Rules of Thermochemistry

RULES OF THERMOCHEMISTRY

As you have learned, thermochemistry helps to describe the energy and heat necessary for a chemical reaction or physical change to occur. The absorption or release of energy may result in a phase change (from solid to liquid, from liquid to gas, etc.). Thermochemistry helps to explain whether or not a reaction will go spontaneously. The rules that govern thermochemistry include the following:

• The amount of enthalpy change will vary proportionally with the amount of substrate or product in the reaction.

• The amount of enthalpy change is the same for opposite reactions so only the sign of the enthalpy amount will change.

• The magnitude of enthalpy change does not depend on the number of steps required to achieve a given end product as long as the beginning substrate and end product are the same.

There is also the second rule of Lavoisier and Laplace, which states that processes that are endothermic have a positive enthalpy and processes that are exothermic have a negative enthalpy. The third rule or Hess’ law states that enthalpy is a state function and does not depend on the path of equation or equations.