Industrial Compromise & Techniques for Investigating Equilibrium

Why must industrial processes involving reversible reactions reach a compromise between rate and yield?

For reactions where the forward reaction is exothermic (e.g. the Haber Process), a lower temperature gives a higher equilibrium yield but a very slow rate — the process would be uneconomical because it would take too long to produce usable quantities of product. A higher temperature increases the rate but shifts the equilibrium to the left, reducing yield. The industrial conditions chosen are therefore a compromise — a temperature that gives an acceptable rate whilst maintaining a reasonable yield, combined with a catalyst to speed up the reaction further, and high pressure where appropriate.

What are the industrial conditions for the Haber Process (N₂ + 3H₂ ⇌ 2NH₃) and explain why each was chosen?

The conditions are approximately 450°C, 200 atm pressure, and an iron catalyst:

• 450°C: a compromise temperature — high enough for the reaction to proceed at an acceptable rate, but not so high that the yield of ammonia falls too drastically (the forward reaction is exothermic so high temperature reduces yield).

• 200 atm: high pressure favours the side with fewer gas moles (products, 2 moles vs 4 moles on the left), increasing yield. Higher pressures would give even better yield but are very expensive to maintain and create safety risks.

• Iron catalyst: increases the rate of reaction and allows equilibrium to be reached more quickly at the chosen temperature, without affecting the position of equilibrium.

How can you investigate the effect of concentration on the position of equilibrium using the chromate/dichromate equilibrium?

The equilibrium is: 2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l). Chromate ions are yellow and dichromate ions are orange. Adding acid (H⁺) shifts equilibrium to the right — the solution turns more orange. Adding alkali (OH⁻) removes H⁺ ions and shifts equilibrium to the left — the solution turns more yellow. The colour change provides direct visual evidence that the position of equilibrium has shifted.

How can you investigate the effect of temperature on the position of equilibrium?

Use a reaction with a visible colour change that depends on the position of equilibrium. For example, the reaction between NO₂ (brown) and N₂O₄ (colourless): 2NO₂(g) ⇌ N₂O₄(g), ΔH = −57 kJ mol⁻¹. Heating the mixture in a warm water bath causes the colour to darken (more brown NO₂ forms) as the endothermic reverse reaction is favoured. Cooling in ice causes the colour to lighten (more colourless N₂O₄ forms) as the exothermic forward reaction is favoured. The colour change gives a qualitative indication of which direction the equilibrium has shifted.