chem question paper jan 2000 unit-6

8/6/2019 Chem Question Paper Jan 2000 Unit-6

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1. The values ofKa for a series of organic acids at 25 C are shown in the table below:

Acid Formula Ka/mol dm3

ethanoic acid CH3COOH 1.75 105

chloroethanoic acid CH2(Cl)COOH 1.38 103

bromoethanoic acid CH2(Br)COOH 1.26 103

iodoethanoic acid CH2(I)COOH 6.76 104

(a) (i) Give the expression for Ka for ethanoic acid.

(1)

(ii) Calculate the pH of 0.100 mol dm3 ethanoic acid.

(3)

(iii) Calculate the pH of a mixture which is 0.100 mol dm3 with respect to ethanoic

acid and 0.125 mol dm3 with respect to sodium ethanoate.

(iv) What property is shown by such a mixture?

....

....

....(2)

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(b) (i) By consideration of bond polarity, suggest why 1-chloroethanoic acid is a strongeracid than ethanoic acid.

....

....

....

(2)

(ii) Suggest a reason for the trend Ka values shown in the table above.

....

....(1)

(iii) Suggest, with an explanation, whether 1 -fluoroethanoic acid is likely to be a

stronger or a weaker acid than 1-chloroethanoic acid.

....

....

....(2)

(c) Ethanoyl chloride, CH3COCl, reacts violently with water, whereas chloroethanoic acid

reacts much more slowly in order to substitute the chlorine atom. Suggest why this is so.

.....

.....

.....(2)

(Total 15 marks)

2. Butenedioic acid has the structure

HOOCCH==CHCOOH

It has two geometric isomers, cis- and trans-butenedioic acid.

(a) Explain in terms of orbitals why butenedioic acid has geometric isomers.

..

..(2)



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(b) The following concerns cis-butenedioic acid.

(i) Draw the structure of the compound formed when cis-butenedioic acid reacts with

bromine dissolved in an organic solvent.

(1)

(ii) On heating to 145 C, cis-butenedioic acid loses water to form a compoundcontaining a 5-membered ring which includes one oxygen atom.

C4H4O4 C4H2O3 + H2O

Draw the structural formula of C4H2O3.

(2)

(c) (i) Draw the structure oftrans-butenedioic acid.

(1)

(ii) Suggest why trans-butenedioic acid does not lose water at 145 C.

....

....(1)



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(d) Ifcis-butenedioic acid is reduced with lithium tetrahydridoaluminate(III) (lithiumaluminium hydride) LiAlH4, the product is cis-but-2-en-1,4-diol:

HOCH2CH==CHCH2OH

(i) State the conditions for the reaction between LiAlH4 and cis-butenedioic acid.

....

....(2)

(ii) How would you test for the presence of the hydroxyl group, and what would yousee as the result of your test?

....

....

....(2)

(iii) Ifcis-but-2-en-1,4-diol is reacted with the compound ClOCCH2COCl a polymer

results. Draw enough of the chain of this polymer to make its repeating structure

clear.

(2)

(iv) Suggest why this polymer would not be suitable for use in strongly alkaline or

acidic conditions.

....

....

....(2)

(Total 15 marks)



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3. When 1,2-dibromoethane is warmed with sodium iodide dissolved in propanone at 50 C,ethene, sodium bromide and iodine are produced:

BrCH2CH2Br + 2NaI CH2==CH2 + I2 + 2NaBr

(a) A study of the kinetics of this reaction at a particular temperature gave thefollowing results

Experiment [BrCH2CH2Br]

/mol dm3

[I]/mol dm3

Initial rate ofreaction

(relative)

A 0.01 0.01 1

B 0.01 0.02 2

C 0.02 0.01 2

D 0.02 0.02 4

(i) What is the order of reaction with respect to 1,2-dibromoethane? Give yourreasoning.

(2)

(ii) What is the order of reaction with respect to sodium iodide? Give your reasoning.(2)

(iii) State the overall order of the reaction.(1)

(iv) Suggest what the first step in the reaction mechanism might be, giving your

reasons. The detailed mechanism for the reaction is not expected.(2)

(v) Explain why the mechanism for the reaction must contain at least two steps.

