t2 Chem Revision Ex 16

8/8/2019 t2 Chem Revision Ex 16

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1 In 1756, Baron Cronstedt, a Swedish geologist, was examining one of his countrys rareminerals. He heated it strongly and to his surprise it bubbled as if it were boiling and gaveoff clouds of steam. He decided to call this strange mineral a zeolite, which is Greek forboiling stone. Later research showed how the chemical structure of a zeolite gives itvaluable catalytic properties.

About forty zeolites are known today, and the names and formulae of some of thecommonest are shown below in Box 1.

The atoms inside the square brackets form the aluminosilicate framework of the mineral

with the negatively charged oxygen. Cations are present to balance the negative charges.

Box 1 Some common natural zeolites

Analcime Na16[(AlO2)16(SiO2)32].16H2O

Chabazite Ca2[(AlO2)4(SiO2)8].13H2O

Clinoptilolite Na6[(AlO2)6(SiO2)30].24H2O

Stilbite NaCa4[(AlO2)9(SiO2)27].30H2O

(a) Examine Box 1 which shows the chemical formulae of five common zeolites.

(i) What is the relationship between the number of aluminium atoms and sodium ions as theonly metal ions in a zeolite?

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(ii) Using this relationship, deduce t he formula of natrolite which has an aluminosilicateframework with the formula of [(AlO 2)16(SiO2)24].16H2O.

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(b) Deduce the formula of the resulting zeolite formed if the sodium ions of clinoptilolite arereplaced with sufficient calcium ions to preserve overall electrical neutrality.

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(c)(i) Draw a dot-and-cross diagram to show the bonding present in a silicate tetrahedron.

Class Reg Number

Candidate Name .......................................................................

Chemistry H2 9746Tutor Tutee

Revision Exercise 16: Integrated Questions


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(ii) Show the ionic nature of the aluminate tetrahedron, assuming it contains aluminium andoxide ions.

(iii) Hence, explain if it would be correct to describe a zeolite as consisting of a giant ioniclattice.

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(d)(i) Aluminium oxide reacts with sodium hydroxide and water to form sodium aluminate,NaAl(OH)4. This is how sodium aluminate is prepared for the laboratory synthesis ofzeolites.

Write an equation to describe this reaction.

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(ii) Sodium aluminate is also formed when aluminium is dissolved in sodium hydroxidesolution. Water is involved in the reaction and hydrogen as is produced.

Write an equation to describe this reaction.

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(iii) Metals and their oxides generally react with dilute acids, but not with alkalis. Explain why itis unusual for aluminium and its oxides to be able to react with acids and alkalis.

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(iv) Draw the structural formula for the silicate anion and deduce its molecular shape. Assumethat the silicate ion is a simple ion.


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2 In the late 19th century the two pioneers of the study of rea ction kinetics, Vernon Harcourtand William Esson, studied the rate of the reaction betwe en hydrogen peroxide and iodideions in acidic solution.

H2O2 + 2I

+ 2H+p 2H2O + I2

This reaction is considered to go by the following steps.

step 1 H2O2 + Ip IO + H2O

step 2 IO

+ H+p HOI

step 3 HOI + H+

+ Ip I2 + H2O

The general form of the rate equation is as follows.

rate = k[H2O2]a[I]b[H+]c

(a) Explain how the rate of reaction can be measured.

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(b) Suggest values for the orders a, b and c in the rate equation for each of the followingcases.

A study was carried out in which both [H 2O2] and [H+] were kept constant at 0.05 mol dm -3,

and [I] was plotted against time. The following curve was obtained.


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(c) Calculate the initial rate of this reaction, indicating its units.

(d) Use half-life data calculated from the graph to show that t he reaction is first order withrespect to [I].

(e) Use the following data to deduce the orders with respect to [H2O2] and [H+], explaining your

reasoning.

(f) From your results, deduce which of the three steps is the slowest (rate determining) step.

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3(a) (4-aminophenyl)ethanoic acid (4 -APEA) and its derivatives are being investigated aspossible drugs to treat chronic inflammation of the intestines. The synthesis of 4 -APEAfrom methylbenzene is shown in the following scheme.

(i) Draw the structures of the compounds G and H in the boxes above.

(ii) Suggest reagents and conditions for the following steps.

Step II

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Step III

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Step V

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(b) Silk from silkworms, used as a fabric shows a different secondary structure to thatproduced by spiders.


