Why should hydrogen:nitrogen be kept in the ratio 3:1? According to Le Chatelier's principle, wouldn't it be better to have an excess of reactants to drive the equilibrium forward?
The ratio doesn't change in the way they react. They will still react in a way which is a 3:1 ratio.
When adding an excess of reactants, the ratio should be the same, because if you add, for example a 4:1 ratio instead, then when all reactants have been depleted, you have an excess of Hydrogen which is not being used for anything, hence a waste of doing so.
So the ratio should always be 3:1, and if you are going to use it in excess, then the ratio should still be 3:1. The ratio does not change.
Source EE