OCR Gateway GCSE Combined Science A Chemistry: reactions and analysis (C3 to C6) overview
An overview of the chemical reactions, predicting and identifying reactions, monitoring and controlling reactions, and global challenges content (topics C3 to C6) in OCR Gateway GCSE Combined Science A (J250), mapping reaction types, energetics, electrolysis, the reactivity series, chemical tests, rates, equilibrium and the Haber process.
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The later chemistry topics of OCR Gateway GCSE Combined Science A (specification J250) cover how reactions happen, how to predict and identify products, and how to control reactions for industry. C3 Chemical reactions, C4 Predicting and identifying reactions and products, C5 Monitoring and controlling chemical reactions and C6 Global challenges are examined across Chemistry Paper 3 (C3) and Chemistry Paper 4 (C4 to C6). This page maps the topics and links to a focused answer page for each.
The reactions and analysis topics
- Types of reactions (C3)
- Word and balanced symbol equations, conservation of mass, reactions of acids, neutralisation, the pH scale, and oxidation and reduction. See Types of reactions.
- Energetics of reactions (C3)
- Exothermic and endothermic reactions and uses, reaction profiles, activation energy, and the energy change from breaking and making bonds. See Energetics of reactions.
- Electrolysis (C3)
- Electrolysis of molten and aqueous compounds, ion movement to the electrodes, the products at each electrode, and extracting reactive metals. See Electrolysis.
- Predicting reactions (C4)
- The reactivity series, displacement reactions, reactions of metals with water and acids, metal extraction by reduction with carbon, and ionic and half equations. See Predicting reactions.
- Identifying substances (C4)
- Tests for hydrogen, oxygen, carbon dioxide and chlorine, flame tests for metal ions, and tests for carbonate, halide and sulfate ions. See Identifying substances.
- Rates and equilibrium (C5 and C6)
- The factors affecting rate and collision theory, the mole and concentration, reversible reactions and dynamic equilibrium, Le Chatelier's principle, and the Haber process. See Rates and equilibrium.
How these topics are examined
Topic C3 is assessed on Chemistry Paper 3, and topics C4 to C6 on Chemistry Paper 4; each paper is 1 hour 10 minutes, worth 60 marks and one sixth of the overall grade. Paper 4 assumes knowledge of C1 to C3. Questions include balancing equations, calculations, chemical tests and six-mark extended responses. Every paper also tests CS7 practical skills, so the rates, titration and chemical-test practicals can be examined.
How to study the reactions and analysis topics
- Work from the specification statements. Each point is a checklist; questions are written from them.
- Drill equations and the reactions of acids. Balancing and the salt-producing reactions come up in nearly every paper.
- Learn the reactivity series and electrolysis rules. They unlock displacement, extraction and electrode products.
- Memorise the chemical tests. Gas, flame and negative-ion tests are easy marks if learned precisely.
- Practise the mole and rate calculations. Quantitative chemistry is a big part of the maths assessment.
For the official specification
OCR publishes the full specification, 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.