Thermochemistry is the branch of chemistry that deals with the heat of reactions. In an introductory chemistry course, thermochemistry will focus mainly on the heat evolved or absorbed in chemical reactions. A student may be asked to calculate the amount of heat evolved when two elements form a covalent bond. They may be asked to determine the specific heat capacity of a metal using information from a calorimeter or they may be asked to use Hess’ Law to determine the overall heat of a reaction. In order to learn these concepts, the student must have a basic understanding of enthalpy, and that is where I would like to begin.
For the beginning chemist, the change in energy of a system can be approximated as the change in enthalpy. The change in enthalpy can be measured as either the heat gained or heat lost in a system. Enthalpy or (H) is measured in Calories or Joules. For most of my problems, we will be using kJ (kiloJoules) or J (Joules).
A glass of hot water has a certain amount of energy or heat. After some time, the water cools. The energy that existed in the form of heat did not disappear, it just transferred to the area surrounding the glass. Energy is never completely lost, rather heat is transferred. This is an example of a system that is exothermic and the enthalpy change of the system will have a negative value.
When ice melts, heat is absorbed from the surrounding environment. Heat is added to the water, increasing the enthalpy of the system. This is an example of a system that is endothermic. Whenever heat is added to a system, the enthalpy value will reflect this change by being positive.