Key term
OCR GCSE Chemistry A J248
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 structuresNanoparticles: size and surface area to volumeUses and risks of nanoparticulate materials
Global Challenges
Topic overviewExtracting metals by reduction and by electrolysisThe 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 polymerisationDNA 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 overviewCarrying out an acid–alkali titrationTheoretical mass and percentage yieldAtom economy and its calculationMeasuring the rate of a reaction, and rate graphsFactors affecting rate, and collision theorySurface area and rateCatalysts and how they workReversible reactions and dynamic 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
Monitoring and Controlling Chemical Reactions
Activation energy, and enzymes as catalysts
- 3 things to know
- Paper 2
Overview
How a catalyst gives a pathway with a lower activation energy, reaction profiles for catalysed reactions, why lowering the activation energy speeds a reaction, calculating activation energies and energy changes from profiles, what enzymes are and why organisms need them, denaturing by heat and pH, enzyme specificity, uses in brewing and washing powders, and how enzymes compare with metal catalysts.
What you need to know
Answer each one before you reveal it.
Your recallNot started
Key term
Explain why lowering the activation energy makes a reaction faster.
Key term
A reaction profile shows the reactants at 100 kJ/mol, the peak at 180 kJ/mol and the products at 40 kJ/mol. Calculate the activation energy and the overall energy change.
Questions from this subtopic
Each one with the model answer underneath.
- 1
Explain how a catalyst increases the rate of a reaction.
Model answer
It provides an alternative reaction pathway that has a lower activation energy.
- 2
What does a reaction profile show?
Model answer
It shows how the energy of the substances changes as the reaction proceeds, with the reactants on the left, the products on the right and a peak between them.
- 3
Describe how the reaction profile for a catalysed reaction differs from that of the same reaction without a catalyst.
Model answer
The peak is lower because the activation energy is smaller, while the energy levels of the reactants and the products are exactly the same.
- 4
Does a catalyst change the overall energy change of a reaction?
Model answer
No, the overall energy change is unaltered; only the activation energy is lowered.
- 5
Explain why lowering the activation energy makes a reaction faster.
Model answer
A greater proportion of the colliding particles then have enough energy to react, so a larger fraction of collisions are successful.
- 6
A reaction profile shows the reactants at 100 kJ/mol, the peak at 180 kJ/mol and the products at 40 kJ/mol. Calculate the activation energy and the overall energy change.
Model answer
The activation energy is 180 − 100 = 80 kJ/mol, and the overall energy change is 40 − 100 = −60 kJ/mol, so the reaction is exothermic.
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