Video Transcript
Which of the following is not a
characteristic of a catalyst? (A) It decreases the amount of
energy needed for a reaction to proceed. (B) It changes the rate of
reaction, but it does not affect the start or end of the reaction. (C) A large amount of catalyst is
often required to make a reaction occur. (D) It can be bonded to the
reactants during the reaction, but it is separated by the end. (E) It does not change chemically
before or after the reaction.
In this question, we need to
identify a statement that describes something that is not a characteristic of a
catalyst. We’ll investigate each statement in
turn to test its validity. The first statement suggests that
the catalyst decreases the amount of energy needed for the reaction to proceed. We can investigate this using an
energy profile diagram. In the energy profile diagram shown
here, R represents the reactants; P represents the products.
It’s true for any chemical reaction
that we have to raise the energy of the reactants to make them react. We can see from the energy profile
diagram that this amount of energy is higher without a catalyst compared with the
situation when a catalyst is present. So, statement (A) does describe a
characteristic of a catalyst, and it’s not the correct answer.
Catalysts do change the rate of a
chemical reaction. A positive catalyst speeds the
reaction up, and negative catalyst will slow it down. Catalysts also change how the
reaction begins and also how the reaction ends. Catalysts offer a special surface
where reactant molecules are concentrated. Reactant molecules form an
intermediate compound on the catalyst surface. They are combined with the catalyst
at this point in the reaction. At the end of the reaction, the
products are finally released from the catalyst surface. Although this process is different
compared with the direct reaction without the catalyst, the reactants and products
are the same either way. Statement (B) does describe the
characteristics of a catalyst and is therefore not the correct answer.
Statement (C) suggests that we need
to use a large amount of catalyst to get a reaction started. In the decomposition of hydrogen
peroxide into water and oxygen gas, we only need to add a tiny amount of manganese
dioxide catalyst to get the reaction started. As soon as a small amount of
manganese dioxide catalyst, which is a black powder, is added to the hydrogen
peroxide, a large amount of oxygen bubbles are observed. Statement (C) is therefore not a
characteristic of a catalyst, and it could be the correct answer. We’ll check whether the remaining
statements are valid to be sure.
We’ve already seen that reactant
molecules are bonded to the catalyst surface during the reaction in order to
concentrate them. This results in the formation of an
intermediate compound. For the catalyst to work properly,
the products must be released at the end of the reaction, so they must be separated
from the catalyst surface. In fact, the catalyst surface is
left unchanged chemically before and after the reaction. This means that the last two
statements are characteristics of a catalyst and then therefore not the correct
answers.
Statement (C) a large amount of
catalyst is often required to make a reaction happen is the correct answer.
