AQA A-Level Chemistry Paper 3 Hard Questions

Paper 3 is the one that students worry most about in my experience.

It has 40 marks of questions on practical chemistry, 20 marks of questions from any area of the syllabus, and ends with 30 multiple choice questions. The latter can really mess with your timing if you don’t have good exam technique, especially the ones that force you to do multiple calculations for a single mark.

Paper 3 questions are often synoptic, requiring you to make connections between different topics. This requires much deeper understanding of concepts.

It’s the practical questions that my students worry most about though. You need to know your apparatus and techniques inside out to score well on this paper, and be prepared to deal with questions on unfamiliar experiments.

The ‘Suggest…’ questions on this paper can be particularly challenging and answering them sometimes requires knowledge that would not normally be gained by doing the required practicals (e.g. understanding the impact of experimental errors). These question are therefore difficult to prepare for.

To help you, I’ve listed below all the areas of recurring difficulty, and the questions in each paper that examiners reported were the hardest. Trying out these questions will definitely stretch your knowledge, teach you a lot, and help you prepare for the worst!

Recurring Topics of Difficulty for Students

Acids and bases

  • Calculations based on interpretation of pH curves.
  • Understanding how acid strength influences enthalpy of neutralisation.
  • Choosing the right indicator for a given titration.
  • Use of the correct reaction arrow in dissociation equations of strong and weak acids.
  • Extended calculations, such as calculating the mass of salt needed to produce a buffer with a given pH.
  • Writing equations for reactions involving acids, including simple ones encountered at GCSE level.
  • Novel, multistep calculations involving rearrangement of equations for pH and Ka.

Amount of substance

  • Calculating percentage yield from organic reactions.
  • Multiple choice questions requiring multistep calculations (e.g. involving the ideal gas equation) proved difficult, perhaps due to time pressure.
  • Surprisingly, students struggled applying GCSE skills to A-Level situation, such as writing ionic equations for unfamiliar reactions.
  • Deducing amount of oxygen needed for combustion reactions.

Bonding and structure

  • Understanding how bond polarity and intermolecular forces influence properties such as solubility, retention times, and physical properties of condensation polymers.
  • Explaining the shapes of molecules in terms of bonding pairs and lone pairs.

Electrode potentials

  • Knowing standard conditions for operation of electrochemical cells.
  • Writing balanced redox equations.
  • Deducing a standard electrode potential from a cell potential.
  • Designing experiments to determine a standard electrode potential.
  • Using standard electrode potentials to explain reaction feasibility, such as the ability of certain transition metal ions to catalyse redox reactions.

Energetics

  • Understanding why calorimetry cannot always to calculate enthalpy changes directly.
  • Plotting graphs accurately to determine temperature changes for calorimetry calculations.
  • Calculating enthalpy changes from experimental data, particularly involving limiting reactants.
  • Knowing when an enthalpy change is considered a standard enthalpy change.
  • Writing chemical equations for standard enthalpy changes, or recognising the latter from a given equation.

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Graphs

  • Plotting graphs accurately and drawing lines of best fit.
  • Determining gradients of straight lines.
  • Recognising that many mathematical formulae can be written to fit the equation of a straight line, y = mx + c, and using this to calculate variables.

Inorganic chemistry

  • Halogen redox chemistry and the reactions of halide ions with concentrated sulfuric acid.
  • Writing the equations for the reactions of period 3 oxides with water.
  • The solubility of group 2 compounds.
  • Relating group 2 compound solubility to pH.

Kinetics

  • The strategy of using a large excess of one reactant so its concentration remains constant (pseudo-first-order kinetics) was not well understood.
  • Understanding zero order reactions and their graphs.
  • Determining rate constants graphically.
  • Knowing which type of line of best fit to draw (curve versus line).
  • Accurately determining the rate from the gradient of a concentration-time graph.
  • Designing experiments to determine rate equations or activation energies.
  • Understanding how first-order behaviour can be demonstrated from graphs.
  • Clock reactions are generally problematic, particularly their chemistry, assumptions, methodology, and mathematical principles.

Organic analysis

  • Understanding retention times and intermolecular forces/polarity.
  • Remembering to state both the wavenumber range and the bond responsible for absorption in IR spectroscopy.

Organic chemistry

  • Predicting and explaining the major product in electrophilic addition reactions.
  • Naming alkene stereoisomers using the E-Z system.
  • Identifying by-products from reactions based on their molecular formulae.
  • Identifying oxidation products, particularly for polyfunctional organic molecules.
  • Making predictions from organic formulae or names, such as chemical behaviour, physical properties, or spectroscopic details (e.g. number of NMR environments).
  • Knowing the roles of reagents in organic reactions (e.g. HNO3 in benzene nitration).
  • The rationale for certain reagent choices in organic synthesis.
  • Finding the number of isomers for a given molecular formulae, particularly if constraints are applied (e.g. must give positive Tollens’ test).
  • The equations showing formation of sulfur and nitrogen oxides (pollutants) from fossil fuel combustion.
  • Understanding hydrolysis of polyesters and polyamides.
  • Explaining how nucleophilic addition to carbonyls leads to stereoisomers and racemic mixtures.
  • The shapes of organic molecules.
  • Understanding the requirements for stereoisomerism in organic molecules.
  • Drawing enantiomers accurately.

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Periodicity

  • The observations and equations for reactions of period 3 elements with oxygen.
  • Comparing physical properties for period 3 elements.
  • Understanding the acid-base properties of period 3 oxides and their reactions.

Practical chemistry

  • Remembering to double the uncertainty when calculating percentage uncertainties for measurements that required two readings (e.g. temperature differences, titres, mass differences).
  • Understanding how apparatus should be used correctly and safely.
  • Designing calorimetry experiments to measure standard enthalpy changes.
  • How to produce and use a calibration curve to find concentrations.
  • Understanding the apparatus and techniques associated with organic synthesis, such as recrystallisation and the distinction between reflux and distillation.
  • Understanding the function of anti-bumping granules.
  • Explaining the post-reaction steps (work-up) for organic reactions, such as why sodium carbonate might be used, or how to use drying agents.
  • Drawing apparatus accurately and ensuring apparatus is not sealed.
  • Drawing apparatus to show the appearance of a meniscus.
  • The steps involved in making a standard solution.
  • Ensuring accurate measurements in volumetric analysis (e.g. correct use of a burette).

Redox

  • Writing balanced redox equations and half-equations.
  • Deducing oxidation states.

Thermodynamics

  • Using the linear graph for ∆G = ∆H – T∆S to calculate ∆S.
  • The factors affecting the difference between experimental and theoretical lattice enthalpy (charge density and ion polarisation).
  • Construction of energy cycles that relate lattice enthalpy with enthalpy of solution and hydration.

Transition metals

  • Identifying complex ion stereoisomers.
  • Explaining the origin of colour.
  • Use of E = hv to calculate energy of absorption.
  • Understanding the action of cisplatin and the process it prevents, including diagrams to show cross-linking of nucleotides.
  • Writing equations for ligand substitution reactions.
  • Correctly deducing charges on complex ions.
  • Recalling observations for aqueous chemistry of complex ions.
  • Recalling the equations and principles behind autocatalysis.
  • Homogeneous and heterogeneous catalysis.

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