Chemical changes

• acids have a pH of less than 7
• alkalis have a ph greater than 7
• another word for an Alkali is a base
• acid + base = salt + water
• acid + metal oxide = salt + water
• acid + metal hydroxide = salt + water
• acid + metal carbonate = salt + water + carbon dioxide
• acid + metal = salt + hydrogen
• metal + water = metal hydroxide + hydrogen
• Acids produce an H+ ion when dissolved in water
• Alkalis produce OH- ions when dissolved in water
• strong acids completely ionise
• weak acids partially ionise
• the reactivity series lists metals in order of how vigorously they react
• The reactivity series -

  • Potassium
  • Sodium
  • Lithium
  • Calcium
  • Magnesium
  • Carbon
  • Zinc
  • Iron
  • Hydrogen
  • Copper
• Oxidation is the gain of Oxygen
• Reduction is the loss of Oxygen
• Oxidation is the loss of electrons
• Reduction is the gain of electrons
• a displacement reaction is when a more reactive metal replaces a less reactive one in its compound
• electrolysis is the process of splitting a compound into its elements by using electricity
• Cathode (-) -

  • attracts metal - positive ion
  • reduction - gain of electrons
• Anode (+) -

  • attracts non-metal - negative ion
  • Oxidation - loss of electrons
• Cryolite is used to lower the melting point of Aluminium Oxide, this reduces costs and saves energy
• Ionic compounds are made molten so that they can conduct electricity, as the ions can move freely
• in aqueous solutions, as well as ions from the ionic compound, there will be Hydrogen and Hydroxide ions from the water
• which ions are discharge at the electrodes when the solution is electrolysed will depend on their relative reactivity
• if H+ ions and metal ions are present, Hydrogen gas will be produced at the Cathode if the metal ions form an element that is more reactive than Hydrogen - eg, Sodium
• if the metal ions form an element that is less reactive than Hydrogen (eg. gold), a solid layer of the pure metal will be produced instead, which will coat the Cathode
• At the Anode, if OH- and Halide ions are present, molecules of halogens will be formed
• if no halide ions are present, then the OH- ions from the water will be discharged and Oxygen gas (and water) will be formed at the Anode
• exothermic reactions give out heat to the surroundings (temperature increases)
• endothermic reactions take in energy from the surroundings (temperature decreases)
• exothermic reactions -

  • combustion
  • respiration
  • neutralisation
• endothermic reactions -

  • evaporation
  • thermal decomposition
  • photosynthesis
• Chemical reactions can only occur when reacting particles collide with each other and with sufficient energy.
• The minimum amount of energy that particles must have to react is called the activation energy
• Exothermic reaction profile -

  
• Endothermic reaction profile -

  
• the overall energy change for an EXOthermic reaction is always NEGATIVE