Key term
OCR GCSE Combined Science Chemistry J250
Chemical Reactions
Topic overviewWriting formulae of elements and compoundsBalanced equations, and deducing formulae from ionsState symbols in equationsConservation of mass, and changes of mass in open systemsExothermic and endothermic reactions, and reaction profilesActivation energy and why reactions need itOxidation and reduction in terms of oxygenAcids, alkalis and neutralisationThe ionic equation for neutralisation, and reactions of acidsThe pH scale, acidity and alkalinityMeasuring pH with indicators and pH metersProducts at the electrodes, and molten ionic compoundsElectrolysis of aqueous solutionsInert and non-inert electrodes
Elements, Compounds and Mixtures
Topic overviewPurity, and melting point dataRelative formula mass and empirical formulaeFormulations, and separating mixturesPaper and thin layer chromatographyInterpreting chromatograms and Rf valuesChoosing a purification methodMetals and non-metals in the Periodic TableElectron arrangement and position in the Periodic TableIonic, covalent and metallic bonding comparedDot and cross diagrams, and the limits of modelsReactions, electron arrangement and Mendeleev's tableCarbon's four bonds, and the variety of organic compoundsDiamond, graphite, fullerenes and grapheneBond strength, intermolecular forces and predicting stateBulk properties of ionic, molecular and giant structures
Global Challenges
Topic overviewExtracting metals by reduction and by electrolysisLife cycle assessmentRecycling, and the decisions behind itFractional distillation of crude oilThe fractions, and crude oil as a feedstockDependence on hydrocarbons, and crackingThe Earth's early atmosphere, and how oxygen developedThe greenhouse effect, and evidence for climate changeEffects of carbon dioxide and methane, and air pollutantsProducing potable water
Monitoring and Controlling Chemical Reactions
Particles
Chemical Reactions
- 14 subtopics
- 42 things to know
- Paper 3 (Chemistry)
What Chemical Reactions covers
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; conservation of mass and changes of mass in open systems; exothermic and endothermic reactions and reaction profiles; activation energy; oxidation and reduction in terms of oxygen; acids, alkalis and neutralisation; the ionic equation for neutralisation and the reactions of acids; the pH scale; measuring pH with indicators and a pH meter; products at the electrodes and molten ionic compounds; electrolysis of aqueous solutions; and inert and non-inert electrodes.
Where it is assessed
Paper 3 (Chemistry)
Combined Science A, Paper 3 (Chemistry): 1 hour 10 minutes, 60 marks, 16.7% of the qualification.
Check the OCR specification ↗The 14 subtopics
Each one has its own page.
- Writing formulae of elements and compoundsWhich 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.Open →
- Balanced equations, and deducing formulae from ionsBalanced 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.Open →
- State symbols in equationsWhat 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.Open →
- Conservation of mass, and changes of mass in open systemsThe 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.Open →
- Exothermic and endothermic reactions, and reaction profilesWhat 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.Open →
- Activation energy and why reactions need itWhat 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.Open →
- Oxidation and reduction in terms of oxygenOxidation 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.Open →
- Acids, alkalis and neutralisationThe 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.Open →
- The ionic equation for neutralisation, and reactions of acidsThe 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.Open →
- The pH scale, acidity and alkalinityWhat 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.Open →
- Measuring pH with indicators and pH metersWhat 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.Open →
- Products at the electrodes, and molten ionic compoundsWhat 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.Open →
- Electrolysis of aqueous solutionsThe 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.Open →
- Inert and non-inert electrodesGraphite 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.Open →
What you need to know
Answer each one before you reveal it.
Your recallNot started
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Using activation energy, explain the spark for methane and air, the effect of heating, bond breaking, and why a candle keeps burning.
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In terms of oxygen, explain what a reducing agent is, name the reducing agent in Fe₂O₃ + 3CO → 2Fe + 3CO₂, say whether magnesium burning in air is oxidised or reduced, and why oxidation and reduction always happen together.
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Which elements exist as molecules of two joined atoms, and how are their formulae written?
