Chemistry 9701/52 — February/March 2017

Suggest why the enthalpy change of reaction, $\Delta H_r$, for the decomposition of sodium hydrogencarbonate cannot be measured directly.

In both experiments the student used a weighing boat. A weighing boat is a small vessel used to contain solid samples when they are weighed.

Experiment 1 Reaction between sodium carbonate, $\text{Na}_2\text{CO}_3\text{(s)}$, and dilute hydrochloric acid, $\text{HCl(aq)}$

step 1 The student added approximately $3\text{ g}$ of $\text{Na}_2\text{CO}_3\text{(s)}$ to a weighing boat and accurately measured the combined mass of the weighing boat and $\text{Na}_2\text{CO}_3\text{(s)}$. This mass was recorded in Table 1.1.

step 2 The student used a measuring cylinder to measure $50\text{ cm}^3$ of $2\text{ mol dm}^{-3}\ \text{HCl(aq)}$.

step 3 The experiment was carried out and the results were recorded in Table 1.2.

step 4 The student reweighed the empty weighing boat and recorded the mass in Table 1.1.

Table 1.1 mass results from Experiment 1

Table 1.2 temperature results from Experiment 1

Outline how the student carried out step 3 of the experiment.
You may find it helpful to write your answer as a series of smaller steps.

Draw a labelled diagram of the apparatus.

The student plotted a graph of the results and drew two lines of best fit which were both extrapolated as shown.

Use the graph to determine the theoretical temperature increase at 4 minutes.

theoretical temperature increase = .............................. °C

Use Table 1.1 on page 2 to determine the mass of $\text{Na}_2\text{CO}_3\text{(s)}$ which was added to the $\text{HCl(aq)}$.
Use this value and your answer to (c) to determine the enthalpy change, $\Delta H_1$, for the reaction shown.

Give your answer to three significant figures.
[Assume that the specific heat capacity of the solution is $4.18\text{ J g}^{-1}\text{ K}^{-1}$.]
[$A_r$: Na, 23.0; C, 12.0; O, 16.0]

Experiment 2 Reaction between sodium hydrogencarbonate, $\text{NaHCO}_3\text{(s)}$, and dilute hydrochloric acid, $\text{HCl(aq)}$

step 1 The student weighed an empty weighing boat and recorded the mass in Table 1.3.

step 2 The student added exactly $4.20\text{ g}$ of $\text{NaHCO}_3\text{(s)}$ to the weighing boat and recorded the mass in Table 1.3.

step 3 The student carried out the same experimental procedure as in steps 2 and 3 of Experiment 1.

Table 1.3 mass results from Experiment 2

Explain why the method of determining the mass of solid added in Experiment 2 is less accurate than the method of determining the mass of solid added in Experiment 1.

The student used the results from Experiment 2 and correctly determined the enthalpy change for the reaction between $\text{NaHCO}_3\text{(s)}$ and $\text{HCl(aq)}$, $\Delta H_2$, to be $+24.2\text{ kJ mol}^{-1}$.

Use the axes to draw a sketch graph of the expected results of Experiment 2.

Use $\Delta H_1$ from (d) and $\Delta H_2$ from (h) to determine the enthalpy change of reaction, $\Delta H_r$, for the decomposition of $\text{NaHCO}_3\text{(s)}$.

An energy cycle has been drawn for you.

If you were unable to calculate $\Delta H_1$ in (d), assume $\Delta H_1 = -26.7\text{ kJ mol}^{-1}$. This is not the correct value of $\Delta H_1$.