The end of year 12 exams are crucial for obtaining a good predicted grade in chemistry. For most students, just equaling your percentages from GCSE will perhaps lead to disappointment - the grade boundaries at GCSE are substantially lower than those at A-level. This discrepancy can be seen in the comparison table for the grade boundaries for the two exams:
| GCSE % | GCSE grade | AS-level % | AS grade |
|---|---|---|---|
| 75% | 9 | >90% | A* |
| 66% | 8 | 73% | A |
| 58% | 7 | 63% | B |
(sources: AQA grade boundaries AS subjects 2025 and AQA grade boundaries GCSE subjects 2025).
If you want a predicted grade of A* for your future university course, aim for an overall percentage of around 90%. Although there is currently no official grade boundary set for A* on AS-level exams, this has historically been the minimum percentage needed in Year 12 for an A* grade at the end of Year 13.
Practicing the core skills and applying yourself to AS-level exam questions will help you to improve. The questions on this page were identified by the examiners as posing particular challenges for students sitting the AQA AS-level chemistry exams between 2016 and 2024.
I also have included some extra practice on tricky questions from the A-level OCR syllabus here, with a few examples from the AQA A-level exams. AQA examiners look for slightly different answers than OCR. Also, note that the questions are from full A-level papers, so not everything there is essential this year.
Inorganic and Physical Chemistry (7404/1)
1. Extended and Unstructured Calculations
Students often struggle to organise their work or handle multi-step problems without guidance. This is a skill that can be taught, but it must also be practiced. I usually suggest that students master the individual skills first before tackling these complex questions; there are some good drills on the basic manipulations in this workbook. Once a student can do these calculations well, I then demonstrate how to breakdown the multi-step questions and create a ‘map’ to follow.
- Multi-step/Unstructured Calculations: Questions requiring students to perform a sequence of calculations proved very difficult in 2016 (Q9) , 2017 (Q3.1), 2023 (Q9.4) and Paper 2 2016 Q23.
- Time of flight (TOF) mass specrometry calculations: 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).
- 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.
2. Redox Reactions and Halide Chemistry
Students often get confused over the correct use of terminology in this topic, such as reducing agent and oxidation number. Constructing equations required for redox processes is also sometimes a challenge. As noted with the unstructured calculations above, it is necessary to master the basic skills first before attempting the past exam papers listed below.
- 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. It is important to know why the steps are taken during a procedure, for example what ‘accuracy’ means and how it can be affected by the choice of method used to make a measurement.
- 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 the basic definitions are often known, applying them to specific trends across the periodic table is more difficult. I put together an overview on ionisation energy with a few worked examples on one of my posts: How to secure the marks on ionisation energy questions.
- 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 by focusing on the number of protons was challenging in 2018 (Q2.4) and 2019 (Q1.1).
5. Molecular Shapes and Intermolecular Forces
Students often learn VSEPR theory incorrectly, so it is no surprise to find them struggling on this topic. Check that you have a robust method before tackling the exam questions listed below.
- 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 is a core skill that will be essential throughout the course.
- 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
Recognising different functional groups and being able to switch between written names for molecules, structural formula, and skeletal diagrams are all fundamental skills for an organic chemist. It is often alarming how little those skills have been practiced ahead of those exams, this appears in the comments from the examiners each year.
- 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).