Chemistry 9701/34 — May/June 2017

Cambridge AS Level · Advanced Practical Skills · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Manipulation, Measurement and Observation · Presentation of Data and Observations · Analysis, Conclusions and Evaluation · Qualitative Analysis

Q1Manipulation, Measurement and ObservationPresentation of Data and ObservationsAnalysis, Conclusions and EvaluationFree sample

Strong acids, such as hydrochloric acid, HCl, are completely ionised in aqueous solution. Weak acids, such as ethanoic acid, $\text{CH}_3\text{COOH}$ , are partially ionised in aqueous solution.

You will investigate the enthalpy change for the reaction of an excess of each of these acids with magnesium and hence determine the energy needed to cause the weak acid to ionise completely.

(a)

Reaction 1: Enthalpy change of a weak acid

FB 1 is ethanoic acid, $\text{CH}_3\text{COOH}$ .
FB 2 is magnesium, Mg.

Method 1

Weigh the strip of magnesium and record the balance reading in the space below.
Support the plastic cup in the $250\text{ cm}^3$ beaker.
Coil the magnesium ribbon loosely so that it fits into the bottom of the plastic cup and then remove the ribbon.
Use the measuring cylinder to transfer $25\text{ cm}^3$ of the acid, FB 1, into the plastic cup.
Place the thermometer in the acid and read the initial temperature. This is the temperature at time zero ( $t = 0$ ).
Start timing and do not stop the clock until the whole experiment has been completed.
Read the temperature of the acid every half minute for two minutes.
At time $t = 2\frac{1}{2}$ minutes drop the magnesium, FB 2, into the acid and stir the mixture.
Measure and record, in the table below, the temperature of the mixture at $t = 3$ minutes and then every half minute until $t = 10$ minutes. Stir the mixture continuously between thermometer readings.
Rinse the plastic cup for use in Method 2. Shake to remove excess water.

Results

Mass of magnesium

Temperature

time/minutes	0	$\frac{1}{2}$	1	$1\frac{1}{2}$	2	$2\frac{1}{2}$	3	$3\frac{1}{2}$	4	$4\frac{1}{2}$	5
temperature/ °C						[shaded]

time/minutes	$5\frac{1}{2}$	6	$6\frac{1}{2}$	7	$7\frac{1}{2}$	8	$8\frac{1}{2}$	9	$9\frac{1}{2}$	10
temperature/ °C

(b)

Plot a graph of temperature on the $y$ -axis against time on the $x$ -axis on the grid below.
The scale for temperature should extend $10\text{ }^\circ\text{C}$ above your highest recorded temperature.
You will use this graph to determine the theoretical maximum temperature rise at $2\frac{1}{2}$ minutes.

Fig. 1

Draw two lines of best fit through the points on your graph. The first line should be for the temperature before adding FB 2 and the second for the cooling of the mixture once the reaction is complete.

Extrapolate the two lines to $2\frac{1}{2}$ minutes, draw a vertical line between the two and determine the theoretical rise in temperature at this time.

\text{theoretical rise in temperature at } 2\frac{1}{2} \text{ minutes} = ............................. \text{ }^\circ\text{C}

(c)

Show your working and appropriate significant figures in the final answer to each step of your calculations.

Magnesium reacts with ethanoic acid according to the equation shown.

\text{Mg(s)} + 2\text{CH}_3\text{COOH(aq)} \rightarrow \text{Mg(CH}_3\text{COO)}_2\text{(aq)} + \text{H}_2\text{(g)}

(i)

Use your answer to (b) to calculate the heat energy, in joules, given out when FB 2 is added to the acid.
[Assume $4.2\text{ J}$ of heat energy raises the temperature of $1.0\text{ cm}^3$ of the mixture by $1.0\text{ }^\circ\text{C}$ .]

\text{heat energy evolved} = ............................. \text{ J}

(ii)

Use the Periodic Table on page 16 and your answer to (i) to calculate the enthalpy change, in $\text{kJ mol}^{-1}$ , when 1 mole of FB 2, Mg, reacts with ethanoic acid.

\text{enthalpy change, } \Delta H = \underset{\text{(sign)}}{\text{......}} \underset{\text{(value)}}{\text{.............................}} \text{ kJ mol}^{-1}

(d)

Reaction 2: Enthalpy change of a strong acid.

FB 3 is hydrochloric acid, $\text{HCl}$ .
The tube labelled FB 4 contains two strips of magnesium, Mg. One strip is longer than the other strip.

