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
Exothermic and endothermic reactions, and reaction profiles
- 3 things to know
- Paper 3 (Chemistry)
Overview
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.
What you need to know
Answer each one before you reveal it.
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Key term
What is an exothermic reaction?
Key term
What is an endothermic reaction?
Questions from this subtopic
Each one with the model answer underneath.
- 1
What is an exothermic reaction?
Model answer
One that transfers energy out to the surroundings, so the temperature of the surroundings rises.
- 2
What is an endothermic reaction?
Model answer
One that takes in energy from the surroundings, so the temperature of the surroundings falls.
- 3
How does a thermometer in the reaction mixture tell you which type of reaction is happening?
Model answer
A rise in temperature shows the reaction is exothermic and a fall shows it is endothermic.
- 4
Give two examples of exothermic reactions.
Model answer
Combustion and neutralisation.
- 5
Give two examples of endothermic changes.
Model answer
Thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate.
- 6
Two solutions are mixed and the outside of the beaker feels cold. What type of reaction is this?
Model answer
Endothermic, because energy is being taken in from the surroundings including the beaker.
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