Entropy (S) is a measure of the number of different ways that a system can store energy.
ΔS for phase changes and warming of solids, liquids and gases can be calculated using relationships based on the equation given at the right.
qrev is the heat exchange in a reversible process.
Examples of reversible processes are phase changes (such as
heating of a substance over a temperature range (provided the temperature changes in in very small steps).
The Third Law of Thermodynamics states that the absolute entropy of any pure crystalline substance at 0 K is 0.
Thus the absolute entropy (S°) of a substance at temperature T can be calculated by summing the entropy changes for the processes occurring on warming the substance from absolute zero to T.
If the absolute entropies for all of the substances in a process are known, the entropy change for a process can be calculated from these.
Entropy is related to probability as it is determined by the number of possible arrangements for a system.
Systems which have more arrangements available to them have higher entropy.
Thus S(gas) > S(liquid) > S(solid)
Systems which have more arrangements available to them have higher entropy.
Thus S(gas) > S(liquid) > S(solid)
| ΔS = | qrev |
| T |
qrev is the heat exchange in a reversible process.
Examples of reversible processes are phase changes (such as
solid 
liquid
) and liquid heating of a substance over a temperature range (provided the temperature changes in in very small steps).
The Third Law of Thermodynamics states that the absolute entropy of any pure crystalline substance at 0 K is 0.
Thus the absolute entropy (S°) of a substance at temperature T can be calculated by summing the entropy changes for the processes occurring on warming the substance from absolute zero to T.
If the absolute entropies for all of the substances in a process are known, the entropy change for a process can be calculated from these.
ΔS° =
where n is the coefficient for that substance in the balanced equation
ΣnS°(products) - ΣnS°(reactants)
where n is the coefficient for that substance in the balanced equation