Absolute Entropy and Entropy Change

Absolute entropy (S) refers to the entropy of a one mole of a substance at a standard state. It includes entropy changes occurring during phase transitions and reactions at standard conditions.
It is a thermodynamic quantity, extensively used in chemical thermodynamics to predict whether a chemical process or reaction will occur spontaneously.
The entropy of a pure crystalline substance at absolute zero is defined as zero in the third law of thermodynamics. This provides a reference point for calculating entropy changes.
Entropy values are always positive. The more disordered a system is, the higher its entropy.
Entropy is measured in joules per mole Kelvin (J/mol K) in the SI unit system.

The entropy change (∆S) is the measure of the degree of randomness or disorder of a system from the initial to final state in a chemical reaction.
It’s calculated as the entropy of the final state minus the entropy of the initial state, ∆S = S(final) - S(initial).
A positive entropy change (∆S > 0) indicates an increase in randomness or disorder in the system and is associated with endothermic reactions.
A negative entropy change (∆S < 0) indicates a decrease in randomness or disorder which is associated with exothermic reactions.
Entropy change can be calculated using the equation, ∆S = q_rev/T, where q_rev is the heat absorbed or released in a reversible process and T is the absolute temperature.

Increasing the temperature normally increases the entropy of a system because it provides molecules with more energy to move.
Changes in physical states can also affect entropy. The entropy increases from solid to liquid and liquid to gas.
Dissolution can increase entropy, especially if a solution form is more disordered than the reactants.
An increase in the number of gas molecules during a reaction often results in a positive entropy change.

The entropy change of the universe (∆Suniverse) must be equal to or greater than zero for any process to be spontaneous based on the second law of thermodynamics.
The ∆Suniverse is the sum of the entropy change of the system (∆Ssystem) and the entropy change of the surroundings (∆Ssurroundings).
A reaction will be spontaneous if the ∆Suniverse is positive, and the reaction will not be spontaneous if the ∆Suniverse is negative.
Most real-world reactions are irreversible and result in an increase in the overall entropy of the universe because entropy is a measure of the dispersal of energy.