Upper secondary · 12 min

Le Chatelier’s principle: shifting a chemical equilibrium

Explore how temperature, pressure, concentration and catalysts affect the N₂O₄ ⇌ 2NO₂ equilibrium.

Goal

Predict the response to a disturbance and distinguish a change in the equilibrium position from a change in K.

Apparatus and reagents

Sealed ampoule of the N₂O₄/NO₂ mixture, warm-water and ice-water baths, pressure demonstration or simulation, goggles.

Procedure

Select the reference equilibrium and read the N₂O₄ and NO₂ amounts and colour.
Choose heating and cooling in turn. Predict the shift using ΔH° = +57 kJ mol⁻¹ for N₂O₄(g) → 2NO₂(g), then compare the colour.
Select compression. Explain why the side with fewer gas molecules is favoured, while the immediate concentration change is not itself an equilibrium shift.
Compare adding N₂O₄ with adding a catalyst. State which perturbation changes K and which changes only the time needed to reach equilibrium.

What to observe

Heating produces a darker brown mixture with more NO₂ at equilibrium; cooling gives a paler mixture richer in N₂O₄. A catalyst does not change the equilibrium composition.
At the reference state the brown NO₂ vessel is shallow; after heating it is both fuller and darker, after cooling paler and richer in N₂O₄.

Explanation

For an endothermic forward reaction, heating favours products and increases K. Compression favours the left side (1 gas mole rather than 2), but K is unchanged at fixed temperature. Adding a reactant drives reaction right; a catalyst speeds both directions without shifting equilibrium.

History of the experiment

In 1884 Henry Louis Le Chatelier generalised earlier observations by van 't Hoff and others into a rule: a system at equilibrium pushed by a change of concentration, pressure or temperature shifts in the direction that counteracts the push. The

\ce{N2O4 <=> 2NO2}

ampoule, darkening when warmed and paling when cooled, became the classic classroom illustration. Fritz Haber later wagered an industry on the same principle, squeezing ammonia out of

\ce{N2 + 3H2}

under high pressure.

Chemists behind it

Jacobus Henricus van ’t Hoff Fritz Haber Wilhelm Ostwald