• The?feasibility?of a reaction can be affected by the?temperature
• The Gibbs equation will be used to explain what will affect the feasibility of a reaction for exothermic and endothermic reactions

Exothermic reactions

• In exothermic reactions, ΔH_reaction^??is?negative
• If the ΔS_system^??is?positive:
  • Both the first and second term will be?negative
  • Resulting in a?negative?ΔG^??so the reaction is?feasible
  • Therefore, regardless of the temperature, an exothermic reaction with a positive ΔS_system^??will?always be feasible
• If the ΔS_system^??is?negative:
  • The first term is?negative?and the second term is?positive
  • At high temperatures, the -TΔS_system^??will be very?large?and?positive?and will overcome ΔH_reaction^?
  • Therefore, at?high temperatures?ΔG^??is?positive?and the reaction is not feasible
  • The reaction is more?feasible?at low temperatures, as the second term will not be large enough to overcome ΔH_reaction^??resulting in a negative ΔG^?
• This corresponds to Le Chatelier’s principle which states that for?exothermic reactions?an increase in temperature will cause the equilibrium to shift position in favour of the reactants, i.e. in the endothermic direction
  • In other words, for exothermic reactions, the products will?not be formed?at high temperatures
  • The reaction is?not feasible?at high temperatures

The diagram shows under which conditions exothermic reactions are feasible

Endothermic reactions

• In endothermic reactions, ΔH_reaction^??is?positive
• If the ΔS_system^??is?negative:
  • Both the first and second term will be?positive
  • Resulting in a?positive?ΔG^??so the reaction is?not feasible
  • Therefore, regardless of the temperature, endothermic with a negative ΔS_system^??will?never be feasible
• If the ΔS_system^??is?positive:
  • The first term is?positive?and the second term is?negative
  • At low temperatures, the -TΔS_system^??will be?small?and?negative?and will not overcome the larger ΔH_reaction^?
  • Therefore, at low temperatures ΔG^??is?positive?and the reaction is less feasible
  • The reaction is?more?feasible?at?high temperatures?as the second term will become negative enough to overcome the ΔH_reaction^??resulting in a negative ΔG^?
• This again corresponds to Le Chatelier’s principle which states that for?endothermic reactions?an increase in temperature will cause the equilibrium to shift position in favour of the products
  • In other words, for endothermic reactions, the products will?be formed?at high temperatures
  • The reaction is therefore?feasible

The diagram shows under which conditions endothermic reactions are feasible