Energy changes

• Energy changes


  During a chemical reaction, energy transfers occur
• Energy can be transferred


  • To the surroundings - exothermic
  • From the surroundings - endothermic
• Energy transfer


  Can cause a temperature change
• Energy is always conserved in chemical reactions
• There is the same amount of energy in the Universe at the start of a chemical reaction as at the end of the chemical reaction
• Surroundings


  Everything that isn't the reaction
• Thermometer records an increase in temperature

  Reaction in the test tube is exothermic
• Thermometer records a decrease in temperature


  Reaction in the test tube is endothermic
• Summary of exothermic and endothermic reactions

  • Exothermic: Energy transfer to the surroundings, temperature increases
  • Endothermic: Energy transfer from the surroundings, temperature decreases
• Exothermic reactions


  • Oxidation, combustion, neutralisation
• Endothermic reactions


  • Thermal decomposition, citric acid and sodium hydrogen carbonate
• Everyday use of exothermic/endothermic reactions

  • Self-heating cans
  • Hand warmers
  • Sports injury packs
• Bonds


  • More energy released when making bonds than required to break bonds - exothermic
  • Less energy released when making bonds than required to break bonds - endothermic
• Energy is always conserved in chemical reactions
• There is the same amount of energy in the Universe at the start of a chemical reaction as at the end of the chemical reaction
• Reaction profile


  Shows whether a reaction is exothermic or endothermic
• Activation energy


  The minimum amount of energy that particles must have to react when they collide
• Exothermic reaction


  1. Activation energy

  2. Reactants

  3. Energy released

  4. Products
• Endothermic reaction


  1. Activation energy

  2. Reactants

  3. Energy absorbed

  4. Products
• Atoms are held together by strong chemical bonds
• Breaking a bond


  Requires energy, so is endothermic
• Making a bond


  Releases energy, so is exothermic
• If a lot of energy is released when making the bonds and only a little energy is required to break them


  The reaction as a whole is exothermic
• If a little energy is released when making the bonds and a lot is required to break them


  The reaction as a whole is endothermic
• Bond calculations


  1. Work out how much energy is required to break all the bonds in the reactants

  2. Work out how much energy is released when making all the bonds in the products

  3. Overall energy transferred = energy required to break bonds - energy required to make bonds
• A positive number means an endothermic reaction
• A negative number means an exothermic reaction
• Endothermic


  Requiring energy from the surroundings to proceed
• Surroundings


  The environment or area around something
• Temperature of the surroundings

  The measure of the average kinetic energy of the particles in the environment or area around something
• Thermal decomposition


  The breaking down of a chemical compound into simpler substances by the action of heat
• Warmers


  Devices that provide heat
• Bonds


  The attractive forces that hold atoms together in a molecule
• Less energy released


  When making bonds than required to break bonds
• Decreases


  Sports injury
• Citric acid and sodium hydrogen carbonate

  Chemicals used in packs
• Chemical cells


  Devices that convert chemical energy into electrical energy
• Metal displacement reaction


  1. One metal is oxidised - it loses electrons

  2. Electrons are transferred to another metal, which gains the electrons and so is reduced

  3. Electron's movement generates a current
• Cell shown


  • Zinc atoms from the electrode lose electrons, turn into ions, and move into the solution
  • Electrons travel through the circuit to the copper electrode, causing the LED to light up
  • Metal ion from the solution will pick the electrons up and become a metal atom
• Potential difference


  The greater the difference in reactivity between the two metals in the cell, the greater the potential difference produced