Entropy (S) is a measure of the number of ways that the energy in a system can be stored in different arrangements of the molecules or atoms in their available states. Systems that have a larger number of allowed states have higher entropy.
Thus for the same substance S°(gas) > S°(liquid) > S°(solid)
Gases have a larger number of accessible energy states due to their greater freedom of movement. Similarly the freedom of movement is greater for the particles in liquids than in solids.
Thus the sign of the entropy change for a process depends on the physical states of the reactants and products.
The third law of thermodynamics states that the entropy of a perfect crystal approaches zero when the temperature in kelvin approaches zero.
Thus for the same substance S°(gas) > S°(liquid) > S°(solid)
Gases have a larger number of accessible energy states due to their greater freedom of movement. Similarly the freedom of movement is greater for the particles in liquids than in solids.
Thus the sign of the entropy change for a process depends on the physical states of the reactants and products.
- If more gases are formed as products, ΔS is positive.
- If gases or liquids are converted to solids, ΔS is negative.
The third law of thermodynamics states that the entropy of a perfect crystal approaches zero when the temperature in kelvin approaches zero.
This means absolute entropies (S°) are always positive (including those for elements).
Absolute entropies are usually reported in J K–1 mol–1 because they are are small relative to enthalpies of formation.
The entropy change (ΔrS°) for a reaction can be calculated by substituting the absolute entropies into the relationship below. Absolute entropies are usually reported in J K–1 mol–1 because they are are small relative to enthalpies of formation.
ΔrS° = ΣΔS°(products) - ΣΔS°(reactants)
where each absolute entropy is multiplied by its coefficient in the balanced equation
where each absolute entropy is multiplied by its coefficient in the balanced equation