What is covered in OCR Gateway Chemistry A topic C3 Chemical reactions?

Topic C3 covers introducing chemical reactions (equations, conservation of mass and the mole) in C3.1, energetics (exothermic and endothermic reactions, reaction profiles and bond energies) in C3.2, types of chemical reactions (acids and bases, neutralisation, the pH scale and redox) in C3.3, and electrolysis in C3.4. It contains a large share of the quantitative chemistry in the course.

Which exam paper assesses C3 in OCR Chemistry A?

Topic C3 is examined on Paper 1 (Foundation, J248/01) or Paper 3 (Higher, J248/03), which covers topics C1 to C3 plus the C7 practical skills. Each paper is 1 hour 45 minutes, worth 90 marks, and is 50 percent of the GCSE. You sit both papers at the same tier.

What kinds of question come up on the Chemical reactions topic?

Expect balancing equations, mole and reacting-mass calculations, percentage yield, bond-energy calculations, reaction profiles, the reactions of acids and naming salts, pH and strong versus weak acids, identifying oxidation and reduction, writing half equations, and predicting electrolysis products. There are six-mark questions on energetics and electrolysis, and a lot of maths.

How much maths is in topic C3?

A lot. The mole, reacting masses, limiting reactants and percentage yield are all here, as are bond-energy calculations. Moles equal mass divided by relative formula mass, reacting masses use mole ratios, percentage yield is actual over theoretical times 100, and bond energy change is bonds broken minus bonds made. Drilling these calculations is essential because at least 20 percent of the marks assess maths.

How should I revise the Chemical reactions topic?

Practise balancing equations until automatic, and drill the mole calculations (moles, reacting masses, limiting reactants, percentage yield, bond energies). Learn the three reactions of acids and the salts they make, the pH scale and the strong versus weak acid idea. Learn the two definitions of oxidation and reduction and the OIL RIG rule, and the electrolysis rules for molten and aqueous compounds. Use OCR past papers and watch the command words such as Calculate, Explain and Describe.

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OCR Gateway GCSE Chemistry A: Chemical reactions (C3) overview

An overview of the Chemical reactions topic (C3) in OCR Gateway GCSE Chemistry A (J248), mapping chemical equations and conservation of mass, the mole and reacting masses, energetics and reaction profiles, the reactions of acids and the pH scale, oxidation and reduction, and electrolysis.

Generated by Claude Opus 4.88 min readJ248 Chemistry C3Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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The Chemical reactions topics
How this topic is examined
How to study the Chemical reactions topic
For the official specification

Topic C3 Chemical reactions of OCR Gateway GCSE Chemistry A (specification J248) is where chemistry becomes quantitative. It covers how reactions are represented and balanced, the mole and reacting masses, the energy changes of reactions, the reactions of acids and the pH scale, oxidation and reduction, and electrolysis. This page maps the topic and links to a focused answer page for each part.

The Chemical reactions topics

Equations and conservation of mass (C3.1): Word and balanced symbol equations, conservation of mass, balancing, ionic equations, and apparent mass changes in open systems. See Equations and conservation of mass.
The mole and reacting masses (C3.1): Relative formula mass, the mole and the Avogadro constant, calculating moles from mass, reacting masses, limiting reactants, and percentage yield. See The mole and reacting masses.
Energetics and reaction profiles (C3.2): Exothermic and endothermic reactions and their uses, reaction profiles, activation energy, and bond-energy calculations. See Energetics and reaction profiles.
Types of reactions: acids and bases (C3.3): The reactions of acids with metals, bases and carbonates, neutralisation and salts, the pH scale, strong and weak acids, and making soluble salts. See Types of reactions: acids and bases.
Oxidation and reduction (C3.3): Oxidation and reduction in terms of oxygen and electrons, redox reactions, oxidising and reducing agents, and half equations. See Oxidation and reduction.
Electrolysis (C3.4): Electrolysis of molten and aqueous compounds, ion movement, the products at each electrode, half equations, and extracting reactive metals such as aluminium. See Electrolysis.

How this topic is examined

Topic C3 is assessed on Paper 1 (Foundation, J248/01) or Paper 3 (Higher, J248/03), alongside topics C1, C2 and the C7 practical skills. Each paper is 1 hour 45 minutes, worth 90 marks and 50% of the GCSE. Questions include balancing equations, mole and bond-energy calculations, the reactions of acids, redox and electrolysis, and six-mark extended responses. The making-salts and electrolysis practicals are part of the C7 practical skills.

How to study the Chemical reactions topic

Work from the specification statements. Each numbered point is a checklist; questions are written from them.
Drill the mole calculations. Reacting masses, limiting reactants, percentage yield and bond energies carry many marks and reappear in C5 and C6.
Master balancing equations. Practise symbol, ionic and half equations until automatic.
Learn the reactions of acids. The three reactions and the salts they make are reliable marks.
Know the electrolysis rules. The products at each electrode for molten and aqueous compounds unlock the whole of C3.4. Use OCR past papers.

For the official specification

OCR publishes the full specification (J248), past papers and mark schemes at ocr.org.uk. Always revise from the current specification and OCR's own past papers, because question style is board-specific.

Sources & how we know this

OCR GCSE (9-1) Gateway Science Suite Chemistry A (J248) specification — OCR (2016)