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(i) What sort of bonding would you expect to occur between adjacent parts of the proteinchains in each form of silk?

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(ii) Suggest two differences in properties that these forms of silk could have. Explain your

answer.

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(iii) Spider dragline silk contains large amounts of the amino acid glycine. How does this affect

the properties of the silk?

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4(a) Explain why most compounds containing transition metals are coloured, whereas compounds of non-transitionmetals are usually colourless.

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(b) When the neutral V(II) complex V(H2O)4Cl2 is added to water, the green complex dissolves toform a violet solution. Explain why this change of colour takes place.

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(c) State the colour and formulae of the aqueous ions of vanadium in oxidation states III and IV.

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(d) Titration of a sample containing 0.0010 mol of an oxychloride of vanadium, VOC lx, required20.0 cm3 of 0.020 mol dm3 KMnO4(aq) for its complete oxidation in acidic solution.

(i) Use EU data from the Data Bookletto predict the final oxidation state of the vanadium after


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the titration.

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(ii) Use the titration data given above to calculate the change in oxidation number undergoneby the vanadium during the titration.

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(iii) Hence deduce a value for x in VOClx.

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5 Suggest explanations for the following observations, writing relevant equations where possible.

(a) Adding aqueous barium chloride to an orange solution of sodium dichromate(VI) causes a yellow solid to beprecipitated, and an acidic solution to remain.

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(b) Adding aqueous ammonia to a solution of copper(II) sulphate produces first a pale blue precipitate, then adeep blue solution.

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(c) Chromium(III) chloride crystallises in three forms, all having the formula CrCl3(H2O)6. One form is violet, andanother is green.

Adding aqueous silver nitrate to a solution of the violet form precipitates all the chloride it contains, but only twothirds of the chloride from the green form is precipitated.

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6 The diagram shows a laboratory illustration of a simple hydrogen-oxygen fuel cell.


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(a) Write the half equation for the reaction occurring at the left hand (oxygen) electrode whenthe cell operates.

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(b) State the polarity (+ or ) of the left hand (oxygen) electrode. .....................................

(c) Use the Data Booklet to calculate the voltage produced by this cell.

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(d) Only a very small current can be drawn from this labo ratory cell. Suggest one way in whichit could be modified to enable a larger current to be drawn from it.

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(e) A fuel cell in an orbiting satellite is required to produce a current of 0.010 A for 400 days.Calculate the mass of hydrogen that will be needed.

(f) State one advantage, and one disadvantage of using fu el cells to power road vehicles

compared to hydrocarbon fuels such as petrol.

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7 The water which runs from waste heaps associated with copper mines contains low


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concentrations of dissolved copper ions. Companie s have begun removing Cu2+

ions fromthe water by the process of ligand exchange solvent extraction.

A compound which is a good ligand for copper ions is dissolved in an orga nic solvent thatis immiscible with water. When this solution is shaken with the water containing the copperions, the following reaction takes place:

Cu2+(aq) + 2LH(organic) CuL2(organic) + 2H+(aq) Equation 7.1

(where L represents the ligand).

(a) Explain why the process represented in Equation 7.1 is classed as a ligand exchangereaction.

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(b) The effect of the process is to remove a low concentration of copp er ions from water and totransfer them, at much higher concent ration, to the organic solvent. The process can bereversed by then shaking the organic solution with moderately concentrated acid. Thispushes Cu

2+ions back into aqueous solution and, again, an increase in concentration can

be achieved.

(i) Use Equation 7.1 to explain why extraction of Cu2+ ions into the organic solvent and thentheir reversal back into the aqueous phase is pH dependent .

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(ii) Suggest how an increase in concentration of copper ions is achieved at each stage of theextraction.

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(c) Research chemists have carried out experiments to find equilibrium constant, K c, values for

reactions like the one in Equation 7.1. Aqueous solutions of Cu2+

ions were shaken withsolutions of a ligand in an organ ic solvent and allowed to reach equilibrium. In oneexperiment, which was maintained at pH = 2 (i.e. [H+] = 1.0 x 102 mol dm3) and 298 Kthroughout, the equil ibrium mixture was analysed and found to correspond to the followingconcentrations:

[Cu2+(aq)] = 0.0020 mol dm3[CuL2(organic)] = 0.045 mol dm

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[LH(organic)] = 0.10 mol dm3

(i) Write an expression, in terms of concentrati ons, for Kc for the reaction in Equation 7.1.

