Predict, using Le Chatelier's principle, the qualitative effects of changes in concentration, temperature, pressure and volume on the equilibrium position of homogeneous reactions

What this dot point is asking

QCAA wants you to apply Le Chatelier's principle to predict the direction of equilibrium shifts under four kinds of disturbance (concentration, temperature, pressure or volume in gaseous systems) and to explain the predictions in terms of how the system opposes the change. The principle reappears in IA1 stimulus, IA2 experimental design (most IA2s involve perturbing equilibrium and observing the response), and the EA.

The answer

Le Chatelier's principle states: when a system at equilibrium is subjected to a change in conditions, the system will shift its equilibrium position in the direction that partially opposes the change.

A shift "to the right" favours product formation; a shift "to the left" favours reactant reformation. The four disturbances examinable in Unit 3 are concentration, temperature, total pressure (or volume) in gaseous systems, and the addition of a catalyst (which is a control case, not a shift).

Concentration changes

For any equilibrium A + B leftrightarrow C + D:

• Increase [A] or [B]. Forward rate rises (more reactant particles colliding). Shift right; [C] and [D] increase, [A] and [B] partially consumed.
• Decrease [A] or [B]. Forward rate falls. Shift left; [C] and [D] partially consumed.
• Increase [C] or [D]. Reverse rate rises. Shift left.
• Decrease [C] or [D]. Reverse rate falls. Shift right.

Important. The shift partially opposes the change. After re-equilibration the disturbed species does not return to its original value; only a fraction of the disturbance is undone. Kc is unchanged.

Temperature changes

Temperature is the only disturbance that actually changes Kc. Whether the equilibrium shifts forward or back depends on the thermicity of the forward reaction.

• Exothermic forward reaction (Delta H negative). Treat heat as a product. Increasing T adds product; shift left. Kc decreases.
• Endothermic forward reaction (Delta H positive). Treat heat as a reactant. Increasing T adds reactant; shift right. Kc increases.

The Haber process N2 + 3H2 leftrightarrow 2NH3 has Delta H = -92 kJ/mol (exothermic forward). Increasing T shifts left and decreases yield. This is why the industrial compromise sets temperature only as high as needed to keep the iron catalyst active.

Pressure and volume changes (gaseous systems only)

These two are linked: at constant temperature, decreasing volume increases pressure and vice versa. The shift direction depends on the relative moles of gas on each side of the equation.

For a gas-phase equilibrium with delta-n moles of gas:

• Increase pressure (decrease volume). Shift toward the side with fewer moles of gas.
• Decrease pressure (increase volume). Shift toward the side with more moles of gas.
• No moles-of-gas difference. No shift in equilibrium position.

For N2 + 3H2 leftrightarrow 2NH3: reactant side has 4 mol gas, product side has 2 mol gas. Increasing pressure favours the product side (fewer mol). This is why industrial Haber reactors operate at high pressure (250 atm).

For H2 + I2 leftrightarrow 2HI: 2 mol gas each side. Pressure change has no effect on position.

Adding an inert gas at constant volume. Total pressure rises but partial pressures of reacting species are unchanged. No shift.

Adding an inert gas at constant pressure. The container expands; partial pressures of reacting species fall. Equivalent to a volume increase; shift toward more moles of gas.

Catalysts

A catalyst speeds the forward and reverse reactions equally. The equilibrium position does not shift and Kc does not change. The system reaches equilibrium faster.

This is examined as a distractor: a stimulus showing equilibrium re-established at the same concentrations after catalyst addition is consistent with a catalyst having no shift effect.

Visualising the shifts

A useful approach is the "what would the system do to undo the change?" test:

• Added a reactant? The system consumes it -> shift right.
• Removed a product? The system replaces it -> shift right.
• Heated an exothermic forward reaction? The system absorbs heat -> shift left (endothermic direction).
• Compressed a gas system? The system reduces gas pressure -> shift toward fewer moles of gas.

Sample concentration-vs-time graph after a disturbance

A typical IA1 stimulus shows a system at equilibrium, then a sudden disturbance (e.g. extra reactant injected), then re-equilibration.

Key features to identify on such a graph:

• The first plateau before the disturbance: original equilibrium.
• The vertical step at the disturbance: a concentration spike (or drop) of the affected species.
• The adjustment curve following the step: the system responding (forward rate rises if reactant was added, etc.).
• The second plateau at the new equilibrium: different concentrations than before but consistent with the same Kc (unless T was changed).

Recognising these features under the criteria "interpretation of stimulus" is exactly what IA1 rewards.

Worked example: applying Le Chatelier in IA2

A common IA2 design uses the iron(III) thiocyanate system. The student designs perturbations and predicts then tests the colour change.

In the IA2 report, the strongest students explicitly state the principle, predict before observing, and then justify the observation against the equilibrium expression. The criterion "claim, evidence, reasoning" rewards each component shown clearly.

Common traps

Saying "the system shifts to make the change go away". It only partially opposes; the affected species does not return to its original concentration.

Applying pressure logic to aqueous systems. Pressure changes affect only gases. The iron(III) thiocyanate equilibrium is aqueous; pressure has no effect.

Forgetting to count moles of gas carefully. Solids and liquids do not count. For 2SO2(g) + O2(g) leftrightarrow 2SO3(g), reactant side has 3 mol gas, product side has 2 mol gas; increasing pressure shifts right.

Confusing kinetics and thermodynamics. A catalyst affects how fast equilibrium is reached, not where it sits. Temperature is the only condition that changes Kc.

Treating heat as a thermal reservoir that absorbs without shifting. Heat is a participant in the equation; raising T shifts in the endothermic direction by adding energy as if it were a reactant or product.

In one sentence

Le Chatelier's principle predicts that a system at equilibrium shifts in the direction that partially opposes any change in concentration (shift away from added species), temperature (shift toward the endothermic side as T rises), or pressure in gaseous systems (shift toward fewer moles of gas as P rises), while catalysts speed both directions equally and do not change the position.