The conversion of sulfur dioxide to sulfur trioxide is an equilibrium reaction, involving a fine delicate balance between a set of compromise conditions which maximise the yield at the smallest cost possible. 2SO2(g) + O2(g) (equilibrium) 2SO3(g) + heat. (catalyst = vanadium oxide). Increasing the temperature or the pressure of the reaction vessel increases the rate of reaction as it provides more kinetic energy for successful collisions between particles to occur. However, increasing temperature, causes equilibrium to shift to the left, favouring the reverse reaction, producing more of the reactants and thus a lower sulfur trioxide yield. Hence why compromise conditions of 450 degrees celsius are enforced. Increasing the pressure not only increases the rate of reaction, but also the yield because of the 3 to 2 mol ratio of gases, with equilibrium shifting to the right hand side as it contains the lesser number of moles of gas. But, due to economical reasons, cost savings and for safety, only pressures of 1-2 atmospheres are enforced in this reaction vessel. The yield of sulfur trioxide could be also increased through the removal of the product, causing equilibrium to shift to the right and produce more sulfur trioxide. One factor that industrial chemists have manipulated and enforced, to acquire maximum yield is through decreasing the temperature while subsequently passing it over catalytic beds of vanadium oxide. Chemists, pass it over a first catalytic bed at 450 degrees celsius and have a 70% conversion rate of sulfur dioxide to sulfur trioxide. Lowering temperatures increases yield however decreases the rate, but passing it over a second catalytic bed counteracts that dilemma, converting sulfur dioxide into 97% sulfur trioxide, and finally a third conversion results in 99.7% sulfur trioxide formation. These steps are necessary as it allows for maximum yield to be produced in a cheap and economically viable manner, thus leading to increased gains and profits.
