Question

2Mg + O2--->2MgO
If more magnesium is added
a.the reaction will shift to the right.
b.the reaction will shift to the left.
c.there will be no shift.
and why im confused on why it would happen.

Answer 1

Is this an equilibrium reaction? If so, then we should write it as
Mg(s) + O2(g) ==> MgO(s)
Adding a solid will have no effect on the reaction at equilibrium.
If you are thinking of Le Chatelier's Principle, the easiest way to think of this is that a system at equilibrium will try to undo what we do to it. Therefore, for a system of A + B ==> C, adding A will cause the reaction to shift to the right. Why? because the reaction will try to undo what we did so it will shift so as to use up the added A and that means shifting to the right; i.e., reacting to use A to form C. B is will decrease, C will increase. I've use A, B, and C so we aren't dealing with solids.