calculate the pH of the reaction mixture at this stage. Propanoic acid has Ka= 1.35 10

3mol dm3,pKa= 4.87.

pH pK gsalt

acidK ressiona= + 1 12

[ ]

[ ]/ exp ( )

at stated point in titration [salt] = [acid] (1)

pH = pKa= 4.87/4.9 (1)

(3)

(c) What criteria must b considered in the selection of an indicator for this titration?pHlnof the indicator / indicator with sharp colour change at this value /

the range of pH over which the indicator changes colour (1)

(2)(Total 6 marks)



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5. When hydrochloric or nitric acid is reacted with sodium hydroxide solution, the enthalpy change

is 57.1 kJ mol1. If propanoic acid is used instead of other acids, the enthalpy change is lessnegative.

(a) Suggest why this is so.propanoic acid weak/partially ionised (1)Hdissociationof the weak acid is endothermic (1)

(2)

(b) What would be the effect on the value of H for the reaction of using ammonia solution

instead of sodium hydroxide with propanoic acid?less negative / more positive / less exothermic (1)not lower/smaller/less

(1)(Total 3 marks)

6. Alkanes and alkenes differ strikingly in their reactivity, for example with chlorine; this is a

consequence of their structure.

(a) Using a charge-cloud representation of the and conds show the structures of ethane

and ethene.

ignore in CH concentrate on CCignore absence of charge cloud here

mark for correct representation of

(2)

(b) (i) Give the equation for the reaction of one mole of ethane with one mole of chlorine.

CH CH + Cl CH CH Cl + HCl allow C H / C H Cl etc(1)3 3 2 3 2 2 6 2 5 (1)

(ii) Give the mechanism for this reaction.

Cl 2Cl

(1)

CH CH + Cl CH CH + HCl

(1)

CH CH + Cl CH CH Cl + Cl

(1)

2

3 3 3 2

3 2 2 3 2 (3)

(iii) Butane is a by-product by this reaction. How does it arise?

2CH CH butane (1)

if start with methane then ( for the Cl 2Cl and(max 2) (1)

(1)for a consequentially correct 2nd propagation step)

3 2

2

if start with methane then (max 2) (1)for the CI22 CIand


(1)



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(c) (i) Give the mechanism for the reaction of chlorine with ethene.

C C

ClCl :Cl

C CC C

Cl Cl Cl

each arrow ()(1)for intermediate(1)for arrow from chlorine the lone pair does not haveto be drawn in but the arrow must notcome from thenegative charge

(3)

(ii) Explain in terms of their structures why the reactions of ethane and of ethene with

chlorine are so different.ethene has more accessible electron density (1)which polarises CI2to form electrophile (1)

in ethane strong bonds need to be broken / bond less strong than (1)(or explain in terms of saturation of ethane)

(3)

(iii) Why are alkenes not used as fuels?

too valuable as synthetic intermediates / used to make polymers (1)not H values as these are about the same

(1)

(d) Alkynes such as ethyne

HCCH

are unsaturated compounds and undergo addition reactions in a similar fashion to alkenes.

Under suitable conditions ethyne will react with water, and the reaction might be expected

to be

HCCH + H2O CH2==CHOH

ethenol

(i) Using the bond enthalpies given below, suggest why the product of this reaction isnot in fact ethenol, but its structural isomer ethanal, CH3CHO.

H/kJ mol1: CH412, C==C612, CO 360, C==O 743, OH 463

CC 348, C837.sum of bond enthalpies of ethenol= 3 412 + 612 + 360 + 463 = 2671 kJ mol1(1)sum of bond enthalpies of ethanal= 4 412 + 348 + 743 = 2739 kJ mol1(1)

production of ethanal more exothermic (1)so thermodynamically favoured / product more stable (1)if answers incorrect but in correct order or calculation not attempted last 2marks can still be scored

(4)



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(ii) The catalyst used for the reaction is mercury(II) sulphate. The process was

responsible for a serious pollution incident in Japan in the 1950s, when mercury(II)sulphate effluent was converted by bacteria in river muds into methylmercury,

