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Chemical reactions

OCR GCSE ChemistryTopic 3 of 632 lessons

Revision notes for Chemical reactions in OCR Chemistry A (J248). Every lesson in BrainCake comes with R.E.C.I.P.E. recall steps, a quick check and spaced reviews.

  1. Formulae of elements and simple compoundsEvery element has a chemical symbol, such as C for carbon, Fe for iron and Na for sodium.
  2. Balanced chemical equationsIn a chemical reaction no atoms are created or destroyed.
  3. Using the Periodic Table to write equationsIn the exam you are given a Periodic Table, so you do not have to memorise every symbol.
  4. Deducing formulae from ionsAn ionic compound has no overall charge.
  5. Balanced ionic equationsIn a reaction between solutions, some ions change and others do not.
  6. State symbolsA symbol equation can also show the physical state of each substance.
  7. The Avogadro constant and the moleAtoms and molecules are far too small to count one at a time, so chemists count them in groups called moles.
  8. Mass and amount in molesThe mass of a substance and the amount of it in moles are linked through its relative formula mass, Mr.
  9. The law of conservation of massThe law of conservation of mass says that no mass is lost or gained in a chemical reaction.
  10. Changes in mass in non-enclosed systemsMass is always conserved, but in a non-enclosed system the mass on a balance can appear to change.
  11. Stoichiometry and limiting reactantsThe numbers in front of the formulae in a balanced equation show the mole ratio of the substances.
  12. Calculating masses from balanced equationsA balanced equation lets you calculate the mass of a reactant needed or a product made.
  13. Distinguish between endothermic and exothermic reactions on the basis of the temperature change of the surroundingsChemical reactions are accompanied by an energy change.
  14. Draw and label a reaction profile for an exothermic and an endothermic reactionA reaction profile is a graph that shows how the energy of the chemicals changes during a reaction.
  15. Explain activation energy as the energy needed for a reaction to occurParticles must collide before they can react.
  16. Calculate energy changes in a chemical reaction by considering bond making and bond breaking energiesIn a chemical reaction, bonds in the reactants are broken and new bonds form in the products.
  17. Oxidation and reduction as gain or loss of oxygenIn a chemical reaction, oxidation is the gain of oxygen and reduction is the loss of oxygen.
  18. Oxidation and reduction as loss or gain of electronsOxidation and reduction can also be explained using electrons.
  19. Hydrogen ions and hydroxide ionsWhen an acid dissolves in water, its molecules ionise and form hydrogen ions, H+(aq).
  20. Neutralisation and making a pure dry saltNeutralisation is the reaction of an acid with an alkali or a base to form a salt and water: acid + alkali → salt + water, and acid + base → salt +…
  21. Neutralisation as hydrogen ions reacting with hydroxide ionsIn an aqueous neutralisation reaction, acid and alkali solutions are mixed.
  22. Reactions of acids with metals and carbonatesAcids react with some metals to form a salt and hydrogen: acid + metal → salt + hydrogen.
  23. Dilute, concentrated, weak and strong acidsThe words concentrated and dilute describe how much acid is dissolved in the water.
  24. Measuring acidity and alkalinity with pHChemists compare how acidic or alkaline solutions are using the pH scale.
  25. pH, neutrality and hydrogen ion concentrationpH tells you about the concentration of hydrogen ions, H+, in a solution.
  26. Each pH unit is a factor of tenThe pH scale is not a simple straight-line scale.
  27. Measuring pH with universal indicator and pH metersThere are two main ways to measure pH.
  28. Products at the electrodes and the terms cations and anionsElectrolysis uses electricity to break down an ionic compound that is molten or dissolved in water.
  29. Predicting products of electrolysis of molten binary compoundsA binary ionic compound is made of two different elements, for example sodium chloride, NaCl, or lead bromide, PbBr2.
  30. Electrolysis of aqueous solutions: competing reactionsAn aqueous solution contains the ions of the dissolved compound and also a few ions from the water itself: hydrogen ions, H+, and hydroxide ions, OH-.
  31. Ions and half equations at the electrodesIn electrolysis the ions carry the current through the electrolyte, while electrons carry it through the wires.
  32. Electrolysis with inert and non-inert electrodesTo carry out electrolysis, two electrodes are dipped into the electrolyte in a beaker and connected to a direct current (d.c.) power supply.

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