Free energy

• some reactions happen, others do not
• a reaction can happen if the products have a lower overall energy than the reactants
• feasibility is used to describe whether a reaction is able to happen and is energetically feasible
• Free energy change - the overall change in energy during a chemical reaction - made up of 2 types of energy:

  • the enthalpy change, delta H - the heat transfer between the chemical system and the surroundings
  • the entropy change at the temperature of the reaction, TdeltaS - the dispersal of energy within the chemical system itself
• The Gibb's equation:

  deltaG = deltaH- TdeltaS
• conditions for feasibility:

  • the feasibility of a reaction depends on the balance between deltaH and TdeltaS in Gibb's equation
  • for a reaction to be feasible there must be a decrease in free energy
  • deltaG is less than 0
• the value for deltaH is usually much larger than for deltaS and often dominates the Gibb's equation
• deltaH - KJmol-1

  deltaS - J K-1 MOL-1
• In the Gibb's equation, the value and units of deltaS must be changes to KJK-1mol-1 by dividing by 1000 - this then matches the KJ in deltaH