9.2 Absolute Entropy and Entropy Change

Absolute Entropy is simply the entropy of a system relative to if it were at absolute zero, which is defined as having no entropy. For a chemical reaction, we can find its change in entropy just like how we would find its change in enthalpy using heats of formation, by taking the sum of all the products and subtracting the sum of all reactants. In this case, ${\Delta S^\circ_{\text{reaction}}}$ is the entropy change for the reaction, ${\sum S^\circ_{\text{products}}}$ is the sum of the standard entropies of the products, and ${\sum S^\circ_{\text{reactants}}}$ reactants is the sum of the standard entropies of the reactants.