NEET UG · Chemistry · Thermodynamics

Enthalpy and Hess's Law: NEET previous year questions

4 questions in ten years of NEET, in 3 of 10 papers. Pattern: Most years. Here is how it is asked and which ideas come back.

Asked in 10 years: 4; about 0.4 per paper
Years asked: 3/10; last asked 2025
Chance of a question next paper: 50%; historical rate, how we compute it
Weighted estimate: 0.5; model; history: 0.4 a paper on average, typically 0–1

Key findings

01
Asked in 3 of 10 papers, 4 questions in all: #161 of 204 topics by ten-year total. It comes back every 3.5 years on average. Last asked in 2025.
02
100% of its questions since 2018 tested an idea never asked before; none repeated an idea from the year before.
03
No idea in this topic has been asked twice: every question tested something new.
04
NEET has asked 4 of the 70 ideas in this topic; 62 have never been asked, directly or indirectly.
05
Mostly asked as numerical (50%); 50% easy, 25% hard, and up from 0.3 to 0.7 questions a paper since 2024.

Questions per year

Up to 2023: 0.3; per paper
Since 2024: 0.7; +0.4
Usual gap: 3.5 yrs; between papers asking it
Swing: ±0.7; questions, year to year

How it is asked

Difficulty, format and skill

Across all 4 questions on Enthalpy and Hess's Law since 2017.

Difficulty

QUESTIONS

211

Easy2 · 50%
Medium1 · 25%
Hard1 · 25%

Format

Numerical · 2 Q50%
Figure-based · 1 Q25%
One-line MCQ · 1 Q25%

Skill tested

Calculation · 2 Q50%
Data or graph · 1 Q25%
Recall · 1 Q25%

Repeats

New idea, or one asked before?

Each column is a paper. Blue questions tested an idea NEET had not asked before; navy ones went back to an idea from an earlier paper. 2017 is the first paper, so everything in it counts as new.

’17’18’19’20’21’22’23’24’25’26

Idea asked for the first time
Idea asked in an earlier paper

Coverage of the topic

NEET has asked 4 of the 70 ideas in this topic. 62 have never been asked yet.

Asked directly4 · 6%
Touched inside another question4 · 6%
Never asked62 · 89%

Ideas come from Lumi’s syllabus map of NCERT. A never-asked idea is not a safe skip: most new questions come from exactly these.

Drill down

Idea, year, question

Every idea NEET tested in this topic, with its ten papers. Open one to read the questions.

ideaAskedShareYears’17’18’19’20’21’22’23’24’25’26

1 · 25%25%1/10
- 2018 · Q85
  The bond dissociation energies of $\mathrm{X_2}$ , $\mathrm{Y_2}$ and XY are in the ratio of 1 : 0.5 : 1. $\Delta H$ for the formation of…
  hardNumericalNew idea
1 · 25%25%1/10
1 · 25%25%1/10
1 · 25%25%1/10

Squares are the ten papers, 2017 to 2026; darker means more questions. Share is of the level above.

Ideas

Every idea inside this topic

4 different ideas were tested; none came more than once. Hess's Law Application is the foundation for 15 other ideas in the syllabus.

Hess's Law Application1×
numerical
Numerical · 6% next · foundation for 15
Reaction Enthalpy from Energy Profiles1×
graph interpretation of reaction enthalpy
Figure-based · 6% next
Relation Between dH and dU1×
relation between thermodynamic quantities
One-line MCQ · 4% next · foundation for 7
Bond Enthalpy Calculations1×
numerical
Numerical · 4% next · foundation for 7

Idea	Times	Format	Chance next	Foundation for
Hess's Law Applicationnumerical	1	NumericalCalculation	6%	15 ideas
Reaction Enthalpy from Energy Profilesgraph interpretation of reaction enthalpy	1	Figure-basedData or graph	6%	–
Relation Between dH and dUrelation between thermodynamic quantities	1	One-line MCQRecall	4%	7 ideas
Bond Enthalpy Calculationsnumerical	1	NumericalCalculation	4%	7 ideas

Chance next is the historical rate at which an idea with this record came back in the following paper. It is low for every idea, which is why learning the topic beats memorising past questions. All repeated ideas

Every question

Enthalpy and Hess's Law questions, year by year

The opening words of each question as it appeared in the official paper, with the idea it tested.

NEET 20252 questions

Q65
$C(s) + 2H_2(g) \to CH_4(g)$ ; $\Delta H = -74.8$ kJ mol $^{-1}$ Which of the following diagrams gives an accurate representation of the…
Idea: Reaction Enthalpy from Energy Profiles
New ideaEasyFigure-based · figure
Q85
The standard heat of formation, in kcal/mol of $Ba^{2+}$ is : [Given : standard heat of formation of $SO_4^{2-}$ ion (aq) = – 216…
Idea: Hess's Law Application
New ideaMediumNumerical

NEET 20231 question

Q88
Which amongst the following options is the correct relation between change in enthalpy and change in internal energy?
Idea: Relation Between dH and dU
New ideaEasyOne-line MCQ

NEET 20181 question

Q85
The bond dissociation energies of $\mathrm{X_2}$ , $\mathrm{Y_2}$ and XY are in the ratio of 1 : 0.5 : 1. $\Delta H$ for the formation of…
Idea: Bond Enthalpy Calculations
New ideaHardNumerical

Questions from the official NEET UG papers published by NTA (CBSE before 2019). Topic and idea tags, counts and notes are Lumi’s.