Thermochemical Equations

Table of Contents
Thermodynamics Gateway Page
In this module:
Hess' Law
GOALS: When you have completed this module, you should be able to:
  • Write thermochemical equations using data obtained from a calorimeter
  • Use thermochemical equations to relate the masses of reactants and products to the amount of enthalpy released or absorbed by a system
  • Use Hess' Law and standard enthalpies of formation to predict the enthalpy changes of unknown reactions

As you have seen in the previous two modules, many (actually, nearly all) changes in the world involve some exchange of energy. Scientists have devised a concise way of describing both the change that takes place and the change in energy or some other thermodynamic quantity that accompanies that change. This shorthand is called a thermochemical equation .

H2O (s) H2O (l)      DH = 6.00 kJ

A thermochemical equation has two parts: a balanced chemical equation and the change in one or more thermodynamic quantities (e.g., temperature, energy, or enthalpy) that occurs when that change occurs. The balanced equation can describe either a physical change (as in the example shown) or a chemical change. In the example above, the equation states that when one mole of solid water melts into liquid water at 0 ºC, the enthalpy of the system increases by 6.00 kJ (the reactants and products in a thermochemical equation will almost always be the system in a thermodynamics problem). This module will show you how thermochemical equations work and how a relatively small number of special thermochemical equations allow you to predict the outcomes of many other reactions.