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
Stoichiometry, and calculating reacting masses
- 3 things to know
- Paper 9 (Chemistry)
Overview
What 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.
What you need to know
Answer each one before you reveal it.
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Key term
48 g of magnesium reacts exactly with 32 g of oxygen. Deduce the equation. Relative atomic masses: Mg 24, O 16.
Key term
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.
Questions from this subtopic
Each one with the model answer underneath.
- 1
What does the stoichiometry of an equation tell you?
Model answer
The ratio in which the substances react and are formed, given by the large numbers in front of the formulae.
- 2
48 g of magnesium reacts exactly with 32 g of oxygen. Deduce the equation. Relative atomic masses: Mg 24, O 16.
Model answer
2 mol of magnesium reacts with 1 mol of oxygen molecules, giving 2Mg + O₂ → 2MgO.
- 3
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.
Model answer
0.1 mol of sodium to 0.05 mol of chlorine molecules, a 2 to 1 ratio, giving 2Na + Cl₂ → 2NaCl.
- 4
What is a limiting reactant?
Model answer
The reactant that is used up first and so stops any more product from being made.
- 5
What is meant by saying a reactant is in excess?
Model answer
More of it is supplied than is needed, so some is left unreacted when the reaction stops.
- 6
Which reactant controls the amount of product that can be made, and why?
Model answer
The limiting reactant, because once it has run out no further product can form however much of the other remains.
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