AimMent
Exam level9 past questions

Entropy and Its Changes

Understand entropy as a measure of disorder, how it changes in physical and chemical processes, and its calculation in various scenarios including phase transitions and isothermal processes.

Why this shows up in the exam

Entropy is central to predicting spontaneity and is a frequent topic in both NEET and JEE.

How NEET tests this

Application · 5 QsStatement analysis · 3 QsNumerical · 3 Qs

Learn the idea

Entropy measures the dispersal of energy or the degree of disorder of a system; the key insight is that for any reversible change ΔS equals the heat exchanged divided by temperature, and for an ideal gas at constant temperature the entropy change depends only on the ratio of initial and final pressures (or volumes).

🧠 Memory hook: Pressure down, entropy up – when a gas expands (pressure falls) its disorder rises, so remember the phrase ‘low pressure, high mess’.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • ΔS = q_rev/T for a reversible process (NCERT Sec 5.2)
  • For an ideal gas at constant T, ΔS = nR ln(V_f/V_i)=nR ln(p_i/p_f)
  • During a phase change at equilibrium ΔS = ΔH_trans/T
  • Entropy is a state function – its change depends only on initial and final states, not the path
  • For an isolated system ΔS_total ≥ 0; equality holds for a reversible process

How to approach it

  1. 1Identify the type of process (isothermal, isobaric, phase change, etc.)
  2. 2Write the appropriate entropy formula from the key ideas
  3. 3Substitute the given data, keeping careful track of the pressure/volume ratio or ΔH/T
  4. 4Check the sign: entropy increases when the system gains heat or expands, decreases otherwise

Worked example — watch it click

For the isothermal expansion of a perfect gas from an initial pressure pᵢ to a final pressure p_f, the entropy change ΔS is given by

  • A)ΔS = nR ln (p_f / pᵢ)
  • ✅ΔS = nR ln (pᵢ / p_f)
  • C)ΔS = nRT ln (p_f / pᵢ)
  • D)ΔS = RT ln (pᵢ / p_f)

The concept behind this problem

The worked example asks for the entropy change of an isothermal expansion of an ideal gas; it tests the student’s ability to apply the state‑function formula ΔS = nR ln(p_i/p_f) and to choose the correct sign based on pressure decrease.

Step by step

  1. 1For isothermal reversible vaporization at equilibrium, ΔS = nR ln(P_f/P_i) where P_f > P_i for liquid→vapor.
  2. 2However, the image shows options (a) and (b) with different forms, and the extraction doesn't match the visible notation exactly.
  3. 3The Clausius-Clapeyron relation gives d ln P/dT = ΔH_v/RT², but the question asks about entropy change directly.

Watch out

Students often write ΔS = nR ln(p_f/p_i), reversing the ratio and getting a negative value instead of the correct positive entropy change.

Common slip-ups that cost marks

  • •Confusing ΔS = nR ln(p_f/p_i) with the correct nR ln(p_i/p_f) for isothermal expansion
  • •Using q/T for irreversible processes – the formula applies only to reversible heat transfer
  • •Mixing up ΔH of vaporisation with ΔS without dividing by the temperature

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 9

For a sample of perfect gas when its pressure changed isothermally from p₁ to pₓ the entro change is given by

Push further

More challenging

43 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 43

A solid melts at 350 K with a molar enthalpy of fusion of 12.25 kJ mol⁻¹. What is the molar entropy of fusion (in J K⁻¹ mol⁻¹) for this process?

More from Thermodynamics