Chapter 7 Equilibrium

Terms in this set (25)

The Haber process is based on the reaction
N2(g) + 3H2(g) s 2NH3(g) ∆H = −93 kJ mol-1

The following information can be derived from this equation:
• all reactants and products are gases
• there is a change in the number of gas molecules as the reaction proceeds: four gas
molecules on the left and two on the right
• the forward reaction is exothermic so releases heat; the backward reaction is endothermic so absorbs heat.

Therefore,
1. Concentration: The product ammonia is removed as it forms, thus helping to pull the equilibrium to the right and increasing the yield.

2. Pressure: as the forward reaction involves a decrease in the number of gas molecules, it will be favoured by a high pressure.
The usual pressure used in the Haber process is about 2 × 107 Pa.

3. Temperature: as the forward reaction is exothermic, it will be favoured by a lower temperature.
However, too low a temperature would cause the reaction to be uneconomically slow, and so a moderate temperature of about 450 °C is used.

4. Catalyst: a catalyst will speed up the rate of production and so help to compensate for the moderate temperature used
A catalyst of finely divided iron is used, with small amounts of aluminium and magnesium oxides added to improve its activity.

After separation of the NH3 product, the unconverted reactants are recycled to the reactor to obtain an overall yield of about 95%.

This recycling of unconverted reactants is commonly used in industrial processes, and allows processes with low equilibrium yield to be made commercially viable.
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