Chemistry 9701/51 — October/November 2021

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

Q1Analysis, Conclusions and EvaluationPlanningFree sample

Potassium hydrogencarbonate, $\text{KHCO}_3$ , decomposes when strongly heated to form potassium carbonate, $\text{K}_2\text{CO}_3$ .

2\text{KHCO}_3(\text{s}) \rightarrow \text{K}_2\text{CO}_3(\text{s}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l})

A student plans to determine the value for the enthalpy change for this reaction, $\Delta H_r$ , which cannot be determined directly. The student carries out two separate experiments using the following apparatus.

Fig. 1.1

Experiment 1 uses solid $\text{KHCO}_3$ .

Experiment 2 uses solid $\text{K}_2\text{CO}_3$ .

The following method is used for both experiments:

Transfer $50.00\text{ cm}^3$ , an excess, of $2\text{ mol dm}^{-3}$ hydrochloric acid into a cup.
After 2 minutes, record the temperature of the acid.
Weigh approximately $0.0250$ moles of solid.
Add the solid to the acid, stir the mixture using a thermometer and record the temperature throughout the reaction.

Hazard information: $2\text{ mol dm}^{-3}$ hydrochloric acid is irritant, solid potassium hydrogencarbonate and solid potassium carbonate may cause irritation to the skin and eyes.

The equations for the two reactions are:

\begin{aligned} \text{reaction 1} &\quad \text{KHCO}_3(\text{s}) + \text{HCl}(\text{aq}) \rightarrow \text{KCl}(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \\ \text{reaction 2} &\quad \text{K}_2\text{CO}_3(\text{s}) + 2\text{HCl}(\text{aq}) \rightarrow 2\text{KCl}(\text{aq}) + \text{CO}_2(\text{g}) + \text{H}_2\text{O}(\text{l}) \end{aligned}

(a)

Suggest why it is not possible to measure $\Delta H_r$ for the decomposition reaction directly.

(b)

(i)

Calculate the mass of $0.0250$ moles of each solid. Give your answers to three decimal places.

[ $A_r$ : K, 39.1; H, 1.0; C, 12.0; O, 16.0]

(ii)

The masses of solid are measured using a three decimal place balance.

Calculate the percentage error in the measurement of the mass of $\text{KHCO}_3$ .

Show your working.

(c)

The student obtained the following results.

solid	initial temperature / $^\circ\text{C}$	maximum/minimum temperature / $^\circ\text{C}$	temperature change, $\Delta T$ / $^\circ\text{C}$
$\text{KHCO}_3$	17.5	14.0
$\text{K}_2\text{CO}_3$	19.0	20.5

(i)

Complete the table by calculating temperature change.

Use the formula $q = mc\Delta T$ to determine the energy change, $q$ , that took place during experiment 1. Use $q$ to calculate the enthalpy change of reaction 1, $\Delta H_1$ , in $\text{kJ mol}^{-1}$ .

Include a sign in your answer.

Assume $1.00\text{ cm}^3$ of solution has a mass of $1.00\text{ g}$ .
$c = 4.18\text{ J g}^{-1}\text{K}^{-1}$

(ii)

Use the formula $q = mc\Delta T$ to determine the energy change, $q$ , that took place during experiment 2. Use $q$ to calculate the enthalpy change of reaction 2, $\Delta H_2$ , in $\text{kJ mol}^{-1}$ .

Include a sign in your answer.

Assume $1.00\text{ cm}^3$ of solution has a mass of $1.00\text{ g}$ .
$c = 4.18\text{ J g}^{-1}\text{K}^{-1}$

(d)

Use the following cycle to calculate $\Delta H_r$ .

Fig. 1.2

(e)

A textbook states the value of the enthalpy change for the decomposition of potassium hydrogencarbonate as $+76.0\text{ kJ mol}^{-1}$ .

Suggest two reasons why the experimental value is different to the actual value.

(f)

Suggest one improvement to the apparatus which would reduce the difference between the experimental value and the actual value.

(g)

Name a suitable piece of apparatus which should be used to measure the volume of acid used in experiment 1.

(h)

Apart from wearing safety glasses and a lab coat, state one safety precaution which must be taken during experiment 1. Explain your answer.

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