(CH3)2Hg. This then entered the food chain via fish caught locally. HgSO4is a

typical inorganic compound, (CH3)2Hg is typically organic and is very much more

toxic. Suggest a reason why?Me2Hg covalent and fat-soluble (. retained in body) (1)

allow similar argument based on solubility of HgSO4

(1)

(e) Alkanes are obtained by the fractional distillation of crude oil; this system is complex, but

it does not form any azeotropes. The boiling-point/composition diagram shown is for apair of liquids Aand Bwhich doe form an azeotrope:

0 1

Temperature

Mole fraction of A

X

(i) State and explain what would happen if a mixture of composition was fractionallydistilled.distillate richer in B / residue richer in A (1)pure B distils off first (1)until azeotropic composition reached (1)when azeotrope distils unchanged (1)

(4)

(ii) Does this system show a positive or negative deviation from Raoults Law?State how this deviation arises.negative (1)interactions between molecules/intermolecular forces in mixture greater thanin either of pure substances (1)

(2)(Total 25 marks)



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7. Alkanes and alkenes differ strikingly in their reactivity, for example with chlorine; this is a

consequence of their structure.

Using a charge-cloud representation of the and conds show the structures of ethane and

ethene.

ignore in CH concentrate on CC ignore absence of charge cloud here

mark for correct representation of

(Total 2 marks)

8. (a) Give the equation for the reaction of one mole of ethane with one mole of chlorine.

CH CH + Cl CH CH Cl + HCl allow C H / C H Cl etc(1)3 3 2 3 2 2 6 2 5 (1)

(b) Give the mechanism for this reaction.

Cl 2Cl

(1)

CH CH + Cl CH CH + HCl

(1)

CH CH + Cl CH CH Cl + Cl

(1)

2

3 3 3 2

3 2 2 3 2 (3)

(c) Butane is a by-product by this reaction. How does it arise?

2CH CH butane (1)

if start with methane then ( for the Cl 2Cl and(max 2) (1)


3 2

2

if start with methane then (max 2) (1)for the CI2 2 CIand


(1)(Total 5 marks)

9. (a) Give the mechanism for the reaction of chlorine with ethene.

C C

ClCl :Cl

C CC C

Cl Cl Cl

each arrow ()(1)for intermediate(1)for arrow from chlorine the lone pair does not haveto be drawn in but the arrow must notcome from thenegative charge

(3)

(b) Explain in terms of their structures why the reactions of ethane and of ethene with

chlorine are so different.ethene has more accessible electron density (1)which polarises CI2to form electrophile (1)

in ethane strong bonds need to be broken/ bond less strong than (1)(or explain in terms of saturation of ethane)

(3)

(c) Why are alkenes not used as fuels?too valuable as synthetic intermediates / used to make polymers (1)



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not H values as these are about the same

(1)(Total 7 marks)

10. Alkynes such as ethyne

HCCH

are unsaturated compounds and undergo addition reactions in a similar fashion to alkenes.

Under suitable conditions ethyne will react with water, and the reaction might be expected to be

HCCH + H2O CH2==CHOH

ethenol

(a) Using the bond enthalpies given below, suggest why the product of this reaction is not infact ethenol, but its structural isomer ethanal, CH3CHO.

H/kJ mol1: CH412, C==C612, CO 360, C==O 743, OH 463

CC 348, C837.sum of bond enthalpies of ethenol= 3 412 + 612 + 360 + 463 = 2671 kJ mol1(1)

sum of bond enthalpies of ethanal= 4 412 + 348 + 743 = 2739 kJ mol1(1)

production of ethanal more exothermic (1)so thermodynamically favoured / product more stable (1)if answers incorrect but in correct order or calculation not attempted last 2marks can still be scored

(4)

(b) The catalyst used for the reaction is mercury(II) sulphate. The process was responsible

for a serious pollution incident in Japan in the 1950s, when mercury(II) sulphate effluentwas converted by bacteria in river muds into methylmercury, (CH3)2Hg.

