What Is Standard Entropy?

According to the third law of thermodynamics, at absolute zero (0K), the entropy value of a perfect crystal of any pure substance is zero, that is, S _m (perfect crystal, 0K) = 0. Now that S _m (perfect crystal, 0K) = 0 can be determined, it is possible to obtain the entropy of the substance in other states, that is, the specified entropy of the substance. At standard pressure and temperature, the specified entropy of a pure substance is called the standard entropy of the substance at T, and the symbol is S (T). At temperature T, the standard entropy of 1 mol pure substance is called the standard molar entropy of the substance at T, the symbol is S _m (T), the unit is J · mol ^-1 · k ^-1 or kJ · mol ^-1 · k ^-1 .

The third law of thermodynamics emphasizes pure matter and perfection The limits of crystals. This is because if the substance is not pure, impurities mixed in the substance will increase the chaos of the entire system, thereby increasing the entropy value of the substance. A perfect crystal means that there is only one way to arrange the particles in the crystal. There may be several arrangements of particles in the crystal of some substances (disordered arrangement). Such disordered arrangement will also increase the degree of chaos in the system, which will increase the entropy value, leading to the entropy value of this crystal at 0K Not zero. ^[1]

Entropy is

Defined by the standard entropy of matter, the following rules can be obtained:

1. For the same substance, the entropy in the gaseous state is greater than that in the liquid state, and the entropy in the liquid state is greater than that in the solid state, that is, S (g)> S (l)> S (s).

2. When the same substance is in the same aggregation state, its entropy value increases with increasing temperature, that is, S _{high temperature} > S _{low temperature} .

3. In general, when the temperature and the aggregation state are the same, the substance with a more complex molecular or crystal structure is larger than the substance with a _{simpler molecular} structure, that is, S _{complex molecule} > S _{simple molecule} .

4. The entropy value of a mixture or solution is often larger than the entropy value of the corresponding pure substance, that is, S _mixture > S _{pure substance} .
Using these rules, we can draw a useful rule for qualitatively determining the entropy change of a process. For physical or chemical changes, there are almost no exceptions. A process leading to an increase in the number of gas molecules increases the entropy value of the reaction, that is, S> 0. If the number of gas molecules decreases, S <0. ^[2]

For example, the following reactions S are all positive values:

CaCO ₃ (s) CaO (s) + CO ₂ (g)

C (graphite) + 1 / 2O ₂ (g) CO (g)