Thermodynamics: Enthalpy

1. Mole
2. Gas Volume
3. States of Matter
4. Heat
5. Enthalpy 🢀
6. Thermodynamics
7. Adiabatic Process
8. Mass Energy Conservation
9. Carnot Engine

Definition of enthalpy

The first law of thermodynamics postulates the conservation of energy of a closed system. The law states that the heat Q added to a closed contributes to the increase ΔU of the inner energy and the work W done by the system on its surroundings. This balance is written as follows:

In typical thermodynamical systems, the most common type of work is pressure-volume (P ⋅ V) work. Let us define the enthalpy H of a system as

The change of enthalpy H during a process is:

For processes with constant pressure we have ΔP = 0, so:

Comparing equations (1) and (4) we see that ΔH = Q for processes at constant pressure, so the change in enthalpy is simply equal to the heat released/absorbed by the process.

Specific heat

Specific heats are defined as:

We will remember:

For ideal gas (1 mol) we have P V = R T, so:

and

Enthalpy in chemistry

Chemical reactions are thermodynamical processes that involve transfer of energy. They usually happen at constant atmospheric pressure of 1 atm. So we can apply the above equations.

• Endothermic reactions require an input of energy to proceed and are signified by a positive change in enthalpy ΔH > 0.
• Exothermic reactions release energy upon completion and are signified by a negative change in enthalpy ΔH < 0.