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Next exam on November 14 – 15, 2026:
All concepts

Chemical Equilibrium

化学平衡

What equilibrium actually is and how to test for it, and which way the position shifts when temperature, pressure or concentration changes (Le Chatelier's principle).

12 real-exam questions · 4 sittings10 in mock papers14 practice

Formulas

  • Equilibrium constant

    Law
    K=cp(C)⋅cq(D)cm(A)⋅cn(B)K=\frac{c^p(\mathrm{C})\cdot c^q(\mathrm{D})}{c^m(\mathrm{A})\cdot c^n(\mathrm{B})}

    Applies when：For mA+nB ⇌ pC+qD; K changes with temperature only. Pure solids and pure liquids are omitted from the expression

How the real exam tests it

Questions from real CSCA papers that test this concept. Work each one out before you open the answer.

  1. CSCA Chemistry December 2025 Past Paper
    When a reaction reaches equilibrium, which of the following must be equal in the reaction system? ( )
    • A. The time atoms spend in reactant molecules equals the time in product molecules
    • B. Reactant concentrations equal product concentrations
    • C. Forward reaction rate equals reverse reaction rate
    • D. The mass of reactants equals the mass of products
    Show answer & solution

    Answer: C

    Answer: C Chemical equilibrium is dynamic: the forward rate equals the reverse rate, so macroscopic properties stay constant while the reaction continues. Reactant and product concentrations (or masses) are constant at equilibrium but need not be equal.
    See it in the full CSCA Chemistry December 2025 Past Paper
  2. CSCA Chemistry March 2026 Past Paper
    When H2( g)+I2( g)⇌2HI(g)\mathrm { H } _ { 2 } ( \mathrm {~g} ) + \mathrm { I } _ { 2 } ( \mathrm {~g} ) \rightleftharpoons 2 \mathrm { HI } ( \mathrm { g } ) reaches equilibrium, which is correct?
    • A．The reaction has stopped
    • B．All concentrations no longer change
    • C．Both forward and reverse rates are zero
    • D．All reactants fully converted to products
    Show answer & solution

    Answer: B

    At equilibrium: concentrations constant ( B∨B \vee ). Reaction continues (A wrong). Rates ≠0\neq 0 ( CC wrong). Not complete conversion (D wrong). Answer: B.
    Take the full CSCA Chemistry March 2026 Past Paper — timed & graded
  3. CSCA Chemistry April 2026 Past Paper
    For the reaction X(g) + 3Y(g) ⇌ 2Z(g) (the forward reaction is endothermic), which of the following operations can shift the equilibrium in the forward direction? ( )
    • A. Lowering the temperature and increasing the pressure
    • B. Lowering the temperature and decreasing the pressure
    • C. Raising the temperature and increasing the pressure
    • D. Raising the temperature and decreasing the pressure
    Show answer & solution

    Answer: C

    Answer: C The forward reaction is endothermic and the number of gas molecules decreases (4 → 2); raising the temperature and increasing the pressure shift the equilibrium forward.
    Take the full CSCA Chemistry April 2026 Past Paper — timed & graded

Real-exam evidence

Recorded past-paper appearances are listed below; online verification is labelled separately.

12 questions across 4 sittings1 of 4 sittings available to verify free online

10 questions across 5 mock papers — our own papers, so a coverage signal rather than real-exam evidence

14 related practice questions availablePractise free: Chemical Reaction Rate and Equilibrium

Sitting evidence comes from the CSCA past papers on record; questions whose original paper is missing are excluded, and sprint papers are deliberately not counted.

Related concepts

Rate of Reaction