3.2.1 - Enthalpy changes

• Activation energy

  Minimum energy required for a reaction to take place
• Average Bond Enthalpy

  Energy required to break one mole of gaseous bonds.
• Enthalpy

  Value that represents the heat content of a system
• Enthalpy Change of Combustion

  Enthalpy change of one mole of substance when completely combusted in excess of oxygen.
• Enthalpy change of formation

  Enthalpy Change associated with the formation of one mole of a substance from its elements.
• Enthalpy change of neutralisation

  Enthalphy change associated with the formation of one mole of water during a neutralisation reaction.
• Enthalphy change of reaction

  Enthalpy change associated with a particular chemical equation.
• Hess’ Law

  The enthalpy change of a reaction is independent of the route it takes.
• Standard conditions

  100 kPa, 298K
• What does a Catalyst do?

  Speed up chemical reactions without being used up in the process
• How does a Catalyst work?

  Lowers activation energy by providing an alternative route
• Homogeneous catalyst

  Catalyst is in the same state as the reactants
• Heterogenous catalyst

  Catalyst is in different state as the reactants.
• Standard states

  Physical states under standard conditions
• Catalysts have economic importance and benefits for increased sustainability.


  Lowers temperature

  • Reduces energy demand from combustion of fossil fuels
  • Reduces CO2 emissions
• Dynamic equilibrium

  Exists in a closed system when the rate of the forward reactions is equal to the rate of the reverse reaction and the concentrations of reactants and products do not change.
• Catalyst increases the rate of both forward and reverse reactions in an equilibrium by the same amount.

  • Results in an unchanged position of equilibrium.
• In the chemical industry, compromises must be made between chemical equilibrium and reaction rate when deciding operational conditions. 


  Balancing effects of equilibrium, rate, safety and economics.