AQA A-Level Chemistry: Synoptic & Practical Skills

Ammonia is manufactured by the Haber process, in which nitrogen and hydrogen reach a dynamic equilibrium over an iron catalyst:

$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \qquad \Delta H = -92\ \text{kJ mol}^{-1}$N2​(g)+3H2​(g)⇌2NH3​(g)ΔH=−92 kJ mol−1

A typical industrial plant operates at about 450 °C and 200 atm, with a finely divided iron catalyst, and continuously condenses ammonia out of the product stream.

Explain why these operating conditions are chosen, making clear why the choice of temperature in particular is described as a compromise. In your answer you should link ideas from chemical equilibrium, reaction kinetics and energetics.

(9 marks)

Iron forms the pale-green aqueous ion $[Fe(H_2O)_6]^{2+}$[Fe(H2​O)6​]2+ and the yellow-brown aqueous ion $[Fe(H_2O)_6]^{3+}$[Fe(H2​O)6​]3+.

A student notes three observations:

• when sodium carbonate solution is added to $[Fe(H_2O)_6]^{3+}$[Fe(H2​O)6​]3+, bubbles of a colourless gas are released, whereas with $[Fe(H_2O)_6]^{2+}$[Fe(H2​O)6​]2+ a green precipitate forms with no gas;
• a solution of $[Fe(H_2O)_6]^{2+}$[Fe(H2​O)6​]2+ slowly turns yellow-brown when left exposed to air;
• both ions are coloured, but a solution of $[Zn(H_2O)_6]^{2+}$[Zn(H2​O)6​]2+ is colourless.

Explain these observations. In your answer you should link ideas about complex-ion formation and bonding, the origin of colour, redox behaviour and the acid-base behaviour of aqueous metal ions.

(9 marks)

A student standardises a solution of sodium hydroxide by titrating it against a standard hydrochloric acid solution of concentration $0.100\ \text{mol dm}^{-3}$0.100 mol dm−3 (assume this concentration is exact).

$HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l)$HCl(aq)+NaOH(aq)→NaCl(aq)+H2​O(l)

Using a pipette, 25.00 cm³ of the hydrochloric acid was placed in a conical flask and titrated with the sodium hydroxide from a burette. The mean titre of sodium hydroxide was 23.50 cm³.

The apparatus uncertainties are:

• burette: $\pm 0.05$±0.05 cm³ on each reading (two readings are taken per titre)
• volumetric pipette: $\pm 0.06$±0.06 cm³

Calculate the concentration of the sodium hydroxide solution, 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 ethanol ($C_2H_5OH$C2​H5​OH, $M_r = 46.0$Mr​=46.0) using a spirit burner to heat water in a copper can.

The data collected were:

• mass of ethanol burned (found by weighing the burner before and after) = 0.46 g
• volume of water in the can = 150 cm³ (measured with a volumetric pipette; take its uncertainty as negligible, density $= 1.00\ \text{g cm}^{-3}$=1.00 g cm−3, $c = 4.18\ \text{J g}^{-1}\,\text{K}^{-1}$c=4.18 J g−1K−1)
• water temperature rose from 19.6 °C to 38.3 °C

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 ethanol from these data, then propagate the percentage uncertainties in the mass of ethanol and the temperature change to give the overall percentage uncertainty (and absolute uncertainty) in your value. The data-book value is $-1367\ \text{kJ mol}^{-1}$−1367 kJ mol−1; explain, in terms of a systematic error, why your value differs.

(6 marks)

A student investigates how the rate of the reaction below depends on the concentration of iodide ions, using the "iodine clock" method (AQA Required Practical 4).

$H_2O_2(aq) + 2I^-(aq) + 2H^+(aq) \rightarrow 2H_2O(l) + I_2(aq)$H2​O2​(aq)+2I−(aq)+2H+(aq)→2H2​O(l)+I2​(aq)

A small, fixed amount of sodium thiosulfate and some starch are added to the mixture. The thiosulfate instantly removes the iodine as it forms; when the thiosulfate is used up, free iodine makes the starch turn blue-black at a sharp, stopwatch-timed instant. The time $t$t for the colour to appear is inversely proportional to the initial rate.

The student varies the volume of potassium iodide solution, keeps the total volume constant by adding water, and records $t$t once for each iodide concentration. They start the stopwatch when the solutions are mixed and judge the end-point by eye.

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)

Sodium forms the chloride $NaCl$NaCl, in which sodium is present as $Na^+$Na+, but the ion $Na^{2+}$Na2+ is never found in its compounds. Magnesium, however, readily forms $Mg^{2+}$Mg2+, and its chloride is $MgCl_2$MgCl2​.

Explain these facts. In your answer link the electron configurations and successive ionisation energies of the two elements to the formulae of their chlorides.

(5 marks)