Gibbs Free Energy Change (College Board AP® Chemistry): Study Guide

Gibbs Free Energy Change

• Gibbs free-energy change is the maximum amount of energy available from any chemical reaction 

  • It is represented by the symbol ΔG°

  • It is for chemical processes in which all the reactants and products are present in a standard state

  • This includes:

    • Pure substances

    • Solutions of 1.0 M 

    • Gases at a pressure of 1.0 atm (or 1.0 bar)

• When ΔG^° is negative, the reaction is thermodynamically favored and likely to occur

• When ΔG^° is positive, the reaction is not thermodynamically favored and unlikely to occur

• When a reaction is described as thermodynamically favorable it refers to whether the reaction takes place of its own accord

• This is an outcome of the second law of thermodynamics which states that any physical or chemical change must result in an increase in the entropy of the universe

• We can see examples of this all around us:

  • Cups fall off tables and spontaneously break into many pieces, never the other way around

  • Hot objects always cool and spread their heat into the surroundings, never the other way around

  • Earthquakes destroy buildings and create chaos and disorder

  • When living things die they decompose and change from complex ordered systems into disordered simple molecules

• The standard Gibbs free energy change for a physical or chemical process can be determined in two ways: 

  • The first is using standard Gibbs free energy of formation of the reactants and products

    • The equation used is:

      ΔG° = ΣΔG_f°_products - ΣΔG_f°_reactants

  • The second is by considering the following factors:

    • Enthalpy change

    • Entropy change

    • These come together to give the following equation:

      ΔG^° = ΔH^° - TΔS^°

  • The units of ΔG^° are in kJ mol^-1

  • The units of ΔH^° are in kJ mol^-1

  • The units of T are in K

  • The units of ΔS^° are in J K^-1 mol^-1(and must therefore be converted to kJ K^-1 mol^-1 by dividing by 1000