# Enthalpy Formula

The following formula is used to calculate the total energy change, also known as a change in Enthalpy, contained within a system. The formula is used specifically for constant pressure situations.

## Enthalpy Definition

Enthalpy is defined as the total energy change of a closed system. Most often this energy is measured in the form of heat, pressure, and volume.

Enthalpy is almost always analyzed as a change in enthalpy, since the singular value of Enthalpy does not hold much value in thermodynamic problems.

## Example Problem

The following is an example of calculating the change in Enthalpy of a system.

1. First, determine the initial and final internal energies from heat of the system. For this value we will say this is 25J and 5J respectively.
2. Next, determine the constant pressure. For this we will assume this value to be 5 Pa.
3. Next, determine the initial and final volumes. In this case that is 10 and 5 respectively.
4. Finally, calculate the change in Enthalpy. (25-5) + 5 * (10-5) = 45.