- Gas Volume
- States of Matter
- Enthalpy 🢀
- Adiabatic Process
- Mass Energy Conservation
- 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 heats are defined as:
We will remember:
For ideal gas (1 mol) we have P V = R T, so:
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.