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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:

Q = ΔU + W

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

H ≡ U + P ⋅ V

The change of enthalpy H during a process is:

ΔH = ΔU + Δ(P ⋅ V) = ΔU + ΔP ⋅ V + P ⋅ ΔV

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

ΔH = ΔU + P ⋅ ΔV

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:

Cv =

     at constant volume v
Cp =

     at constant pressure p

We will remember:

ΔU = Cv ΔT
ΔH = Cp ΔT

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

H = U + P V = U + R T


Cp =


∂ (R T)

= Cv + R

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.