Hard A-Level Chemistry Questions: AQA AS Paper 1 and 2

• Amount of substance calculations account for a high proportion (33%) of difficult questions
• Questions on practical chemistry consistently pose a significant challenge
• Novel questions not seen on previous papers are a common source of difficulty
• 6-mark level of response questions are frequently among the hardest questions on AS exam papers

Contents

“Read examiner reports” is standard advice in online guides about mastering A-Level chemistry, including mine here.

Why should you read examiner reports? First, they tell you what students did well in the exam. Second, they tell you what things students did badly. And last of all, they tell you which questions caused students problems, and why.

If you’re aiming to master AQA A-Level chemistry, this is all valuable information for you.

Knowing common exam mistakes are obviously things you want to avoid. Difficult questions extend your understanding and improve your analytical skills, making them vital preparation for the exam. Topics that are repeatedly identified as problem areas are ones you want to prioritise when studying.

In this article, I’ve analysed the available examiner reports for AQA AS chemistry and listed the questions that were described in various ways as difficult. These are typically questions with a low success rate (i.e. a small percentage of students got them right).

If you’re an AQA student wanting to test your knowledge against the hardest AS questions, this article is for you.

You can use this article in several ways:

• Use it to find and attempt all the difficult questions on a given topic
• Use it to structure your revision – devote extra time to mastering the difficult topics that I’ve listed
• Use it to find all the hard questions in each AS paper so that you use them to extend your knowledge and skills

Not studying AQA A-Level chemistry? Comment below if you’d like to see a similar article for CAIE, Edexcel, or OCR A-Level chemistry.

These are the topics that tend to come up repeatedly in the hardest AS questions:

Amount of Substance Calculations

• Multistep calculations, especially those involving the ideal gas equation.
• Practical data-based calculations, novel problems not seen in previous papers, and limiting reactants.
• Calculations involving density, non-standard units, and either very large or very small quantities.
• The use of algebra in various calculations and the application of Avogadro’s constant.
• Writing ionic equations and half-equations.

Atomic Structure

• Transition metal ions and their electron configurations.
• The principles of Time-of-Flight (TOF) mass spectrometry and predicting mass spectra, especially m/z peak ratios.
• Multistep TOF mass spectrometry calculations.

Bonding & Structure

• Intermolecular forces, in particular, relating intermolecular forces to physical properties, such as melting and boiling points.
• The common misconception that melting or boiling simple covalent substances involves breaking covalent bonds was frequently seen.
• Determining the shapes of unfamiliar molecules and ions.

Energetics

• Writing equations for specific standard enthalpy changes.
• Calculations based on calorimetry data, particularly involving limiting reactants.
• Unfamiliar applications of Hess’s Law.
• Bond enthalpy calculations, especially requiring rearrangement of equations.

Equilibrium

• Calculation of moles at equilibrium from Kc.
• The effects of temperature on equilibrium position and yield.
• Understanding compromise conditions in industrial processes.

Group 17 Halogens

• The redox chemistry of halide ions with concentrated sulfuric acid.
• The ability to write redox equations or half-equations for halogen chemistry.

Organic Chemistry

• Understanding mechanisms involving unsymmetrical alkenes and cyclic molecules.
• Interpreting skeletal formulae.
• Deducing the identity and number of isomers from molecular formulae.
• Drawing repeating units and monomers in polymers.

Practical Chemistry

• A clear understanding of apparatus and their purposes, practical techniques, and the principles behind certain techniques was often found lacking.
• Accurately describing experimental procedures.
• Suggesting improvements to experiments.
• Selecting the most appropriate technique for a given practical requirement.

Qualitative Analysis

• Identifying or distinguishing organic functional groups using tests.
• Tests for ions and gases to distinguish/identify substances.

Redox

• Writing balanced redox equations and deducing half-equations.

Level of Response Questions

• These 6-mark questions demand a well-structured and reasoned answer, encompassing multiple marking points. They have been significant discriminators in all AQA AS exams.

Here are the difficult questions from each AS paper, arranged by year:

Based on the recurring challenges identified by examiners, here are some specific recommendations to help you better prepare for AQA AS chemistry exams:

• Set aside time to practice a variety of multistep calculations, especially those involving the ideal gas equation, practical contexts, and Avogadro’s constant.
• Familiarise yourself with different units and practice converting between them.
• Ensure you have mastered the important skill of writing electron configurations, especially for transition metal ions.
• Practise rearranging the equations for TOF-MS calculations.
• Solve a range of TOF-MS calculations from AS papers and have worked examples of these in your notes – although they are challenging, there are only so many question types and they tend to be repeated.
• Work hard to understand intermolecular forces and how they affect physical properties – these difficult questions are very common.
• Make sure you are clear about the types of bonds broken when melting or boiling substances – don’t confuse breaking covalent bonds with breaking intermolecular forces!
• Challenge yourself to work out the shapes for less common molecules and ions not found in textbooks.
• Get into the habit of translating enthalpy definitions into chemical equations, and vice versa.
• Ensure you understand limiting reactants and how they influence calorimetry calculations.
• Try to draw Hess’s cycles for unfamiliar reactions.
• Practise the ‘algebraic’ bond enthalpy calculations that require you to calculate a bond enthalpy from enthalpy changes.
• Ensure you understand the effects of temperature, pressure and concentration changes on the position of equilibrium and the value of Kc.
• Learn how to write redox equations and half-equations for all the halogen and halide ion redox reactions.
• For the reactions of bromide and iodide with concentrated sulfuric acid, try to understand how the equations are constructed in terms of oxidation state changes.
• Ensure you have a good understanding of electrophilic addition to unsymmetrical alkenes. Explaining the major and minor product in terms of carbocation stability is one of the most difficult organic chemistry topics. It is also common on the A-Level organic paper 2, so work hard to understand it in year 1.
• Practise drawing organic mechanisms for cyclic molecules.
• Get familiar with interpreting skeletal formulae – harder organic questions frequently use skeletal formula.
• Practise deducing the identity and number of isomers from molecular formulae. There is no mathematical trick here to go from a formula to a number of isomers. You just have to practise drawing isomers.
• Make sure you are comfortable drawing repeating units and monomers for addition polymers.
• Ensure you have a clear understanding of practical apparatus and their purposes, practical techniques, and the principles behind them.
• Make sure you can accurately describe the experimental procedures for all the required practicals.
• For each required practical, understand common errors and how they can impact experimental outcomes and calculated quantities.
• Understand how to select the most appropriate technique for a given practical requirement.
• Make it a priority to learn all the organic functional group tests, and the test-tube reaction tests for ions and gases. That means knowing all the reagents, and the observations for a positive test.

Summing up, it’s clear that calculations, practical chemistry, and 6-markers are priority areas if you’re aiming to excel in AQA AS chemistry.

I knew maths was important in A-Level chemistry exams, but I must admit, even I was surprised to discover that so many of the most difficult chemistry questions are calculations.

While the maths is relatively ‘easy’ by A-Level maths standards, it’s the need to translate chemical information into mathematical information that poses the challenge. Doing this correctly is the essential step in seeing your way through to the answer. Get it wrong, and your whole approach will be wrong.

If you’ve found this article useful, or have any comments/questions, please leave them below.

Happy studying!