Chemistry · Thermodynamics · NEET
Entropy is a number that tells you how spread out or disordered the energy and particles of a system are. A neat, ordered solid has LOW entropy. A gas whose molecules move everywhere has HIGH entropy. More randomness = more entropy. Its unit is J K⁻¹ mol⁻¹. For NEET, remember that anything that increases randomness (melting, boiling, mixing, more gas molecules) increases entropy.
It says that for any spontaneous process, the total entropy of the universe increases. Total means system PLUS surroundings: ΔS(total) = ΔS(system) + ΔS(surroundings) > 0. The system alone can lose entropy (for example, water freezing), but only if the surroundings gain even MORE entropy. NEET loves the phrase 'entropy of the universe is always increasing.'
ΔS is POSITIVE when disorder goes up: solid→liquid (melting), liquid→gas (boiling/evaporation), solid→gas (sublimation), or when the number of gas molecules increases. ΔS is NEGATIVE when disorder goes down: gas→liquid, liquid→solid (freezing), cooling toward 0 K, or when gas molecules combine into fewer molecules. Quick rule: count gas moles — if gas moles go UP, ΔS is positive.
Two separate gas atoms have more ways to move around than one combined molecule. Going from 2 moles of gas to 1 mole of gas means Δn(gas) = 1 − 2 = −1, so the number of free particles drops and disorder decreases. That makes ΔS negative. This exact reaction was the NEET 2019 answer for 'entropy change is negative.'
For an isothermal (constant T) change of an ideal gas: ΔS = nR ln(V_f / V_i). Since pV = constant at fixed T, V is inversely related to p, so you can also write ΔS = nR ln(p_i / p_f). When the gas expands (V_f > V_i, or p_f < p_i), ΔS is positive. This is the basis of the NEET 2016 and ReNEET 2026 questions.
Entropy is a STATE FUNCTION. It depends only on the initial and final states, not on the path taken. That is why in free expansion you can still calculate ΔS(system) using ΔS = nR ln(V₂/V₁), even though the process is irreversible. Contrast this with heat (q) and work (w), which are path functions.
In which case is the change in entropy negative?
In which of the following processes does entropy increase? A. A liquid evaporates to vapour. B. Temperature of a crystalline solid is lowered from 130 K to 0 K. C. 2NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(g). D. Cl₂(g) → 2Cl(g). Choose the correct option.
For a sample of a perfect gas when its pressure is changed isothermally from p_i to p_f, the entropy change is given by:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The SI unit of entropy is joule per kelvin (J K⁻¹). For molar entropy it is J K⁻¹ mol⁻¹. Notice it has kelvin in the denominator because entropy relates to heat divided by temperature (ΔS = q(rev)/T).
No. Only the entropy of the UNIVERSE (system + surroundings) always increases in a spontaneous process. The system alone can lose entropy, for example when water freezes, as long as the surroundings gain more.
A process is spontaneous when the total entropy of the universe increases (ΔS(total) > 0). This idea leads to the Gibbs free energy equation ΔG = ΔH − TΔS, which is a more convenient way to test spontaneity using only the system's properties.
Entropy change is defined using the heat exchanged in a REVERSIBLE path, because reversible heat transfer is the maximum and gives a unique, path-independent value. Since entropy is a state function, you use the reversible path to calculate it even for an irreversible process.
According to the Third Law of Thermodynamics, the entropy of a perfect crystalline substance is zero at 0 K. This is because a perfect crystal at absolute zero has only one possible arrangement, meaning zero disorder.
