AS & A Level Chemistry - 9701 Paper 3 2018 Summer Zone 2

Many metal hydroxides decompose when heated to produce water vapour and the metal oxide as residue.
In this experiment, you will heat a metal hydroxide $M(OH)_2$. You will then identify the metal M.

$M(OH)_2(s) \rightarrow MO(s) + H_2O(g)$

FB 1 is the hydroxide of a metal in Group 2 of the Periodic Table, $M(OH)_2$. You are supplied with approximately 2 g of FB 1.

(a) Method

Experiment 1

• Weigh a crucible with its lid and record the mass.
• Add between 0.5 and 0.7 g of FB 1 to the crucible. Weigh the crucible with FB 1 and lid and record the mass.
• Place the crucible on the pipe-clay triangle and remove the lid.
• Heat the crucible and contents strongly for about four minutes.
• Replace the lid and leave the crucible and residue to cool.
• While the crucible is cooling, begin work on a different question.
• Once the crucible is cool, reweigh the crucible and contents with the lid on. Record the mass.
• Calculate and record the mass of FB 1 used and the mass of residue obtained.

• Repeat the method used in Experiment 1, using between 0.8 and 1.0 g of FB 1 in the second crucible.
• Calculate and record the mass of FB 1 used and the mass of residue obtained.

In this experiment you will determine the enthalpy change, $\Delta H_r$, for the decomposition of calcium hydroxide to calcium oxide.
$$\text{Ca(OH)}_2(s) \rightarrow \text{CaO}(s) + \text{H}_2\text{O}(l)$$
To do this, you will determine the enthalpy changes for the reactions of calcium hydroxide and calcium oxide with hydrochloric acid. Excess acid will be used for both experiments.
You will then use Hess' Law to calculate the enthalpy change for the reaction above.
FB 2 is 3.0 mol dm$^{-3}$ hydrochloric acid, HCl.
FB 3 is calcium hydroxide, Ca(OH)$_2$.
FB 4 is calcium oxide, CaO.
(a) Determination of the enthalpy change for the reaction of calcium hydroxide, FB 3, with hydrochloric acid, FB 2.
(i) Method
• Support a plastic cup in the 250 cm$^3$ beaker.
• Weigh the container with FB 3. Record the mass.
• Use the measuring cylinder to transfer 30 cm$^3$ of FB 2 into the 100 cm$^3$ beaker.
• Place the beaker on the tripod and gauze and heat FB 2 gently until its temperature is between 35 ºC and 40 ºC. Turn off the Bunsen burner.
• Carefully transfer all FB 2 from the 100 cm$^3$ beaker into the plastic cup.
• Measure and record the temperature of FB 2 in the plastic cup in the space below.
• Immediately add all the FB 3 from the container to the FB 2 in the plastic cup.
• Stir constantly until the maximum temperature is reached.
• Measure and record the maximum temperature.
• Weigh and record the mass of the container with any residual solid.
• Calculate and record the mass of FB 3 used.
• Calculate and record the temperature rise.
Results

(ii) Calculate the energy produced during this reaction. [Assume that 4.2 J of heat energy changes the temperature of 1.0 cm$^3$ of solution by 1.0 ºC.]
energy produced = ............................... J [1]
(iii) Calculate the number of moles of calcium hydroxide, FB 3, used in the experiment.
moles of Ca(OH)$_2$ = ............................... mol [1]
(iv) Calculate the enthalpy change, in kJ mol$^{-1}$, for reaction 1 below, $\Delta H_1$.
$$\text{Ca(OH)}_2(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + 2\text{H}_2\text{O}(l)$$
$\Delta H_1$ = ...... ........................... kJ mol$^{-1}$
(sign) (value) [1]

