Chlorine gas, $\text{Cl}_2$, is slightly soluble in water, approximately $5\text{ g dm}^{-3}$ at $25^\circ\text{C}$. The molar enthalpy of solution of a gas is defined as the enthalpy change when one mole of the gas is dissolved in water. (i) Predict how the solubility of chlorine in water changes as the temperature is increased. Explain this prediction using Le Chatelier’s Principle in terms of the equilibrium between the gaseous chlorine and the aqueous solution, as shown…

Chemistry 9701/53 — May/June 2013

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Q1Analysis, Conclusions and EvaluationPlanningFree sample

Chlorine gas, $\text{Cl}_2$ , is slightly soluble in water, approximately $5\text{ g dm}^{-3}$ at $25^\circ\text{C}$ . The molar enthalpy of solution of a gas is defined as the enthalpy change when one mole of the gas is dissolved in water.

(a)

(i)

Predict how the solubility of chlorine in water changes as the temperature is increased. Explain this prediction using Le Chatelier’s Principle in terms of the equilibrium between the gaseous chlorine and the aqueous solution, as shown in the equation.

\text{Cl}_2\text{(g)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{HClO(aq)} + \text{HCl(aq)} \quad \Delta H_{\text{soln}} = -23.4\text{ kJ mol}^{-1}

Predict how the solubility will change as the temperature is increased.

Explanation

(ii)

Display your prediction in the form of a sketch graph between $0^\circ\text{C}$ and $100^\circ\text{C}$ .
Label the axes with units and give numerical values on the axes to ensure that the line clearly shows the solubility at $25^\circ\text{C}$ and $100^\circ\text{C}$ .

Fig. 1.1

(b)

If you were to carry out an experiment to investigate how the solubility of chlorine varies as the temperature increases name,

(i)

the independent variable,

(ii)

the dependent variable.

(c)

You are to plan an experiment to determine as accurately as possible the concentration of a saturated aqueous solution of chlorine by titration. You are reminded that the approximate solubility of chlorine is $5\text{ g dm}^{-3}$ at $25^\circ\text{C}$ .

The following information gives some of the hazards associated with chlorine, iodine and sodium thiosulfate.


Saturated chlorine water is low hazard but chlorine gas escapes, which is harmful. Iodine is harmful by inhalation and in contact with skin or eyes. Solutions more concentrated than or equal to $1\text{ mol dm}^{-3}$ are harmful. Sodium thiosulfate is non-hazardous.

Aqueous chlorine, $\text{Cl}_2$ , displaces iodine, $\text{I}_2$ , from aqueous potassium iodide.

\text{Cl}_2\text{(aq)} + 2\text{KI(aq)} \rightarrow \text{I}_2\text{(aq)} + 2\text{KCl(aq)}

Therefore if a solution of chlorine is mixed with an excess of aqueous potassium iodide, iodine is displaced in a $1 : 1$ molar ratio with chlorine.

The concentration of chlorine in the original solution can therefore be calculated from the concentration of the displaced iodine.

\text{I}_2\text{(aq)} + 2\text{Na}_2\text{S}_2\text{O}_3\text{(aq)} \rightarrow 2\text{NaI(aq)} + \text{Na}_2\text{S}_4\text{O}_6\text{(aq)}

You are provided with the following materials:

saturated aqueous chlorine,
solid sodium thiosulfate $\text{Na}_2\text{S}_2\text{O}_3\cdot5\text{H}_2\text{O}$ ,
concentrated aqueous potassium iodide. This will be used in excess.

Give a step-by-step description of how you would carry out the experiment by including:

(i) a list of apparatus with volumes where appropriate,
(ii) a suitable indicator with relevant colours,
(iii) a calculation of the approximate concentration of saturated aqueous chlorine in $\text{mol dm}^{-3}$ at $25^\circ\text{C}$ ,
$[A_r: \text{Cl}, 35.5]$
(iv) a detailed description of the method for preparing a solution of aqueous sodium thiosulfate that can be used in the titration. In a titration, it is usual for the two reacting volumes to be approximately equal at the end-point. Calculate the mass of sodium thiosulfate, $\text{Na}_2\text{S}_2\text{O}_3\cdot5\text{H}_2\text{O}$ , which will produce a solution suitable for use in this titration. The relevant calculations and reasoning must be shown in full,
$[A_r: \text{H}, 1.0; \text{O}, 16.0; \text{Na}, 23.0; \text{S}, 32.1]$
(v) a detailed method for carrying out sufficient titrations to allow an accurate end-point to be obtained,
(vi) an outline calculation to show how the results are to be used to determine the accurate concentration of the aqueous chlorine.

(d)

State one hazard that must be considered when planning the experiment and describe a precaution that should be taken to keep risks from this hazard to a minimum. You should use the information in (c).

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