Chemistry

1. 1.

   For N2(g) + 3H2(g) <=> 2NH3(g), the forward reaction is exothermic. Explain how increasing pressure and increasing temperature affect equilibrium yield of ammonia.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Treat pressure and temperature as separate changes. Compare gas mole counts for pressure and heat direction for temperature.
   2. Increasing pressure favours the side with fewer gas molecules.
   3. The product side has 2 moles of gas versus 4, so ammonia yield increases.
   4. Increasing temperature favours the endothermic reverse reaction, reducing ammonia yield.

   Marking points

   • Increasing pressure favours the side with fewer gas molecules.
   • The product side has 2 moles of gas versus 4, so ammonia yield increases.
   • Increasing temperature favours the endothermic reverse reaction, reducing ammonia yield.

   Examiner tip: A catalyst changes the rate of reaching equilibrium, not its equilibrium yield.

2. 2.

   Calculate the pH of 0.020 mol dm^-3 hydrochloric acid, assuming complete dissociation.

   [3 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. For this strong monoprotic acid, complete dissociation makes hydrogen-ion concentration equal to acid concentration.
   2. HCl dissociates completely, so [H+] = 0.020 mol dm^-3.
   3. pH = -log10[H+].
   4. pH = 1.70 to two decimal places.

   Marking points

   • HCl dissociates completely, so [H+] = 0.020 mol dm^-3.
   • pH = -log10[H+].
   • pH = 1.70 to two decimal places.

   Examiner tip: Use base-ten logarithms for pH, not natural logarithms.

3. 3.

   Explain why refluxing ethanol with excess acidified potassium dichromate(VI) forms ethanoic acid, while distilling the product early can yield ethanal.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Follow the oxidation sequence from primary alcohol to aldehyde to carboxylic acid; then ask whether the intermediate stays in contact with oxidant.
   2. Ethanol is a primary alcohol; its first oxidation product is ethanal.
   3. Reflux keeps volatile compounds in the reaction mixture.
   4. Excess oxidant allows ethanal to oxidise further to ethanoic acid.
   5. Distillation removes ethanal as it forms, limiting further oxidation.

   Marking points

   • Ethanol is a primary alcohol; its first oxidation product is ethanal.
   • Reflux keeps volatile compounds in the reaction mixture.
   • Excess oxidant allows ethanal to oxidise further to ethanoic acid.
   • Distillation removes ethanal as it forms, limiting further oxidation.

   Examiner tip: Reflux retains volatile material; distillation removes it. Conditions determine whether further oxidation occurs.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.