NEET Chem — Chemical Equilibrium Deep Dive

Chapter Overview & Weightage

Chemical Equilibrium is a perennial favourite for NEET. Direct questions appear every year, plus the concepts power Ionic Equilibrium and Electrochemistry. Total contribution: 8-12 marks across the paper.

Year	Direct Qs	Marks
2024	3	12
2023	2	8
2022	3	12
2021	2	8
2020	2	8

NEET Chemistry is heavily mark-weighted toward chapters with direct numerical or conceptual MCQs. Equilibrium fits the bill perfectly.

Key Concepts You Must Know

Reversible reactions: forward and reverse rates equal at equilibrium.
Equilibrium constant: $K_c$ (in concentrations), $K_p$ (in partial pressures).
Relation: $K_p = K_c (RT)^{\Delta n_g}$ .
Le Chatelier’s principle: equilibrium shifts to minimise applied stress.
Effects of concentration, pressure, temperature, catalysts on equilibrium.
Reaction quotient $Q$ : predicts direction of shift.
Coupled reactions: $\Delta G$ adds, $K$ multiplies.
Heterogeneous equilibria: pure solids and liquids excluded from $K$ .

Important Formulas

For reaction $a A + b B \rightleftharpoons c C + d D$ :

$K_c = \frac{[C]^c [D]^d}{[A]^a [B]^b}$

$K_p = K_c (RT)^{\Delta n_g}$ where $\Delta n_g = (c + d) - (a + b)$ (gaseous moles only).

$\Delta G° = -RT \ln K$

$\Delta G° < 0 \Rightarrow K > 1$ (forward favoured). $\Delta G° > 0 \Rightarrow K < 1$ (backward favoured).

Stress	Shift
Add reactant	Forward
Add product	Backward
Increase pressure	Toward fewer moles of gas
Increase $T$ (endo)	Forward
Increase $T$ (exo)	Backward
Catalyst	No shift, but reach equilibrium faster

Solved Previous Year Questions

PYQ 1 (NEET 2024)

For $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$ at equilibrium, $K_p$ at $400\text{ K}$ is $1.6 \times 10^{-3}$ . What is $K_c$ ?

$\Delta n_g = 2 - 4 = -2$ . $K_p = K_c (RT)^{\Delta n_g}$ , so $K_c = K_p / (RT)^{-2} = K_p \cdot (RT)^2$ .

$RT = 0.0821 \times 400 = 32.84$ .

$K_c = 1.6 \times 10^{-3} \times 32.84^2 = 1.6 \times 10^{-3} \times 1078.5 \approx 1.726$ .

PYQ 2 (NEET 2023)

Which of these favours the forward reaction in $\text{N}_2 + \text{O}_2 \rightleftharpoons 2\text{NO}$ , $\Delta H > 0$ ?

(a) Decrease temperature (b) Increase pressure (c) Increase temperature (d) Add a catalyst.

For endothermic, increase $T$ shifts forward. $\Delta n_g = 0$ , so pressure has no effect. Catalyst speeds equilibrium but does not shift it.

Answer: (c) Increase temperature.

PYQ 3 (NEET 2022)

At $500\text{ K}$ , $K_c = 4$ for $\text{H}_2 + \text{I}_2 \rightleftharpoons 2\text{HI}$ . If $1\text{ mol}$ each of $\text{H}_2$ and $\text{I}_2$ is taken in a $1\text{ L}$ vessel, find equilibrium concentration of HI.

Let $x$ moles dissociate. At equilibrium: $[\text{H}_2] = 1-x$ , $[\text{I}_2] = 1-x$ , $[\text{HI}] = 2x$ .

$K_c = (2x)^2 / (1-x)^2 = 4 \implies 2x/(1-x) = 2 \implies x = 1/2$ (taking positive root).

$[\text{HI}] = 2(1/2) = 1\text{ M}$ .

Difficulty Distribution

Sub-topic	Easy	Medium	Hard
Definitions	70%	25%	5%
$K_c$ vs $K_p$	30%	60%	10%
Le Chatelier	50%	40%	10%
ICE-table problems	20%	50%	30%

ICE-table problems are the hardest — be comfortable with quadratic factoring.

Expert Strategy

Always set up an ICE (Initial, Change, Equilibrium) table for unknowns. The discipline alone earns 1-2 marks in NEET subjective stages and saves errors.

For Le Chatelier, write down the sign of $\Delta H$ first. Forgetting whether the reaction is endo or exo is the most common error.

Memorise: increasing $T$ favours the endothermic direction; increasing pressure favours fewer moles of gas. Two rules cover most NEET MCQs.

Common Traps

Including pure solids/liquids in $K$ . They have constant activity (= 1) and do not appear. NEET MCQs love listing solids in the expression as a wrong option.

Using the wrong $\Delta n_g$ in $K_p = K_c (RT)^{\Delta n_g}$ . Count only gases — solids and liquids do not enter $\Delta n_g$ .

Thinking a catalyst shifts equilibrium. It does not — it just speeds up both directions equally, reaching equilibrium faster.