energetics

• Exothermic reaction

  Chemical reaction that transfers energy to the surroundings, increasing the temperature of the surroundings
• Exothermic reactions

  • Combustion
  • Many oxidisation reactions
  • Neutralisation
• Everyday examples of exothermic reactions

  • Self-heating cans (e.g. for coffee)
  • Hand warmers
• Endothermic reaction

  Chemical reaction that takes in energy from the surroundings, decreasing the temperature of the surroundings
• Examples of endothermic reactions

  • Thermal decomposition
  • Reaction of citric acid and sodium hydrogencarbonate
• Sports injury packs based on

  • Endothermic reactions
• Salts dissolving in water can be either exothermic or endothermic
• Neutralisation reaction is exothermic
• Displacement is an exothermic or endothermic reaction
• Combustion is an exothermic reaction
• Calculating heat energy change from temperature change

  Q = mc∆T
• ΔE

  Energy supplied by water (joules)
• m

  Mass of water (grams)
• c

  Specific heat capacity of water (4.2 J/g/°C)
• ΔT

  Change in temperature of the water (°C)
• Calculating enthalpy change (ΔH)

  ΔH = -mc∆T
• Enthalpy change is commonly given per mole, in kilojoules per mole
• Calculating molar enthalpy change (ΔH)

  Q/kJ divide by moles to get ΔH/kJ/mol
• Energy level diagram

  Can show the energy of reactants compared to products in exothermic and endothermic reactions
• Exothermic reaction

  Reactants have more energy than products, energy is released to surroundings
• Endothermic reaction

  Reactants have less energy than products, energy is taken in from surroundings
• Bond-breaking is an endothermic process and bond-making is an exothermic process
• Using bond energies to calculate enthalpy change

  Add bond energies of reactants (energy in)

  2. Add bond energies of products (energy out)

  3. Calculate energy change: energy in - energy out
• Energy needed to BREAK bonds > energy RELEASED when bonds are formed (endothermic)
• Energy needed to BREAK bonds < energy RELEASED when bonds are formed (exothermic)
• Practical: investigate temperature changes accompanying dissolving salts in water, neutralisation reactions, displacement reactions, combustion reactions