I often get students telling me that they know the content but don’t do well on exam questions. If you are doing well on topic tests but not on your mock exams, it is likely that you might be missing a key skill or detail, or that you simply haven’t faced sufficiently challenging questions often enough.

What makes a question difficult? Synoptic questions (those that combine or test ideas from different topics), those covering practical chemistry and unusual calculations feature frequently as the most difficult on the exam. Quite often, the range of potential questions on these topics is not fully explored in previous exams, or sometimes that type of question has only appeared once during the current exam series.

The key to doing well in your end of Year 12 mock exams is to know your content in detail, to anticipate slight variations in the questions, and to get more practice on the genuinely hard skills. I have collated and listed these more challenging parts of the AQA AS-level Chemistry exams below alongside links to the exams papers they came from.

I also have more practice on difficult questions from the OCR syllabus here with a few examples from the AQA A-level exams if you fancy more practice, but please be careful with your answers to the descriptive questions because OCR look for slightly different answers to AQA.

Hard questions from AQA AS-level exams, 2016-2024

Inorganic and Physical Chemistry (7404/1)

1. Extended and Unstructured Calculations

Mathematical complexity is a recurring theme where many students struggle to organize their work or handle multi-step problems without guidance.

  • Multi-step/Unstructured Calculations: Questions requiring students to perform a sequence of calculations proved very difficult in 2016 (Q9) , 2017 (Q3.1) , and 2023 (Q9.4).

  • Rearranging Formulae: Rearranging the kinetic energy expression for Time of Flight (TOF) mass spectrometry calculations was a major hurdle in 2019 (Q2.4), 2020 (Q3)

  • Unit Conversions and Precision: Students often failed to convert units correctly (e.g., mg to kg) or provide answers to the correct number of significant figures in 2018 (Q3.1) and 2024 (Q7.1, 7.2).

2. Redox Reactions and Halide Chemistry

Students often struggle with the precise terminology and equations required for redox processes.

  • Ionic Equations and Half-Equations: Combining half-equations for halide reactions was poorly done in 2016 (Q8), 2017 (Q2.1) , and 2024 (Q1.1) where only about 25% of students were successful.

  • Group VII Redox Trends: Understanding which halides can reduce concentrated sulfuric acid remained challenging, for example in 2024 (Q19). Distinguishing between the oxidising ability of halogens and the reducing ability of halide ions is also a common point of confusion, as noted in 2019 (Q19) and 2024 (Q1.2).

3. Practical Skills and Titrations

Practical procedures are often “learned by rote” without an understanding of the underlying principles.

  • Uncertainty and Accuracy: Explaining how to reduce percentage uncertainty (e.g., by using a lower concentration of alkali to increase the titre volume) was only understood by a minority in 2018 (Q22).

  • Titration Procedures: Common misconceptions involve rinsing equipment; for example, many incorrectly believe washing a pipette with water between titrations improves accuracy in 2018 (Q20) and 2017 (Q16).

  • Identifying Mistakes: Only the strongest students were capable of suggesting improvements to a method and explaining why a mistake affects the results, for example in 2022 (Q2.3).

4. Atomic Structure and Ionisation Energy

While basic definitions are often known, applying them to specific trends is more difficult.

  • Successive Ionisation Energies: Explaining the “jump” in energy levels (removing an electron from a shell closer to the nucleus) was a separator for top students in 2023 (Q1.3) and 2022 (Q1.3).

  • Isoelectronic Ions: Explaining the difference in size between isoelectronic ions (e.g., vs ) by focusing on the number of protons was challenging in 2018 (Q2.4) and 2019 (Q1.1).

5. Molecular Shapes and Intermolecular Forces

Visualizing 3D structures and their resulting properties remains a demanding area.

  • Bond Angles and Lone Pairs: Students often forget the impact of lone pairs on bond angles, such as in the ion in 2019 (Q1.4) or the ion in 2022 (Q3.2).

  • Dipoles and Symmetry: Explaining that a molecule is non-polar because individual bond dipoles cancel out due to symmetry was only achieved by a few in 2024 (Q3.4) and 2018 (Q5.2).

  • Extended Responses on Bonding: Organizing thoughts into a logical sequence to explain intermolecular forces was highlighted as a “best students only” task in 2016 (General Comments) and 2023 (Q4.2).

Organic and Physical Chemistry (7404/2)

1. Energetics and Hess’s Law Calculations

Calculations involving enthalpy changes were frequent stumbling blocks, particularly regarding the correct application of formulas and signs.

  • Unfamiliar Hess Cycles: Less common or non-standard Hess cycles proved particularly difficult, such as in 2024 (Q20). Students lacked a robust method for solving problems using Hess’s law.

  • Energy per Molecule: Questions requiring the use of the Avogadro constant to calculate the heat released by a single molecule were very challenging (2019 Q24, 2024 Q22).

2. Moles at Equilibrium (Kc)

Deducing the quantities of substances present at equilibrium remains a high-level skill.

  • Using ICE tables correctly: Students struggled to deduce the amount in moles of species at equilibrium using ICE tables, then struggled to find Kc 2017 (Q21) and 2022 (Q19).

3. Practical Procedures and Accuracy

Stronger students are distinguished by their ability to explain why procedures are performed, rather than just knowing what to do.

  • Reducing Uncertainty: Understanding how to reduce percentage uncertainty in titrations (e.g., by increasing the titre volume) was poorly understood by many (2018 Q22).
  • Experimental Temperature: Calculating or determining a “mean temperature” during a reaction (related to Required Practical 3) was a procedure few students were familiar with in 2022 (Q1.3).
  • Titre Precision: Using the correct number of significant figures and understanding the precision of experimental data were recurring general weaknesses (2016, 2017, 2022).

4. Organic Analysis and Mechanisms

Visualizing structures and correctly identifying functional groups in skeletal forms were common points of failure.

  • Recognising Functional Groups on Skeletal Strucures: Misinterpreting skeletal structures led students to suggest incorrect chemical tests (e.g., testing for an aldehyde instead of a ketone) in 2019 (Q1.1).
  • High-Resolution Mass Spectrometry: Using high-res data to distinguish between compounds with similar molecular masses required precise molecular formula deduction, which many found difficult in 2024 (Q18).
  • Stereoisomerism Rules: Applying the Cahn-Ingold-Prelog (CIP) priority rules to name E-Z stereoisomers with complex substituents (like ethyl groups) was a discriminator in 2023 (Q18).
  • Substitution vs. Elimination: Deducing which products could be formed by either substitution or elimination with hydroxide ions was a challenging task in 2022 (Q21).

5. Mathematical Complexity

  • Unstructured Calculations: Multi-step calculations that lacked guided sub-steps were “very poorly answered,” with only the best students scoring full marks, particularly when unit conversions were involved (2016 Q23).
  • Combining Gas Volumes, Unbalanced Equations, and Ratios: Calculations involving gas volumes were often found very challenging for the majority of candidates ((2022 Q4, 2023 Q17).