NEET ChemistryUpdated 19 Oct 2026

Thermodynamics: NEET PYQs and weightage

Thermodynamics carries 4.2 of 100 Chemistry marks in NEET by past-paper share: 30 of the 707 tagged NEET Chemistry PYQs in the ViJEEta bank (NEET 2013–2026). It ranks 9 of 26 Chemistry chapters.

Take the 25-question Thermodynamics test Formula sheet ↓All NEET Chemistry chapters

4.2/100 Weight in Chemistry

Exam importance (3/5, by past-paper share)

30 NEET PYQs

517 Practice questions

498 With measured difficulty

How the weight is computed: this chapter's tagged NEET PYQs divided by all tagged NEET Chemistry PYQs in the bank (30 of 707), times 100, with repeated questions counted once. The bank does not store the year of each NEET question, so none is printed. Computed from the bank at build time. Updated 19 Oct 2026.

Thermodynamics: formula sheet for NEET

14 formulas, each with when to use it. Copy it onto one page of your notebook: that page is this chapter's handbook.

First law
$Δ U = q + w$
Use it when: heat and work both cross the boundary. $q$ and $w$ are $+$ when energy goes into the system
Work against constant external pressure
$w = - p_{ex} Δ V = - p_{ex} (V_{f} - V_{i})$
Use it when: a gas expands or is compressed in one step against fixed $p_{ex}$ . Free expansion ( $p_{ex} = 0$ ) gives $w = 0$
Reversible isothermal work (ideal gas)
$w_{rev} = - n R T ln \frac{V _{f}}{V _{i}} = - 2.303 n R T lo g \frac{V _{f}}{V _{i}}$
Use it when: $T$ is constant and the change is reversible. Here $Δ U = 0$ , so $q = - w$
Heat at constant volume or pressure
$Δ U = q_{V}$
$Δ H = q_{p}$
Use it when: a bomb calorimeter (fixed $V$ ) gives $Δ U$ , and an open vessel (fixed $p$ ) gives $Δ H$
Enthalpy
$H = U + p V$
$Δ H = Δ U + p Δ V (at constant p)$
Use it when: you need the bridge between $H$ and $U$ for a constant-pressure change
ΔH and ΔU for gas reactions
$Δ H = Δ U + Δ n_{g} R T$
Use it when: converting bomb-calorimeter $Δ U$ to $Δ H$ . $Δ n_{g}$ = gaseous moles of products $-$ gaseous moles of reactants. Put $R = 8.314 \times 1 0^{- 3} kJ K^{- 1} mo l^{- 1}$ when $Δ H$ is in kJ
Heat from a temperature change
$q = C Δ T = c m Δ T$
Use it when: you find heat from a temperature rise. A calorimeter's heat is equal and opposite to the reaction's heat
Ideal gas heat capacities
$C_{p} - C_{V} = R (per mole)$
Use it when: you are given one molar heat capacity of an ideal gas and need the other
Reaction enthalpy from formation data
$Δ_{r} H^{\circ} = \sum a_{i} Δ_{f} H^{\circ} (products) - \sum b_{i} Δ_{f} H^{\circ} (reactants)$
Use it when: $Δ_{f} H^{\circ}$ values are given. $Δ_{f} H^{\circ}$ of an element in its reference state $= 0$
Hess's law
$Δ_{r} H = Δ_{r} H_{1} + Δ_{r} H_{2} + Δ_{r} H_{3} + \dots$
Use it when: you add steps to reach the target equation. Reversing an equation flips the sign of $Δ H$ , and multiplying it scales $Δ H$
Bond enthalpy route
$Δ_{r} H^{\circ} = \sum bond enthalpies (reactants) - \sum bond enthalpies (products)$
Use it when: all species are gases and $Δ_{f} H^{\circ}$ is not given. Note the order is reactants $-$ products
Entropy
$Δ S = \frac{q _{rev}}{T}$
$Δ S_{total} = Δ S_{sys} + Δ S_{surr} > 0 (for a spontaneous change)$
$Δ S_{surr} = - \frac{Δ H _{sys}}{T}$
Use it when: judging spontaneity by entropy, or finding $Δ S$ of a phase change at its transition temperature
Gibbs equation
$Δ G = Δ H - T Δ S (Δ G < 0 spontaneous, Δ G > 0 non-spontaneous, Δ G = 0 equilibrium)$
Use it when: at constant $T$ and $p$ . When $Δ H$ and $Δ S$ are both $+$ (or both $-$ ), the crossover temperature is $T = \frac{Δ H}{Δ S}$ . Use $T$ in kelvin and the same energy unit for $Δ H$ and $T Δ S$
Gibbs energy and K
$Δ_{r} G^{\circ} = - R T ln K = - 2.303 R T lo g K$
Use it when: converting between $Δ_{r} G^{\circ}$ and the equilibrium constant. $K > 1$ means $Δ_{r} G^{\circ} < 0$

