Question

What is the difference between enthalpy and entropy?

Answer 1

Almost... everything. Letter, units... everything except that they're state functions and expressions of energy.

Enthalpy:

`color(blue)(DeltaH) = DeltaU + Delta(PV)`

`= q + w + Delta(PV)`

`= q cancel(- PDeltaV + PDeltaV) + VDeltaP + DeltaPDeltaV`

`= color(blue)(q + VDeltaP + DeltaPDeltaV)`

where `U` is internal energy, `q` is heat flow, `w` is expansion/compression work, and of course, `P` is pressure and `V` is volume.

The units for the generic enthalpy are usually in `"kJ"`.

This makes enthalpy the heat flow in a closed system, plus the amount of expansion/compression work done via a change in pressure.

Entropy:

`color(blue)(DeltaS >= q/T)`

`color(blue)(DeltaS = (q_"rev")/T)`

The units for the generic entropy are usually in `"J/K"`.

This makes entropy equal to the amount of reversible heat flow possible per unit temperature.