(2)

(b) Propanone is chosen as the solvent because iodine, 1,2-dibromoethane and sodium iodideare soluble in it; sodium bromide is not.

(i) By considering this information and the equation for the reaction say what you

would expect to see as the reaction proceeds.(2)

(ii) In such a reaction, 15.7 g of 1,2-dibromoethane (Mr = 188) was used initially.

Assuming complete reaction, what volume of ethene (in cm3) would be produced?

The volume of 1 mole of gas at the temperature and pressure of the experiment is24 dm3 Assume that ethene is not significantly soluble in propanone.

(3)



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(iii) The progress of the reaction could be followed by titrating the liberated iodine withsodium thiosulphate solution. Describe how you would perform such titrations on

portions of the reaction mixture.(6)

(iv) Calculate the volume of 1.00 mol dm3 sodium thiosulphate solution required totitrate one-tenth of the reaction mixture in (ii) after the reaction was completed.

(3)

(v) The solubility of sodium iodide in propanone suggests that it has somewhat more

covalent character than sodium bromide, which is not soluble. Suggest why thebonding in the two salts is different.

(2)

(Total 25 marks)

4. Chlorine forms oxyacids such as HOCl. Until 1973 HOF was believed not to exist.

(a) (i) Give the equation for the reaction of chlorine with water at room temperature.

By considering oxidation states explain why this is a disproportionation reaction.

(3)

(ii) If sodium hydroxide solution is added to the solution of chlorine in water from (i)and the mixture is heated, a further disproportionation reaction occurs. Write an

ionic equation to represent this further reaction, and hence show that chlorine reacts

with hot sodium hydroxide solution to give the two chlorine-containing products ina mole ratio of 5 to 1.

(3)

(b) HOF is a covalent substance.

(i) Draw a dot-and-cross diagram for HOF.(1)

(ii) State and explain the shape of the molecule.(2)

(iii) Suggest why the oxidation state of F in HOF cannot be the same as thatof Cl in HOCl.

(2)

(c) At room temperature HOF decomposes as follows:

HOF(g) HF(g) + O2(g)

The reaction is first order with respect to HOF and has a half-life of 30 minutes.

(i) Write the rate equation for the reaction,

(ii) Approximately how long would it take for the concentration of HOF to fall to 1.5%of its initial value?

(2)



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(d) HOF is made by the following reaction:

F2(g) + H2O(s)40C

HOF(g) + HF(g)

HOF must be rapidly removed from the reaction site since it reacts with HF, F 2 and water.

The apparatus is made from poly(l-chloro-1,2,2-trifluoroethene).

(i) Draw the structural formula of 1-chloro-1,2,2-trifluoroethene. (1)

(ii) Draw enough of the polymer chain formed by this material, to make the repeatingstructure clear.

(2)

(iii) Suggest, in terms of the nature of polyhalogenated hydrocarbons, why this polymer

is used for the apparatus for synthesis of HOF.(3)

(e) Organic fluorine compounds are often different from their analogues with other halogens.

(i) Chloromethane reacts with a solution of hydroxide ions in an SN2 reaction. Give the

mechanism for this reaction.(3)

(ii) Given the following bond lengths

CCl 0. 177 nm CF 0.139 nm

suggest why fluoromethane does not react with a solution of hydroxide ions.(2)

(Total 25 marks)

5. (a) Silver ions are used in the qualitative and quantitative analysis of solutions of chlorides,bromides and iodides. The analysis depends on the low solubility of the silver salts.

Solubility at 298 K/mol dm3: AgCl 1.35 105AgBr 7.19 107

AgI 1.11 107

(i) Explain why the solution to be analysed in the qualitative tests is acidified withnitric acid before silver nitrate solution is added.

(2)

(ii) The lattice enthalpies of the silver halides change by only about 13% between AgCl

and AgI. Suggest reasons in terms of enthalpy changes why their solubility is so

different.(2)

(iii) Explain the reasons for the difference between the experimental lattice enthalpy ofsilver iodide, which is 889 kJ mol1, and the lattice enthalpy calculated

theoretically which is 778 kJ mol1.(2)



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(b) The electrochemical cell shown below can be used to measure the solubility of silverchloride in water; the solution in the right-hand half-cell stands over a small amount of

solid silver chloride.

