Question

Will increasing the amount of one substance in an equilibrium reaction favor the reaction that produces more or less of that substance?

Answer 1

The reaction that is favoured will be the one that uses up some of the added substance.

Explanation:

Le Chatelier's principle tells us this:

When a change is made to a system at equilibrium, the system will shift in such a manner as to counteract the change that was applied to it.

So, if I raise the concentration of a substance in the equilibrium system, the shift in equilibrium will result in a new equilibrium that has consumed some of the added material.

But be careful here! Adding more of a substance is not necessarily the same as raising its concentration. For example, if you add more of a pure liquid or a solid, the concentration is not increased, and there is no shift in equilibrium.