Class 11 Chemistry Chapter 6 Equilibrium Notes PDF

These equilibrium class 11 notes gather every equilibrium law, constant, and pH formula that the CBSE Boards, JEE Main, JEE Advanced, NEET and CUET papers actually test in 2026-27. Revise physical and chemical equilibrium, K_c and K_p, Le Chatelier's principle, and the full ionic equilibrium block in one place.

This is one of the most scoring chapters in the Class 11 syllabus, and its ionic equilibrium half feeds straight into Class 12 electrochemistry and salt analysis.

• CBSE Weightage: 7 to 9 marks, usually one K_c/K_p or pH numerical plus one Le Chatelier short answer.
• Topics covered: physical and chemical equilibrium, law of mass action, K_c and K_p, reaction quotient, Le Chatelier's principle, acids and bases, K_a, K_b, pH, buffers, common ion effect, K_sp, and salt hydrolysis.
• Key formulas: K_p = K_c(RT)^Δn, pH = -log[H⁺], the Henderson equation, and K_sp from solubility.

These equilibrium class 11 notes are curated by subject experts, based on the 2026-27 NCERT textbook, and checked against the last five years of CBSE Board, JEE Main and NEET papers.

Topic-by-Topic Summary of Equilibrium

The chapter has two clear halves. The first builds physical and chemical equilibrium and the equilibrium constant, and the second applies the same idea to acids, bases, and sparingly soluble salts in water. Here is the quick map of what each topic gives you.

• Physical and chemical equilibrium: the dynamic state where forward and reverse rates become equal.
• Law of mass action and K_c: the rule that fixes the equilibrium constant from active masses.
• K_p and the reaction quotient: the constant in terms of pressure, and the test for reaction direction.
• Le Chatelier's principle: how concentration, pressure, and temperature shift the position of equilibrium.
• Acids, bases and pH: the Arrhenius, Bronsted-Lowry and Lewis views, plus K_a, K_b and pH.
• Buffers, common ion effect and K_sp: ionic equilibrium in real solutions, including salt hydrolysis.

Revise the two halves in order, because the ionic half is just the equilibrium constant applied to water. Master K_c and Le Chatelier first, and the entire pH block becomes easy. These equilibrium class 11 notes follow the same sequence as the NCERT textbook.

Physical and Chemical Equilibrium and the Law of Mass Action

Equilibrium is a dynamic state, not a stopped reaction. In a closed system, the forward and reverse reactions keep running, but at equal rates, so the amounts of reactants and products stop changing. This idea holds for both physical changes and chemical reactions.

• Physical equilibrium: a phase change in balance, such as water in equilibrium with its vapour in a closed flask.
• Chemical equilibrium: a reversible reaction where forward and reverse rates are equal, such as N₂ + 3H₂ ↔ 2NH₃.
• Dynamic nature: both reactions continue at the molecular level even though nothing changes on the outside.
• Characteristics: equilibrium is reached only in a closed system and can be approached from either side.

The law of mass action says the rate of a reaction is proportional to the product of the active masses of the reactants. Applying it to both directions and setting the rates equal gives the equilibrium constant. For a A + b B ↔ c C + d D, the constant is K_c = [C]^c[D]^d / [A]^a[B]^b, with every concentration measured at equilibrium.

Equilibrium Constants: K_c, K_p and Their Relation

For gaseous reactions, concentration can be replaced by partial pressure. This gives a second equilibrium constant, K_p, written from the partial pressures of the gases. K_c and K_p describe the same equilibrium and are linked by a simple gas-law relation.

• K_c: the equilibrium constant in terms of molar concentrations of reactants and products.
• K_p: the equilibrium constant in terms of partial pressures, used only for gaseous species.
• Relation: K_p = K_c(RT)^Δn, where Δn is moles of gaseous products minus moles of gaseous reactants.
• Units: K depends on Δn, so it has units unless the total moles of gas stay the same.

When Δn = 0, K_p equals K_c and both are unitless, as in H₂ + I₂ ↔ 2HI. A large K (much greater than 1) means the products dominate at equilibrium, and a small K means the reactants dominate. Pure solids and pure liquids are left out of the K expression because their active mass is taken as one. Keep the K_c-K_p relation at the centre, since most numericals start from it.

Reaction Quotient Q and Le Chatelier's Principle

The reaction quotient Q uses the same expression as K, but with concentrations at any moment, not just at equilibrium. Comparing Q with K tells you which way the reaction must move. Le Chatelier's principle then predicts how a disturbance shifts the equilibrium.

• Q < K: too few products, so the forward reaction runs until Q rises to K.
• Q = K: the system is at equilibrium and no net change happens.
• Q > K: too many products, so the reverse reaction runs until Q falls to K.

