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9476 / 2027
Chemical Equilibria

Lesson 2 of 4

Concentration, pressure and temperature

Predict the response and distinguish a shifted position from a changed constant.

A-Level 9476 (2026-2027)

  • I can apply Le Chatelier Principle to disturbances.Syllabus

    Syllabus 9476, 9(b). Apply Le Chatelier Principle to disturbances.

  • I can distinguish changes in K from changes in composition.Syllabus

    Syllabus 9476, 9(c). Distinguish changes in K from changes in composition.

A disturbed equilibrium responds against the imposed change

Name the actual change first, then the direction that partially opposes it.

Make a guess. You are not marked.

A sealed syringe holds N2O4(g) <=> 2NO2(g) at equilibrium. The plunger is pushed in at constant temperature. Which way does the position of equilibrium move?

  1. Towards N2O4, the side with fewer gas molecules
  2. Towards NO2, as more molecules can absorb pressure
  3. It does not move, because K stays constant
Show the answer

Towards N2O4, the side with fewer gas molecules

The total pressure rises, and the system partly opposes it by favouring the side with fewer gas molecules: 1 mol of N2O4 against 2 mol of NO2.

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.

Apply the principle to N2(g) + 3H2(g) ⇌ 2NH3(g), an exothermic forward reaction
DisturbanceEquilibrium responseReason
Add H2 at fixed volume and temperatureShift towards NH3.Consume some of the added reactant.
Remove NH3Shift towards NH3.Replace some of the removed product.
Reduce volume at fixed temperatureShift towards NH3.There are fewer gas moles on the product side: two rather than four.
Raise temperatureShift 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 exampleInterpret 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?

Try it first. Then show the worked stepsHide the worked steps
  1. Heating favours the endothermic direction, which absorbs energy.
  2. More NO2 is present at the new equilibrium.
  3. At a controlled optical path length and with the volume constraint specified, increased NO2 concentration gives a darker brown colour.
Answer

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.

Make a guess. You are not marked.

A catalyst is added to a reaction already at equilibrium. What happens to the equilibrium constant K?

  1. K decreases, because the catalyst is used up
  2. K increases, because products form faster
  3. K stays the same, as both directions speed up
Show the answer

K stays the same, as both directions speed up

A catalyst speeds up the forward and reverse reactions equally, so it changes how fast equilibrium is reached, not where it lies.

Equilibrium position and equilibrium constant are different ideas
ChangeCan the equilibrium composition change?Does K change?
Concentration or reacting-gas pressureYes, when the change disturbs the equilibrium ratio.No, at constant temperature.
TemperatureUsually yes.Yes. Heating increases K for an endothermic forward reaction and decreases K for an exothermic forward reaction.
CatalystNo change to the final equilibrium composition at the same temperature.No. Both directions approach equilibrium faster.
Amount of a pure solid already presentNot 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.

Check your understanding

Compressing an equilibrium mixture increases the amount of ammonia. Has Kp increased?

  1. No. Kp falls on compression, which is why more ammonia forms to restore it.
  2. Yes. More ammonia at equilibrium means a larger equilibrium constant.
  3. No. At the same temperature, the mixture shifts until it again matches the same Kp.
Show the answer

No. At the same temperature, the mixture shifts until it again matches the same Kp.

Compression changes every partial pressure at first, so the ratio no longer equals Kp. The equilibrium shifts towards fewer gas moles until the ratio equals Kp again. Only temperature changes Kp.