Entropy Change

For a reversible process, the entropy change of a system is defined by the heat transferred in a tiny reversible step divided by the absolute temperature at which that step occurs, so for a constant-temperature reversible transfer this becomes ΔS = Q/T. This works because entropy measures how heat is dispersed among microscopic states, and dividing by Kelvin temperature captures the fact that the same amount of heat causes a smaller entropy change at higher temperature. The formula is only valid directly for reversible heat transfer; for irreversible processes you must compute ΔS from a reversible path between the same states.

Use this when you know the heat exchanged reversibly at a fixed temperature, such as isothermal expansion/compression, phase changes at equilibrium, or idealized heat-transfer steps in thermodynamic cycles. It is also useful as a building block for finding total entropy change by summing many small reversible contributions.

• Using Celsius instead of Kelvin for temperature
• Applying ΔS = Q/T to an irreversible process without using a reversible path
• Forgetting that the sign of Q matters: heat absorbed gives positive ΔS, heat released gives negative ΔS