(b) Determination of the enthalpy change for the reaction of calcium oxide, FB 4, with hydrochloric acid, FB 2.
(i) Method
• Support the second plastic cup in the 250 cm$^3$ beaker.
• Weigh the container with FB 4. Record the mass.
• Use the measuring cylinder to transfer 30 cm$^3$ of FB 2 into the 100 cm$^3$ beaker.
• Place the beaker on the tripod and gauze and heat FB 2 gently until its temperature is approximately 35 ºC.
• Carefully transfer all FB 2 from the 100 cm$^3$ beaker into the plastic cup.
• Measure and record the temperature of FB 2 in the plastic cup in the space below.
• Immediately add all the FB 4 from the container to the FB 2 in the plastic cup.
• Stir constantly until the maximum temperature is reached.
• Measure and record the maximum temperature.
• Weigh and record the mass of the container with any residual solid.
• Calculate and record the mass of FB 4 used.
• Calculate and record the temperature rise.
Results

(ii) Calculate the enthalpy change, in kJ mol$^{-1}$, for reaction 2 below, $\Delta H_2$.
$$\text{CaO}(s) + 2\text{HCl}(aq) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l)$$
$\Delta H_2$ = ...... ........................... kJ mol$^{-1}$
(sign) (value) [2]

(c) Use your values for $\Delta H_1$ and $\Delta H_2$ to calculate the enthalpy change for the decomposition of calcium hydroxide, $\Delta H_r$.
Show clearly how you obtained your answer by drawing a Hess' Law energy cycle.
(If you were unable to calculate the enthalpy changes, assume that $\Delta H_1$ is −129 kJ mol$^{-1}$ and $\Delta H_2$ is −150 kJ mol$^{-1}$. Note: these are not the correct values.)
$$\text{Ca(OH)}_2(s) \rightarrow \text{CaO}(s) + \text{H}_2\text{O}(l)$$
$\Delta H_r$ = ...... ........................... kJ mol$^{-1}$
(sign) (value) [2]

(d) (i) Give a reason why FB 2 was heated before FB 3 or FB 4 were added to it.
.........................................................................................................................................................
[1]
(ii) The procedure in (b) was repeated using the same mass of calcium oxide, FB 4. However, 30 cm$^3$ of 4.0 mol dm$^{-3}$ HCl was used instead of 30 cm$^3$ of 3.0 mol dm$^{-3}$ HCl.
How would the temperature rise compare with the one you obtained in the experiment in (b)?
Explain your answer.
.........................................................................................................................................................
[1]

(a) \textbf{FB 5, FB 6 and FB 7 are all aqueous solutions.}
Each solution contains one cation and one anion.
The cation in \textbf{FB 6} is listed in the Qualitative Analysis Notes, but the other cations are not.
The anions present are chloride, nitrate and sulfate (but not necessarily in that order).

Use a 1 cm depth of each solution in a test-tube for the following tests.
Record all your observations in the table.

| test | observations | |------|------------------| | | FB 5 | FB 6 | FB 7 | | Add a 2 cm strip of magnesium ribbon. | | | | | Add several drops of aqueous sodium carbonate. | | | | | Add aqueous sodium hydroxide. | | | | | Add several drops of aqueous barium chloride or aqueous barium nitrate. | | | |

| test | observations | |------|------------------| | | FB 5 | FB 6 | FB 7 | | Add a 1 cm depth of \textbf{FB 5.} | | | | | Add a 1 cm depth of \textbf{FB 6.} | | | | | Add a 1 cm depth of aqueous potassium iodide. | | | |

(b) (i) From your observation of the reaction of \textbf{FB 7} with aqueous potassium iodide, suggest the identity of the cation in \textbf{FB 7.}
..................................................................................................................................................... [1]

(ii) Give the ionic equation for the reaction of magnesium with \textbf{FB 5.}
Include state symbols.
..................................................................................................................................................... [1]

(iii) What type of reaction takes place when \textbf{FB 6} reacts with sodium carbonate?
..................................................................................................................................................... [1]

(iv) Give the ionic equation for the reaction between \textbf{FB 6} and \textbf{FB 7.}
Include state symbols.
..................................................................................................................................................... [1]