Chemistry 9701/52 — October/November 2013

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

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Topics Analysis, Conclusions and Evaluation · Planning

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

Ammonium nitrate, $\text{NH}_4\text{NO}_3$ , is soluble in water (approximately $2.5 \text{ mol} / 100 \text{ g}$ at $25 \text{ }^\circ\text{C}$ ). The molar enthalpy of solution of a solid is defined as the enthalpy change when one mole of the solid is dissolved in water.

\text{NH}_4\text{NO}_3(\text{s}) \rightleftharpoons \text{NH}_4^+(\text{aq}) + \text{NO}_3^-(\text{aq}) \quad \Delta H_{\text{soln}} = +26.5 \text{ kJ mol}^{-1}

(a)

(i)

Predict how the temperature of water, initially at $25 \text{ }^\circ\text{C}$ , would change as ammonium nitrate is dissolved. Explain this prediction in terms of lattice energy and the enthalpy of hydration of ions.

Prediction of the temperature change:

Explanation:

(ii)

In the space below, sketch a graph to show your prediction of temperature change with concentration. Use two labelled axes and include an origin.

(b)

If you were to carry out an experiment to investigate how the temperature change of the solution varies as the concentration changes name,

(i)

the independent variable,

(ii)

the dependent variable.

(c)

You are to plan an experiment to determine as accurately as possible how the temperature change varies when different solutions are made, each with different concentrations of ammonium nitrate. You are reminded that the approximate solubility of ammonium nitrate is $2.5 \text{ mol} / 100 \text{ g}$ at $25 \text{ }^\circ\text{C}$ .

The following information gives some of the hazards associated with ammonium nitrate.

Ammonium nitrate $\text{NH}_4\text{NO}_3$ . Contact with combustible material may cause fire. Explosive when mixed with combustible material.
Do not allow the salt to become contaminated with organic matter and do not grind it.
Solutions should be diluted to less than $0.5 \text{ mol dm}^{-3}$ for disposal.

You should use only standard apparatus found in a school or college laboratory. Draw a diagram of the apparatus and experimental set up you would use showing clearly the following:

(i) the apparatus used, such as the reaction vessel, and how the thermometer will be positioned in order to measure the temperature of the solution as accurately as possible,
(ii) how the apparatus will be insulated.

Label each piece of apparatus used, indicating its size or capacity and both the temperature range and the precision of the thermometer.

(d)

Using the apparatus shown in (c) design an experiment to test your prediction in (a)(ii) of how the temperature change of the solution varies with solutions of different concentration.

In addition to the apparatus normally found in a laboratory you are provided with the following materials;

a supply of solid ammonium nitrate,
distilled (deionised) water.

Give a step-by-step description of how you would carry out the experiment to include;

(i) the number of experiments you would do,
(ii) the temperature measurements you would take,
(iii) the volume of water you would use,
(iv) a calculation to show the maximum mass of ammonium nitrate you could use for your volume of water in (iii) and a range of masses for the other experiments.

[ $A_r$ : H, 1.0; N, 14.0; O, 16.0]

(e)

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 may use the information in (c) if you wish.

(f)

In order to test your prediction in (a)(ii), you would need to plot a graph. In the space below, draw a table with appropriate headings, in which you would record all your experimental data and calculated values necessary for the construction of the graph. The headings must include the appropriate units.

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