**Enthalpy, H,** is the heat contenet of a system. Enthalpy
can always be calculate by,

The change in enthalpy is reference to be the heat transfer at constant pressure. We can calculate enthalpy change this way for any system because enthalpy is a state function. More generally enthalpy change calculated in terms of internal energy change and a pressure-volume correction,

and setting PV = nRT we get,

Where delta n_{g} is the change in moles of gas (final - initial).

Lets calculated entropy changes for different type of processes.

**Phase transitions such as fusion and vaporization. e.g. entropy change for
vaporization would be,*

Most liquids have nearly the same molar entropy of vaporization. **Trouton's
rule** estimates that it is 83-93 J mol^{-1}K^{-1}. For freezing
we calculate entropy change in a similar way except we use the heat of fustion
and the freezing temperature.

**Reversible heating/cooling at constant V (reversible isochoric).* When a system cools, its entropy
decreases. The entropy change is negative (-) with a decrease in temperature and
positive (+) with an increase in temperature. We calculate isochoric entropy
change by,

**Reversible heating/cooling at constant P (reversible isobaric).* We
calculate entropy change for isobaric processes in a similar way to isochoric
processes.

**Reversible expansion/compression at constant T(isothermal).* The entropy
of the system increases with its size according to,

and since P is inversely related to V,

All these formulas assume ideal behavior.

Reversibility is an ideal concept. In genergal most real world processes are

- q

-w_{irrev} < -w_{rev}

The work done by the system irreversibly is less than work done by the system reversibly.

*In a irreversible process the total entropy of a system plus its
surrounding increase. delta S_{univ} > 0.

*
*The universe tends toward disorder or randomness. delta S cannot be
less than 0.*

The following is a list of things that increase or decrease entropy. 1) Gases have higher entropy than liquids, and liquids higher than solids.

2) Low pressures have higher entropy than high pressures.

3) Dilute solutions have higher entropy than concentrated solutions.

4) Dissolved substance have higher entropy then its undissolved form.

5) Entropy increases with increasing temperature.