Thermochemistry

The mathematical representation for thermal energy contains many terms. We can, however, simplify it based on how we perform the experiments. Most chemical reactions take place under atmospheric pressure, which is (for the most part) constant. When a thermal energy change is measured under constant pressure conditions, it is called a change in "enthalpy." The symbol for enthalpy is H. Since only a difference in enthalpy can be measured, the difference is called "delta H." When describing different kinds of changes, we can indicate the difference as part of the name by using subscripts after the H. However, reactions can be done under different pressure and temperature conditions. For the sake of uniformity, a standard state is defined as the state of a substance at one atmosphere pressure. For solids and liquids, the standard state is the pure solid or liquid at one atmosphere pressure. For gases, the standard state is the ideal gas at a partial pressure of one atmosphere. Once the standard state is defined, some simplifications can be made. The enthalpy of formation of an element in its standard state is zero. If there is more than one form of the element under the defined conditions, the most stable form is given an enthalpy of formation of zero. Carbon, for example, has two forms at one atmosphere, graphite and diamond. Graphite is more stable and is assigned an enthalpy of formation of zero. Diamond does not have an enthalpy of formation of zero.

Enthalpy has a special property. Its value is determined based on the initial state of the system and the final state of the system. It does not depend on how the system gets from the initial state to the final state. A function that has this property is called a "state function." The fact that enthalpy is a state function makes it possible to calculate enthalpies for some compounds without having to measure them experimentally. By combining different reactions with known enthalpies, it is possible to calculate the unknown enthalpy. Hess' Law summarizes this observation by stating that the thermal energy absorbed or released in a change is the same whether the change occurs in a single step or in multiple steps.

Those enthalpies that cannot be calculated using Hess' law can be measured experimentally. An apparatus called a calorimeter is used to measure the quantity of thermal energy gained or lost in a chemical change. A simple calorimeter can be constructed using two nested styrofoam cups with lids and a thermometer. A more complex type of calorimeter is the bomb calorimeter, which measures thermal energy changes under constant volume conditions.