Le Chatelier's principle states that when a system at equilibrium is disturbed, it shifts to oppose the change. Adding a reactant shifts the equilibrium forward, and raising the pressure shifts it towards the side with fewer gas moles. Raising the temperature favours the endothermic direction, while a catalyst changes neither K nor the position of equilibrium. A catalyst only helps the system reach equilibrium faster, which is a favourite assertion-reason trap.

Ionic Equilibrium: Acids, Bases and the Ionization of Water

Ionic equilibrium applies the equilibrium constant to ions in water. It starts with three definitions of acids and bases, then adds the strength constants K_a and K_b, and finally the self-ionization of water that fixes the pH scale.

Water ionizes slightly, giving the ionic product K_w = [H⁺][OH^-] = 1.0 × 10^-14 at 298 K. From this comes the pH scale, where pH = -log[H⁺] and pH + pOH = 14. A neutral solution has pH 7, an acidic one below 7, and a basic one above 7. For a weak acid, K_a and K_b of its conjugate base are linked by K_a × K_b = K_w, a relation these equilibrium class 11 notes use throughout the ionic block.

Buffer Solutions, Common Ion Effect and Solubility Product

The last block covers real solutions where more than one equilibrium runs at once. It brings in buffers, the common ion effect, the solubility product for sparingly soluble salts, and the hydrolysis of salts. Each idea is the equilibrium constant applied to a specific mixture.

• Buffer solution: a mixture of a weak acid and its salt (or a weak base and its salt) that resists pH change on adding small amounts of acid or base.
• Henderson-Hasselbalch equation: pH = pK_a + log([salt] / [acid]), used to find the pH of an acidic buffer.
• Common ion effect: adding an ion already present in an equilibrium suppresses the ionization of a weak electrolyte.
• Solubility product: K_sp is the equilibrium constant for a sparingly soluble salt dissolving into its ions.
• Salt hydrolysis: the reaction of a salt's ions with water, which makes the solution acidic, basic, or neutral.

For a salt A_xB_y, the solubility product links to solubility s, so a salt like AgCl has K_sp = [Ag⁺][Cl^-] = s². A precipitate forms only when the ionic product exceeds K_sp. A salt of a strong acid and weak base gives an acidic solution, while a salt of a weak acid and strong base gives a basic one. The common ion effect is why adding a bit of the salt lowers a weak acid's ionization and raises its pH.

Important Formulas and Values for Equilibrium

Every formula and value you need for the chapter sits in one table below, with what it means. Learn the K_p-K_c and pH rows first, since those carry the most marks in both Boards and entrance papers.

Carry the unit on every line of your working. Losing a unit or a log step is a silent way to drop the final mark even when the number is right. Keep this table open while you solve the back-exercise numericals in these revision notes.

Key Definitions in Equilibrium

Board short-answer questions often ask for a clean definition in one or two lines. Learn these word-for-word, because a vague definition loses easy marks. Each one also sets up a numerical you can be asked to solve.

A common numerical asks you to find the pH of a weak acid or a buffer. Use K_a for the weak acid, then the Henderson equation for the buffer, and keep the log values handy. Learning these definitions makes the wording of every board question familiar.

Common Mistakes Students Make in Equilibrium

These slips happen while setting up or calculating, not because the concept is unclear. Each one costs 1 to 3 marks in the paper, so watch for them at the exact step.

Equilibrium Weightage in CBSE Boards, JEE and NEET

This chapter is large and consistently high-scoring. It carries both a numerical and a reasoning question every year, and its ionic half is a favourite in entrance papers. Here is how the marks split across the main exams for 2026-27.

Ionic equilibrium, pH and buffers together carry more questions than the K_c-K_p block in NEET and JEE Main. Master the equilibrium constant first, then Le Chatelier, then the full pH and K_sp block, in that order of return on effort.

How to Revise Equilibrium Quickly

Use these equilibrium class 11 notes for a fast, ordered recap the night before a test. The checklist below takes about 30 minutes and hits every marks-heavy idea.

• First 10 minutes: write the K_c expression, the K_p = K_c(RT)^Δn relation, and the Q-versus-K rule from memory.
• Next 10 minutes: list the three shifts of Le Chatelier's principle and solve one pH problem for a weak acid.
• Last 10 minutes: write the Henderson equation and one K_sp expression, then decide the nature of two hydrolysed salts.

Close the loop by predicting the direction of one disturbed equilibrium. If you can do all three blocks without notes, the chapter is exam-ready. Keep the Important Formulas table beside you for the first pass only, then try it closed-book.

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