C4

• Displacement reaction


  A more reactive metal displaces a less reactive metal from a solution of its salt, e.g. magnesium displacing copper from copper sulfate solution
• Reduction


  Positively charged ions gain electrons at the negative electrode
• Oxidation


  Negatively charged ions lose electrons at the positive electrode
• In a redox reaction both reduction and oxidation occur
• Reactions that take place at the electrodes during electrolysis can be represented by half-equations
• Metals that are more reactive than carbon can be extracted from their ores using electrolysis
• Electrolysis requires lots of heat and electrical energy, making it an expensive process
• Cryolite


  A compound of aluminium that lowers the melting point of aluminium oxide, meaning less energy is needed
• Electrolysis of aluminium oxide

  1. Aluminium forms at the negative electrode

  2. Oxygen gas forms at the positive carbon electrode and reacts with the carbon, forming carbon dioxide
• When ionic compounds are dissolved in water to form aqueous solutions, it is slightly harder to predict the products of electrolysis
• Electrolysis of aqueous solutions

  1. Water molecules break down to form hydroxide ions, OH, and hydrogen ions, H+

  2. At the negative electrode: Hydrogen is produced if the metal is more reactive than hydrogen, the metal is produced if the metal is less reactive than hydrogen

  3. At the positive electrode: Oxygen is produced unless the solution contains halide ions, if halide ions are present then the halogen is produced
• In the electrolysis of sodium chloride solution: Hydrogen is released at the negative electrode, Chlorine gas is released at the positive electrode
• A low voltage must be used to prevent an electric shock, the room must be well ventilated, and the experiment must only be carried out for a short period of time, to prevent exposure to dangerous levels of chlorine gas
• When electrolysing aqueous solutions, you may be asked to suggest a hypothesis to explain given data
• When extracting metals, the positive electrode is often made of carbon
• Electrolysis can be used to separate ionic compounds into elements, for example lead bromide can be split into lead and bromine
• Ionic substances can only conduct electricity when they are molten or dissolved in water
• Oxidation is loss of electrons and reduction is gain of electrons (OILRIG)
• The balanced symbol equation for the reaction between magnesium and oxygen can be split into two ionic equations
• When substances dissolve in water, they dissociate into their individual ions
• Hydroxide ions, OH-(aq), make solutions alkaline, hydrogen ions, H+(aq), make solutions acidic
• The pH scale is a measure of the acidity or alkalinity of an aqueous solution
• A solution with a pH of 7 is neutral, aqueous solutions with a pH less than 7 are acidic, aqueous solutions with a pH more than 7 are alkaline
• The pH of a solution can be measured using a pH probe or universal indicator
• Soluble bases are called alkalis, acids are neutralised by bases
• Acids contain hydrogen ions, H+(aq), alkalis contain hydroxide ion, OH-(aq)
• When an acid reacts with an alkali, the H+ and OH- ions react together to produce water, H2O which has a pH of 7, this type of reaction is called neutralisation
• Acids can also be neutralised by metal oxides and metal carbonates
• A salt is produced when the hydrogen in the acid is replaced by a metal ion, the name of the salt produced depends on the acid used
• Soluble salts can be made by reacting acids with insoluble bases, such as metal oxides, metal hydroxides and metal carbonates
• Strong acids are completely ionised in water and weak acids are only partially ionised in water
• The pH of a solution is a measure of the concentration of H+ ions, a pH decrease of one unit indicates that the concentration of hydrogen ions has increased by a factor of 10
• For a concentration of acid, a strong acid will have a higher concentration of hydrogen ions and therefore a lower pH
• Neutralisation practical

  1. Use a measuring cylinder to add dilute hydrochloric acid to a beaker

  2. Dip a clean glass rod into the contents of the beaker, use it to transfer a drop of liquid to a piece of paper universal indicator paper on a white tile, wait 30 secs then match the colour to a pH colour chart and record the estimated pH

  3. Add a level spatula of calcium hydroxide powder to the beaker

  4. Stir thoroughly, then estimate and record the pH of the mixture

  5. Repeat steps 3 and 4 until there are no more changes in pH