2.3.2—Equilibrium law

$Kc = [C]^r[D]^s / ([A]^p[B]^q) for pA + qB ⇌ rC + sD$

For a homogeneous reaction, place product concentrations over reactant concentrations and use each balanced-equation coefficient as the exponent.

Build the expression only after balancing the equation, and use equilibrium rather than initial concentrations. The numerical value of K changes with temperature; changing starting amounts can move the equilibrium composition without changing K.

Representative question

[Maximum number: 1]

Deduce the $K_{\mathrm{c}}$ expression for the reaction in part (d)(i).

$\left[\mathrm{NH}_{3}\right]^{2} /\left(\left[\mathrm{N}_{2}\right]\left[\mathrm{H}_{2}\right]^{3}\right)$

Retrieve the route: define dynamic equilibrium, write K, interpret its magnitude, predict Le Châtelier shifts, compare Q with K, solve a RICE table, and connect K with ΔG.

Check closed-system and equal-rate language, exponents and direction, whether a change affects K, current versus equilibrium concentrations, stoichiometric x changes, and kelvin/unit consistency in ΔG calculations.

2.3.2—Equilibrium law

The Equilibrium Law

Writing Kc Expressions

Assessment in practice

Extent of Chemical Change Summary

Question 1