Effect of Surface Area of the Reactant

Surface area only matters for heterogeneous (solid-involving) reactions. When a solid reacts with a liquid or a gas, only the molecules sitting right at the solid's exposed SURFACE can actually take part in a collision with the other reactant -- material buried in the interior of a large lump is chemically inert until the reaction has eaten its way down to it. For a FIXED total mass of solid, breaking it into smaller particles dramatically increases the total exposed surface area, which means more collisions per litre per second can occur, and the reaction speeds up.

A lab demonstration. A known mass of marble chips (lump $CaCO_3$) is reacted with a known volume of dilute HCl, $CaCO_3(s)+2HCl(aq)\rightarrow CaCl_2(aq)+H_2O(l)+CO_2(g)$; since $CO_2$ gas escapes as the reaction proceeds, the FLASK'S MASS drops steadily, and tracking that mass loss over time indirectly tracks the reaction's progress. The identical experiment is repeated with the SAME mass of marble, now finely POWDERED, under otherwise identical conditions. Plotting mass loss against time for both gives two rising curves that both level off once the reaction is complete -- but the POWDERED marble's curve reaches its final (completed …