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.

- 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.
- Next, determine the constant pressure. For this we will assume this value to be 5 Pa.
- Next, determine the initial and final volumes. In this case that is 10 and 5 respectively.
- Finally, calculate the change in Enthalpy. (25-5) + 5 * (10-5) = 45.

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