Thermodynamic Favorability (College Board AP® Chemistry) : Study Guide

Thermodynamic Favorability

• Gibbs free energy provides an effective way of focusing on a reaction system at constant temperature and pressure to determine whether the reaction is thermodynamically favorable 

• A thermodynamically favorable reaction is one that occurs without the need for additional energy input after the reactants are mixed

  • A thermodynamically favored reaction will have a negative ΔG^o value 

• For a reaction to be spontaneous, Gibbs free energy must be a negative value (ΔG^o ≤ 0)

Thermodynamic Favorability & Temperature Changes

• The temperature conditions for a process to be thermodynamically favored (whereby ΔG^o is negative and less than 0) can be predicted from the signs of ΔH^o and ΔS^o

• If ΔH^o is negative (exothermic reaction) and ΔS^o is positive:

  • This will result in a negative  ΔG^o  

  • The reaction is always thermodynamically favored

  • No calculation of ΔG^o  would be necessary to determine this

• If ΔH^o is negative (exothermic reaction) and ΔS^o is negative:

  • At high temperatures -TΔS^o will be very large and positive and will overcome ΔH^o

  • At high temperatures ΔG^o is positive

  • The reaction is thermodynamically favored at low temperatures

ΔG^o and Exothermic Reactions

If ΔH^o is negative and If ΔS^o is negative the reaction will only be thermodynamically favored at low temperatures

• If ΔH^o is positive (endothermic reaction) and ΔS^o is negative:

  • Both ΔH^o and -TΔS^o will be positive

  • This results in a positive ΔG^o 

  • Therefore, regardless of the temperature, endothermic with a negative ΔS^o is not thermodynamically favored

  • No calculation of ΔG^o is necessary to determine that the process is thermodynamically unfavored

• If ΔH^o is positive (endothermic reaction) and ΔS^o is positive
  

  •  ΔH^o is positive and -TΔS^o will be negative

  • At low temperatures -TΔS^o cannot overcome the larger ΔH^o

  • At low temperatures, ΔG^o is positive and the reaction is thermodynamically favored 

  • At higher temperatures, the second term will become negative enough to overcome the ΔH resulting in a negative ΔG^o

ΔG^o and Endothermic Reactions

If ΔH^o is positive and If ΔS^o is positive the reaction will only be thermodynamically favored at a high temperatures

Summary table showing the relationship between ΔH^o , ΔS^o and ΔG^o