A reaction gives out or takes in energy. A physical transformation also gives out or takes in energy. Thermochemistry looks at these energy changes, particularly on a system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities at all times during a given reaction. Thermochemists do this by using data, including entropy determinations. Thermochemists will tell if a reaction is spontaneous or non-spontaneous, favorable or unfavorable. 