Reactions

• Exothermic reactions

  Reactions where the total chemical energy of the products is less than the total chemical energy of the reactants, so energy is released to the surroundings
• Endothermic reactions


  Reactions where the total chemical energy of the products is greater than the total chemical energy of the reactants, so energy is absorbed from the surroundings
• Exothermic reaction


  1. Energy released

  2. Reaction system loses energy

  3. Reaction is exothermic
• Exothermic reactions


  • Production of water from hydrogen and oxygen
  • Combustion of methane
• Endothermic reaction


  1. Energy absorbed

  2. Reaction system gains energy

  3. Reaction is endothermic
• In an endothermic reaction, the container may feel cold to the touch as the reaction system is absorbing heat from the surroundings, leaving the environment cooler
• The energy gained by the system in an endothermic reaction is the 'unobservable' stored chemical energy that cannot be experienced or directly measured
• Endothermic reaction equation

  Energy written on reactant side of equation arrow
• If total chemical energy of products is less than total energy of reactants


  Energy will be released from the system into the surroundings (exothermic reaction)
• If total chemical energy of products is greater than total energy of reactants

  Energy will be absorbed from the surroundings (endothermic reaction)
• Changes of state (phase changes)

  • Physical changes, not chemical changes
  • Involve energy being absorbed or released
• Endothermic phase changes


  • Melting of ice
  • Boiling of water
• Exothermic phase changes


  • Condensing a gas to a liquid
  • Freezing a liquid to form a solid