How do chemists make sure that they get the best possible yield?
Why is it important to get the best possible yield?
What happens to waste products in chemical processes? #
The reaction N2 + 3H2 >>>2NH3 is exothermic (Delta H = -92 KJ mol-1) so an increase in temperature will decrease the yield of ammonia. The position of equilibrium is thus favoured by low temperature.
If the temperature is too low however, the rate of reaction will be so slow that the ammonia is not produced quickly enough for the reaction to be commercially viable. It is therefore necessary to choose a compromise temperature; high enough for the reaction to proceed at a reasonable rate, but low enough for the yield to be reasonable.
The reaction involves a reduction in the number of gas moles, and so an increase in pressure will increase the yield of ammonia. The position of equilibrium is thus favoured by high pressure.
High pressure also favours the rate of reaction. However the cost of maintaining conditions of high pressure is significant, and this limits the maximum pressure that can be applied. Chemists need to balance the conditions to favour the best and most economical yield (and don't forget the catalyst!)