Enthalpy, Entropy, 2nd Law, and 3rd Law

Enthalpy, Entropy, 2nd Law, and 3rd Law

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 ng is the change in moles of gas (final - initial).

Entropy, S, is a state function and is a measure of disorder or randomness. A positive (+) entropy change means an increase in disorder. The universe tends toward increased entropy. All spontaneous change occurs with an increase in entropy of the universe.

The sum of the entropy change for the system and the surrounding must be positive(+) for a spontaneous process.

Lets calculated entropy changes for different type of processes.

Reversibility is an ideal concept. In genergal most real world processes are irreversible. The work and the heat transfer (because they are path dependent) will differ between reversible and irreversible processes. The following summarizes the differences:

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

The Second Law states that

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.

The Third Law sets the zero of entropy for a pure perfect crystal of a substance at 0 K. We can then reference this zero of entropy can calculate standard state entropy changes. Some of these values are listed in Appendix D of Oxtoby. We calculate the standard state entropy changes as we did with standard state enthalpy changes.