Topic 2 of 4
Concentration, pressure and temperature
Predict the response and distinguish a shifted position from a changed constant.
A-Level 9476 (2026-2027)
A disturbed equilibrium responds against the imposed change
Name the actual change first, then the direction that partially opposes it.
Le Chatelier's Principle: when a system at equilibrium is disturbed, its position changes in the direction that tends to oppose that disturbance. The response usually offsets only part of the imposed change. Adding a reactant does not mean that all of the added material disappears.
| Disturbance | Equilibrium response | Reason |
|---|---|---|
| Add H2 at fixed volume and temperature | Shift towards NH3. | Consume some of the added reactant. |
| Remove NH3 | Shift towards NH3. | Replace some of the removed product. |
| Reduce volume at fixed temperature | Shift towards NH3. | There are fewer gas moles on the product side: two rather than four. |
| Raise temperature | Shift towards N2 and H2. | The endothermic reverse direction absorbs some added energy. |
Compression affects a gaseous equilibrium through the changed partial pressures. If both sides have the same total gaseous coefficient, as in H2(g) + I2(g) ⇌ 2HI(g), compression does not shift the ideal-gas equilibrium position. Pure solids and liquids are not counted as gaseous particles.
Adding an inert gas at constant volume and temperature raises total pressure but leaves each reacting gas partial pressure unchanged, so it causes no shift in the ideal-gas model. At constant total pressure, adding inert gas expands the mixture and lowers reacting partial pressures; the side with more gas moles is then favoured. State the constraint rather than using total pressure alone.
Worked example
Interpret colour after a temperature change
N2O4(g) ⇌ 2NO2(g) has an endothermic forward direction. N2O4 is colourless and NO2 is brown. What does warming a sealed sample favour?
- Heating favours the endothermic direction, which absorbs energy.
- More NO2 is present at the new equilibrium.
- At a controlled optical path length and with the volume constraint specified, increased NO2 concentration gives a darker brown colour.
Warming favours dissociation to NO2. For a rigid sealed vessel, the equilibrium colour becomes darker.
On a concentration-time graph, adding a species produces an immediate jump in its concentration, followed by a gradual reaction-driven adjustment. A volume decrease immediately increases all gaseous concentrations before any shift occurs. Distinguish that instantaneous physical change from the subsequent change in composition.
Temperature changes the constant; other disturbances change the route back to it
For a fixed written reaction, K is fixed at a specified temperature.
| Change | Can the equilibrium composition change? | Does K change? |
|---|---|---|
| Concentration or reacting-gas pressure | Yes, when the change disturbs the equilibrium ratio. | No, at constant temperature. |
| Temperature | Usually yes. | Yes. Heating increases K for an endothermic forward reaction and decreases K for an exothermic forward reaction. |
| Catalyst | No change to the final equilibrium composition at the same temperature. | No. Both directions approach equilibrium faster. |
| Amount of a pure solid already present | Not merely from adding more of the same solid phase, provided relevant phases remain present. | No, at constant temperature. |
A catalyst changes kinetics through an alternative mechanism. It does not alter the thermodynamic free-energy difference between the reactants and products and therefore does not change K. An industrial catalyst may allow a different operating temperature; it is that temperature change, not the catalyst itself, that can alter the equilibrium yield.