Key term
OCR GCSE Chemistry A J248
Chemical Reactions
Topic overviewWriting formulae of elements and compoundsBalanced equations, and deducing formulae from ionsIonic equations, and state symbolsThe mole and the Avogadro constantConservation of mass, and changes of mass in open systemsStoichiometry, and calculating reacting massesExothermic and endothermic reactions, and reaction profilesActivation energy, and bond energy calculationsOxidation and reduction, by oxygen and by electronsAcids, alkalis and neutralisationThe ionic equation for neutralisation, and reactions of acidsDilute and concentrated, weak and strongThe pH scale, and hydrogen ion concentrationpH as a factor of ten, and measuring pHProducts 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 structuresNanoparticles: size and surface area to volumeUses and risks of nanoparticulate materials
Global Challenges
Topic overviewExtracting metals by reduction and by electrolysisBiological methods of metal extractionRate against yield in industry, and reaction conditionsThe Haber process and fertiliser productionNitrogen, phosphorus and potassium in agricultureLife cycle assessmentRecycling, and the decisions behind itAlloys, and the corrosion of metalsComparing glass, ceramics, polymers, composites and metalsFunctional groups and homologous seriesPredicting the products of organic reactionsAddition polymerisationCondensation polymerisation, and making a polymerDNA and other natural polymersFractional distillation of crude oilThe fractions, and crude oil as a feedstockDependence on hydrocarbons, and crackingChemical cells and fuel cellsThe 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
Topic overviewConcentration in mol/dm³, and titrationTitration calculations, and the mass of a soluteMolar gas volume and the volumes of gasesTheoretical mass and percentage yieldAtom economy, and choosing a reaction pathwayMeasuring the rate of a reaction, and rate graphsFactors affecting rate, and collision theorySurface area and rateCatalysts and how they workReversible reactions and dynamic equilibriumChanging the conditions of an equilibriumActivation energy, and enzymes as catalysts
Particles
Predicting and Identifying Reactions
Topic overviewGroups 1, 7 and 0, and why they behave as they doTransition metalsPredicting reactivity from the Periodic TableReactivity of metals with water and acidsTests for oxygen, hydrogen, carbon dioxide and chlorineTests for aqueous cations and anionsFlame tests, and identifying metal ionsIdentifying a species from test resultsInstrumental methods of analysis
Chemical Reactions
- 17 subtopics
- 45 things to know
- Paper 3
What Chemical Reactions covers
Chemical Reactions is examined in Chemistry A Paper 3. It covers writing formulae of elements and compounds; balanced equations and deducing formulae from ions; ionic equations and state symbols; the mole and the Avogadro constant; conservation of mass and changes of mass in open systems; stoichiometry and calculating reacting masses; exothermic and endothermic reactions and reaction profiles; activation energy and bond energy calculations; oxidation and reduction by oxygen and by electrons; acids, alkalis and neutralisation; the ionic equation for neutralisation and the reactions of acids; dilute and concentrated, weak and strong acids; the pH scale and hydrogen ion concentration; pH as a factor of ten and measuring pH; 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 A Paper 3: 1 hour 45 minutes, 90 marks, 50% of the GCSE.
Check the OCR specification ↗The 17 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, the formulae of common acids, sodium hydroxide, calcium carbonate and ammonia, and half equations for aluminium and bromine.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 →
- Ionic equations, and state symbolsWhat an ionic equation and a spectator ion are, why charge must balance, ionic equations for the precipitates copper(II) hydroxide, silver chloride and barium sulfate and for magnesium with acid, what the state symbols s, l, g and aq mean, and adding them to equations such as zinc or marble chips with acid.Open →
- The mole and the Avogadro constantWhat the Avogadro constant and a mole are, counting the atoms, molecules and ions in a number of moles, the mass of a single atom or molecule, why chemists use the mole, and converting between mass and moles with relative formula mass for carbon dioxide, calcium carbonate, magnesium, water and sodium hydroxide.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 →
- Stoichiometry, and calculating reacting massesWhat stoichiometry shows, deducing an equation from reacting masses, limiting reactants and reactants in excess, and calculating masses of product or reactant from balanced equations for magnesium oxide, calcium oxide, hydrogen, water and sodium chloride.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 bond energy calculationsWhat activation energy is, why heating, a spark or a flame starts a reaction, why bond breaking is endothermic and bond making exothermic, and calculating energy changes from bond energies.Open →
- Oxidation and reduction, by oxygen and by electronsOxidation and reduction as the gain and loss of oxygen, oxidising and reducing agents in reactions such as copper oxide with carbon and iron oxide with carbon monoxide, why the two always happen together, oxidation and reduction as the loss and gain of electrons, OIL RIG, half equations for magnesium and chlorine, and the reaction of zinc with copper ions.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 →
- Dilute and concentrated, weak and strongWhat concentrated and dilute mean, what strong and weak mean, hydrochloric, sulfuric and ethanoic acids as examples, why an acid can be strong and dilute, and which solution holds more hydrogen ions.Open →
- The pH scale, and hydrogen ion concentrationWhat pH measures, the 0 to 14 scale and its acidic, neutral and alkaline values, how pH relates to hydrogen ion concentration, and the shape of a pH curve as alkali is added to acid.Open →
- pH as a factor of ten, and measuring pHHow pH changes by one unit for each tenfold change in hydrogen ion concentration, calculating those factors between pH values, what universal indicator and litmus show and their colours, using, rinsing and calibrating a pH meter and its advantage over indicator, 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 is an ionic equation?
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What is the value of the Avogadro constant?
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State the law of conservation of mass.
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What does the stoichiometry of an equation tell you?
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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 it mean to say that an acid is concentrated?
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What does pH measure?
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What happens to the pH of a solution when its hydrogen ion concentration increases by a factor of ten?
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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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What is a spectator ion?
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What is a mole of a substance?
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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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48 g of magnesium reacts exactly with 32 g of oxygen. Deduce the equation. Relative atomic masses: Mg 24, O 16.
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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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What does it mean to say that an acid is dilute?
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Which pH values show that a solution is acidic?
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What happens to the pH when the hydrogen ion concentration falls by a factor of ten?
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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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Besides the atoms, what else must balance on the two sides of an ionic equation?
Numbers
How many atoms are there in one mole of carbon?
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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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2.3 g of sodium reacts exactly with 3.55 g of chlorine. Deduce the ratio and the equation. Relative atomic masses: Na 23, Cl 35.5.
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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?
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 Chemistry A?
- 17 subtopics: writing formulae of elements and compounds, balanced equations, and deducing formulae from ions, ionic equations, and state symbols, the mole and the avogadro constant, conservation of mass, and changes of mass in open systems, stoichiometry, and calculating reacting masses, exothermic and endothermic reactions, and reaction profiles, activation energy, and bond energy calculations, oxidation and reduction, by oxygen and by electrons, acids, alkalis and neutralisation, the ionic equation for neutralisation, and reactions of acids, dilute and concentrated, weak and strong, the ph scale, and hydrogen ion concentration, ph as a factor of ten, and measuring ph, 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 J248 specification, subtopic by subtopic, so it matches the order you are taught it in.
Every question in Chemical Reactions, on a schedule.
Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.
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