Chemistry 9701/53 — October/November 2021

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

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Topics Planning · Analysis, Conclusions and Evaluation

Q1PlanningAnalysis, Conclusions and EvaluationFree 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:

\text{reaction 1: } \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: } \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})

(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]

mass of $\text{KHCO}_3$ = .............................. g

mass of $\text{K}_2\text{CO}_3$ = .............................. g

(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.

percentage error = ..............................

(c)

The student obtained the following results.

solid	initial temperature/ °C	maximum/minimum temperature/ °C	temperature change, $\Delta T$ / °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 $1.00\text{ g}$ .
$c = 4.18\text{ J g}^{-1}\text{K}^{-1}$

$\Delta H_1$ = .............................. $\text{kJ mol}^{-1}$

(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 $1.00\text{ g}$ .
$c = 4.18\text{ J g}^{-1}\text{K}^{-1}$

$\Delta H_2$ = .............................. $\text{kJ mol}^{-1}$

(d)

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

Fig. 1.2

$\Delta H_r$ = .............................. $\text{kJ mol}^{-1}$

(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.

1 .................................................................................................................................................

2 .................................................................................................................................................

(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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