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.
