The Contact Process

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  • Contact Process
    • 1.    In the first stage of the Contact process, sulphur is burned in air to make sulphur dioxide: sulphur + oxygen ? sulphur dioxide S (l) + O2 (g) ? SO2¬ (g)
      • 2.    In the second stage, sulphur dioxide reacts with more oxygen to make sulphur trioxide: sulphur dioxide + oxygen ? sulphur trioxide 2SO2 (g) + O2 (g) ? 2SO3 (g)
        • 3.    In the final stage, sulphur trioxide reacts with water to make sulfuric acid: H2O (l) + SO3 (g) ? H2SO4 (aq)
        • This reaction is reversible. The conditions needed for it are:  a catalyst of vanadium(V) oxide, V2O a temperature of around 450° & atmospheric pressure
      • This is not a reversible reaction. 
    • The position of equilibrium moves to: the right if a product is removed, or extra reactant is added; the left if a reactant is removed, or extra product is added. For example, the position of equilibrium moves to the right if extra sulphur dioxide and oxygen is added, or if sulphur trioxide is removed.
    • If the pressure is increased, the position of equilibrium moves in the direction of the least number of moles of gas. If the pressure is increased, the position of equilibrium moves to the right (where there are fewer moles) - making more sulphur trioxide at equilibrium. In this example the position of equilibrium moves to the left if the pressure is reduced.
    • If the temperature is increased, the position of equilibrium moves in the direction of the endothermic reaction. In the second stage of the Contact process: the forward reaction 2SO2 + O2 ? 2SO3 is exothermic (it gives out heat);  the backward reaction 2SO3 ? 2SO3 + O2 is endothermic (it takes in heat) This means that if the temperature is increased, the position of equilibrium moves to the left - making less sulphur trioxide at equilibrium.
    • In the Contact process, the vanadium (V) oxide (V2O5) catalyst increases the rate of reaction - but it does not change the position of equilibrium. The position of equilibrium is already on the right, even at low pressure, so high pressure is not needed. This saves money. A compromise temperature of around 450°C is used. This is low enough to keep the position of equilibrium towards the right, but high enough to get a reasonable rate of reaction.

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