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Question Video: Identifying the Characteristic Properties of a Catalyst Science

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.

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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.

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