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Write the balanced equation for sodium burning in chlorine.
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What do the state symbols s, l, g and aq stand for?
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State the law of conservation of mass.
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Which ion do all acids produce when they dissolve in water?
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Which single reaction is common to every neutralisation that happens in water?
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What does pH measure?
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What colour is universal indicator in a strongly acidic, a neutral and a strongly alkaline solution?
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What is a cation, and which electrode does it move to?
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Which extra ions are present in any aqueous solution besides those of the dissolved compound?
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Which two materials are usually chosen for inert electrodes, and why?
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Why is the formula of neon written as Ne and not Ne₂?
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Write the balanced equation for the complete combustion of methane.
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Add state symbols to the reaction of zinc metal with dilute sulfuric acid.
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Why does a reaction carried out in a closed container show no change in the balance reading even when a gas is made?
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What is an exothermic reaction?
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What is activation energy?
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Define oxidation in terms of oxygen.
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Which ion is present in every alkaline solution?
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Write the general equation for neutralisation in solution, with state symbols.
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Which pH values show that a solution is acidic?
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Why is universal indicator unsuitable for testing a strongly coloured solution?
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What is an anion, and which electrode does it move to?
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What decides whether a metal or hydrogen is produced at the cathode in an aqueous solution?
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What is a non-inert electrode?
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What is the formula of water, and how many atoms of each element does one molecule contain?
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Write the balanced equation for potassium reacting with water.
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Which state symbol shows that a precipitate has formed in a solution?
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Why can the mass of a product never be greater than the total mass of the reactants that were used?
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What is an endothermic reaction?
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Why does a mixture of methane and air not react until a spark is applied?
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Define reduction in terms of oxygen.
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Name three common acids and state the ion they all release in water.
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In terms of ions, what is left in the beaker after an acid has been exactly neutralised by an alkali?
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Which pH values show that a solution is alkaline?
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How is a pH meter used to measure the pH of a solution?
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Which electrode is the negative one, and which is the positive one?
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What decides which product forms at the anode in an aqueous solution?
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How are non-inert electrodes used to purify copper?
Take Chemical Reactions further
The same content, four other ways.
Commonly confused
Pairs that lose marks when they are blurred together.
Cation vs Anion
Cation: positive ion attracted to the negative electrode. Anion: negative ion attracted to the positive electrode.
Oxidation vs Reduction
Oxidation: loss of electrons, or gain of oxygen. Reduction: gain of electrons, or loss of oxygen.
Exothermic reaction vs Endothermic reaction
Exothermic reaction: change that gives out energy and warms the surroundings. Endothermic reaction: change that takes in energy and cools the surroundings.
Questions from this topic
Each one with the model answer underneath.
- 1
Which elements exist as molecules of two joined atoms, and how are their formulae written?
Model answer
Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine, written H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.
- 2
Why is the formula of neon written as Ne and not Ne₂?
Model answer
Neon atoms already have a full outer shell, so they do not bond to one another and exist as separate atoms.
- 3
What is the formula of water, and how many atoms of each element does one molecule contain?
Model answer
H₂O, containing two hydrogen atoms and one oxygen atom.
- 4
Give the formula of methane and state how many covalent bonds the carbon atom forms.
Model answer
CH₄, with the carbon atom forming four covalent bonds.
- 5
What is the formula of carbon dioxide, and what does the subscript tell you?
Model answer
CO₂, where the subscript 2 shows that each molecule contains two oxygen atoms.
- 6
Sodium chloride is written NaCl even though it has no molecules. What does this formula represent?
Model answer
The simplest whole-number ratio of the ions in the giant lattice, one sodium ion to one chloride ion.
What is inside Levo
Six ways to work the same material.
Chemical Reactions revision questions
- What does Chemical Reactions cover in OCR GCSE Combined Science Chemistry?
- 14 subtopics: 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. Each one has its own page with questions and model answers.
- How is Chemical Reactions organised?
- By the OCR J250 specification, subtopic by subtopic, so it matches the order you are taught it in.
Every question in Chemical Reactions, on a schedule.
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