Lesson 12 of 12 · 17 min
Chapter review
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Must-know facts
20 facts
- 01Equilibrium is dynamic: forward and backward rates are equal, not zero.
- 02Normal boiling point: vapour pressure = 1.013 bar (1 atm).
- 03Reverse reaction: K' = 1/K; equation multiplied by n: K' = Kⁿ.
- 04Kp = Kc(RT)^Δn with Δn = gaseous products − gaseous reactants; R = 0.0831 bar L mol⁻¹ K⁻¹.
- 05Pure solids and liquids are omitted from K; CaCO₃ ⇌ CaO + CO₂ has Kp = p(CO₂).
- 06Q < K forward; Q > K backward; Q = K equilibrium.
- 07ΔG° = −RT ln K = −2.303 RT log K.
- 08Only temperature changes K; a catalyst changes neither K nor composition.
- 09Inert gas added at constant volume: no shift.
- 10Haber process: N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92.38 kJ mol⁻¹.
- 11Kw = 1.0 × 10⁻¹⁴ at 298 K; it rises with temperature.
- 12pH + pOH = 14 at 298 K.
- 13Ka × Kb = Kw; pKa + pKb = 14.
- 14Weak acid (small α): [H⁺] = √(Ka c).
- 15Lewis acids: BF₃, AlCl₃, Co³⁺, Mg²⁺; Lewis bases: NH₃, OH⁻, F⁻.
- 16Acid strength: HF < HCl < HBr < HI (bond strength); CH₄ < NH₃ < H₂O < HF (electronegativity).
- 17Salt of weak acid + weak base: pH = 7 + ½(pKa − pKb).
- 18Buffer: pH = pKa + log([salt]/[acid]); pH = pKa when they are equal.
- 19Ksp for M₂X or MX₂ = 4S³; for MX = S².
- 20Common ion lowers solubility; NaCl is purified by HCl gas.
Common traps
Where marks are lost
Formulas
12 to know
Equilibrium constant (concentrations)
Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ
For aA + bB ⇌ cC + dD; pure solids and liquids omitted.
Kp and Kc
Kp = Kc(RT)^Δn
Δn = gaseous product moles − gaseous reactant moles; R = 0.0831 bar L mol⁻¹ K⁻¹ with p in bar.
Gibbs energy and Q
ΔG = ΔG° + RT ln Q
At equilibrium ΔG = 0 and Q = K.
Gibbs energy and K
ΔG° = −RT ln K = −2.303 RT log K
K = e^(−ΔG°/RT).
Ionic product of water
Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ (298 K)
pKw = 14 at 298 K.
pH
pH = −log[H₃O⁺] ; pH + pOH = 14
The sum is 14 only at 298 K.
Weak acid ionisation
Ka = cα² / (1 − α) ≈ cα² ; [H₃O⁺] = cα ≈ √(Ka c)
Approximation valid when α is small.
Conjugate pair
Ka × Kb = Kw ; pKa + pKb = pKw
Applies to an acid and its own conjugate base.
Salt of weak acid and weak base
pH = 7 + ½(pKa − pKb)
At 298 K; independent of salt concentration.
Acidic buffer
pH = pKa + log([A⁻]/[HA])
Henderson-Hasselbalch equation.
Basic buffer
pOH = pKb + log([BH⁺]/[B])
Then pH = 14 − pOH at 298 K.
Solubility product
Ksp = xˣ yʸ S^(x+y) for MₓXᵧ
MX: S²; M₂X or MX₂: 4S³; S in mol L⁻¹.
Key terms
13 terms
- Dynamic equilibrium
- State where opposing processes run at equal rates so the composition stays constant.
- Law of mass action
- Reaction rate is proportional to the product of reactant active masses, leading to the K expression.
- Reaction quotient (Q)
- The K-type ratio evaluated at any moment, used to predict the direction of change.
- Heterogeneous equilibrium
- An equilibrium involving species in more than one phase.
- Le Chatelier's principle
- A disturbed equilibrium shifts to partly undo the disturbance.
- Conjugate acid-base pair
- Two species differing by one proton.
- Lewis acid
- A species that accepts an electron pair.
- Amphoteric
- Able to act as either an acid or a base, as water does.
- Degree of ionisation (α)
- Fraction of a weak electrolyte present as ions at equilibrium.
- Common ion effect
- Suppression of a weak electrolyte's ionisation, or of a salt's solubility, by an added shared ion.
- Hydrolysis
- Reaction of a salt's ions with water that makes the solution acidic or basic.
- Buffer solution
- A solution that resists pH change on adding small amounts of acid or base.
- Solubility product (Ksp)
- Equilibrium constant for a sparingly soluble salt dissolving into its ions.