Key term
OCR GCSE Combined Science Chemistry J250
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 structures
Global Challenges
Topic overviewExtracting metals by reduction and by electrolysisBiological methods of metal extractionLife 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
Topic overviewMass of solute, concentration and dilution calculationsMeasuring 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
Chemical Reactions
Conservation of mass, and changes of mass in open systems
- 3 things to know
- Paper 9 (Chemistry)
Overview
The 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.
What you need to know
Answer each one before you reveal it.
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Key term
Why does a reaction carried out in a closed container show no change in the balance reading even when a gas is made?
Key term
Why can the mass of a product never be greater than the total mass of the reactants that were used?
Questions from this subtopic
Each one with the model answer underneath.
- 1
State the law of conservation of mass.
Model answer
Mass is neither created nor destroyed in a chemical reaction, so the total mass of the products equals the total mass of the reactants.
- 2
Explain conservation of mass in terms of atoms.
Model answer
The same atoms are present at the end of the reaction as at the start; they have only been rearranged into different substances.
- 3
In a sealed container, 24 g of magnesium reacts completely with 16 g of oxygen. What mass of magnesium oxide is formed?
Model answer
40 g.
- 4
A sealed flask holding 10.0 g of calcium carbonate is heated until 5.6 g of calcium oxide remains. What mass of carbon dioxide was produced?
Model answer
4.4 g.
- 5
12 g of carbon burns completely in 32 g of oxygen. Calculate the mass of carbon dioxide formed.
Model answer
44 g.
- 6
Why does a reaction carried out in a closed container show no change in the balance reading even when a gas is made?
Model answer
The gas cannot escape, so it is still inside the container and is still being weighed.
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