Chemical/energy changes, rate of reaction,reversible reactions

• Exothermic reaction

  Reactions that transfer energy to the surroundings and result in an increase in temperature
• Endothermic reaction

  Reactions that take in energy from the surroundings and result in a decrease in temperature
• Examples of exothermic and endothermic reactions

  • Exothermic: combustion, rusting, neutralisation

  Endothermic: thermal decomposition, reaction between citric acid and sodium hydrogen carbonate
• Energy transfers in chemical reactions

  • Energy is transferred from the chemicals to or from the surroundings, resulting in temperature changes
• Exothermic reactions are accompanied by a temperature rise
• Exothermic reactions transfer heat energy from the chemicals to the surroundings
• Examples of exothermic reactions used in products

  • Self-heating cans (for coffee), hand warmers
• Endothermic reactions are accompanied by a fall in temperature
• Endothermic reactions transfer heat energy from the surroundings to the chemicals
• Examples of endothermic reactions used in products

  • Sports injury packs
• If a reversible reaction is exothermic in one direction, it is endothermic in the opposite direction
• The same amount of energy is transferred in each case for a reversible reaction
• Activation energy

  The minimum amount of energy that the particles must have for a reaction to take place
• Energy level diagrams

  • Show the energy changes in chemical reactions

  Exothermic: energy is given out to the surroundings, products have less energy than reactants

  Endothermic: energy is taken in from the surroundings, products have more energy than reactants
• Catalyst

  A substance that increases the rate of a chemical reaction without being used up in the process
• Catalysts reduce the activation energy needed for a reaction, making the reaction go faster
• Electrolysis

  The use of an electrical current to break down compounds containing ions into their constituent elements
• Electrolyte

  The substance being broken down during electrolysis
• Electrodes

  • Made from solids that conduct electricity
• During electrolysis


  1. Negatively charged ions move to the anode (positive electrode)

  2. Positively charged ions move to the cathode (negative electrode)
• Electrolysis can be used to separate ionic compounds into elements
• Electrolysis of lead bromide

  1. Lead bromide is heated until it melts

  2. Positively charged lead ions move to the negative electrode (cathode) and gain electrons to form lead atoms

  3. Negatively charged bromide ions move to the positive electrode (anode) and lose electrons to form bromine atoms, which join together to form bromine molecules
• 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
• Ionic substances can only conduct electricity when they are molten or dissolved in water
• Electrolysis of molten copper chloride

  1. Copper is deposited at the negative electrode

  2. Chlorine gas is given off at the positive electrode
• Electrodes used in electrolysis of molten ionic compounds

  • Must be inert so that they do not react with the electrolyte or the products
• Metals that are more reactive than carbon can be extracted from their ores using electrolysis
• Extraction of aluminium

  1. Aluminium oxide is mixed with cryolite (a compound of aluminium)

  2. Cryolite lowers the melting point of the aluminium oxide, meaning less energy is needed

  3. Aluminium forms at the negative electrode

  4. Oxygen gas forms at the positive carbon electrode and reacts with the carbon, forming carbon dioxide
• 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, otherwise the metal is produced

  3. At the positive electrode: Oxygen is produced unless the solution contains halide ions, in which case the halogen is produced
• In the electrolysis of sodium chloride solution, hydrogen is released at the negative electrode and chlorine gas is released at the positive electrode
• A low voltage must be used in electrolysis to prevent an electric shock, and the room must be well ventilated to prevent exposure to dangerous levels of gases