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Equilibrium: questions with answers

Class 11 Chemistry, Chapter 6 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Dynamic physical and chemical equilibrium

Question 1

Why does water in an open watch glass fail to establish liquid-vapour equilibrium?

  1. (a)Equilibrium can be reached only from reactants
  2. (b)Constant macroscopic properties imply no molecular change
  3. (c)Vapour escapes, so condensation cannot balance evaporation
  4. (d)The result is independent of the stated equilibrium conditions
Show answer

(c) Vapour escapes, so condensation cannot balance evaporation

Matter loss prevents a stable opposing-rate balance.

Question 2

The same reversible reaction is started once with reactants and once with products at the same temperature. What final relationship is expected?

  1. (a)Constant macroscopic properties imply no molecular change
  2. (b)Equilibrium can be reached only from reactants
  3. (c)Both systems approach compositions satisfying the same equilibrium constant
  4. (d)The result is independent of the stated equilibrium conditions
Show answer

(c) Both systems approach compositions satisfying the same equilibrium constant

K depends on temperature and reaction definition, not the side used to start.

Equilibrium composition and stoichiometric change tables

Question 3

An ICE calculation gives roots x = 0.20 and x = 1.20 mol from an initial 1.00 mol reactant. Which root can be physical?

  1. (a)A small K means exactly zero product
  2. (b)Percent dissociation need not obey stoichiometry
  3. (c)The result is independent of the stated equilibrium conditions
  4. (d)x = 0.20 mol
Show answer

(d) x = 0.20 mol

Reaction extent cannot consume more reactant than is available.

Question 4

When may equilibrium moles be substituted directly into a Kc expression?

  1. (a)When the common volume factors cancel exactly after concentration conversion
  2. (b)Percent dissociation need not obey stoichiometry
  3. (c)The result is independent of the stated equilibrium conditions
  4. (d)A small K means exactly zero product
Show answer

(a) When the common volume factors cancel exactly after concentration conversion

Kc is fundamentally written in molar concentrations.

Gibbs energy and the equilibrium constant

Question 5

Which equation gives the reaction Gibbs energy when the composition is not standard?

  1. (a)ΔG=ΔG∘+RTln⁡Q\Delta G=\Delta G^\circ+RT\ln Q
  2. (b)K or ΔG predicts reaction speed
  3. (c)The result is independent of the stated equilibrium conditions
  4. (d)Common log replaces ln without 2.303
Show answer

(a) ΔG=ΔG∘+RTln⁡Q\Delta G=\Delta G^\circ+RT\ln Q

The Q term accounts for the current composition.

Question 6

At equilibrium with K not equal to one, which quantity is zero?

  1. (a)Common log replaces ln without 2.303
  2. (b)K or ΔG predicts reaction speed
  3. (c)The actual reaction Gibbs energy ΔG
  4. (d)The result is independent of the stated equilibrium conditions
Show answer

(c) The actual reaction Gibbs energy ΔG

Actual ΔG is zero at equilibrium; ΔG° need not be.

Heterogeneous equilibria and the relation between Kp and Kc

Question 7

For CaCO3(s)⇌CaO(s)+CO2(g)\mathrm{CaCO_3(s)\rightleftharpoons CaO(s)+CO_2(g)}, what is KpK_p?

  1. (a)Kp always equals Kc
  2. (b)Kp=pCO2K_p=p_{\mathrm{CO_2}}
  3. (c)Pure-liquid concentration varies in K
  4. (d)Pure solids must appear in K expressions
Show answer

(b) Kp=pCO2K_p=p_{\mathrm{CO_2}}

Pure solid activities are constant and absorbed into K.

Law of mass action and equilibrium-constant expressions

Question 8

A reaction with equilibrium constant K is reversed. What is the new constant?

  1. (a)K changes when initial amounts change
  2. (b)K−1K^{-1}
  3. (c)Scaling an equation leaves K unchanged
  4. (d)The result is independent of the stated equilibrium conditions
Show answer

(b) K−1K^{-1}

Reversal inverts the activity quotient.

