AS & A Level Chemistry - 9701 Paper 5 2013 Winter Zone 3

Air, which is 99% nitrogen and oxygen, is slightly soluble in water. At 25°C a saturated solution of air in water has a concentration of 19 cm³ dm⁻³. When water is boiled all the dissolved air is boiled out of solution.

(a) (i) The molar enthalpies of solution for nitrogen and oxygen are:
ΔH_{soln} N₂ = -1.04 kJ mol⁻¹ and ΔH_{soln} O₂ = -1.20 kJ mol⁻¹

Predict how the solubility of air in water will change as the temperature is increased. Explain this prediction using Le Chatelier’s principle in terms of the equilibrium between air and the aqueous solution as the temperature is increased.

Prediction ........................................................................................................................
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Explanation ......................................................................................................................
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(ii) Display your prediction in the form of a sketch graph for the solubility of air between 0°C and 100°C, labelling clearly the axes. Include labelled points to indicate the solubility of air at 25°C and 100°C.
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(b) If you were to carry out an experiment to investigate how the solubility of air varies as the temperature increases name,

(i) the independent variable, .................................................................................................

(ii) the dependent variable. ................................................................................................
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When heated, aqueous hydrogen peroxide, $H_2O_2$, decomposes to form oxygen and water.
$$2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$$
The decomposition can also occur at room temperature if a suitable catalyst is added. Both of the solids, manganese(IV) oxide and lead(IV) oxide, will catalyse the decomposition.
The following information gives some of the hazards associated with manganese(IV) oxide and lead(IV) oxide.

You are provided with a 0.300 mol dm$^{-3}$ solution of hydrogen peroxide and a syringe with a capacity of 100 cm$^3$.
(a) Provide the following information about experiments you would carry out to collect oxygen from the decomposition of hydrogen peroxide and to determine, using identical masses, which of the two catalysts was the most efficient at promoting this decomposition:
- a fully labelled diagram of the apparatus to be used that would ensure that no oxygen would be lost when the experiment was carried out,
- a calculation of the maximum volume in cm$^3$ of the aqueous hydrogen peroxide that could be used such that the oxygen produced would not exceed the volume of the syringe,
- a statement of the measurements you would take that would allow you to say which of the catalysts was most efficient.
The molar volume of a gas at 25°C is 24.0 dm$^3$.
Please continue in the space provided on the next page if necessary. [6]
(b) What other feature of the catalyst should be controlled?
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(c) If one of the experiments takes 2 minutes to complete, draw a sketch graph with labelled axes showing how the volume of oxygen produced will vary with time between 0 and 3 minutes. [2]
(d) State the hazards that might be encountered when using the solids required in this experiment and give the one essential precaution you would take to make sure these chemicals were handled safely during the experiments.
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(a) The results of several of these experiments are recorded below.

[Table_1]

Calculate the relative molecular masses $\left( M_{r} \right)$ of $CCl_4$ and $C_2H_2Cl_4$.

$[A_r$: H, 1.0; C, 12.0; Cl, 35.5]

(b) (i) The vapour from the equilibrium at $108.5\degree C$ was analysed and found to consist of 7.22 g of $CCl_4$ and 8.92 g of $C_2H_2Cl_4$. Show clearly by calculation that this gives a mole fraction of 0.469 for $CCl_4$ vapour.

(ii) The vapour from the equilibrium at $83.3\degree C$ was analysed and found to consist of 14.14 g of $CCl_4$ and 1.38 g of $C_2H_2Cl_4$. Show clearly by calculation that this gives a mole fraction of 0.918 for $CCl_4$ vapour.

(c) On the same axes, plot two graphs, one for the liquid and one for the vapour, to show the variation in temperature (y-axis) with the mole fraction compositions (x-axis) of both the liquid and the vapour. Draw two lines of best fit. Each line could be either a curved line or a straight line.



(d) Circle and label on the graph the point you consider to be the most anomalous. Do not circle or label any other point. If it is assumed that the analysis was carried out accurately, suggest a reason why the point might be anomalous.

(e) By drawing an appropriate construction line on your graphs, determine the mole fraction of $CCl_4$ in the vapour which is in equilibrium with a liquid with a mole fraction of 0.500 $CCl_4$.

(f) The temperatures were measured using a thermometer calibrated in 0.1 $\degree C$ graduations. If the thermometer had only been calibrated in 1.0 $\degree C$ graduations, calculate the percentage errors which would result from the determination of the boiling points of each of the two pure liquids.

(g) (i) Use your graphs to state whether $CCl_4$ or $C_2H_2Cl_4$ distills first from a mixture of the two liquids.

(ii) Explain your answer (i).