This then entered the food chain via fish caught locally. HgSO4is a typical inorganic

compound, (CH3)2Hg is typically organic and is very much more toxic. Suggest a

reason why?Me2Hg covalent and fat-soluble (. retained in body) (1)

allow similar argument based on solubility of HgSO4

(1)(Total 5 marks)



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11. Alkanes are obtained by the fractional distillation of crude oil; this system is complex, but it

does not form any azeotropes. The boiling-point/composition diagram shown is for a pair ofliquids Aand Bwhich doe form an azeotrope:

0 1

Temperature

Mole fraction of A

X

(a) State and explain what would happen if a mixture of composition was fractionally

distilled.distillate richer in B / residue richer in A (1)pure B distils off first (1)until azeotropic composition reached (1)when azeotrope distils unchanged (1)

(4)

(b) Does this system show a positive or negative deviation from Raoults Law?State how this deviation arises.

negative (1)interactions between molecules/intermolecular forces in mixture greater thanin either of pure substances (1)

(2)(Total 6 marks)

12. Phenylamine, C6H5NH2, can be made in several ways from benzene. The flowchart shows two

of these in outline.

NO2

2

Step 1

Step 2

Step 3

NH

C H8 10

C H O7 6 2

C H OCl7 5

C H NO7 7

A

B

C

D

MnO /OH4

phenylamine

AlCl /C H Cl3 2 5



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(a) (i) Give the reagents and conditions needed for step 1.concentrated nitric + (concentrated) sulphuric (1)40 60 C/heat/warm to max 60 C (1)

(2)

(ii) Write the mechanism for this reaction.

HNO + 2H SO 2HSO + H O + NO

or HNO + H SO HSO + H O + NO (1)

(1)3 2 4 4 3 2

3 2 44 2 2 +

NO2

HNO2 NO2

+

+H

(4)

(iii) Give the reagents and conditions for step 2.tin / iron (1)+ concentrated HCI (1)allow ethanoic acid for ironheat (1)not refluxnot LiAIH4

(3)

(b) (i) In the alternative pathway identify the compounds Aand Dinclusive.

CH CH

COCl

COOH

CONH

2

2

3A

C

B

D

(1)each = (4) allow correct unambiguous names ieA ethylbenzene / phenylethane B benzenecarboxylic acid/benzoic acidC benzenecarbonyl chloride/benzoyl chloride D benenecarboxamide/benzamide

(4)

(ii) State the reagents and conditions needed for step 3.bromine (1)alkali / NaOH (1)heat (1)

(3)

(iii) Why is the first pathway commercially preferable for the manufacture of

phenylamine?quicker formation of product/simplest path best because yields higher / moreeconomic (1)

(1)



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(c) Phenylamine reacts with nitrous acid in the presence of concentrated hydrochloric acid at

05C to produce benzenediazonium chloride.

If the temperature of the solution rises, this compound reacts with the solvent water togive phenol, C6H5OH, and nitrogen gas. The rate of the reaction can be followed by

measuring the volume of nitrogen produced at various times, pressure and temperatureremaining constant. The amount of benzenediazonium chloride remaining in the solution

is proportional to V Vt, where Vtis the volume of gas at time tand Vis that at the end

of the reaction.

(i) Give the equation for the reaction between benzenediazonium chloride and water.

N Cl2 OH

2 2+ H O + N + HCl

+

allow C6H5or kekule

if no charges (1)

(2)

(ii) Some results of this experiment are shown below.

250

200

150

100

50

00 500 1000

t/s

)/cm

V

V

3

t

gets half-life from graph at least two readings (2)half-life constant (1)characteristic of first order reactions (1)if say not constant but then say not1st order award the last mark

Deduce the order of reaction with respect to benzenediazonium chloride fromthe graph.

(4)

(iii) Is the reaction necessarily of this order overall? Explain your answer.the other reagent/water present (1)which may / may not contribute to theoverall order (1)

(2)(Total 25 marks)



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13. (a) Phenylamine, C6H5NH2, can be made in several ways from benzene. The flowchart

shows two of these in outline.