Trap

Chemistry follows the IUPAC sign convention: work done on the system is +, so expansion work is w = −p_ex ΔV and ΔU = q + w. Physics books still use the opposite convention, where w is work done by the system and ΔU = q − w. Putting the chemistry w into ΔU = q − w reverses the sign of the work term, so stick to one convention throughout a problem.

How does NEET ask Thermodynamics?

8 real previous-year questions from this chapter, as printed in the paper, each with its worked solution. Answer the first one here with no sign-in.

Q1ChemistryNEET UG PYQGibb's Energy and Spontaneity

Try right here · No sign-in

For a given reaction, ΔH = 35.5 kJ mol

^{- 1}

and ΔS = 83.6 JK

^{- 1}

mol

^{- 1}

. The reaction is spontaneous at: (Assume that ΔH and ΔS do not vary with temperature)

Answer key (D) & worked solution

Correct answer: Option D

∵ Δ G = Δ H - T Δ S

For a reaction to be spontaneous, ΔG = −ve i.e., ΔH < TΔS

∴ T > \frac{Δ H}{Δ S} = \frac{35.5 \times 10 ^{3}}{83.6}

i.e.,

T > 425 K

Q2ChemistryNEET UG PYQEntropy and Spontaneity

For the reaction,

2 C l (g) \to C l_{2} (g),

the correct option is:

Δ_{r} H 0

Δ_{r} H < 0 an d Δ_{r} S < 0

Δ_{r} H > 0 an d Δ_{r} S > 0

Δ_{r} H > 0 an d Δ_{r} S < 0

Answer key (B) & worked solution

Correct answer: Option B

2 C l (g) \to C l_{2} (g) + H e a t

Due to bond formation stability increases which results in release of heat.

∴

Δ H_{r} = -

ve or exothermic process \[\Delta {H_{\mathbf{r}}}{\mathbf{

Q3ChemistryNEET UG PYQGibb's Energy and Spontaneity

For the reaction: X

_{2}

_{4}

( l ) → 2 XO

_{2}

( g ) ΔU = 2.1kcal, ΔS = 20cal K− 1 at 300 K, Hence, ΔG is

A9.3 k cal

B-9.3 k cal

C2.7 k cal

D-2.7 k cal

Answer key (D) & worked solution

Correct answer: Option D

ΔH = ΔU + Δn

_{g}

RT = 2.1 × 1000 + 2 × 2 × 300 = + 3300 cal ΔG = ΔH − TΔS = 3300 − 300(20) = 3300 − 300 × 20 = -2700 cal = -2.7 k cal

Q4ChemistryNEET UG PYQBond Dissociation Enthalpy

The standard enthalpy of formation of NH

_{3}

is -46kJ mol

^{- 1}

. If the enthalpy of formation of H

_{2}

from it's atoms is -436kJ mol

^{- 1}

and that of N

_{2}

is -712 kJ mol

^{- 1}

, the average bond enthalpy of N − H bond in NH

_{3}

is:

A-964 kJ mol

^{- 1}

B+352 kJ mol

^{- 1}

C+1056 kJ mol

^{- 1}

D-1102 kJ mol

^{- 1}

Answer key (B) & worked solution

Correct answer: Option B

\frac{1}{2} N_{2 (g)} + \frac{3}{2} H_{2}_{(g)} \to N H_{3 (g)} Δ H_{f}^{0} = - 46 k j m o l^{- 1}

2 H_{(g)} \to H_{2}_{(g)} Δ H_{f}^{0} = - 436 k j m o l^{- 1}

2 N_{(g)} \to N_{2}_{(g)} Δ H_{f}^{0} = - 712 k j m o l^{- 1}

- 46 = [- \frac{1}{2} \times 712 - \frac{3}{2} \times 436] - 3 N - H

3N − H = 1056 kJ mol − 1 N − H = 352 kJ mol –

^{1}

Q5ChemistryNEET UG PYQEntropy and Spontaneity

In which case change in entropy is negative?

AExpansion of a gas at constant temperature.