(ii) Calculate the ratio of the concentration of Cu2+

in the organic phase to the concentration ofCu

2+in the aqueous phase, and hence the percentage of copper extracted in the

experiment.

(iii) Calculate a value for K c at 298 K from the results of this experiment.

8 It has recently been reported that an incident occurr ed at the Dounreay fast -reactor nuclear

plant in Scotland in May 1977 in which a bout 2 kg of sodium were dumped down a shaftwhich had earlier been used for the disposa l of radioactive waste. (Liquid sodium is usedas a coolant in this type of reactor.) The shaft was partially flooded wi th seawater, and theviolent reaction between sodium and water led to an explosion which scattered radioactivematerial over the nearby area.

(a) A number of highly exothermic reactions occur when sodium comes into contact with water.The principal reaction is

2Na(s) + 2H2O(l) p 2NaOH(aq) + H2(g) Equation 8.1

(i) Suggest why the reaction of sodium with water, in the restricted situation of the old mineshaft, gave rise to an explosion.

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(ii) Calculate the standard enthalpy change for the reaction in Equation 8.1 using the standardenthalpy change of formation values which follow.


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(HfU /kJ mol1: H2O(l) = 286, NaOH(aq) = 470

(b) Sodium and magnesium are neighbours in the P eriodic Table. Sodium hydroxide isconsiderably more soluble in water than magnesium hydroxide, Mg(OH) 2. In part, solubility

is controlled by the enthalpy chang e of solution ((Hsoln). This is itself determined by other

enthalpy changes: for example, the enthalpies of hydration ((Hhyd) of the cations andanions. Some data for the cations, Na

+and Mg

2+, are given in the table below.

(i) Explain why water molecules are able to interact with both cations and anions.

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(ii) Explain, in terms of the three quantities: charge, ionic radius and extent of hydration, why

(Hhyd for Mg2+ is more negative than (Hhyd for Na

+.

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(c) Solubility is also controlled by the entropy change ( (S) which accompanies solution.


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(i) In terms of the number of ions per mole of each compound, explain why this entropychange would be expected to be mor e positive for Mg(OH)2 than for NaOH.

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(ii) In terms of the arrangement of water molecules, explain why this entropy change would beexpected to be more negative for Mg(OH) 2 than for NaOH.

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(iii) Name another entropy change which contribu tes to the total entropy change accompanying

solution, and explain how it arises.

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9 New Zealand has no oilfields of its own and until recently relied completely on imported oilto meet its need for liquid fuels. The co untry does, however, have large reserves of naturalgas (which is largely methane) and, since 1985, much of the petro l needed in New Zealandhas been produced by chemical conversion of methane into liquid, hydrocarbon fuel.

The first stage in this process involves production of methanol from methane using the

reactions in Equations 3.1 and 3.2. Data about these reactions are shown in the tablebelow.

CH4(g) + H2O(l) CO(g) + 3H2(g) Equation 3.1

CO(g) + 2H2(g) CH3OH(g) Equation 3.2

(a) Catalysts play a key role in increasing the rates of Reactions 3.1 and 3.2.

Briefly describe two other ways in which the conditions above are chosen to increase therates of Reactions 3.1 and 3.2.


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(b) The conditions used for Reactions 3.1 and 3.2 ar e chosen to give optimum yields atequilibrium from these reactions. For each react ion, explain why the conditions oftemperature and pressure chosen give an optimum yield of products.

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(c) In the second stage of the process, methanol is converted into a mixture of hydrocarbonsby Reactions 3.3 and 3.4. ( In Equation 3.4, the mixture of hydrocarbons (petrol) isrepresented by octene, C 8H16.) The reactions take place at 600 K in the presence of a

zeolite catalyst.

2CH3OH(g) CH3OCH3(g) + H2O(g) Equation 3.3

4CH3OCH3(g) C8H16(g) + 4H2O(g) Equation 3.4

(i) In the space below, write an expression for Kp for the equilibrium in Equation 3.3 in terms ofthe partial pressures of the gases involved.

(ii) Under the conditions used in the industria l process, Reaction 3.3 reaches equilibrium.

Calculate the equilibrium par tial pressure of methoxymethane (CH3OCH3) when the partialpressure of methanol at equilibrium is 0.142 atm. (Kp = 9.00)

(d) Zeolites are crystalline aluminosilicate materials with structures containing a network of linked channels throughwhich molecules can pass. The channels restrict the size of the hydrocarbon molecules produced and their


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passage out of the zeolite.

