QCE Chemistry - Unit 3 - Chemical equilibrium systems

Equilibrium Constants | QCE Chemistry

Learn how equilibrium constants describe QCE Chemistry equilibrium systems, including Kc expressions, reaction quotients and common exam traps.

Updated 2026-05-17 - 4 min read

The equilibrium constant, $K_c$, is a number that describes the composition of an equilibrium mixture at a particular temperature. It turns the idea of "products are favoured" or "reactants are favoured" into a concentration ratio.

Writing a Kc expression

For the general reversible reaction:

$ aA + bB \rightleftharpoons cC + dD $

the concentration expression is:

$ K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b} $

The coefficients become powers. Products go on top. Reactants go on the bottom.

Example:

$ \mathrm{CaCO_3(s)} \rightleftharpoons \mathrm{CaO(s)} + \mathrm{CO_2(g)} $

$ K_c = [\mathrm{CO_2}] $

The two solids are not included.

What Kc tells you

• $K_c \gg 1$: products are strongly favoured.
• $K_c \ll 1$: reactants are strongly favoured.
• $K_c$ near $1$: both reactants and products are present in significant amounts.

$K_c$ does not tell you the speed of the reaction. A reaction can be product-favoured and still take a long time to reach equilibrium.

Temperature and Kc

For a given reaction, $K_c$ is constant only at a fixed temperature. Concentration, pressure and catalysts can change the current mixture, but they do not change $K_c$. Temperature changes do.

Reaction quotient, Q

$Q$ uses the same expression as $K_c$, but with current concentrations rather than equilibrium concentrations.

• If $Q < K_c$, there are too many reactants compared with equilibrium, so the system shifts forward.
• If $Q > K_c$, there are too many products compared with equilibrium, so the system shifts reverse.
• If $Q = K_c$, the system is at equilibrium.

The important difference is timing. $K_c$ uses equilibrium concentrations only. $Q$ can be calculated at any moment. That makes $Q$ useful when a mixture has just been disturbed or when the question gives you concentrations and asks which way the system will move.

Heterogeneous equilibria and Ksp

For heterogeneous equilibria, pure solids and pure liquids are omitted from the expression. Their concentrations are treated as effectively constant, so including them would not help compare the mixture.

Solubility product, $K_{sp}$, is the equilibrium constant used for sparingly soluble ionic solids. For example:

$ \mathrm{AgCl(s)} \rightleftharpoons \mathrm{Ag^+(aq)} + \mathrm{Cl^-(aq)} $

$ K_{sp} = [\mathrm{Ag^+}][\mathrm{Cl^-}] $

This tells you about the concentration of ions in a saturated solution, not the mass of undissolved solid sitting at the bottom. Adding more solid $\mathrm{AgCl}$ does not appear in the expression.

Worked example

Simple calculation example

For:

$ \mathrm{A(g)} \rightleftharpoons 2\mathrm{B(g)} $

At equilibrium, $[\mathrm{A}] = 0.20\ \mathrm{mol\ L^{-1}}$ and $[\mathrm{B}] = 0.60\ \mathrm{mol\ L^{-1}}$.

$ K_c = \frac{[\mathrm{B}]^2}{[\mathrm{A}]} $

$ K_c = \frac{(0.60)^2}{0.20} = 1.8 $

The value is greater than 1, so products are favoured, but not overwhelmingly.

Approximation questions

Some equilibrium calculations give a very small $K_c$. In that case, only a tiny amount of reactant changes into product. QCE may allow the approximation that the equilibrium concentration of a reactant is almost the same as its initial concentration.

For example, if $x$ is very small compared with $0.100$, then:

$ 0.100 - x \approx 0.100 $

Only use that shortcut when the question or value of $K_c$ makes it reasonable, and state the assumption. If the calculated $x$ is not small compared with the initial concentration, the approximation is not reliable.

Exam traps

Other traps:

• forgetting powers from coefficients
• including solids or pure liquids
• flipping the fraction
• saying large $K_c$ means a fast reaction
• treating $K_c$ as fixed when temperature changes

Quick check

Sources

• QCAA Chemistry subject page
• QCAA Chemistry 2025 syllabus
• OpenStax Chemistry 2e: Equilibrium Constants
• OpenStax Chemistry 2e: Shifting Equilibria