How Do You Calculate Delta G?

Delta G, or Gibbs free energy, is calculated by the equation ?G = ?H - T?S. ?G describes the change in free energy and is the difference between the change in enthalpy (?H) and the product of temperature and change in entropy (T?S). It is also worth noting that if ?G is positive, then the reaction is non-spontaneous, but if ?G is negative, then it is spontaneous.

  1. Find ?H

    ?H is the change in enthalpy and is usually given for every element in the reaction. To find ?H, multiply the enthalpy of the element by the amount of moles in the reaction. Repeat this process for every element, then sum the enthalpies of each side of the reaction. Subtract the reactant's enthalpy from the product to yield the overall enthalpy.

  2. Calculate T?S

    T?S is the product of temperature and the change in entropy. Keep in mind that temperature must always be measured in Kelvin. ?S is calculated in the same way as ?H. Once ?S is found, multiply it by the temperature measured in Kelvin.

  3. Simplify the terms

    Since T?S has already been simplified into one term, all that is left is to subtract this term from ?H. The result is ?G, or Gibbs free energy.