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NEET Chemical Thermodynamics: 5 practice MCQs with answers

By the PrepMatic team · Updated 9 Oct 2026 · 4 min read

Chemical Thermodynamics asks how much heat a reaction releases or absorbs, and whether it can happen at all. The five MCQs below test the link between enthalpy and internal energy, the temperature at which a reaction becomes spontaneous, Hess's law, entropy changes and standard enthalpy of formation, at NCERT Class 11 level. The Physics chapter on heat engines is separate: see NEET Thermodynamics (Physics).

What this chapter covers in the NEET syllabus

The NMC unit is Chemical Thermodynamics, taught as a Class 11 NCERT chapter. The topics are:

• System, surroundings, state functions, and the types of processes.
• The first law: internal energy, work, heat and enthalpy; heat capacity.
• Enthalpy changes of formation, combustion, atomisation, bond dissociation, solution and phase changes.
• Hess's law of constant heat summation.
• The second law: spontaneity, entropy and Gibbs energy, and the link between ΔG° and the equilibrium constant.
• The third law, in outline.

Five practice MCQs on Chemical Thermodynamics

Question 1

For which of the following reactions (all species are gases) is ΔH equal to ΔU?

• A. N₂ + 3H₂ → 2NH₃
• B. PCl₅ → PCl₃ + Cl₂
• C. H₂ + I₂ → 2HI
• D. 2SO₂ + O₂ → 2SO₃

Question 2

For a reaction ΔH = +40 kJ/mol and ΔS = +100 J/(mol K). Assuming both stay constant, the reaction becomes spontaneous above:

• A. 400 K
• B. 300 K
• C. 100 K
• D. 4000 K

Question 3

Given C(graphite) + O₂ → CO₂, ΔH = −393.5 kJ, and CO + ½O₂ → CO₂, ΔH = −283.0 kJ, the enthalpy change for C(graphite) + ½O₂ → CO is:

• A. −676.5 kJ
• B. −110.5 kJ
• C. +110.5 kJ
• D. −283.0 kJ

Question 4

Which of the following processes shows a decrease in the entropy of the system?

• A. Melting of ice
• B. Evaporation of water
• C. Dissolution of sugar in water
• D. Condensation of steam to water

Question 5

Which of the following has a standard enthalpy of formation of zero at 298 K?

• A. Br₂(l)
• B. C(diamond)
• C. Br₂(g)
• D. O₃(g)

Answers and explanations

1. C. ΔH = ΔU + Δng RT, so ΔH = ΔU when the number of moles of gas does not change. For H₂ + I₂ → 2HI, Δng = 2 − 2 = 0. The others have Δng of −2, +1 and −1.
2. A. ΔG = ΔH − TΔS. The reaction turns spontaneous when ΔG is negative, i.e. T is greater than ΔH/ΔS = 40000 J/100 J K⁻¹ = 400 K. Convert kJ to J before dividing.
3. B. Subtract the second equation from the first: C + O₂ − (CO + ½O₂) gives C + ½O₂ → CO. So ΔH = −393.5 − (−283.0) = −110.5 kJ. Option A wrongly adds the two values.
4. D. Condensation turns a high-entropy gas into a lower-entropy liquid, so ΔS of the system is negative. Melting, evaporation and dissolving all increase disorder.
5. A. The standard enthalpy of formation is zero for an element in its most stable form at 298 K and 1 bar. For bromine this is the liquid, Br₂(l). Diamond is not the stable form of carbon (graphite is), and Br₂(g) and O₃(g) are not the stable forms of their elements.

How to prepare Chemical Thermodynamics

• Stick to the NCERT sign convention. Work and heat are positive when added to the system, so ΔU = q + w, and work done on the system by compression is positive. This differs from the Physics chapter, where work done by the gas is positive. Note which one the question follows.
• Master Hess's law by algebra. Write the target reaction, then add, subtract or multiply the given equations (and their ΔH values) until it matches. Check that every species cancels as it should.
• Use ΔH = ΔU + Δng RT and ΔG = ΔH − TΔS together. The four sign combinations of ΔH and ΔS give always spontaneous, never spontaneous, or temperature-dependent cases. Draw this as a 2 × 2 table.
• Know the standard states and definitions. Enthalpy of formation, combustion and bond dissociation each refer to a precise reaction. A common question type asks which substance has zero enthalpy of formation.
• Remember ΔG° = −RT ln K and ΔG° = −nFE°cell, which link this chapter to equilibrium and electrochemistry.

A short daily set of five problems, with Hess's law cycles drawn out, is better than long cramming. See the NEET Chemistry preparation guide for how physical chemistry fits your plan, and the NEET syllabus for the unit list.

Take a full Chemical Thermodynamics test on PrepMatic

Take a topic test on PrepMatic (about 15 questions in 30 minutes) after you have worked through a set of Hess's law and Gibbs energy problems by hand. The score uses NEET's negative marking, and the free Basic plan includes topic tests. Related chapters are on the practice hub.

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