Dynamic equilibrium

• Dynamic equilibrium is reached in a closed system when the rate of the forward reaction = rate of the backward reaction.

• At equilibrium, concentrations/macroscopic properties stay constant, but particles are still reacting.

• Applies to both physical equilibrium and chemical equilibrium.

• In exam wording, constant concentration does not mean the reaction has stopped.

This figure shows dynamic equilibrium: particles continue changing in both directions, but the overall amounts remain constant because the forward and reverse rates are equal. It is a useful visual reminder that equilibrium is dynamic, not static. Source

Equilibrium constant, $K$

• The equilibrium law gives an expression for $K$ from the stoichiometric coefficients in the balanced equation.

• For a general homogeneous reaction, $aA + bB \rightleftharpoons cC + dD$:

  • $K = \dfrac{[C]^c[D]^d}{[A]^a[B]^b}$

• Products go on top, reactants on the bottom.

• Each concentration is raised to the power of its coefficient.

• In this topic, you must be able to deduce the equilibrium expression from the equation.

• For IB exam technique: write the balanced equation first, then build the $K$ expression carefully from coefficients.

What the size of $K$ means

• The magnitude of $K$ shows the extent of reaction at equilibrium.

• $K \ll 1$ → equilibrium lies far to the left; mostly reactants.

• $K < 1$ → reactants favored.

• $K = 1$ → appreciable amounts of both reactants and products.

• $K > 1$ → products favored.

• $K \gg 1$ → equilibrium lies far to the right; mostly products.

• $K$ is temperature dependent.

• For the reverse reaction at the same temperature: $K_{\text{reverse}} = \dfrac{1}{K_{\text{forward}}}$.

Le Châtelier’s principle

• Le Châtelier’s principle predicts how an equilibrium system responds to a stress.

• The system shifts to oppose the change and re-establish equilibrium.

• You must apply this to changes in concentration, pressure, and temperature.

These diagrams summarize how equilibrium shifts when concentration, pressure, or temperature changes. They are helpful for building a fast decision process in exam questions. Source

Changing concentration

• Increasing a reactant shifts equilibrium to the right.

• Increasing a product shifts equilibrium to the left.

• Removing a reactant shifts equilibrium to the left.

• Removing a product shifts equilibrium to the right.

• $K$ does not change when only concentration changes.

• The equilibrium composition changes, but the value of $K$ stays the same.

Changing pressure

• Pressure changes matter only when gases are involved.

• Increasing pressure favors the side with fewer moles of gas.

• Decreasing pressure favors the side with more moles of gas.

• If both sides have the same total moles of gas, changing pressure causes no shift.

• $K$ does not change when only pressure changes.

• Always compare gaseous moles only; ignore solids and liquids.

Changing temperature

• Treat heat as a reactant or product.

• For an endothermic forward reaction, increasing temperature shifts equilibrium to the right.

• For an exothermic forward reaction, increasing temperature shifts equilibrium to the left.

• Temperature change is the one common stress that changes the value of $K$.

• If the forward reaction is endothermic, increasing temperature increases $K$.

• If the forward reaction is exothermic, increasing temperature decreases $K$.

Catalysts and equilibrium

• A catalyst speeds up both the forward and backward reactions.

• A catalyst helps the system reach equilibrium faster.

• A catalyst does not change the value of $K$.

• A catalyst does not change the equilibrium composition.

Heterogeneous equilibria

• Le Châtelier’s principle can also be applied to heterogeneous equilibria, for example: $X(g) \rightleftharpoons X(aq)$

• Focus on how changing conditions affects the position of equilibrium.

• In equilibrium expressions, remember that in many cases pure solids and pure liquids are omitted.

Exam traps to avoid

• Do not say equilibrium means equal concentrations; it means equal rates.

• Do not include substances incorrectly in a $K$ or $Q$ expression.

• Do not say a catalyst shifts equilibrium; it only makes equilibrium faster to reach.

• Do not confuse changing concentration/pressure with changing $K$; only temperature changes $K$.

• Do not forget that $K$ for the reverse reaction is the reciprocal of the forward reaction.

Fast exam summary

• Equilibrium = closed system, forward rate = backward rate, constant macroscopic quantities.

• Large $K$ means products favored; small $K$ means reactants favored.

• Concentration and pressure can change the position of equilibrium, but not $K$.

• Temperature changes both the position of equilibrium and $K$.

• HL: use $Q$, ICE tables, and $\Delta G^\circ = -RT\ln K$ to quantify equilibrium.