NO2

2

Step 1

Step 2

Step 3

NH

C H8 10

C H O7 6 2

C H OCl7 5

C H NO7 7

A

B

C

D

MnO /OH4

phenylamine

AlCl /C H Cl3 2 5

(i) Give the reagents and conditions needed for step 1.concentrated nitric + (concentrated) sulphuric (1)40 60 C/heat/warm to max 60 C (1)

(2)

(ii) Write the mechanism for this reaction.

HNO + 2H SO 2HSO + H O + NO

or HNO + H SO HSO + H O + NO (1)

(1)3 2 4 4 3 2

3 2 44 2 2 +

NO2

HNO2 NO2

+

+H

(4)

(iii) Give the reagents and conditions for step 2.tin / iron (1)+ concentrated HCI (1)allow ethanoic acid for ironheat (1)not refluxnot LiAIH4

(3)



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(b) (i) In the alternative pathway identify the compounds Aand Dinclusive.

CH CH

COCl

COOH

CONH

2

2

3A

C

B

D

(1)each = (4) allow correct unambiguous names ieA ethylbenzene / phenylethane B benzenecarboxylic acid/benzoic acidC benzenecarbonyl chloride/benzoyl chloride D benenecarboxamide/benzamide

(4)

(ii) State the reagents and conditions needed for step 3.bromine (1)alkali / NaOH (1)heat (1)

(3)

(iii) Why is the first pathway commercially preferable for the manufacture of

phenylamine?quicker formation of product/simplest path best because yields higher / moreeconomic (1)

(1)(Total 17 marks)

14. Phenylamine reacts with nitrous acid in the presence of concentrated hydrochloric acid at 0

5C to produce benzenediazonium chloride.If the temperature of the solution rises, this compound reacts with the solvent water to give

phenol, C6H5OH, and nitrogen gas. The rate of the reaction can be followed by measuring the

volume of nitrogen produced at various times, pressure and temperature remaining constant.

The amount of benzenediazonium chloride remaining in the solution is proportional to V Vt,

where Vtis the volume of gas at time tand Vis that at the end of the reaction.

(a) Give the equation for the reaction between benzenediazonium chloride and water.

N Cl2 OH

2 2+ H O + N + HCl

+

allow C6H5or kekule

if no charges (1)

(2)



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(b) Some results of this experiment are shown below.

250

200

150

100

50

00 500 1000

t/s

(

)/cm

V

V

3

t

gets half-life from graph at least two readings (2)half-life constant (1)characteristic of first order reactions (1)if say not constant but then say not1st order award the last mark

Deduce the order of reaction with respect to benzenediazonium chloride from the graph.(4)

(c) Is the reaction necessarily of this order overall? Explain your answer.the other reagent/water present (1)which may / may not contribute to theoverall order (1)

(2)

(Total 8 marks)

15. The elements in Group 4 of the periodic table below a trend from non-metallic to metallic

behaviour as atomic number increases. At the same time oxidation state + 2 becomes morestable compared with oxidation state + 4.

(a) Explain why metallic character increases with increasing atomic number in Group 4.atomic radius increases (1)ionisation energies fall (1)must be present

covalent bond strengths fall (1)ions less polarising (1)

other possible4 valid points which are explanations of trends of metals in element orcompounds notstatements or properties of element or compoundif candidate deals in detail with only one property can obtain (3 max)



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(b) Lead forms oxides in the +2 and +4 oxidation state.(4)

(i) State the acid-base character of lead(I1) oxide, illustrating your answer

with equations.amphoteric (1)balanced equation for reaction as acid lead ion Pb(OH)42 / Pb02 (1)

balanced equation for reaction as base (1)any valid molecular or ionic equationif write 2 equations which do not balance but correct lead compounds then(1)

(3)

(ii) Write an equation for the reaction of lead(JY) oxide with concentrated {TCI atroom temperature, and explain the difference between this reaction and that in part

(b)(i)Pb02+ 4HCI PbCI2+ 2H20 (1)

Pb(IV) oxidising / redox reaction (1)since o.s. (II) is more stable (1)inert pair effect with brief explanation (1)

(4)

(c) Tetrachloromethane does not react with cold water, whereas silicon tetrachloride

hydrolyses rapidly. In contrast, in Group 3 both boron trichloride and aluminiumtrichloride hydrolyse rapidly.

