Equilibrium — Class 11 MCQs with Answers

Equilibrium — important questions & MCQs with answers (Class 11 Chemistry)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Chemical equilibrium is:

   AStatic state with no reactionsBAll productCDynamic — forward and reverse rates equalDAll reactant
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   Answer: Dynamic — forward and reverse rates equal

   Explanation: Equilibrium is dynamic, not static: forward and reverse reactions both keep running, but at equal rates so concentrations stay constant. The trap is calling it ‘no reaction’ — nothing stops, it only stops changing.

2. Q2 of 6Easy

   pH of pure water at 25°C:

   A1B0C14D7
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   Answer: 7

   Explanation: Pure water self-ionises: Kw = [H⁺][OH⁻] = 1×10⁻¹⁴, and since [H⁺] = [OH⁻] here, each equals 1×10⁻⁷ M, giving pH = −log(10⁻⁷) = 7. Picking 14 confuses pH with pOH’s scale limit.

3. Q3 of 6Medium

   Le Chatelier's principle states equilibrium shifts to:

   AStopBSide with more atomsCRandom sideDCounteract the disturbance
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   Answer: Counteract the disturbance

   Explanation: Le Chatelier’s principle: when a system at equilibrium is disturbed by concentration, pressure, or temperature change, it shifts in the direction that partially counteracts that disturbance — not one that stops or randomises the reaction.

4. Q4 of 6Medium

   An acid by Brønsted definition:

   ADonates a protonBAccepts a protonCDonates electron pairDAccepts electron pair
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   Answer: Donates a proton

   Explanation: Brønsted–Lowry defines an acid as a proton (H⁺) donor. Accepting a proton instead describes a Brønsted base, while donating or accepting an electron pair are the separate Lewis acid/base definitions.

5. Q5 of 6Hard

   For aA + bB ⇌ cC + dD, Kc =

   A[A][B][C][D]B[A]^a[B]^b / [C]^c[D]^dC[A]+[B] / [C]+[D]D[C]^c[D]^d / [A]^a[B]^b
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   Answer: [C]^c[D]^d / [A]^a[B]^b

   Explanation: The law of mass action puts products over reactants, each raised to its own stoichiometric coefficient: Kc = [C]^c[D]^d/[A]^a[B]^b. Writing reactants on top instead inverts the whole expression.

6. Q6 of 6Hard

   A buffer solution resists:

   AChanges in pH on addition of small amount of acid/baseBHeatCLightDPressure
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   Answer: Changes in pH on addition of small amount of acid/base

   Explanation: A buffer pairs a weak acid/base with its conjugate, so added H⁺ or OH⁻ is consumed by that reserve instead of changing free [H⁺] much — it resists pH shifts from small acid or base addition, not heat, light, or pressure.

Equilibrium notes: key concepts (Class 11 Chemistry)

Reactions rarely go to completion; they settle into a dynamic balance. This chapter develops chemical equilibrium and the constants Kc and Kp, the reaction quotient and Le Chatelier's principle, then ionic equilibrium: acid–base theories, pH, ionisation constants Ka and Kb, buffers and the solubility product Ksp.

Dynamic equilibrium

The forward and reverse reactions proceed at equal rates, so measurable concentrations stay constant even though both reactions continue at the molecular level.

Equilibrium constant Kc

For a reaction at equilibrium, Kc is the product concentrations divided by the reactant concentrations, each raised to its stoichiometric coefficient.

Kp and its link to Kc

For gaseous equilibria Kp is written with partial pressures; Kp = Kc(RT)^Δn, where Δn is moles of gaseous products minus gaseous reactants.

Reaction quotient Q

Q has the same form as K but for any instant; Q < K drives the reaction forward, Q > K drives it backward, and Q = K means equilibrium.

Le Chatelier's principle

A system at equilibrium responds to a disturbance in concentration, pressure or temperature by shifting in the direction that partly opposes that change.

Arrhenius and Brønsted–Lowry

Arrhenius: acids furnish H⁺ and bases OH⁻ in water. Brønsted–Lowry: acids donate protons and bases accept them, forming conjugate acid–base pairs.

Lewis acids and bases

A Lewis acid accepts an electron pair while a Lewis base donates one, broadening acidity beyond proton transfer — BF₃ is a Lewis acid, NH₃ a base.

Ionic product of water Kw

Water self-ionises so that Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C; a neutral solution therefore has [H⁺] = [OH⁻] = 10⁻⁷ M.

pH scale

pH = −log[H⁺] and pOH = −log[OH⁻], with pH + pOH = 14 at 25 °C; a pH below 7 is acidic and above 7 is basic.

Ka, Kb and strength

Ka and Kb are the ionisation constants of weak acids and bases; larger values mean stronger species, and for a conjugate pair Ka × Kb = Kw.

Degree of ionisation

The fraction α of a weak electrolyte that ionises; by Ostwald's dilution law α rises on dilution, with Ka ≈ Cα² when α is small.

Buffer solution

A weak acid with its salt (or weak base with its salt) resists pH change; its pH follows the Henderson equation, pH = pKa + log([salt]/[acid]).

Solubility product Ksp

For a sparingly soluble salt, Ksp is the product of its ion concentrations at saturation; a precipitate forms when the ionic product exceeds Ksp.

Common-ion effect

Adding an ion already present in an equilibrium suppresses ionisation or dissolution, since the system shifts to consume the extra ion (Le Chatelier).