AQA A-Level Chemistry Paper 3 Hard Questions

• Most of the difficult paper 3 questions are on practical chemistry.
• Organic chemistry questions make up the next highest category.
• Acids and bases, kinetics, and transition metals account for an approximately equal proportion of difficult questions.
• Students’ weaknesses in understanding and explaining practical chemistry are repeatedly stated throughout paper 3 examiner reports.
• Organic practical techniques prove most problematic, particularly the principles behind recrystallisation, reflux, and distillation.
• Questions involving graphs consistently cause problems for students.

Contents

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!

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.

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.

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.

“Students should now be familiar with the fact that this paper contains a significant proportion of marks related to the description of practical scenarios. The key to success with such questions is to be familiar with a wide range of practical techniques and to be able to write concise, accurate descriptions.” – AQA Examiner Report, Paper 3, 2020

That quote sums it up well – know your practical chemistry!

My guide to paper 2 has strategies for preparing for practical chemistry questions, as well as questions on organic chemistry and organic analysis.

My other guide on paper 1 has hints and tips to help you prepare for difficult physical and inorganic topics.

• Read the question carefully! This is particularly important on this paper, which contains many lengthy and data-based questions. There were far too many instances where students lost marks on this paper due to simply misreading the question.
• Learn the entire syllabus, not just what has come up on previous papers.
• Ensure at least 50% of the available space is used when plotting graphs.
• Don’t rewrite the question as part of your answer, as it uses up space and time.
• Don’t include apparatus lists for practical descriptions as they will be mentioned when used.
• Only give one answer to avoid negating a correct answer with an incorrect one.
• Cross out mistakes and incorrect answers.
• Clearly label intermediate calculations so examiners understand what the number refers to.
• Number extra pages clearly and cross out rough work.
• Follow instructions carefully for filling in answers in Section B to avoid issues with scanning the papers.

What are the key topics for AQA A-Level Chemistry Paper 3?

• Practical chemistry, organic chemistry, acids and bases, kinetics, transition metals, electrode potentials, periodicity, energetics, amount of substance, redox, inorganic chemistry (halogens especially), thermodynamics, bonding and structure, and organic analysis (NMR) make up the highest proportion of challenging paper 3 question.
• Knowing your required practicals (apparatus, techniques, common mistakes, sources of uncertainty) is vital.

How can I effectively revise for AQA A-Level Chemistry Paper 3?

• Ensure you have thorough and up to date notes for each topic.
• Notes on the required practicals are especially important. These must include labelled apparatus diagrams, explanations for apparatus choice, step-by-step methods, explanations for steps, common errors and their impact, uncertainties and how to reduce them.
• Do topical questions first to ensure complete coverage of each topic.
• Do complete paper 3s under timed conditions to get used to the time pressure.
• Practise multiple choice questions separately and learn to spot the multistep ones that are best left until last.
• Do relevant practical questions and multiple choice questions from other exam boards to further your knowledge.
• Practise plotting graphs and determining gradients! It’s a bit of a lost art, so you can easily take far too long if you’re out of practise.

What are the best resources for preparing for AQA A-Level Chemistry Paper 3?

For practical chemistry, I recommend the AQA Practical Chemistry book (this is the ‘official’ guide)

I also recommend the excellent Chemrevise guide, which Neil Goalby regularly updates

How can I improve my problem-solving skills for AQA A-Level Chemistry Paper 3?

Solving and analysing difficult problems is really the best way. This develops your analytical problem-solving skills, and reveals connections between concepts that increases your understanding.

Use this guide, and my other guides linked above, to find and answer the most difficult questions from AS Papers 1 and 2, and A-Level Papers 1 to 3.