In the zeolite used as a catalyst for Reactions 3.3 and 3.4, only molecules with up to 12 carbons atoms can beformed and pass through the channels. Despite this, up to 200 hydrocarbon compounds are present in thereaction product. This large number is due to the fact that, for most hydrocarbons, there are several ways inwhich the carbon and hydrogen atoms can be arranged for any given formula.

(i) Give three ways in which the arrangement of carbon and hydrogen atoms can give rise todifferent molecules of the same formula.

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(ii) Describe another industrial use for zeolites.

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10 This question is regarding transition metal chemistry.

(a) What do you understand by the term ligand?

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(b) Choose two neutral molecules and one ion that can act as ligands. Draw a diagram toshow clearly how one of these ligands co mbines with a metal ion such as Cr3+.

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(c) The salt tetrasodium ethylenediaminetetra -acetate, EDTA, is a polydentate ligand thatforms six bonds to a metal.

With many metal ions EDTA forms strong complexes that are usually very soluble;consequently it has been used to counteract the effects of poisoning by heavy metals suchas cadmium. The complexed cadmium can then be excreted in urine.


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(ii) What features of both compound A and compound B make them suitable to act as ligands?

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(iii) Suggest the co-ordination number in, and the shape of, the complex formed between A andSr2+ ions.

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(iv) Suggest the co-ordination number in, and the shape o f, the complex formed between B andNi2+ ions.

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11 This question is about the properties and reactions of the o xides of some elements in their+4 oxidation state.

(a) Chlorine dioxide, ClO2, is an important industrial chemical, used to bleach wood pulp formaking paper, and to k ill bacteria in water supplies. However, it is unstable anddecomposes into its elements as follows.

2ClO2(g) p Cl2(g) + 2O2(g)

(i) The chlorine atom is in the middle of the Cl O2 molecule. Using the chlorine -oxygen bondenergy as 278 kJ mol

1, and other values from the Data Booklet, calculate (H for the above

reaction.

(ii) Assuming the ClO bonds in chlorine dioxide are double bonds, predict the shape of theClO2 molecule. Explain your answer.

(iii) ClO2 can be made in the laboratory by reacting KClO3 with concentrated H2SO4. Otherproducts are K2SO4, KClO4 and H2O.


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Construct a balanced equation for this reaction.

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(b) All the oxides of the elements in Group IV in their +4 o xidation state are high melting pointsolids except CO2.

(i) Explain this observation by describing the bonding in CO 2, SiO2 and SnO2.

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(ii) State the difference in the thermal stabilities of SnO2 and PbO2. Illustrate your answer withan equation.

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CO2 dissolves in water to form a weakly acidic solution containing the hydrogencarbonateion.

(iii) Write an equation for the reaction of CO 2 with water, and write an expression for theequilibrium constant, K c.

(iv) Explain the role of the hydrogencarbonate ion in controlling the pH of blood, illustrating youranswer with relevant equations.

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12 The following account describes the preparation of Pligots sal t, named after the 19thcentury French chemist who first made it.


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Place 6.0 g of potassium dichromate(VI) in a 100 cm3

beaker and add 8.0 g ofconcentrated hydrochloric acid and 1.0 cm 3 water. Warm the mixture gently; if carefullydone the dichromate(VI) will dissolve without the evolution of chlorine. On cooling thebeaker in an ice bath the solution will dep osit long orange-red crystals of Pligots salt.

An analysis of Pligots salt showed that it contained the following percentages by mass:

K, 22.4%; Cr, 29.8%; Cl, 20.3%; O, 27.5%.

(a) Calculate the empirical formula of Pligots salt.

(b) Suggest a balanced equation for the formation of Pligots salt.

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(c) The instructions suggest that strong heating might cause chlorine to be evolved.

(i) What type of reaction would produce chlorine in this system?

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(ii) Use the Data Booklet to identify relevant half e quations and EU values for the production ofchlorine from the reaction between K 2Cr2O7 and HCl.

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Use these equations to write the overall full ionic equation for this reaction.

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(iii) The use of dilute HCl (aq) does not result in the production of chlorine. Suggest why this isso.

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(iv) Use the Data Booklet to suggest a reason why it is not possible to prepare the bromineanalogue of Pligots salt by using HBr(aq) instead of HCl (aq).

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End of Paper

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t2 Chem Revision Ex 16