Video Transcript
Which of the following best defines
a catalyst? (A) A substance that provides
energy to the reactants to increase the rate of reaction. (B) A substance that can make any
chemical reaction occur. (C) A substance that increases
either the concentration or the surface area of reactants to increase the rate of
reaction. (D) A substance that changes the
rate of reaction but is chemically unchanged by the end of the reaction. Or (E) a substance that increases
the amount of product produced by a chemical reaction.
To determine which statement best
defines a catalyst, let’s take a look at a biological catalyst, lactase. Biological catalysts like lactase
are also called enzymes. Each enzyme has its own specific
function. The job of lactase is to break down
lactose. Lactose is a large sugar molecule,
which is difficult for our body to digest. So it needs to be broken down into
smaller sugar molecules. This process on its own is slow
because it requires a lot of energy. That’s where lactase comes in. Lactose binds to lactase, and the
enzyme breaks the lactose apart. This reaction is faster because
using the catalyst has lowered the amount of energy needed for the reaction to
occur.
Once the lactose has been broken
apart, the enzyme lactase looks the same as it did before the reaction started. So it can be used again to break
more and more lactose molecules. So lactase, the enzyme or catalyst,
doesn’t get used up during the reaction but remains chemically unchanged. Now that we’ve seen how a catalyst
works, let’s take a look at the statements provided.
Statement (A) states that a
catalyst is a substance that provides energy to the reactants to increase the rate
of reaction. While catalysts do lower the amount
of energy needed for a reaction to occur, they do not supply energy to the
reactants. So answer choice (A) cannot be the
answer to this question.
Answer choice (B) states that a
catalyst is a substance that can make any chemical reaction occur. Looking at the figure for lactase,
we can see that its active site, the place where the reaction takes place, is
specifically designed to fit the molecule lactose. In order to break down starches,
we’d need a different enzyme with a different-shaped active site. And to break down lipids, we’d need
yet another enzyme. So a catalyst cannot be used to
make any reaction occur. Thus, answer choice (B) cannot be
the answer to this question.
Answer choice (C) states that a
catalyst is a substance that increases either the concentration or the surface area
of reactants to increase the rate of reaction. While increasing the concentration
or the surface area of the reactants are ways to increase the rate of reaction, a
catalyst increases rate of reaction by lowering the amount of energy needed for the
reaction to occur. So answer choice (C) does not
define a catalyst.
Answer choice (D) states that a
catalyst is a substance that changes the rate of reaction but is chemically
unchanged by the end of the reaction. We saw in the example that lactose
was broken into smaller sugars faster when lactase, a catalyst, is used. We also saw that after breaking
apart lactose, lactase remains unchanged despite having participated in the
reaction. Answer choice (D) does seem to
define a catalyst. But just to be sure, let’s take a
look at answer choice (E).
Answer choice (E) states that a
catalyst is a substance that increases the amount of product produced by a chemical
reaction. While a catalyst does speed up a
chemical reaction, the amount of product produced primarily depends on the amount of
reactants we have to start with. So a catalyst does not increase the
amount of product produced. Thus, answer choice (E) cannot be
the answer to this question.
Therefore, the statement which best
defines a catalyst is answer choice (D), a substance that changes the rate of
reaction but is chemically unchanged by the end of the reaction.