(i) State the shape of a molecule of boron trichloride and justify it in terms of electronstructures.trigonal planar (1)3 pairs electrons as far apart as possible (1)

(2)

(ii) Write the equation for the reaction of boron trichloride with water. Explain whyboron trichloride reacts with water whereas tetrachloromethane, CCI4, does not.

BCI3is electron deficient / B only 6 electron when C has 8/

BCI3has vacant (p-)orbital via which attacking lone pair on water

can coordinate (1)if ref to d orbital then (0) or to expansion of octet (0)CCI4has to break bonds first, since it is saturated (or words to that effect) (1)

(3)

(iii) Explain how you would use solutions of silver nitrate and ammonia to show that thereaction product from boron trichloride and water contains chloride ions.(+AgNO3(solution)) white (1)precipitate (1)

soluble in dilute ammonia (1)

(3)

(d) Silver nitrate can also be used to show the presence of iodide ions in solution. Silveriodide is sparingly soluble in water.

(i) Given the thermochemical data below, suggest why silver iodide is so insoluble.lattice enthalpy of AgI 876 kJ mol1

hydration enthalpies: Ag+ 464 kJ mol1

I 293 kJ mol1

+876

Ag (g) + I (g)

464293

AgI(s) Ag (aq) + I (aq)

+

+

H solution



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cycle (1)

Hsolution=876 464 293 kJ mol1

=+119 kJ mol1 (1)

or

lattice energy > sum of hydration energy (1)some ref to how this relates to the value of Hsolutionand its effect on

solubility

(this point could be covered in (ii)) (1)

(2)

(ii) Explain why your result from (i) only suggestswhy AgI is so insoluble.

H may not control the feasibility of the reaction /

H is not the only factor that needs to be considered /there may be some other important factor /

+ H not mean reaction will not go (1)reference to entropy accepted but not expected

(1)

(iii) The following equilibrium is established when excess solid silver iodide is incontact with a saturated solution of silver iodide:

AgI(s) Ag+(aq) + I(aq)

Silver nitrate is used to precipitate halides in quantitative analysis, where theprecipitate is weighed to find the amount of halide. Why, in such experiments, is an

excess of silver nitrate added to the test solution?if [Ag+] rises (1)equilibrium driven to LHS (1)so precipitation is complete (with excess Ag+) (1)

(3)(Total 25 marks)

16. (a) What is

(i) a d-block elementlast electron going into a d-orbital (1)

(ii) a transition element?element having incomplete/partially-filled d-orbitals in one or more of its ions(1)

(2)

(b) Transition elements characteristically have several oxidation states; iron commonly has

oxidation states of +2 and +3, but these are not the only ones.

(i) Explain why, in normal conditions, iron(II1) is more stable than iron(II).Fe3+d5Fe2+d6 (1)d5more stable (1)

(2)



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(ii) Oxidation of iron with potassium nitrate in alkaline solution gives a red compound,

R which contains potassium, iron and oxygen only. Ris a powerful oxidising agent,and will oxidise iron(IT) to iron(III).

25.0cm3of a 0.100 mol dm solution of Roxidised 37.5 cm3of a solution of

iron(I1) containing 55.6 g dm3of FeSO4.7H20.

Calculate the oxidation state of the iron in R.FeSO47H2O has RMM 278

concentrated FeSO47H20 =556

2780 2 13

.. ( )= mol dm

moles oxidised = 0 237 5

10007 5 10 13 2.

