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Playbook

Equilibrium

Kc and Kp, Le Chatelier, then ionic equilibrium: pH, buffers, salt hydrolysis and solubility product. The ionic half carries most of the calculations.

Questions in the bank
131
q/paper in 2025–26
1.31
Calculation
60%
Notes pages
8

Strand: Calculate

When you’ll see it

Kc, Kp or a degree of dissociation for a gas reaction, or a pH, a buffer, a salt solution or a solubility product in water.

How this chapter is tested

Nearly every question writes one equilibrium expression and solves it for a concentration, a partial pressure, a degree of dissociation, a pH or a solubility. The gas-phase pages come first because the ionic pages reuse their tools: Ka, a buffer and Ksp are the same expression written for ions in water.

Time goes on ICE tables and roots. The change row follows the coefficients, an inert gas counts only in the total pressure, and a solid adds nothing to K or to the pressure. On the ionic side, the formula is easy and the marks go on the concentration that enters it: two H⁺ from H₂SO₄, two lactate ions from calcium lactate, halved concentrations after mixing equal volumes.

The recall that remains is short but exact: what pressure, an inert gas or a catalyst does to an equilibrium, and which indicator suits which titration. Numeric answers are often a pH or a power of ten in a solubility, so keep log 2 = 0.301 and log 3 = 0.477 at hand.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Writing and combining K

    Leave out solids and pure liquids; reverse gives 1/K, scaling by n gives Kⁿ, adding equations multiplies K; Kp = Kc(RT)^Δn.

  • ICE tables

    Check Q against K for the direction, follow the coefficients in the change row, and convert moles to partial pressures through the total.

  • Degree of dissociation and ΔG°

    For PCl₅ ⇌ PCl₃ + Cl₂, Kp = α²P/(1 − α²); raising P lowers α; ΔG° = −2.303RT log K.

  • Le Chatelier's principle

    Pressure and concentration move the mixture, not K; only temperature changes K; a catalyst moves nothing; an inert gas at constant volume moves nothing.

  • pH of acids and bases

    Strong acids and bases by net moles over total volume; weak acids by [H⁺] = √(KaC), pH = ½(pKa − log C).

  • Buffers

    pH = pKa + log([salt]/[acid]); part-neutralising a weak acid with n_b of strong base leaves salt n_b and acid n_a − n_b.

  • Salt hydrolysis and indicators

    Salt of a weak acid and strong base: pH = 7 + ½pKa + ½log C; pick the indicator whose range covers the end-point pH.

  • Solubility product

    Ksp = xˣyʸs^(x+y) for AₓBᵧ; a common ion divides by its concentration to the power of its count; precipitate when Q > Ksp.

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • Scaling K as a factor

    Dividing every coefficient by 3 turns K into K^(1/3), not K/3. Adding equations multiplies their K; it never adds them.

  • The sign of Δn

    For CO + ½O₂ ⇌ CO₂, Δn = 1 − 3/2 = −½, so Kp/Kc = 1/√(RT). Reactants minus products gives √(RT), which is always offered.

  • Dropping the ion counts

    For Ag₂CrO₄, [Ag⁺] = 2s and Ksp = 4s³; for Zn(OH)₂ in NaOH the common ion is squared. Writing s² · s or dividing once gives the printed wrong answer.

  • Diluting an acid past neutral

    HCl diluted to 10⁻⁸ M does not have pH 8. Add water's own 10⁻⁷ M of H⁺; an acid stays acidic.

  • Henderson ratio inverted

    pH = pKa + log(salt/acid). For a basic buffer written for pH the ratio is base over salt; mixing the forms puts the pH on the wrong side of pKa.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Equilibrium notes

Drill every Equilibrium question

131 questions from the bank, across 8 subtopics.

Drill the 131 questions

Drill one subtopic at a time

The 8 subtopics, in teaching order.

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