- 1). Determine the change in enthalpy, which is a measure of a system's total energy. This value is symbolized as H for enthalpy (in joules) or ?H for change in enthalpy. In a closed system at constant pressure, this value is zero. The values for the enthalpy of formation for any common chemical has been measured and cataloged by chemists. Check the resources section for a link containing tables of enthalpies. Subtract the enthalpy of the reactants from the enthalpy of the products to calculate ?H.
- 2). Determine the temperature of the system. This can either be measured with a thermometer or will be provided if you are solving a theoretical problem. Make sure your units are in kelvins.
- 3). Determine the change in entropy of the system. Subtract the entropy of the entropy of the reactants from the entropy of the products. This value is ?S and is measured in joules.
- 4). Calculate Gibbs free energy. The equation is ?G = ?H - T?S for systems at constant temperature. Multiply the temperature, in kelvins, by the change in entropy and subtract this value from the change in enthalpy. The answer is the Gibbs free energy of the reaction.
- 5). If ?G is less than zero, then the products are thermodynamically favored, and the reaction will proceed forward spontaneously. If ?G is greater than zero, the reactants are thermodynamically favored, and it will require the input of energy to cause the reaction to proceed forward. If ?G is zero, neither products nor reactants are favored, and the system is in equilibrium.
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