How does increasing the surface area of a reactant affect the rate of a reaction

Question

How does increasing the surface area of a reactant affect the rate of a reaction?

a) There are more frequent collisions with more surface area exposed.

            b) Each collision is more energetic.

            c) The fraction of collisions with the proper orientation increases with more surface area exposed.

            d) The reaction pathway changes to one with a lower energy of activation when more surface area is exposed.

**The answer is A. I'm hoping for an explanation so I can fully understand the concept. What if I decreased the surface area? Any other scenarios? Thanks

Explanation / Answer

Answer.
a) There are more frequent collisions with more surface area exposed.

First, let's consider the next chemical reaction as an example:
CaCO3(s) + 2HCl(aq) --> CaCl2(aq) + H2O(l) + CO2(g)

The chemical reaction occurs when the particles of HCl collide with the solid CaCO3.

For instance, consider a big solid cube of calcium carbonate. Only the external faces of the cube are in direct contact with the acid particles and only those particles will react, while the inner part of the cube remains intact.
Now imagine you pulverize the same solid cube into small bits, as small as the molecule size, and are added to the same acidic solution. Each molecule will be available to collide with the acid particles, yielding a higher rate of reaction.

Increasing the surface area of the solid increases the chances of collision.
Increasing the number of collisions per second increases the rate of reaction.
Decreasing the surface area has the opposite effect.

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