# Consider following reaction: $Mg(s)+2HCl(aq)\rightarrow H_2(g)+MgCl_2(aq)$.The rate of this reaction increases when more magnesium is added.This change is due to

$\begin{array}{1 1}(a)\;adding\;of\;catalyst\\(b)\;increase\;in\;surface\;area\\(c)\;change\;in\;nature\;of\;reactants\\(d)\;increase\;in\;concentration\;of\;reactants\end{array}$

You are only going to get a reaction if the particles in the gas or liquid collide with the particles in the solid. Increasing the surface area of the solid increases the chances of collision taking place.
Increasing the number of collisions per second increases the rate of reaction.
As amount of magnesium is increased surface area increases and hence more useful collisions can take place and hence rate of reaction will increase.
edited Jul 24, 2014