Chemistry 9701/34 — May/June 2010

Method

• Weigh the bottle/tube containing the sodium hydrogencarbonate labelled FB 1.
• Support the plastic cup in the $250\text{ cm}^3$ beaker.
• Use the measuring cylinder to transfer $30\text{ cm}^3$ of FB 4 into the plastic cup.
• Place the thermometer in the acid in the plastic cup and record the steady temperature of the acid.
• Add the contents of the weighed tube, FB 1, to the acid in the plastic cup, a little at a time with constant stirring.
• You should add the solid as quickly as possible – taking care to minimise any acid spray from the plastic cup.

Avoid breathing any fumes from the experiment.

• Record the minimum temperature obtained in the reaction.
• Reweigh the emptied tube, FB 1, containing any remaining solid that was not tipped from the tube.
• Empty and rinse the plastic cup. Rinse the thermometer. Shake dry the plastic cup.
• Repeat the experiment using tubes labelled FB 2 and FB 3. In each experiment use $30\text{ cm}^3$ of FB 4.

Carry out two further experiments.
Using the empty weighing bottles/tubes, labelled FB 5 and FB 6, weigh two further masses of sodium hydrogencarbonate. Choose masses to enable you to plot an appropriate graph of temperature change against mass of sodium hydrogencarbonate.

Results
Record your results in an appropriate form showing, for each experiment, the measurements of mass and temperature and the calculated temperature fall.

Use the grid below to plot a graph of decrease in temperature ($y$-axis) against the mass of sodium hydrogencarbonate added ($x$-axis).
Draw a line of best fit through the plotted points.
You should consider if the best-fit line passes through the origin (0,0) of the graph.

Explain why the mass of $\text{NaHCO}_3$ is plotted on the $x$-axis rather than on the $y$-axis.

Construct the balanced equation for the reaction of $\text{NaHCO}_3$ with hydrochloric acid.

Calculate the gradient of your graph.
Show all of your working clearly, both construction lines on the graph and working in the calculation.

Although there is insufficient acid in $30\text{ cm}^3$ of FB 4 to neutralise $1\text{ mol}$ of $\text{NaHCO}_3$ it is possible to calculate the theoretical fall in temperature for this reaction.
Use your answer from (e) to calculate this value.
[$A_r$: C, 12.0; H, 1.0; Na, 23.0; O, 16.0]

The theoretical fall in temperature for $1\text{ mol}$ of $\text{NaHCO}_3$ = .................... $^\circ\text{C}$

Calculate the theoretical enthalpy change for the neutralisation of $1\text{ mol}$ of $\text{NaHCO}_3$ by hydrochloric acid. Give your answer in $\text{kJ mol}^{-1}$ and include the correct sign for the reaction.
[$4.3\text{ J}$ are absorbed or released when the temperature of $1\text{ cm}^3$ of solution changes by $1\text{ }^\circ\text{C}$.]

Suggest two ways in which your apparatus could be modified to reduce transfer of heat from the surroundings to the solution in the apparatus.

modification 1 ..................................................................................................................

modification 2 ..................................................................................................................

State and explain why the experiment would be more accurate if the volumes of FB 4 had been measured using a burette instead of a measuring cylinder.

The mass of $\text{NaHCO}_3$ used in a further experiment and its associated temperature change are shown in the tables below.

• The mass was obtained on a balance reading to 1 decimal place.
• The thermometer used was graduated at $1\text{ }^\circ\text{C}$.

Complete the table to show the errors in these results.

Two students add $6.0\text{ g}$ of sodium carbonate to $50.00\text{ cm}^3$ of $2.0\text{ mol dm}^{-3}$ hydrochloric acid. Each student repeats the experiment a number of times.
The thermometer readings and temperature changes obtained consistently by each student are shown below.

Suggest the type of error shown by these results.