AS & A Level Chemistry - 9701 Paper 2 2011 Summer Zone 2

Ethanoic acid can be reacted with alcohols to form esters, an equilibrium mixture being formed.

$$\text{CH}_3\text{CO}_2\text{H} + \text{ROH} \rightleftharpoons \text{CH}_3\text{CO}_2\text{R} + \text{H}_2\text{O}$$

The reaction is usually carried out in the presence of an acid catalyst.

(a) Write an expression for the equilibrium constant, $K_c$, for this reaction, clearly stating the units.
$$K_c = \text{.....................}$$
units .................................................. [2]

In an experiment to determine $K_c$ a student placed together in a conical flask 0.10 mol of ethanoic acid, 0.10 mol of an alcohol ROH, and 0.005 mol of hydrogen chloride catalyst. The flask was sealed and kept at 25°C for seven days.
After this time, the student titrated all of the contents of the flask with $2.00 \text{mol dm}^{-3}$ NaOH using phenolphthalein indicator.
At the end-point, $22.5 \text{cm}^3$ of NaOH had been used.

(b) (i) Calculate the amount, in moles, of NaOH used in the titration.

(ii) What amount, in moles, of this NaOH reacted with the hydrogen chloride?

(iii) Write a balanced equation for the reaction between ethanoic acid and NaOH.

(iv) Hence calculate the amount, in moles, of NaOH that reacted with the ethanoic acid. [4]

(c) (i) Use your results from (b) to calculate the amount, in moles, of ethanoic acid present at equilibrium. Hence complete the table below.

[Table_1]

(ii) Use your results to calculate a value for $K_c$ for this reaction. [3]

(d) Esters are hydrolysed by sodium hydroxide. During the titration, sodium hydroxide reacts with ethanoic acid and the hydrogen chloride, but not with the ester. Suggest a reason for this.
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(e) What would be the effect, if any, on the amount of ester present if all of the water were removed from the flask and the flask kept for a further week at 25°C?
Explain your answer.
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(a) Use relevant bond energies from the Data Booklet, and the equation below to calculate a value for the bond energy of the C–F bond.

CH$_2$ = CH$_2$(g) + HF(g) ⟶ CH$_3$CH$_2$F(g) ΔH$^∘$ = –73 kJ mol$^{-1}$

C–F bond energy = .......................................... kJ mol$^{-1}$

(b) Another halogenoalkane which was used as a refrigerant, and also as an aerosol propellant, is dichlorodifluoromethane, CCl$_2$F$_2$.
State two reasons why compounds such as CH$_3$CH$_2$F and CCl$_2$F$_2$ have been used as aerosol propellants and refrigerants.

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(c) By using relevant data from the Data Booklet, and your answer to (a) suggest why CCl$_2$F$_2$ is responsible for damage to the ozone layer in the stratosphere whereas CH$_3$CH$_2$F is not.

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(d) (i) What is meant by the term enhanced greenhouse effect?

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(d) (ii) Water vapour is the most abundant greenhouse gas. What is the second most abundant greenhouse gas?

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(e) What shape is the SF$_6$ molecule?

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(a) State the formula of each of the barium compounds R to W.

R .................................... S ....................................
T .................................... U ...................................
V .................................... W ...................................


(b) (i) Write balanced equations for the following reactions.
compound T to compound W
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the roasting of V in air
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(b) (ii) Suggest a gaseous reagent for the conversion of T into V and write a balanced equation for the reaction.
reagent .................................................................................................
equation .................................................................................................


(c) Suggest the formula of an aqueous reagent, other than an acid, for reaction 1.
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(d) (i) Calculate the empirical formula of X.
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(ii) Suggest the identity of the solid Y.
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(iii) Use your answers to (i) and (ii) to construct an equation for the reaction of X with H$_2$SO$_4$.
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(a) (i) Describe, with the aid of a fully labelled diagram, the industrial electrolysis of brine in a diaphragm cell. State what each electrode is made of and show clearly the inlet for the brine and the outlets for the products.

(ii) Write a half-equation, with state symbols, for the reaction at each electrode.

anode .........................................................................................................
cathode .......................................................................................................

(iii) Name the chemical that is produced in solution in this electrolytic process.
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Although there are many different types of food eaten around the world, animal fats and/or vegetable oils are commonly used in cooking.
Animal fats and vegetable oils are usually glyceryl esters, that is esters of glycerol, propane-1,2,3-triol.

CH2OH
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CHOH
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CH2OH

Many animal fats contain esters of stearic acid, $\text{CH}_3(\text{CH}_2)_{16}\text{CO}_2\text{H}$.
Vegetable oils often contain esters of oleic acid, $\text{CH}_3(\text{CH}_2)_7\text{CH} = \text{CH}(\text{CH}_2)_7\text{CO}_2\text{H}$.
(a) Draw the structural formula of the glyceryl ester formed when one molecule of glycerol is completely esterified with stearic acid.

(b) What reagent(s) would you use, in a school or college laboratory, to obtain a small sample of oleic acid, $\text{C}_{17}\text{H}_{33}\text{CO}_2\text{H}$, from the glyceryl ester present in a vegetable oil?

Oleic acid is the cis isomer and elaidic acid the trans isomer of $\text{CH}_3(\text{CH}_2)_7\text{CH} = \text{CH}(\text{CH}_2)_7\text{CO}_2\text{H}$.
(c) By using this formula, draw the structural formula of elaidic acid, clearly showing the stereochemistry.

(d) (i) Suggest the meaning of the term polyunsaturated fatty acid.
(ii) What reagent and condition(s) are used for the hydrogenation of an unsaturated fatty acid?
reagent ..................................................
condition(s) ............................................

(e) In cooking, unsaturated fats are often oxidised to give aldehydes or ketones.
(i) Give the structural formulae of the two aldehydes formed by the partial oxidation of the unsaturated fat below.
In the structure, X, represents the rest of the fat molecule.

$\text{CH}_3(\text{CH}_2)_7\text{CH} = \text{CH}(\text{CH}_2)_7\text{X}$
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(ii) Name the reagent you would use to show that the product contained either an aldehyde or a ketone. What change would be seen?
reagent ..................................................
observation ..............................................

(iii) What reagent would you use to confirm the presence of an aldehyde? What change would be seen?
reagent ..................................................
observation ..............................................

Animal fats and vegetable oils can become rancid because of oxidation. The rancid fat or oil has an unpleasant smell and taste.
Antioxidants are used to prevent the spoilage of many foodstuffs by oxidation.
One antioxidant that is widely used is vitamin C, ascorbic acid.

[Image_
ascorbic acid drawing]

(f) (i) How many chiral carbon atoms are present in one molecule of ascorbic acid?
If none, write 'none'.
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(ii) The ascorbic acid molecule contains three functional groups.
Two of these are alcohol (primary and secondary) and alkene.
What is the name of the third functional group?
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