Top 10 Topics That Always Come Out In A-Level Chemistry Exams

---

If you're revising for OCR A A-Level Chemistry, you’ve probably wondered: What topics actually come up every year? After analysing the past five years of exam papers, I’ve identified the 10 most frequently examined topics - and I’m giving you brief model answers to typical questions you might encounter so you can study smarter and score higher.

Whether you're aiming for an A* or just trying to boost your confidence, this guide is a starting point to strategic revision.

1. Atomic Structure & Periodicity

What it’s about: Electron configuration, periodic trends, and ionisation energy patterns.

You need to know:

• Subatomic particles

• Successive ionisation energies

• Electron shells and shielding

• Periodicity across periods

Example question: Explain why the 2nd ionisation energy of strontium is greater than the 1st.

Model answer: The second ionisation energy of strontium is greater than the first because the second electron is removed from a positively charged ion (Sr⁺), which has a smaller radius and a stronger nuclear attraction to its remaining electrons. Additionally, the second electron is removed from a lower energy level closer to the nucleus (less shielded), therefore requires more energy to remove.

✅ Top marks for: Clear terminology and dialogical flow.

2. Amount of Substance (Moles & Stoichiometry)

What it’s about: Calculating quantities in chemical reactions using the mole concept.

You need to know:

• Mole calculations

• Empirical and molecular formulae

• Ideal gas equation

• Titration calculations

Example question: A student reacts 25.0 cm³ of 0.20 mol dm⁻³ HCl with excess calcium carbonate. Calculate the mass of CO₂ produced.

Model answer:

1. Volume of HCl used = 25 / 1000 = 0.025 dm³

2. Moles of HCl = 0.20 × 0.025 = 0.005 mol

3. Balanced equation: 2HCl + CaCO₃ → CaCl₂ + CO₂ + H₂O

4. Moles of CO₂ = 0.005 ÷ 2 = 0.0025 mol (1:2 molar ratio between HCl and CO₂)

5. Mass of CO₂ = 0.0025 × 44.0 = 0.110 g

✅ Top marks for: Step-by-step clarity, correct units, balanced equation.

3. Enthalpy Changes & Hess’s Law

What it’s about: Energy changes in reactions and how to calculate them.

You need to know:

• ΔH definitions (formation, combustion…)

• Bond enthalpies

• Hess cycles

• Calorimetry (q = mcΔT)

Example question: Using the following data, calculate the enthalpy change for the reaction: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g).

ΔHf (kJ mol⁻¹) values:

• CH₄: - 74.8

• CO₂: -393.5

• H₂O: -286

Model answer:

ΔH = ΣΔHf(products) – ΣΔHf(reactants)

ΔH = (-393.5 + 2(-286)) – (-74.8) = -890.7 kJ mol⁻¹

✅ Top marks for: Correct formula, correct sign, final unit.

4. Rates of Reaction

What it’s about: How fast reactions occur and what affects them.

You need to know:

• Rate equations

• Orders of reactants and overall order

• Rate constant (k) and unit derivation

• Graphical analysis

Example question: The rate of a reaction is first order with respect to H₂ and second order with respect to O₂. Write the rate equation and determine the units of the rate constant.

Model answer:

Rate = k[H₂][O₂]²

Units of rate = mol dm⁻³ s⁻¹

k = rate / [H₂][O₂]²

Units of k = dm⁶ mol⁻² s⁻¹

✅ Top marks for: correct rearranging, clarity.

5. Chemical Equilibria & Le Chatelier’s Principle

What it’s about: Reversible reactions and how conditions affect equilibrium.

You need to know:

• Kc and Kp calculations

• Le Chatelier’s Principle

• Effect of temperature, pressure, concentration

Example question: Predict and explain the effect of increasing pressure on the yield of ammonia in the Haber process.

Model answer:

N₂ + 3H₂ ⇌ 2NH₃

Increasing pressure shifts equilibrium to the right, favouring ammonia formation because there are fewer moles of gas on the product side (2 vs 4). Yield increases.