BSublimation of solid to gas

C2H(g) → H

_{2}

(g)

DEvaporation of water

Answer key (C) & worked solution

Correct answer: Option C

H_{2} O (l) ⇌ H_{2} O (v), Δ S > 0

* Expansion of gas at constant temperature, ∆S > 0 * Sublimation of solid to gas, ∆S > 0 *

2 H (g) \to H_{2} (g), Δ S < 0 (∵ Δ n_{g} < 0)

Q6ChemistryNEET UG PYQGibb's Energy and Spontaneity

The correct thermodynamic conditions for the spontaneous reaction at all temperature is:

AΔH < 0 and ΔS < 0

BΔH < 0 and ΔS = 0

CΔH > 0 and ΔS < 0

DΔH 0

Answer key (D) & worked solution

Correct answer: Option D

ΔG = ΔH – TΔS For spontaneous process (ΔG = −ve) at all temperature, ΔH 0 are favourable factors ΔG = ΔH − TΔS = (−ve) − T(+ve)

Q7ChemistryNEET UG PYQEntropy and Spontaneity

In which of the following processes entropy increases? A. A liquid evaporates to vapour. B. Temperature of a crystalline solid lowered from 130 K to 0 K C. 2 NaHCO

_{3}

(s) → Na

_{2}

_{3}

(s) + CO

_{2}

(g) + H

_{2}

O(g) D. Cl

_{2}

(g) → 2Cl(g) Choose the correct answer from the options given below:

AA, B and D

BA, C and D

CC and D

DA and C

Answer key (B) & worked solution

Correct answer: Option B

A. l → vap. Entropy in c B. temperature decreases entropy decreases C. 2 NaHCO

_{3}

(s) → Na

_{2}

_{3}

(s) + CO

_{2}

(g) + H

_{2}

O(g) More number of moles of gas on product side →entropy increases D. Cl

_{2}

(g) → 2Cl(g) Entropy increases Option (b) is correct

Q8ChemistryNEET UG PYQFirst Law of Thermodynamics

At a certain temperature,

T (K)

, during a process, 500 J is absorbed by the system and work of 200 J is done by the system. Then change in internal energy of the system is :

A700 J

B300 J

C400 J

D500 J

Answer key (B) & worked solution

Correct answer: Option B

From first law of thermodynamics

Δ U q w Δ U = q + w = + 500 J = - 200 J = 500 - 200 = 300 J

Which topics in Thermodynamics carry the most questions?

The 12 busiest of 24 topics, by tagged PYQs, then practice questions. Counted from the bank. Updated 19 Oct 2026.

Topic	PYQs	Practice questions
First Law of Thermodynamics	7	84
Entropy and Spontaneity	6	61
Gibb's Energy and Spontaneity	5	53
Reversible and Irreversible Work	3	33
Enthalpy of Reactions/processes	2	64
Bond Dissociation Enthalpy	2	32
Concept of Heat and Work	2	19
Relationship Between ΔH and ΔU	1	24
Types of Processes	1	13
Enthalpy of a System	1	11
Internal Energy	0	27
Introduction to Thermodynamics	0	27

Why does Thermodynamics show "–" until you answer 3 questions?

Unknown is not zero. Until you answer at least 3 graded questions in Thermodynamics, ViJEEta shows – instead of a zero or a 50% default. Book exercises can raise chapter mastery up to 0.60, adaptive practice up to 0.75, and only a timed chapter test or mock can certify 0.80 and above.

How ViJEEta scores you

What happens when you test Thermodynamics?

Every chapter test is 25 questions. It starts at moderate difficulty and adapts after each saved answer. If you open a solution before answering, that attempt earns no credit, so your mastery stays honest.

Start the Thermodynamics test

Chapters either side of Thermodynamics by weight

d- and f-Block Elements Weight 4.4/100 Electrochemistry Weight 4.2/100 Amines Weight 4.1/100 Biomolecules Weight 4.1/100

Thermodynamics: NEET PYQs and weightage

Thermodynamics: formula sheet for NEET

First law

Work against constant external pressure

Reversible isothermal work (ideal gas)

Heat at constant volume or pressure

Enthalpy

ΔH and ΔU for gas reactions

Heat from a temperature change

Ideal gas heat capacities

Reaction enthalpy from formation data

Hess's law

Bond enthalpy route

Entropy

Gibbs equation

Gibbs energy and K

How does NEET ask Thermodynamics?

Which topics in Thermodynamics carry the most questions?

Why does Thermodynamics show "–" until you answer 3 questions?

What happens when you test Thermodynamics?

Chapters either side of Thermodynamics by weight