Chemical equilibria are quantified by writing algebraic expressions that represent reactant and product amounts. In AP Chemistry, you must correctly build Q and K expressions from a balanced equation using concentrations or partial pressures.

Law of mass action: the blueprint for Q and K

For a general reversible reaction:

• $aA + bB \rightleftharpoons cC + dD$

the law of mass action states that the reaction’s composition can be summarized by a ratio:

• Products in the numerator

• Reactants in the denominator

• Each term raised to a power equal to its stoichiometric coefficient in the balanced equation

This same structure is used for both the reaction quotient (Q) and the equilibrium constant (K); only when you evaluate it differs.

Reaction quotient Q (any time) vs equilibrium constant K (at equilibrium)

A system’s Q value changes as the reaction proceeds, because the amounts of reactants and products change.

K is fixed for a given reaction at a fixed temperature, but Q can take on many values as conditions change.

These plots show Gibbs free energy $G$ versus reaction progress, with the minimum corresponding to equilibrium where $Q=K$. If the mixture starts with nonequilibrium amounts ($Q\neq K$), the reaction proceeds in the direction that decreases $G$ until it reaches that minimum. The figure reinforces that the “drive to equilibrium” is fundamentally an energetic tendency toward minimum $G$. Source

Writing $Q_c$ and $K_c$ (concentration-based)

Use $Q_c$ or $K_c$ when amounts are expressed as molar concentrations (typical for dissolved species, and sometimes gases when concentration data are provided).

The exponents (like $c$ and $d$) come directly from the balanced equation coefficients, so balancing must happen before writing any Q or K expression.

Checklist for writing $Q_c$ or $K_c$

• Balance the chemical equation.

• Write brackets for each species: $[,]$.

• Multiply product terms together in the numerator.

• Multiply reactant terms together in the denominator.

• Raise every term to its coefficient power.

Writing $Q_p$ and $K_p$ (partial-pressure-based)

Use $Q_p$ or $K_p$ for gaseous equilibria when amounts are expressed as partial pressures.

When writing $Q_p$ or $K_p$, pressures must be partial pressures for each gas (not total pressure), and you must be consistent with units within the expression.

Choosing between concentration and pressure forms

• Use $Q_c$ / $K_c$ when the problem gives concentrations ($[,]$).

• Use $Q_p$ / $K_p$ when the problem gives partial pressures ($P$).

• The structure (products over reactants with coefficient exponents) stays the same in all cases.