Reaction quotient, direction, and extent of reaction

Question 9

For a reaction mixture, Q is smaller than K. Which net direction follows?

  1. (a)Q below K drives the reverse reaction
  2. (b)Q and K use different reaction expressions
  3. (c)Forward, because product formation raises Q toward K
  4. (d)Q greater than K means equilibrium
Show answer

(c) Forward, because product formation raises Q toward K

Q below K means too little product relative to equilibrium.

Le Chatelier responses to concentration, pressure, and temperature

Question 10

A product is removed from an equilibrium mixture. Which immediate quotient change and shift follow?

  1. (a)A catalyst changes K or equilibrium composition
  2. (b)Q decreases and the reaction shifts forward
  3. (c)Adding inert gas at constant volume shifts equilibrium
  4. (d)Pressure shifts every gas equilibrium the same way
Show answer

(b) Q decreases and the reaction shifts forward

Product removal lowers the numerator of Q.

Arrhenius, Brønsted-Lowry, and Lewis acid-base models

Question 11

In NH3+H2O⇌NH4++OH−\mathrm{NH_3+H_2O\rightleftharpoons NH_4^++OH^-}, what role does ammonia play?

  1. (a)Conjugate pairs differ by an electron
  2. (b)A Brønsted base donates a proton
  3. (c)Brønsted base because it accepts a proton
  4. (d)A Lewis acid donates an electron pair
Show answer

(c) Brønsted base because it accepts a proton

A Brønsted base is a proton acceptor.

Water ionization, pH, and strong electrolytes

Question 12

A solution has [H3O+]=10−3 M[\mathrm{H_3O^+}]=10^{-3}\,M. What is its pH?

  1. (a)pOH is positive log[OH−]
  2. (b)Neutral pH is always exactly 7
  3. (c)3.00
  4. (d)pH equals hydrogen-ion concentration
Show answer

(c) 3.00

pH is the negative base-10 logarithm of hydrogen-ion concentration.

Weak-acid/base ionization, Ka, Kb, and common-ion effects

Question 13

What relation connects a conjugate acid-base pair at a fixed temperature?

  1. (a)Conjugate Ka and Kb add to Kw
  2. (b)KaKb=KwK_aK_b=K_w
  3. (c)A common ion increases weak-electrolyte ionization
  4. (d)The result is independent of the stated equilibrium conditions
Show answer

(b) KaKb=KwK_aK_b=K_w

Conjugate ionization constants multiply to the water ion product.

Salt hydrolysis and buffer solutions

Question 14

Which pair forms an acid buffer?

  1. (a)CH3COOH and CH3COO−
  2. (b)The result is independent of the stated equilibrium conditions
  3. (c)Buffer pH never changes
  4. (d)The Henderson conjugate ratio may be inverted
Show answer

(a) CH3COOH and CH3COO−

A weak acid and its conjugate base constitute an acid buffer.

Solubility product, common ions, and selective precipitation

Question 15

For AX2(s)⇌A2++2X−\mathrm{AX_2(s)\rightleftharpoons A^{2+}+2X^-} with molar solubility s, what is Ksp?

  1. (a)Ksp=4s3K_{sp}=4s^3
  2. (b)Precipitation occurs when Qsp is below Ksp
  3. (c)Every salt produces ions one-to-one
  4. (d)Ksp always equals molar solubility
Show answer

(a) Ksp=4s3K_{sp}=4s^3

The ion concentrations are s and 2s.

Common misconception in Equilibrium

Open full misconception guide →

Does chemical equilibrium mean the reactions stop?

Wrong

The forward and reverse reactions stop at equilibrium.

Why it feels right. Concentrations stay constant, so nothing seems to be happening.

Right

At equilibrium, forward and reverse rates are equal.

Worked example

Ten molecules convert in each direction per second. Both totals stay steady while exchange continues.
  1. Imagine ten molecules converting each way per second.
  2. The totals remain constant even though molecules keep changing form.