Edexcel A-Level Chemistry: Synoptic & Practical Skills

Sulfur trioxide is manufactured in the Contact process, the key step in making sulfuric acid, by the catalysed oxidation of sulfur dioxide:

$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \qquad \Delta H = -196\ \text{kJ mol}^{-1}$2SO2​(g)+O2​(g)⇌2SO3​(g)ΔH=−196 kJ mol−1

A typical plant runs this step at about 450 °C, at a pressure only slightly above atmospheric (about 1-2 atm), over a vanadium(V) oxide catalyst, and achieves a conversion of around 98 per cent.

Explain why these operating conditions are chosen, making clear why the choice of temperature is described as a compromise and why a high pressure is not used here even though more gas molecules appear on the left. In your answer you should link ideas from chemical equilibrium, reaction kinetics and energetics.

(9 marks)

Copper(II) sulfate dissolves in water to give the pale-blue ion $[Cu(H_2O)_6]^{2+}$[Cu(H2​O)6​]2+.

A student makes three observations:

• when an excess of aqueous ammonia is added, the solution turns a deep royal-blue and forms $[Cu(NH_3)_4(H_2O)_2]^{2+}$[Cu(NH3​)4​(H2​O)2​]2+;
• when concentrated hydrochloric acid is added, the solution turns yellow and forms $[CuCl_4]^{2-}$[CuCl4​]2−;
• when solid copper(I) sulfate is stirred into water, a brown solid forms and the solution turns pale blue.

Explain these observations. In your answer you should link ideas about complex-ion formation and bonding (including any change in coordination number), the origin of colour, and the redox behaviour and variable oxidation states of copper.

(9 marks)

A student standardises a solution of sulfuric acid by titrating it against a standard sodium carbonate solution of concentration $0.0500\ \text{mol dm}^{-3}$0.0500 mol dm−3 (assume this concentration is exact).

$Na_2CO_3(aq) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + H_2O(l) + CO_2(g)$Na2​CO3​(aq)+H2​SO4​(aq)→Na2​SO4​(aq)+H2​O(l)+CO2​(g)

Using a pipette, 25.00 cm³ of the sodium carbonate solution was placed in a conical flask and titrated with the sulfuric acid from a burette. The mean titre of sulfuric acid was 22.40 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 sulfuric acid, 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 combustion of propan-1-ol ($C_3H_7OH$C3​H7​OH, $M_r = 60.0$Mr​=60.0) using a spirit burner to heat water in a copper can.

The data collected were:

Quantity	Value
Mass of propan-1-ol burned (burner weighed before and after)	0.60 g
Volume of water in the can (pipette; uncertainty negligible)	200 cm³
Initial water temperature	18.0 °C
Final water temperature	39.4 °C

Take the density of water 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. 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 combustion of propan-1-ol from these data, then propagate the percentage uncertainties in the mass of fuel and the temperature change to give the overall percentage uncertainty (and absolute uncertainty) in your value. The data-book value is $-2021\ \text{kJ mol}^{-1}$−2021 kJ mol−1; explain, in terms of a systematic error, why your value differs.

(6 marks)

In an Edexcel Core Practical, a student investigates the order of reaction with respect to propanone in the acid-catalysed iodination of propanone:

$CH_3COCH_3(aq) + I_2(aq) \rightarrow CH_3COCH_2I(aq) + HI(aq)$CH3​COCH3​(aq)+I2​(aq)→CH3​COCH2​I(aq)+HI(aq)

The method described is: mix known volumes of propanone, dilute sulfuric acid and aqueous iodine in a flask and start a stopwatch. At intervals, withdraw a 10 cm³ sample with a measuring cylinder, run it into sodium hydrogencarbonate solution to quench the reaction, then titrate the remaining iodine against sodium thiosulfate using a starch end-point judged by eye. From a graph of iodine concentration against time the initial rate is found. The student carries out the run once for each initial propanone concentration and keeps the temperature "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)

The successive ionisation energies of aluminium, in $\text{kJ mol}^{-1}$kJ mol−1, are:

Ionisation	1st	2nd	3rd	4th
Energy / $\text{kJ mol}^{-1}$kJ mol−1	578	1817	2745	11577

In its compounds aluminium is found as $Al^{3+}$Al3+ (for example in the chloride $AlCl_3$AlCl3​), but the ion $Al^{4+}$Al4+ is never formed.

Explain these facts. In your answer link the electron configuration and the successive ionisation energies of aluminium to the formula of its chloride.

(5 marks)