V

Ag Ag

Salt bridge

Ag (aq)

AgCl(s)

+

0.001 mol dm Ag (aq)3 +

The potential of a silver/silver ion electrode is given by:

EAg+/Ag = E Ag+/Ag + 0.06 lg [Ag+1]

At the temperature of the experiment E Ag+/Ag = +0.80 V, and the potential of the cellabove is 0.12 V.

(i) Using the expression given above find the potential of the left-hand half cell.(2)

(ii) Find the potential of the right-hand half-cell, and hence find the solubility of silver

chloride in mol dm3 under the conditions of the experiment.(3)

(iii) Why is it important to have a small amount of solid silver chloride in the right-hand

beaker?(1)

(iv) Salt bridges are usually made from solutions (or gels) containing potassium

chloride or potassium nitrate. State which you would choose for this experiment,

and explain why.(2)

(c) The reaction taking place in a lead-acid storage cell on discharge is:

PbO2 + Pb + 2H2SO4 2PbSO4 + 2H2O

(i) Give the half-equation for the reaction occurring at the positive pole of the cell(PbO2 electrode).

(2)

(ii) Give the half-equation for the reaction occurring at the negative pole of the cell.(1)



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(iii) Pure sulphuric acid has a density of 1.98 g cm3. Explain what happens to thedensity of the electrolyte as the cell is discharged.

(2)

(iv) A similar storage cell based on tin(IV) oxide is not effective since tin(IV)) oxide

is not a strong oxidising agent. Explain in terms of electronic structures andbond strengths why lead(IV) oxide is a strong oxidising agent whereas tin(IV)

oxide is not.

(6)(Total 25 marks)

6. (a) Enthalpy changes of reaction can be obtained using average bond enthalpies, enthalpies offormation, or enthalpies of combustion.

(i) Show how enthalpies of combustion could be used to determine the enthalpychange for the reaction

CH3CH==CH2 + H2 CH3CH2CH3

(2)

(ii) The enthalpy change for this reaction could also be found by using average bond

enthalpies. Explain why in general the use of average bond enthalpies is not as

accurate a method as the use of enthalpies of formation or enthalpies of combustionin finding enthalpies of reaction.

(3)

(b) (i) The equation for the thermit reaction is

2Al(s) + Fe2O3(s) 2Fe(l) + A12O3(s)

It has been used for many years for welding railway lines or ironwork on canallocks often in areas inaccessible to normal welding equipment. Use the following

enthalpies of formation to find the enthalpy change Hfor this reaction.

Hf/kJ mol1 : Fe2O3 824 A12O3 1676

(2)

(ii) The heat exchange from this reaction originates mainly in the high lattice enthalpy

of aluminium oxide compared to that of iron(III) oxide. Suggest why the latticeenthalpy of aluminium oxide is more negative than that of iron(III) oxide.

(2)

(iii) Explain why the reaction, although strongly exothermic, requires a fuse of burning

magnesium to start it.(1)



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(c) The thermit reaction is a very expensive way of making iron and makes no commercialsense on a large scale.

(i) State why the thermit reaction is so expensive.(1)

(ii) Iron is produced from either Fe2O3 orFe3O4 and coke. What are the two functions

of the coke? Show by suitable calculations that the costs involved in using either

Fe2O3 (Mr = 160) or Fe3O4 (Mr = 232) as the ore are virtually the same.

(5)

(iii) Iron from the blast furnace contains about 4% carbon; steel has about 0.5%.

Explain why the carbon content of the iron needs to be decreased formost purposes.

(2)

(d) (i) Iron has two main oxidation states. By means of equations, including state symbols,

show how you could make the anhydrous chlorides of iron in each of these two

oxidation states.(2)

(ii) A third oxidation state shown by iron is exemplified in the ion FeO42.Write a half-equation to show the reduction of FeO42 to Fe3+ in acidic solution.

(2)

(iii) Solutions of the FeO42 ion are red. Explain with a reason whether you

would use an indicator if you were to use K2FeO4 as an oxidising agent in

volumetric analysis.(3)

(Total 25 marks)

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chem question paper jan 2000 unit-6

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