Chemical Reactions

• Electrolysis is the breakdown of an ionic compound, molten or in aqueous solution, by the use of electricity.
• Electrolytes are liquids that conduct electricity by movement of ions.
• In a physical change, the substances present remain chemically the same; no new substances are formed.
• Physical changes are often easy to reverse. Any mixtures produced are usually easy to separate.
• The major feature of a chemical change, or reaction, is that new substance(s) are made during the reaction.
• During a chemical reaction, energy can be given out or taken in:

  • When energy is given out, the reaction is EXOTHERMIC
• During a chemical reaction, energy can be given out or taken in:

  • When energy is taken in, the reaction is ENDOTHERMIC
• WORD EQUATION: the simplest way to describe a chemical reaction. 
• REACTANTS: substances that react
• PRODUCTS: new substances formed
• Synthesis - occurs when two or more substances react together to form just ONE product.
• In synthesis, heat is required to start the reaction, but once started, it continues exothermically.
• Decomposition - have just one reactant, which breaks down to give two or more simpler products. 
• Decomposition reactions are endothermic. Decomposition caused by heat energy is called thermal decomposition.
• Neutralisation - when acids react with bases or alkalis and salt is produced.
• Precipitation - the sudden formation of a solid, either when two solutions are mixed or when a gas is bubbled into a solution
• A Displacement reaction occurs because a more reactive element will displace a less reactive one from a solution of one of its compounds.
• Combustion -  the reaction of a substance with oxygen causing the release of energy; it is exothermic and often involves a flame.
• Burning - combustion in which a flame is produced.
• Oxidation -  when a substance gains oxygen during a reaction. We say, it is oxidised
• Reduction -  if a substance loses oxygen during a reaction, it is reduced.
• Reducing agent - an element or compound that will remove oxygen from the other substances.
• Oxidising agent - a substance that will add oxygen to another substance.
• Two common examples of oxidation reactions are:

  • Corrosion
  • Rancidity
• Electrolysis: the breakdown of an ionic compound, molten or in aqueous solution, by the use of electricity.
• Electrical conductor: a substance that conducts electricity but is not chemically changed in the process.
• Electrolytes: liquids that conduct electricity by movement of ions.
• Non-electrolytes: liquids that do not conduct electricity.
• Electrodes: rods in an electrolytic cell which carry the current into and out of the solution. Usually made up of graphite or platinum.
• Cathode: negative electrodes

  Anode: positive electrodes
• During electrolysis:

  Positive ions (metal ions/cations) move towards the cathode (negative electrode)

  Negative ions (non-metal ions/anions)move towards the anode (positive electrode)
• Electroplating - coating a metal with another metal
• The most commonly used metals for electroplating are:

  copper

  chromium

  silver

  tin