AS & A Level Chemistry - 9701 Paper 3 2016 Summer Zone 5

In this experiment you will determine the concentration of a solution of sulfuric acid by titration.
FA 1 is sulfuric acid, $H_2SO_4$.
FA 2 is aqueous sodium hydroxide, containing 4.20 g NaOH dissolved in 1.00 dm$^3$ of water.
Thymolphthalein indicator
(a) Method
Dilution of FA 1

• Pipette 10.0 cm$^3$ of FA 1 into the 250 cm$^3$ volumetric flask.
• Make the solution up to the mark using distilled water.
• Shake the flask thoroughly.
• This diluted solution of sulfuric acid is FA 3. Label the flask FA 3.

• Fill the burette with FA 2.
• Pipette 25.0 cm$^3$ of FA 3 into a conical flask.
• Add a few drops of thymolphthalein indicator.
• Perform a rough titration and record your burette readings in the space below. The end point is reached when the solution turns a permanent pale blue colour. The rough titre is ...................... cm$^3$.
• Carry out as many accurate titrations as you think necessary to obtain consistent results.
• Make sure any recorded results show the precision of your practical work.
• Record in a suitable form below all of your burette readings and the volume of FA 2 added in each accurate titration.

In this experiment you will determine the enthalpy change, $\Delta H$, for the decomposition of magnesium carbonate to magnesium oxide.
$\text{MgCO}_3(s) \rightarrow \text{MgO}(s) + \text{CO}_2(g)$

In order to do this, you will determine the enthalpy changes for the reactions of magnesium carbonate and magnesium oxide with sulfuric acid. Excess of the two magnesium compounds will be used in each experiment. Then you will use Hess’ Law to calculate the enthalpy change for the reaction above.

$\text{FA 1 is sulfuric acid, H}_2\text{SO}_4$.
$\text{FA 4 is magnesium carbonate, MgCO}_3$.
$\text{FA 5 is magnesium oxide, MgO}$.

(a) Determination of the enthalpy change for the reaction of magnesium carbonate, $\text{FA 4}$, with sulfuric acid, $\text{FA 1}$
(i) Method
• Support the plastic cup inside the $250 \text{cm}^3$ beaker.
• Use a measuring cylinder to transfer $25 \text{cm}^3$ of $\text{FA 1}$ into the plastic cup.
• Measure and record the initial temperature of the $\text{FA 1}$ in the space below.
• Add all the $\text{FA 4}$ from the container to the $\text{FA 1}$ in the plastic cup.
• Stir constantly until the maximum temperature is reached.
• Measure and record the maximum temperature of the contents of the cup.
• Rinse out the plastic cup and shake to dry for use in (b).
• Calculate and record the temperature rise.

Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(ii) Calculate the energy produced during this reaction. [Assume that $4.2\,\text{J}$ are needed to raise the temperature of $1.0 \text{cm}^3$ of solution by $1.0^\circ\text{C}$.]

energy produced = $\text{.......................}$ J

(iii) Use your answer to 1(c)(v) to calculate the number of moles of sulfuric acid in $25\text{cm}^3$ of $\text{FA 1}$. (If you were unable to calculate the concentration of sulfuric acid in $\text{FA 1}$, assume that it is $1.27\,\text{mol dm}^{–3}$. This is not the true value.)

moles of $\text{H}_2\text{SO}_4$ = $\text{.......................}$ mol

(iv) Calculate the enthalpy change, in $\text{kJ mol}^{–1}$, for the reaction below.
$$\text{MgCO}_3(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{MgSO}_4(aq) + \text{CO}_2(g) + \text{H}_2\text{O}(l)$$
enthalpy change = $\text{......}$ $\text{.......................}$ $\text{kJ mol}^{–1}$
(sign) (value)

(b) Determination of the enthalpy change for the reaction of magnesium oxide, $\text{FA 5}$, with sulfuric acid, $\text{FA 1}$
(i) Method
• Use the measuring cylinder to transfer approximately $40\text{cm}^3$ of $\text{FA 1}$ into the $100\text{cm}^3$ beaker.
• Place the beaker on a tripod and gauze.
• Heat $\text{FA 1}$ in the beaker until the temperature is between $40^\circ\text{C}$ and $50^\circ\text{C}$.
• Support the plastic cup in the $250 \text{cm}^3$ beaker.
• Use the measuring cylinder to transfer $25\text{cm}^3$ of hot $\text{FA 1}$ into the plastic cup. CARE.
• Measure and record, in the space below, the initial temperature of $\text{FA 1}$ in the plastic cup.
• Immediately, add all the $\text{FA 5}$ from the container to the $\text{FA 1}$ in the plastic cup.
• Stir constantly until the maximum temperature is reached.
• Measure and record the maximum temperature.
• Calculate and record the temperature rise.

Calculations
Show your working and appropriate significant figures in the final answer to each step of your calculations.
(ii) Calculate the energy produced during this reaction. [Assume that $4.2\,\text{J}$ are needed to raise the temperature of $1.0 \text{cm}^3$ of solution by $1.0^\circ\text{C}$.]
energy produced = $\text{........................}$ J

(iii) Use your answer to (a)(iii) to calculate the enthalpy change, in $\text{kJ mol}^{–1}$, for the reaction below.
$$\text{MgO}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{MgSO}_4(aq) + \text{H}_2\text{O}(l)$$
enthalpy change = $\text{......}$ $\text{........................}$ $\text{kJ mol}^{–1}$
(sign) (value)

(c) Use your values for the enthalpy changes calculated in (a)(iv) and (b)(iii) to calculate the enthalpy change for the reaction below.
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 the value of the enthalpy change in (a)(iv) is $–58.7\text{kJ mol}^{–1}$ and the value in (b)(iii) is $–140.3\text{kJ mol}^{–1}$. Note: these are not the correct values.)
$$\text{MgCO}_3(s) \rightarrow \text{MgO}(s) + \text{CO}_2(g)$$
enthalpy change = $\text{......}$ $\text{............................}$ $\text{kJ mol}^{–1}$
(sign) (value)

(d) (i) Calculate the maximum percentage error in the temperature rise in (b)(i).
percentage error = $\text{........................}$ %

(ii) The magnesium oxide, $\text{FA 5}$, was weighed with a balance measuring to one decimal place. A student suggested that the accuracy of the experiment in (b)(i) would be improved by weighing $\text{FA 5}$ using a balance measuring to two decimal places. State and explain whether or not the student is correct.
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(a) (i) FA 6 is a salt containing one cation and one anion from those listed on pages 14 and 15. Transfer a small spatula measure of FA 6 into a hard-glass test-tube. Heat gently at first, then heat strongly until no further change occurs.

Record all your observations below.
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(ii) Dissolve the remainder of FA 6 in an approximate depth of 5 cm of distilled water in a boiling tube for use in the following tests. Record your observations in the table below.

[Table_1]