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Chemical reactions

OCR GCSE Combined ScienceTopic 9 of 1834 lessons

Revision notes for Chemical reactions in OCR Combined Science A (J250). Every lesson in BrainCake comes with R.E.C.I.P.E. recall steps, a quick check and spaced reviews.

  1. Writing formulae of elements and simple compoundsEvery element has a chemical symbol of one or two letters.
  2. Conservation of mass and balanced equationsIn a chemical reaction atoms are rearranged, but none are created or destroyed.
  3. Using the Periodic Table to write formulae and equationsIn the exam you are given a Periodic Table.
  4. Deducing formulae from common ionsYou need to know the formulae and charges of some common ions.
  5. Balanced ionic equationsWhen an ionic compound dissolves in water, its ions separate and move freely.
  6. State symbolsState symbols tell us the physical state of each substance in an equation.
  7. Tests for oxygen, hydrogen, carbon dioxide and chlorineMost gases are colourless, so we identify them with simple chemical tests.
  8. The Avogadro constant and the moleAtoms and molecules are far too small to count one at a time, so chemists count them in huge groups called moles.
  9. Mass and amount in molesThe amount of a substance is measured in moles (mol).
  10. Concentration of solutionsA solution is made when a solute dissolves in a solvent, usually water.
  11. The law of conservation of massThe law of conservation of mass says that no mass is lost or gained in a chemical reaction.
  12. Changes in mass in open systemsIn a non-enclosed system the reaction vessel is open to the air, so gases can enter or leave.
  13. Stoichiometry and limiting reactantsThe stoichiometry of an equation is the ratio of moles of each substance that react or form.
  14. Calculating reacting masses from equationsA balanced equation tells you the ratio of moles, so you can use it to find the mass of a reactant or product.
  15. Exothermic and endothermic reactionsEvery chemical reaction transfers energy between the chemicals and their surroundings.
  16. Reaction profilesA reaction profile is a graph that shows how the energy of the chemicals changes during a reaction.
  17. Activation energyReactions do not start just because reactants are mixed.
  18. Calculating energy changes from bond energiesIn a reaction, bonds in the reactants are broken and new bonds form in the products.
  19. Oxidation and reduction as gain or loss of oxygenMany reactions can be described as oxidation or reduction.
  20. Oxidation and reduction as loss or gain of electronsOxidation and reduction can also be explained using electrons.
  21. Hydrogen ions and hydroxide ions in acids and alkalisAn acid is a substance that forms hydrogen ions, H+, when it dissolves in water.
  22. Neutralisation: acid and alkali or base forming a salt and waterNeutralisation is the reaction of an acid with an alkali or a base to form a salt and water.
  23. The ionic equation for aqueous neutralisationIn aqueous neutralisation an acid reacts with an alkali in solution.
  24. Reactions of acids with carbonates and metalsAcids react with carbonates to form a salt, water and carbon dioxide.
  25. Dilute, concentrated, weak and strong acidsTwo different ideas are easy to mix up.
  26. Measuring acidity and alkalinity with pHAcids and alkalis are not all equally acidic or alkaline, so we need a way to compare them.
  27. Hydrogen ions and the pH valueThe pH of a solution depends on the concentration of hydrogen ions, H+, in it.
  28. How pH changes with hydrogen ion concentrationThe pH scale is not a simple straight-line scale.
  29. Measuring pH: universal indicator and pH metersThere are two main ways to measure the pH of a solution.
  30. Products at the electrodes in electrolysisElectrolysis uses electricity to break down an ionic compound.
  31. Electrolysis of molten binary ionic compoundsA binary ionic compound contains just two elements, one metal and one non-metal, for example sodium chloride or lead bromide.
  32. Electrolysis of aqueous solutionsAn aqueous solution has more ions than a molten compound.
  33. Half equations at the electrodesIn electrolysis, ions are turned into atoms or molecules by gaining or losing electrons.
  34. Inert and non-inert electrodesTo carry out electrolysis, the electrolyte is placed in a beaker and two electrodes are dipped in.

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