Chemical Reactions | OCR GCSE Combined Science Chemistry, Foundation tier (J250)

Chemical Reactions

193 questions
14 subtopics
Paper 3 (Chemistry)
Paper 3 (Chemistry)

Chemical Reactions is examined in Paper 3 (Chemistry).

It covers writing formulae of elements and compounds, balanced equations, and deducing formulae from ions, state symbols in equations, conservation of mass, and changes of mass in open systems, exothermic and endothermic reactions, and reaction profiles, activation energy and why reactions need it, oxidation and reduction in terms of oxygen, acids, alkalis and neutralisation, the ionic equation for neutralisation, and reactions of acids, the pH scale, acidity and alkalinity, measuring pH with indicators and pH meters, products at the electrodes, and molten ionic compounds, electrolysis of aqueous solutions and inert and non-inert electrodes.

Sample questions from Chemical Reactions

Answer each one closed book first, then open the answer.

Writing formulae of elements and compounds

What is the formula of carbon dioxide, and what does the subscript tell you?

Show the answer
CO₂, where the subscript 2 shows that each molecule contains two oxygen atoms.
Balanced equations, and deducing formulae from ions

Deduce the formula of sodium sulfate from the ions Na⁺ and SO₄²⁻.

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Na₂SO₄, because two sodium ions are needed to balance one sulfate ion.
Conservation of mass, and changes of mass in open systems

4 g of hydrogen reacts completely with 32 g of oxygen. What mass of water is formed?

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36 g.
Exothermic and endothermic reactions, and reaction profiles

Between which two points on a reaction profile is the activation energy measured?

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From the energy level of the reactants up to the top of the curve.
Oxidation and reduction in terms of oxygen

In 2CuO + C → 2Cu + CO₂, state which substance is oxidised and which is reduced.

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Carbon is oxidised because it gains oxygen, and copper oxide is reduced because it loses oxygen.
Acids, alkalis and neutralisation

What decides the name of the salt made in a neutralisation?

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The first part of the name comes from the metal in the base or alkali and the second part comes from the acid used.
The pH scale, acidity and alkalinity

What pH value does a neutral solution have?

Show the answer
7.
Measuring pH with indicators and pH meters

Name an indicator that shows only whether a solution is acidic or alkaline, and give its colours.

Show the answer
Litmus, which is red in acidic solutions and blue in alkaline ones.

The 14 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Writing formulae of elements and compounds	Which elements exist as molecules of two atoms and why neon does not, the formulae of water, methane, carbon dioxide, magnesium oxide and carbon monoxide, what NaCl represents, how a symbol differs from a formula, working out a formula from ion charges, and the formulae of common acids, sodium hydroxide, calcium carbonate and ammonia.	13
Balanced equations, and deducing formulae from ions	Balanced equations for sodium in chlorine, methane burning, potassium in water and calcium carbonate decomposing, why only the large numbers change, and deducing formulae such as Al₂O₃ and Ca(NO₃)₂ from ion charges.	16
State symbols in equations	What the state symbols s, l, g and aq stand for, why the water made in neutralisation is a liquid, the symbols for a precipitate and a molten salt, and adding state symbols to the reactions of zinc and marble chips with acid.	8
Conservation of mass, and changes of mass in open systems	The law of conservation of mass and what it means for atoms, reacting-mass calculations in sealed containers, why a balance reading falls when a carbonate gives off gas and rises when magnesium burns or a nail rusts, showing that mass is conserved, and using y = mx + c and the gradient of a magnesium oxide mass graph, including a line that misses the origin.	21
Exothermic and endothermic reactions, and reaction profiles	What exothermic and endothermic reactions are, how a temperature change shows which is happening, examples including combustion, neutralisation, hand warmers and cold packs, drawing and labelling reaction profiles with the activation energy and overall energy change, and what accurate, precise, repeatable and reproducible results and random and systematic errors mean when measuring a temperature change.	22
Activation energy and why reactions need it	What activation energy is, why the bonds in the reactants must break first, why heating, a spark or a flame starts a reaction, why an exothermic reaction still needs activation energy, and why a candle keeps burning once lit.	8
Oxidation and reduction in terms of oxygen	Oxidation and reduction as the gain and loss of oxygen, identifying what is oxidised and reduced when carbon reduces copper oxide, oxidising and reducing agents, carbon monoxide as the reducing agent for iron oxide, magnesium burning in air, and why oxidation and reduction always happen together.	8
Acids, alkalis and neutralisation	The hydrogen ions that acids release and the hydroxide ions in alkalis, common acids and alkalis with their formulae, what neutralisation, a base and an alkali are, naming salts and writing equations such as sodium chloride and copper(II) sulfate, and making a pure dry salt by adding an insoluble base in excess to warm dilute acid.	21
The ionic equation for neutralisation, and reactions of acids	The reaction of hydrogen ions with hydroxide ions common to every neutralisation, spectator ions, what is left at the end point, the products and equations of acids with carbonates and metals, and the limewater test for carbon dioxide.	18
The pH scale, acidity and alkalinity	What pH measures, the 0 to 14 scale and its acidic, neutral and alkaline values, comparing how acidic or alkaline two solutions are, and why a pH value says more than simply calling a solution acidic.	8
Measuring pH with indicators and pH meters	What universal indicator does and its colours in acidic, neutral and alkaline solutions, why it fails in coloured solutions and gives only approximate values, litmus, using and rinsing a pH meter and its advantage over indicator, calibrating a meter to prevent a systematic error, and what the resolution of a meter means.	10
Products at the electrodes, and molten ionic compounds	What cations and anions are and which electrode each moves to, what forms at the cathode and the anode, inert electrodes, why a compound must be molten or dissolved, and predicting the products of electrolysing molten sodium chloride, lead bromide, magnesium oxide and potassium iodide, with the difficulties of electrolysing molten sodium chloride.	16
Electrolysis of aqueous solutions	The hydrogen and hydroxide ions from water, the rules for what forms at each electrode, the products from sodium chloride and copper(II) sulfate solutions, and the half equations for hydrogen, chlorine, oxygen and copper.	16
Inert and non-inert electrodes	Graphite and platinum as inert electrodes, what a non-inert electrode is, how copper electrodes change in copper(II) sulfate solution, purifying copper, setting up the cell, and why direct current is used.	8

Chemical Reactions is 193 of the 797 questions in the guide.Get the guide, £7

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.