Method 2

Read the whole method before starting any practical work and prepare a table for your results in the space below.

Weigh the longer strip of magnesium and record the balance reading.
Support the plastic cup in the $250\text{ cm}^3$ beaker.
Coil the magnesium ribbon loosely so that it fits into the bottom of the plastic cup and then remove the ribbon.
Use the measuring cylinder to transfer $25\text{ cm}^3$ of the acid, FB 3, into the plastic cup.
Place the thermometer in the acid and measure and record the initial temperature of the acid.
Add the piece of magnesium into the acid in the cup.
Stir constantly until the maximum temperature is reached.
Measure and record the maximum temperature.
Rinse the plastic cup for use in the next experiment.
Calculate and record the temperature rise.
Repeat this experiment using the shorter strip of magnesium and record all results.

(e)

Show your working and appropriate significant figures in the final answer to each step of your calculations.

Use your results from (d) for the longer strip of magnesium and the Periodic Table on page 16 to calculate the enthalpy change, in $\text{kJ mol}^{-1}$ , when 1 mole of FB 4, Mg, reacts with hydrochloric acid.
[Assume $4.2\text{ J}$ of heat energy changes the temperature of $1.0\text{ cm}^3$ of the mixture by $1.0\text{ }^\circ\text{C}$ .]

\text{enthalpy change, } \Delta H = \underset{\text{(sign)}}{\text{......}} \underset{\text{(value)}}{\text{.............................}} \text{ kJ mol}^{-1}

(f)

(i)

A student suggested that the experiment carried out in (d) could be improved by using a catalyst.

Would the use of a catalyst improve the accuracy of the results in this experiment? Give a reason for your answer.

(ii)

Another student could not find the hydrochloric acid, FB 3, so used sulfuric acid, $\text{H}_2\text{SO}_4$ , instead. He used the same volume and the same concentration as the hydrochloric acid in FB 3.

What effect would this change have on the temperature rise in the experiment? Give a reason for your answer.

(g)

Ethanoic acid is a weak acid. It is partially ionised in aqueous solution.

\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}^+\text{(aq)}

You are to determine the energy needed to cause the molecules of ethanoic acid to ionise completely.

\text{CH}_3\text{COOH(aq)} \rightarrow \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}^+\text{(aq)}

Hydrochloric acid is a strong acid; it is fully ionised in aqueous solution.

The values for the enthalpy changes you obtained in (c)(ii) and (e) could be used to calculate the energy change for the ionisation but more accurate experiments give the results in Table 1.

Table 1

reaction	equation	$\Delta H / \text{kJ mol}^{-1}$
1	$\text{Mg(s)} + 2\text{CH}_3\text{COOH(aq)} \rightarrow \text{Mg(CH}_3\text{COO)}_2\text{(aq)} + \text{H}_2\text{(g)}$	$-460.3$
2	$\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$	$-464.1$

(i)

Write the ionic equation, including state symbols, for the reaction of magnesium with aqueous hydrochloric acid.

(ii)

Use the data in Table 1 to calculate the enthalpy change for the ionisation of ethanoic acid.

\text{CH}_3\text{COOH(aq)} \rightarrow \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}^+\text{(aq)}

Show clearly how you obtained your answer.

\Delta H = \underset{\text{(sign)}}{\text{......}} \underset{\text{(value)}}{\text{.............................}} \text{ kJ mol}^{-1}

(h)

The experiment in (a) was repeated using trichloroethanoic acid instead of ethanoic acid.

\text{Mg(s)} + 2\text{CCl}_3\text{COOH(aq)} \rightarrow \text{Mg(CCl}_3\text{COO)}_2\text{(aq)} + \text{H}_2\text{(g)} \quad \text{reaction 3}

Trichloroethanoic acid, $\text{CCl}_3\text{COOH}$ , is a weak acid that is however stronger than ethanoic acid.

The enthalpy change for reaction 3 is between the two values given in Table 1.

Table 1

reaction	equation	$\Delta H / \text{kJ mol}^{-1}$
1	$\text{Mg(s)} + 2\text{CH}_3\text{COOH(aq)} \rightarrow \text{Mg(CH}_3\text{COO)}_2\text{(aq)} + \text{H}_2\text{(g)}$	$-460.3$
2	$\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$	$-464.1$

(i)

Explain why the enthalpy change for reaction 3 is more exothermic than the enthalpy change for reaction 1.

(ii)

Explain why the enthalpy change for reaction 3 is less exothermic than the enthalpy change for reaction 2.

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