OCR A-Level Chemistry: Synoptic & Practical Skills

Chromium can be extracted from chromium(III) oxide by the thermite-type reduction with aluminium:

$Cr_2O_3(s) + 2Al(s) \rightarrow 2Cr(s) + Al_2O_3(s)$Cr2​O3​(s)+2Al(s)→2Cr(s)+Al2​O3​(s)

Some data for this reaction at 298 K are given below.

Quantity	Value
Standard enthalpy change, $\Delta H^{\circ}$ΔH∘	$-536\ \text{kJ mol}^{-1}$−536 kJ mol−1
Standard entropy change of the system, $\Delta S^{\circ}_{\text{system}}$ΔSsystem∘​	$-12.5\ \text{J K}^{-1}\,\text{mol}^{-1}$−12.5 J K−1mol−1

A student notes that the standard electrode potentials also suggest the reaction should occur:

$Al^{3+}(aq) + 3e^- \rightleftharpoons Al(s) \qquad E^{\circ} = -1.66\ \text{V}$Al3+(aq)+3e−⇌Al(s)E∘=−1.66 V $Cr^{3+}(aq) + 3e^- \rightleftharpoons Cr(s) \qquad E^{\circ} = -0.74\ \text{V}$Cr3+(aq)+3e−⇌Cr(s)E∘=−0.74 V

Explain, using the data, why this extraction is thermodynamically feasible, and why aluminium rather than chromium acts as the reducing agent. In your answer you should link ideas from electrode potentials and from thermodynamics (enthalpy, entropy and Gibbs free energy), and comment on what the entropy data imply about feasibility at higher temperatures.

(9 marks)

A student oxidises propan-1-ol with acidified potassium dichromate(VI). Heating under reflux gives a single organic liquid X, which has the molecular formula $C_3H_6O_2$C3​H6​O2​.

The student records spectroscopic data for X:

Technique	Key data
Mass spectrum	Molecular ion at $m/z = 74$m/z=74; prominent fragment at $m/z = 45$m/z=45
Infrared	Strong, broad absorption 2500-3300 cm⁻¹; strong absorption at 1710 cm⁻¹
¹H NMR	Three peaks; one at about $\delta\ 11.5$δ 11.5 (1H), a quartet at about $\delta\ 2.4$δ 2.4 (2H), a triplet at about $\delta\ 1.1$δ 1.1 (3H)

Explain how this evidence shows that X is propanoic acid, and explain why X, rather than an aldehyde, is the product of the reaction described. In your answer you should link ideas from organic synthesis (oxidation of alcohols) and from spectroscopic analysis (mass spectrometry, infrared and NMR).

(9 marks)

A student determines the concentration of iron(II) ions in an acidified solution by titrating it against potassium manganate(VII) of concentration $0.0200\ \text{mol dm}^{-3}$0.0200 mol dm−3 (assume this concentration is exact). The manganate(VII) is its own indicator, the end-point being the first permanent pale-pink colour.

The ionic equation is:

$MnO_4^-(aq) + 8H^+(aq) + 5Fe^{2+}(aq) \rightarrow Mn^{2+}(aq) + 4H_2O(l) + 5Fe^{3+}(aq)$MnO4−​(aq)+8H+(aq)+5Fe2+(aq)→Mn2+(aq)+4H2​O(l)+5Fe3+(aq)

Using a pipette, 25.00 cm³ of the iron(II) solution was placed in a conical flask. The mean titre of potassium manganate(VII) from the burette was 24.60 cm³.

The apparatus uncertainties are:

Apparatus	Uncertainty
Burette (each reading; two readings per titre)	$\pm 0.05$±0.05 cm³
Volumetric pipette	$\pm 0.06$±0.06 cm³

Calculate the concentration of the iron(II) ions, then calculate the percentage uncertainty in each measured volume and combine them to find the overall percentage uncertainty in the calculated concentration. Quote the concentration with its absolute uncertainty.

(6 marks)

A student determined the enthalpy of solution of anhydrous calcium chloride, $CaCl_2$CaCl2​ ($M_r = 111.1$Mr​=111.1), by dissolving it in water in an expanded-polystyrene cup.

The data collected were:

Quantity	Value
Mass of $CaCl_2$CaCl2​ dissolved (weighing bottle weighed before and after)	2.78 g
Volume of water in the cup (pipette; uncertainty negligible)	50.0 cm³
Initial temperature	20.0 °C
Final (maximum) temperature	28.5 °C

Take the density of the solution as $1.00\ \text{g cm}^{-3}$1.00 g cm−3 and its specific heat capacity as $c = 4.18\ \text{J g}^{-1}\,\text{K}^{-1}$c=4.18 J g−1K−1, and base $q$q on the mass of water. The apparatus uncertainties are: balance $\pm 0.005$±0.005 g on each weighing; thermometer $\pm 0.1$±0.1 °C on each reading.

Calculate the enthalpy of solution of calcium chloride from these data, then propagate the percentage uncertainties in the mass of solute and the temperature change to give the overall percentage uncertainty (and absolute uncertainty) in your value. The data-book value is $-82.8\ \text{kJ mol}^{-1}$−82.8 kJ mol−1; explain, in terms of a systematic error, why your value differs.

(6 marks)

In a PAG (Practical Activity Group) on reaction rates, a student investigates the order of reaction with respect to iodide ions in the 'iodine clock' reaction between peroxodisulfate(VI) ions and iodide ions:

$S_2O_8^{2-}(aq) + 2I^-(aq) \rightarrow 2SO_4^{2-}(aq) + I_2(aq)$S2​O82−​(aq)+2I−(aq)→2SO42−​(aq)+I2​(aq)

The method described is: into a beaker, place fixed volumes of potassium iodide solution, a small fixed amount of sodium thiosulfate solution and starch indicator; add the potassium peroxodisulfate solution and start a stopwatch; stop the stopwatch when the mixture suddenly turns blue-black. The small amount of thiosulfate first removes the iodine produced, so the blue colour appears at a fixed, known extent of reaction and the time is taken as a measure of the initial rate ($\text{rate} \propto 1/t$rate∝1/t). To vary the iodide concentration, the student uses different volumes of potassium iodide made up to the same total volume with water. The student carries out one run at each iodide concentration, measures all volumes with measuring cylinders, and works "at room temperature".

Evaluate this experimental method. In your answer identify the independent, dependent and control variables, distinguish random from systematic error, comment on reliability, and suggest improvements that would reduce the uncertainty.

(6 marks)

Carbon dioxide and sulfur dioxide are both simple molecular oxides, but their boiling points are very different:

Molecule	$M_r$Mr​	Boiling point
$CO_2$CO2​	44.0	$-78$−78 °C (sublimes)
$SO_2$SO2​	64.1	$-10$−10 °C

Explain why sulfur dioxide has the much higher boiling point. In your answer link the shapes of the two molecules and their polarity to the intermolecular forces between the molecules.

(5 marks)