Video: Describing the Effect of Surface Area on Reaction Rates

An insoluble solid is reacted with dilute acid. Why does breaking up the solid into smaller particles increase the rate at which it can react? [A] Reducing the particle size of the solid allows more molecules to dissolve and collide with acid molecules. [B] Weakening the interactions between molecules in the solid produces gaps to accommodate acid molecules. [C] Breaking bonds between molecules in the solid liberates bonding electrons to react with acid molecules. [D] Splitting the solid into sm

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Video Transcript

An insoluble solid is reacted with dilute acid. Why does breaking up the solid into smaller particles increase the weight at which it can react? A) Reducing the particle size of the solid allows more molecules to dissolve and collide with acid molecules. B) Weakening the interactions between molecules and the solid produces gaps to accommodate acid molecules. C) Breaking bonds between molecules and the solid liberates bonding electrons to react with acid molecules. D) Splitting the solid into smaller particles increases the kinetic energy of colliding molecules. Or E) increasing the surface area of the solid results in more collisions with acid molecules.

When we have two chemical species, a reaction between them occurs when particles of one of the species collides with particles of the other with enough energy. In this problem, we want to understand why breaking an insoluble solid into smaller pieces would cause the reaction to occur more quickly.

If we had our solid in one large piece, the acid molecules would be able to collide with the particles of solid on the surface of the shape. Since the reaction occurs when molecules collide, the molecules of acid would be able to react with the particles of the solid on the surface. But the acid particles wouldn’t be able to collide with the particles of the solid on the inside of the shape. So a reaction couldn’t occur there.

Since the acid molecules can’t reach the particles of the solid on the inside until after the particles of the solid on the outside are gone, the reaction would occur relatively slowly if the solid was in a large piece. If we were to break the solid up into smaller pieces, we wouldn’t have the same problem. So the reaction would occur more quickly.

So a reaction can occur most quickly when there’s the most particles on the surface available to react from the beginning. And another way of saying that we have more particles on the surface is just saying that we have a larger surface area. So to answer this question, we just have to figure out which answer choice matches what we’ve been talking about. So let’s go through the answer choices one by one.

A says if we reduce the particle size of the solid, more molecules will dissolve and collide with the acid molecules. Well, it’s true that more collisions with the acid molecules will cause a faster reaction. The problem tells us that the solid is insoluble and reducing the particle size isn’t going to make it soluble. So A is not the correct answer choice.

B says that if we weaken the interactions between molecules and the solid, it will produce gaps to accommodate acid molecules. While it’s true that having more gaps to accommodate acid molecules would increase the number of collisions and therefore increase the rate of reaction. Breaking a solid into smaller pieces doesn’t weaken the interaction between molecules and the solid. So B is not the correct answer choice.

C states that breaking bonds between molecules and the solid liberates bonding electrons to react with acid molecules. We’re looking for an answer choice that deals with increasing the number of collisions. So C is not the correct answer.

D says that splitting the solid into smaller particles increases the kinetic energy of colliding molecules. It’s true that particles need to collide with a certain amount of energy in order for them to react. But they react faster because there’s more frequent collisions, not more energetic collisions. So D is also not the correct answer.

E says that increasing the surface area of the solid results in more collisions with the acid molecules. This is the answer choice that we’re looking for. As when we break up the solid, the acid molecules can collide with more particles of the solid, resulting in more collisions and a faster reaction. So an insoluble solid will react more quickly when we break it into smaller pieces because that increases the surface area, which results in more collisions with the acid molecules.

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