Enthalpy is a measure of total energy in a system. This is typically in the form of heat, but also a form of volume and pressure. Since enthalpy is a measure of the state of a system, it does not change at equilibrium. That is why we look at the change in enthalpy of a system from one state to another. The state of the system has to change in order for the enthalpy to change. This normally happens when work or energy is transferred to a system, typically through heat. Enthalpy changes in both endothermic or exothermic reactions. An endothermic reaction is an act of absorbing energy to change states, and an exothermic reaction is an act of releasing energy or heat. The change in enthalpy will be positive for endothermic and negative for exothermic.
The formula to calculate the change in enthalpy (ΔH) is:
\[ \Delta H = (Q_2 - Q_1) + p \cdot (V_2 - V_1) \]
Where:
Let's say the initial internal energy (Q1) is 500 J, the final internal energy (Q2) is 800 J, the initial volume (V1) is 0.1 m3, the final volume (V2) is 0.2 m3, and the pressure (p) is 1000 Pa. Using the formula:
\[ \Delta H = (800 - 500) + 1000 \cdot (0.2 - 0.1) = 300 + 100 = 400 \text{ J} \]
So, the change in enthalpy (ΔH) is 400 J.