Chemical Equilibrium for JEE: Kp, Kc, Le Chatelier's Principle Complete Guide

Q: How does temperature affect K?

K changes with temperature. For exothermic reactions (ΔH 0): K increases as T increases. This follows from ΔG° = -RT ln K and ΔG° = ΔH° - TΔS°.

The direct answer

Master the core concepts and apply them consistently to JEE PYQs. This guide covers everything you need for Chemical Equilibrium — from fundamentals to exam strategy.

Section 1

Equilibrium Constants

aA + bB ⇌ cC + dD: Kc = [C]^c[D]^d/[A]^a[B]^b. Kp = (p_C^c·p_D^d)/(p_A^a·p_B^b). Kp = Kc(RT)^(Δn). Δn = (c+d) - (a+b). Pure solids and liquids are excluded from K expressions.

Section 2

Le Chatelier's Principle

System at equilibrium responds to stress to minimize it. Temperature: increase T shifts toward endothermic direction. Pressure: increase P shifts toward fewer gas molecules. Concentration: increase reactant shifts forward. Catalyst: no effect on equilibrium position.

Section 3

Degree of Dissociation

α = moles dissociated/initial moles. For PCl₅ ⇌ PCl₃ + Cl₂ with initial moles n: at equilibrium: n(1-α), nα, nα. Total moles = n(1+α). Kp = (α²P)/(1-α²) for this reaction.

Section 4

ICE Table Method

I: initial, C: change, E: equilibrium. For A ⇌ B: I = [A]₀, 0. C = -x, +x. E = [A]₀-x, x. Substitute into Kc expression and solve for x. Useful for all equilibrium problems.

Section 5

Relationship Between K and ΔG

ΔG° = -RT ln K. If K > 1: ΔG° < 0, products favored. If K < 1: ΔG° > 0, reactants favored. If K = 1: ΔG° = 0, at equilibrium. K is temperature dependent.

Section 6

PYQ Patterns

JEE asks: Kp/Kc conversion, Le Chatelier applications, degree of dissociation, ICE table problems, simultaneous equilibria, and pressure/density based questions.

Rapid recall

Memory Tricks for jee chemical equilibrium guide

🧠 Key memory techniques:

1. Create mnemonics for formulas and sequences

2. Use active recall — test yourself, don't just re-read

3. Review at spaced intervals: 1, 3, 7, 14, 30 days

4. Write formulas from memory every morning

5. Connect new concepts to what you already know

6. Use the Feynman technique — explain concepts aloud

Avoid these

Common JEE Mistakes in jee chemical equilibrium guide

❌ Skipping NCERT basics and jumping to advanced problems

❌ Not practicing PYQs regularly

❌ Ignoring error log analysis after mocks

❌ Memorizing without understanding the concepts

❌ Not revising at spaced intervals

❌ Spending too much time on one topic and neglecting others

Your action plan

Next Steps to Master jee chemical equilibrium guide

1

Today

Read this guide thoroughly and create your own notes. Write down all key formulas and concepts.

2

This Week

Solve 20+ PYQs on this topic. Categorize every error in your error log.

3

This Month

Take a full mock test. Analyze results. Revise weak areas using this guide.

4

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Common doubts answered

Frequently Asked Questions

What is the relationship between Kp and Kc?

Kp = Kc(RT)^(Δn), where Δn = (moles of gaseous products) - (moles of gaseous reactants). If Δn = 0 (no change in gas moles): Kp = Kc. If Δn > 0: Kp > Kc. If Δn < 0: Kp < Kc.

How does temperature affect K?

K changes with temperature. For exothermic reactions (ΔH < 0): K decreases as T increases. For endothermic reactions (ΔH > 0): K increases as T increases. This follows from ΔG° = -RT ln K and ΔG° = ΔH° - TΔS°.

What is the effect of a catalyst?

A catalyst increases both forward and reverse rates equally. It does not change the equilibrium position or K value. It only helps reach equilibrium faster.

What is the difference between Kc and Kp?

Kc uses molar concentrations, Kp uses partial pressures. Relationship: Kp = Kc(RT)^(Δn), where Δn = gas moles products - gas moles reactants. When Δn = 0, Kp = Kc.

What is the reaction quotient Q?

Q has the same expression as K but uses non-equilibrium concentrations. Compare Q with K: Q < K: reaction proceeds forward. Q > K: reaction proceeds backward. Q = K: at equilibrium.

What is the degree of dissociation?

Degree of dissociation α = (moles dissociated)/(initial moles). For weak acid HA: α = √(Ka/C) for dilute solutions. For PCl₅: α is related to Kp by Kp = α²P/(1-α²).

How do pressure changes affect equilibrium?

Increasing pressure shifts equilibrium toward fewer gas molecules (Le Chatelier). If Δn_g = 0, pressure has no effect. Inert gas at constant volume: no effect. Inert gas at constant pressure: shifts toward more gas molecules.

What is the effect of adding an inert gas?

At constant volume: no effect on equilibrium (partial pressures unchanged). At constant pressure: volume increases, equilibrium shifts toward more moles of gas.

What is the difference between homogeneous and heterogeneous equilibrium?

Homogeneous: all reactants and products in same phase (N₂ + 3H₂ ⇌ 2NH₃, all gases). Heterogeneous: different phases present (CaCO₃(s) ⇌ CaO(s) + CO₂(g)). For heterogeneous, pure solids/liquids excluded from K.

What is the equilibrium constant for reversed reaction?

If a reaction is reversed, K_new = 1/K_original. If the reaction is multiplied by n, K_new = K^n. If two reactions are added, K_total = K₁ × K₂.

What is the relationship between ΔG and equilibrium?

At equilibrium, ΔG = 0. Also, ΔG = ΔG° + RT ln Q. At equilibrium: ΔG° = -RT ln K. ΔG° < 0: K > 1 (products favored). ΔG° > 0: K < 1 (reactants favored).

What is the ionic product of water?

Kw = [H⁺][OH⁻] = 10⁻¹⁴ at 25°C. pH + pOH = 14. For a weak acid: pH = ½(pKa - log C). For a weak base: pOH = ½(pKb - log C).

What is the solubility product?

Ksp is the equilibrium constant for dissolution of a sparingly soluble salt. AgCl(s) ⇌ Ag⁺ + Cl⁻: Ksp = [Ag⁺][Cl⁻] = 1.77 × 10⁻¹⁰. Compare Qsp with Ksp: Qsp > Ksp: precipitation. Qsp < Ksp: no precipitation.

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