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
Writing formulae of elements and compounds
- 3 things to know
- Paper 3
Overview
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, the formulae of common acids, sodium hydroxide, calcium carbonate and ammonia, and half equations for aluminium and bromine.
What you need to know
Answer each one before you reveal it.
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Key term
Why is the formula of neon written as Ne and not Ne₂?
Key term
What is the formula of water, and how many atoms of each element does one molecule contain?
Questions from this subtopic
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.
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