.. ( ) = +mol Fe

moles R used =25

100001 2 5 10 1

3 =

. . ( )mol

R : Fe2+= 3 : 1 (1)3 electron change

Fe(VI) (1)if get ratio based on their figures can get consequential mark

(5)

(iii) Suggest a formula for R.K2Fe04(1)consequential on (ii) within reason ie >3

(1)

(c) (i) It is claimed that the cyanide complex of iron(II) is more easily oxidise (1 by air, in

acidic solution, than the hydrated complex. What information would you seek froma data book, or reference book, to consider this claim, and what relative numerical

values would you expect to find if the claim was justified? Write ionic halfequations appropriate.Evalues (1)forFe(CN63 (+ e ) Fe(CN)64 (1)

Fe(H2O)63+(+ e ) Fe(H2O)62+(1)

if claim true cyanide complex less easily reduced (III II) / or reverse (1)so E0for this half reaction more negative / or reverse (1)

(5)

(ii) Predictions from E~ values concerning the feasibility of a reaction are not realisedin practice. Suggest reasons for this.kinetic factors (1)non standard conditions (1)

if E values very close / difference in E values small (1)(max 2)

(2)



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18. Transition elements characteristically have several oxidation states; iron commonly has

oxidation states of +2 and +3, but these are not the only ones.

(a) Explain why, in normal conditions, iron(II1) is more stable than iron(II).Fe3+d5Fe2+d6(1)d5more stable (1)

(2)

(b) Oxidation of iron with potassium nitrate in alkaline solution gives a red compound, Rwhich contains potassium, iron and oxygen only. Ris a powerful oxidising agent, and will

oxidise iron(IT) to iron(III).

25.0cm3of a 0.100 mol dm solution of Roxidised 37.5 cm3of a solution of iron(I1)containing 55.6 g dm3of FeSO4.7H20.

Calculate the oxidation state of the iron in R.FeSO47H2O has RMM 278

concentrated FeSO47H2O =556

2780 2 13

.. ( )= mol dm

moles oxidised = 0 237 5

10007 5 10 13 2.

.. ( ) = +mol Fe

moles R used =25

100001 2 5 10 1

3 =

. . ( )mol

R : Fe2+= 3 : 1 (1)3 electron change

Fe(VI) (1)if get ratio based on their figures can get consequential mark

(5)

(c) Suggest a formula for R.K2FeO4(1)consequential on (b) within reason ie > 3

(1)(Total 8 marks)

19. (a) It is claimed that the cyanide complex of iron(II) is more easily oxidise (1 by air, in acidic

solution, than the hydrated complex. What information would you seek from a data book,or reference book, to consider this claim, and what relative numerical values would you

expect to find if the claim was justified? Write ionic half equations appropriate.Evalues (1)forFe(CN63 (+ e ) Fe(CN)64 (1)

Fe(H2O)63+(+ e ) Fe(H2O)62+(1)

if claim true cyanide complex less easily reduced (III II) / or reverse (1)so EOfor this half reaction more negative / or reverse (1)

(5)

(b) Predictions from E~ values concerning the feasibility of a reaction are not realised in

practice. Suggest reasons for this.kientic factors (1)non standard conditions (1)

if E values very close / difference in E values small (1)(max 2)

(2)(Total 7 marks)



21/21


20. Rusting of iron is commercially very important, both to those who lose articles to rust and those

who manufacture their replacements.

(a) Show how rusting occurs at a wet iron or steel surface, making clear its electrochemical

nature.

O

O

O

Fe

Fe OH

Fe

2

2

2

O + 2H O + 4e 4OH2 2

3+

3+

3+

Fe2+

Feanodic cathodic

(1)

(1)

(1)

(1)

e

rust

(1)

(1)

or words to that effect involving the stages:conversion of Fe Fe2+(1)

Fe2+Fe Fe2+(1)

Fe2+Fe3 (1)

O2absorbed (1)

O2gains electrons from Fe (1)

O2+ 2H2O + 4 e 4OH (1)

or O2+ 4H++ 4e 2H2O followed by H2O H++ OH

moves to RHS to replace H+lost

Fe3++ 3OHrust (1)

(6)

(b) Explain why rusting is usually faster in marine or industrial environments.

electrolyte more acidic/ionic (1)higher conductivity (1)so reactions faster since cell resistance less (1)change of electrode potential of Fe (1)

(2)(Total 8 marks)

Documents

Mark Scheme Jan 1997 Unit-6