✅ Top marks for: Mole ratio, principle application, concise explanation.

6. Redox & Electrode Potentials

What it’s about: Electron transfer reactions and electrochemical cells.

You need to know:

• Oxidation states

• Half equations

• Standard electrode potentials

• Cell diagrams and feasibility

Example question: Using standard electrode potentials, predict whether Zn will reduce Fe²⁺ ions to Fe.

Model answer:

Zn²⁺/Zn = –0.76 V, Fe²⁺/Fe = –0.44 V

Zinc has a more negative electrode potential, so it is a stronger reducing agent. Therefore, zinc will reduce Fe²⁺ to Fe.

✅ Top marks for: Comparative terminology, clear prediction.

7. Organic Mechanisms (Substitution, Addition, Elimination)

What it’s about: Reaction pathways in organic chemistry.

You need to know:

• Curly arrow mechanisms

• Reaction conditions and reagents

• Functional group transformations

Example question: Outline the curly arrow mechanism for the reaction that converts bromomethane (CH₃Br) to methanol (CH₃OH).

Model answer:

Nucleophilic substitution:

– δ+ label on C in C-Br bond; δ- label on Br in C-Br bond

– Curly arrow from :OH⁻ to carbon in C-Br bond

– Curly arrow from C–Br bond to Br

– Products: CH₃OH + Br⁻

✅ Top marks for: Arrow placement, correct dipoles, correct product.

8. Analytical Techniques (IR, Mass Spec, NMR)

What it’s about: Identifying compounds using instrumental methods.

You need to know:

• How to determine empirical formulae

• Common IR absorption peaks

• How to interpret MS

• NMR chemical shifts and splitting

Example question: Use the elemental composition, IR and mass spectrum provided to identify the organic compound.

Model answer:

IR shows broad peak at ~2800 cm⁻¹ → O–H group (for carboxylic acids)

MS molecular ion peak at m/z = 60 indicates molecular mass of 60

Fragment at m/z = 43 → CH₃CO⁺

Likely compound: ethanoic acid

✅ Top marks for: matching IR data to data sheet, correct m/z and fragment ion(s), justified conclusion.

9. Periodicity & Group Trends (Group 2 & Group 7)

What it’s about: Trends in physical and chemical properties across the periodic table.

You need to know:

• Group 2 reactions with water

• Group 7 displacement

• Solubility and thermal stability

Example question: Explain the trend in reactivity of Group 2 metals with water.

Model answer:

Reactivity increases down Group 2 because atomic radius increases and ionisation energy decreases. This makes it easier for the metal to lose electrons and form M²⁺ ions, speeding up the reaction with water with a more vigorous reaction as you proceed down the group.

✅ Top marks for: Trend explanation, atomic structure link, clear terminology.

10. Transition Metals & Complex Ions

What it’s about: Properties and reactions of d-block elements.

You need to know:

• Various oxidation states of metal complexes

• Ligand substitution reactions

• Colour changes and precipitation

Example question: Describe what is observed when aqueous ammonia is added dropwise to [Cu(H₂O)₆]²⁺.

Model answer:

Initially, a pale blue precipitate of Cu(OH)₂ forms.

On addition of excess ammonia, the precipitate dissolves to form a deep blue solution of [Cu(NH₃)₄(H₂O)₂]²⁺.

✅ Top marks for: Colour change and precipitation observations, complex ion naming.

Final Tip: Practise With Purpose

Use these topics to guide your revision - but don’t just read them. Practise past papers, draw out mechanisms, and practise teaching the concepts to others or to yourself in the mirror. Active recall and spaced repetition are your best friends.

While these topics are a great place to begin revising strategically, do remember that this list is only a reflection of past exams and not meant to offer any assurance that they will appear in upcoming exams.

Need Further Help?

I offer exam focused resources and tutoring for GCSE and A-Level students. Whether you're aiming for top grades or a top university, I’ll help you study smarter and succeed.

For more personalised tutoring, get in touch with me or book a free trial session today