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Chemistry · Unit 6

Chemical Equilibrium: MDCAT Notes and MCQs

Equilibrium

  • In a reversible reaction, equilibrium is reached when rate of forward = rate of reverse reaction. It is dynamic; concentrations stay constant (not necessarily equal); macroscopic properties (colour, pressure, density) become constant.
  • Can be approached from either side; needs a closed system.
  • For aA + bB ⇌ cC + dD: Kc = [C]c[D]d / [A]a[B]b. Kp = Kc(RT)Δn; Kp = Kc when Δn = 0.
  • Large K → products favoured. K changes only with temperature.

Le Chatelier's principle

If a stress is applied to a system at equilibrium, the system shifts to partially undo the stress.

  • Concentration: adding reactant → shifts forward.
  • Pressure: increase → shifts to side with fewer gas moles; no effect if Δn = 0 (e.g. H2 + I2 ⇌ 2HI).
  • Temperature: increase favours endothermic direction (and changes K).
  • Catalyst: no shift — equilibrium reached faster only.

Haber process

N2 + 3H2 ⇌ 2NH3; ΔH = −92.4 kJ/mol (exothermic, Δn = −2). Conditions: ≈ 200–300 atm, ≈ 400–450 °C, iron catalyst (promoters K2O/Al2O3). High pressure favours NH3; low temperature favours yield but is too slow → compromise (optimum) temperature. NH3 removed by liquefaction.

Solubility product (Ksp)

For a sparingly soluble salt AgCl ⇌ Ag+ + Cl−: Ksp = [Ag+][Cl−]. For PbCl2: Ksp = [Pb2+][Cl−]2 = 4s3. Precipitation occurs when ionic product > Ksp.

Common ion effect

Suppression of ionization/solubility of a weak electrolyte or sparingly soluble salt by adding a strong electrolyte with a common ion. E.g. passing HCl gas into saturated NaCl solution precipitates pure NaCl; adding CH3COONa to CH3COOH suppresses its ionization; NH4Cl suppresses NH4OH ionization (used in group III analysis).

Buffers

  • Solutions that resist change in pH on adding small amounts of acid or base.
  • Acidic buffer: weak acid + its salt with strong base — CH3COOH + CH3COONa (pH < 7). Basic buffer: weak base + its salt with strong acid — NH4OH + NH4Cl (pH > 7).
  • Henderson equation: pH = pKa + log([salt]/[acid]); pOH = pKb + log([salt]/[base]). Maximum buffer capacity when [salt] = [acid] → pH = pKa.
  • Blood pH 7.35–7.45 maintained by H2CO3/HCO3− buffer.
PMDC learning outcomes for this unit
  • Chemical Equilibrium: 6.1 Define chemical equilibrium in terms of reversible reaction.
  • Chemical Equilibrium: 6.2 Write both forward and reverse reactions; describe the macroscopic characteristics of each
  • Le Chatelier's principle: 6.3 State Le Chatelier's principle and be able to apply it to systems in equilibrium with changes in concentration, pressure, temperature or addition of catalyst.
  • Solubility Products: 6.4 Define and explain solubility products.
  • Common Ion Effect: 6.5 Define and explain the common ion effect by giving suitable examples.
  • Buffer Solution: 6.6 Define buffer solution and explain types of buffers.
  • Haber's Process: 6.7 Explain synthesis of ammonia by Haber's process.

Chemical Equilibrium MCQs with answers

Practise and track your score →
Q1
At chemical equilibrium:
ARates of forward and reverse reactions are equal
BForward reaction stops
CConcentrations of reactants and products are equal
DReaction is complete
Show answer
Answer: A (Rates of forward and reverse reactions are equal). Equilibrium is dynamic — both reactions continue at equal rates.
Q2
For N₂ + 3H₂ ⇌ 2NH₃, increasing pressure will:
AHave no effect
BShift equilibrium backward
CShift equilibrium forward
DDecrease Kc
Show answer
Answer: C (Shift equilibrium forward). The forward reaction reduces gas moles (4 → 2).
Q3
Addition of a catalyst to a system at equilibrium:
AShifts it backward
BIncreases K
CDoes not change the equilibrium position
DShifts it forward
Show answer
Answer: C (Does not change the equilibrium position). A catalyst speeds both reactions equally; equilibrium is reached sooner but not shifted.
Q4
Kp = Kc when:
AΔn = 1
BTemperature is 273 K
CΔn = −1
DΔn = 0
Show answer
Answer: D (Δn = 0). Kp = Kc(RT)^Δn, so equal when Δn = 0 (e.g. H2 + I2 ⇌ 2HI).
Q5
The catalyst used in Haber's process is:
APlatinum
BIron
CNickel
DV₂O₅
Show answer
Answer: B (Iron). Finely divided iron with promoters (K2O, Al2O3).
Q6
The Ksp expression for CaF₂ is:
A[Ca²⁺][2F⁻]
B[Ca²⁺][F⁻]
C[Ca²⁺]²[F⁻]
D[Ca²⁺][F⁻]²
Show answer
Answer: D ([Ca²⁺][F⁻]²). CaF2 ⇌ Ca²⁺ + 2F⁻, so Ksp = [Ca²⁺][F⁻]².
Q7
Passing HCl gas through a saturated solution of NaCl precipitates NaCl due to:
AHydrolysis
BLe Chatelier's temperature effect
CBuffer action
DCommon ion effect
Show answer
Answer: D (Common ion effect). Extra Cl⁻ ions make the ionic product exceed Ksp.
Q8
A mixture of CH₃COOH and CH₃COONa acts as:
ABasic buffer
BNeutral solution
CAcidic buffer
DStrong acid
Show answer
Answer: C (Acidic buffer). Weak acid + its salt with a strong base forms an acidic buffer.
Q9
When [salt] = [acid] in an acidic buffer, the pH equals:
A7
B14 − pKa
CpKa
DpKb
Show answer
Answer: C (pKa). pH = pKa + log 1 = pKa.
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