AS & A Level Chemistry - 9701 Paper 3 2011 Summer Zone 2

Many solid salts exist as hydrates. One example is washing soda - hydrated sodium carbonate, $Na_2CO_3.xH_2O$.
You are to determine the value of x in $Na_2CO_3.xH_2O$ by titration with hydrochloric acid.

FB 1 is hydrated sodium carbonate, $Na_2CO_3.xH_2O$.
FB 2 is 0.200 mol dm^-3 hydrochloric acid, HCl.
methyl orange indicator

The equation for the reaction between hydrated sodium carbonate and hydrochloric acid is shown below.

Na$_2$CO$_3$.xH$_2$O + 2HCl → 2NaCl + CO$_2$ + (x + 1) H$_2$O

(a) Method

• Weigh the tube containing FB 1, the hydrated sodium carbonate. Record the mass in the space below.
• Add all the FB 1 into a 250cm^3 glass beaker. Reweigh the tube containing any residual FB 1. Record the mass in the space below.
• Calculate and record the mass of FB 1 used.
  mass of FB 1 used = ................. g

• Use the 50cm^3 measuring cylinder to add, in total, about 100cm^3 of distilled water to the beaker.
• Stir with a glass rod until all the solid has dissolved.
• Pour the solution from the beaker into the 250cm^3 graduated (volumetric) flask.
• Wash out the beaker thoroughly with distilled water and add the washings to the graduated flask.
• Make up the contents of the graduated flask to the 250cm^3 mark with distilled water.
• Shake the flask to mix the solution of FB 1.
• Pipette 25.0cm^3 of your solution of FB 1 into a conical flask.
• Add to the flask a few drops of methyl orange indicator and place the flask on a white tile.
• Fill the burette with hydrochloric acid, FB 2.
• Titrate the solution of FB 1 with the acid until the end-point is reached.

• Carry out as many accurate titrations as you think necessary to obtain consistent results.
• Make certain any recorded results show the precision of your practical work.
• Record, in an appropriate form below, all your burette readings and the volume of FB 2 added in each accurate titration.

You are to determine the enthalpy change for the reaction of hydrated sodium carbonate, $Na_2CO_3 \cdot xH_2O$, with hydrochloric acid, $HCl (aq)$.

The equation for this reaction is shown below.
$$Na_2CO_3 \cdot xH_2O(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + CO_2(g) + (x+1)H_2O(l)$$
FB 3 is hydrated sodium carbonate, $Na_2CO_3 \cdot xH_2O$.
FB 4 is 4.00 mol $dm^{-3}$ hydrochloric acid, $HCl$.

Make sure that in this experiment you use the hydrochloric acid labelled FA 4.

(a) Method

• Support the plastic cup in a 250 $cm^3$ beaker.
• Use a measuring cylinder to transfer 25 $cm^3$ of FB 4 into the plastic cup.
• Tilt the beaker so that the bulb of the thermometer is covered by the solution.
• Measure and record the temperature of the solution.
• Measure and record the mass of the tube containing FB 3.
• Carefully tip all the hydrated sodium carbonate from the weighed tube into the plastic cup.
• There will be effervescence. Add the solid in small portions with constant stirring using the thermometer.
• Record the lowest temperature obtained.
• Reweigh the tube containing any residual FB 3.

In the space below, record, in an appropriate form,
• both balance readings,
• both temperature measurements,
• the mass of FB 3 used in the experiment,
• the fall in temperature.

(b) Calculation

Show your working and express your answers to three significant figures.
(i) Calculate the heat energy change involved in the reaction.
(You may assume that 4.3 J are required to change the temperature of 1.0 $cm^3$ of any solution by 1.0°C.)

heat energy produced = ........................................... J

(ii) Calculate the number of moles of $Na_2CO_3 \cdot xH_2O$ you used in 2(a).
You will need to use the relative formula mass you calculated in 1(c)(v).
If you were unable to calculate the relative formula mass in 1(c), assume it is 259 but note that this is not the correct value.

.................... mol of $Na_2CO_3 \cdot xH_2O$

(iii) Calculate the enthalpy change, in $kJ \cdot mol^{-1}$, for the following reaction.

$$Na_2CO_3 \cdot xH_2O(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + CO_2(g) + (x+1)H_2O(l)$$

enthalpy change = ........ ................ $kJ \cdot mol^{-1}$

(c) In experiments carried out to determine enthalpy changes, heat transfer between the surroundings and the reactants is a significant source of error. This problem can be limited by improved insulation.

Apart from modifications made to minimise heat transfer, suggest one possible improvement you could make to the apparatus or procedure to make the determination of the enthalpy change more accurate.

(a) (i) You are provided with three solutions FB 5, FB 6 and FB 7, each of which contains a single cation. One of these cations is aluminium, one is magnesium and the other is lead.
Use the information in the Qualitative Analysis Notes on page 11 to select reagents that would enable you to determine the cation in FB 5, FB 6 and FB 7.
reagents ............................................................
Carry out suitable tests and record the results of your experiments in an appropriate form in the space below.
(ii) Complete the table below.
| | FB 5 | FB 6 | FB 7 |
| cation | | | |
What is the minimum evidence from your observations, that enables you to identify these cations?
The minimum evidence for the cation in FB 5 is ...................................
The minimum evidence for the cation in FB 6 is ...................................
The minimum evidence for the cation in FB 7 is ...................................

(b) You are provided with solid FB 8.
Carry out the tests and complete the following table.
| test | observations |
|------|--------------|
| (i) To a spatula measure of FB 8, in a test-tube, add about a 1 cm depth of distilled water to make a solution. To this solution add 4 pieces of magnesium ribbon. | |
| (ii) To a small spatula measure of FB 8, in a boiling tube, add 3 cm depth of aqueous sodium hydroxide. Warm gently and carefully. | |
| (iii) To a spatula measure of FB 8, in a test-tube, add about a 1 cm depth of distilled water to make a solution. To this solution add an equal volume of aqueous sodium hydroxide.
To this mixture add a small volume of hydrogen peroxide. | |
Identify the metal ion present in FB 8.
metal ion = .......................................................
State the change in oxidation number (state) of this metal ion that is occurring in test (i).
oxidation number (state) changes from ............................ to ............................
State the change in oxidation number (state) of this metal ion that is occurring in test (iii).
oxidation number (state) changes from ............................ to ............................