Which of the following best describes the rate of catalytic reaction?

In this explainer, we will learn how to describe and explain the effect catalysts, including enzymes, have on the rate of chemical reactions.

Catalysts are substances that can speed up or slow down chemical reactions. They can do this without being used up or chemically changed as the reactants turn into products. They have essentially the same structure and properties before and after a chemical reaction. The following figure shows how a metal catalyst surface can speed up a reaction by making materials join together and form product molecules.

Definition: Catalyst

A catalyst is a substance that increases the rate of a reaction without undergoing a permanent chemical change.

Example 1: Determining the Best Definition for a Catalyst

Which of the following best defines a catalyst?

1. A substance that provides energy to the reactants to increase the rate of reaction
2. A substance that can make any chemical reaction occur
3. A substance that increases either the concentration or the surface area of reactants to increase the rate of reaction
4. A substance that changes the rate of reaction but is chemically unchanged by the end of the reaction
5. A substance that increases the amount of product produced by a chemical reaction

Answer

Catalysts are substances that can speed up or slow down chemical reactions. They can do this without being changed during the process. Catalysts have essentially the same structure and properties before and after a chemical reaction. This explanation is summarized in option D, and we can use this statement to determine that option D is the correct answer for this question.

Catalysts can speed up chemical reactions because they have the ability to change how reactants turn into products. Chemical reactants can be made to react with less energy in the presence of the right catalyst. The catalysts can make the chemical reactions happen more easily by binding to the reactant molecules during the chemical reaction. The catalyst breaks away from the reactant molecules as they turn into products.

Catalysts have essentially the same chemical properties before and after any chemical reaction. Most chemists use a relatively small amount of catalyst material to change a reaction rate because the same catalyst surface can be used again and again without being used up. The following figure shows how a catalyst surface can be used to speed up a reaction between green and yellow molecules.

Example 2: Identifying the Characteristic Properties of a Catalyst

Which of the following is not a characteristic of a catalyst?

1. It decreases the amount of energy needed for a reaction to proceed.
2. It changes the rate of reaction, but it does not affect the start or end of the reaction.
3. A large amount of a catalyst is often required to make a reaction occur.
4. It can be bonded to the reactants during the reaction, but it is separated by the end.
5. It does not change chemically before or after the reaction.

Answer

Catalysts are substances that change the rate of a chemical reaction while not undergoing any permanent chemical change. They have the capacity to activate chemical reactant substances and make them react with a relatively small amount of energy. Chemists usually use a relatively small amount of catalytic material to change a reaction rate because catalysts are not used up during a chemical reaction. One catalyst surface can be used again and again to speed up or slow down a chemical reaction. The catalysts usually change the reaction rate by binding to reactant molecules during a chemical reaction. The catalysts and reactant molecules are separated from each other by the end of the chemical reaction. All of these statements can be used to determine that options A, B, D, and E are characteristic properties of a catalyst but option C is not a property of a catalyst. This suggests that option C is the answer to this question.

Enzymes are biological catalysts that are made by almost every type of cell. Enzymes tend to be large proteins with a complex shape. They catalyze lots of important reactions. Some enzymes make long molecules break down into small molecules at a very fast rate. Others make small molecules react together and form larger molecules at a very fast rate. The following image shows how an enzyme can rapidly break down long starch molecules into smaller sugar units.

Definition: Enzyme

An enzyme is a biological catalyst that speeds up the rate of reactions without being used up.

Example 3: Identifying the Correct Term for a Biological Catalyst

Which of the following names is given to the class of biological catalysts?

1. Enzymes
2. Substrates
3. Minerals
4. Gametes
5. Alleles

Answer

Biological catalysts are called enzymes and they are almost always large proteins that have complex shapes and structures. Enzymes can speed up lots of different types of chemical reactions. They can make large molecules rapidly break down into smaller molecules and they can make small molecules rapidly bind with each other and form larger molecules. These statements can be used to determine that option A is the correct answer to this question.

Definition: Positive Catalyst

Positive catalysts are substances that speed up the rate of a chemical reaction without being used up or significantly changed as the reactants turn into products.

Positive catalysts are substances that have the capacity to speed up a chemical reaction. They can be added to a reaction mixture to make a chemical reaction happen faster. Negative catalysts are substances that have the capacity to slow down or retard a chemical reaction. They can be added to a reaction mixture to make a chemical reaction happen less rapidly.

Definition: Negative Catalyst

Negative catalysts are substances that slow down the rate of a chemical reaction without being used up or significantly changed as the reactants turn into products.

The following image shows how the rate of a chemical reaction changes if positive and negative catalysts are added to a reaction mixture. The green line shows how reactants turn into products when they are not combined with a catalyst. The blue line shows how the same reactants turn into products when they are combined with a positive catalyst. The red line shows how the reactants turn into products when they are combined with a negative catalyst. The amount of product produced per second is significantly changed if the reaction occurs in the presence of a catalyst.

Example 4: Identifying the Name of a Catalyst That Slows Down a Chemical Reaction

What name is given to a catalyst that can slow down a chemical reaction?

1. Block catalyst
2. Negative catalyst
3. Stop catalyst
4. Slow catalyst
5. Decelerating catalyst

Answer

Negative catalysts are substances that have the capacity to slow down or retard a chemical reaction. Negative catalysts can be added to a reaction mixture to make a chemical reaction happen less rapidly. These statements can be used to determine that option B is the correct answer for this question.

Energy level diagrams can be used to understand why catalysts make chemical reactions happen more rapidly. The following image shows that a minimum amount of energy has to be added to reactants to make them react with each other and turn into product molecules. The catalyst changes the minimum amount of energy that needs to be added to the reactant molecules to make them react and produce product molecules. Less energy is needed to make the chemical reaction happen if the reaction happens in the presence of the catalyst.

Example 5: Determining How a Catalyst Affects the Energy Profile of a Chemical Reaction

The diagram below shows the energy needed for a chemical reaction to occur. How does a positive catalyst affect this energy?

1. It will increase it.
2. It will not affect it.
3. It will decrease it.

Answer

The image shows that a certain amount of energy must be added to the reactants to make them undergo a chemical reaction. The double-sided red arrow represents how much energy must be added to the reactants to make them transform into product molecules. A catalyst can decrease the size of this arrow. It can change the reaction process so that less energy is needed to make the reactants transform into products. This line of reasoning can be used to determine that option C is the correct answer to this question.

The action of positive catalysts can be understood with some simple experiments. A simple scientific equipment can be used to show that hydrogen peroxide molecules decompose more rapidly if they are in the presence of the right type of catalyst.

The following image shows how a student can use relatively simple scientific equipment to understand how a manganese dioxide (MnO2) catalyst can speed up the decomposition of hydrogen peroxide liquid into water and oxygen gas.

How To: Determining How a Manganese Dioxide Catalyst Affects the Decomposition of Hydrogen Peroxide Molecules

The setup for this reaction is shown below.

The steps to perform this experiment are as follows:

1. Attach two identical test tubes to clamp stands.
2. Fill the test tubes with 10 cm3 of hydrogen peroxide liquid.
3. Add a small amount of manganese dioxide powder to one of the test tubes.
4. Observe the test tubes to determine which sample releases oxygen bubbles at a faster rate.

The experiment should show that hydrogen peroxide decomposes more rapidly in the presence of the manganese dioxide catalyst. The manganese dioxide powder is a positive catalyst in this experiment, and it makes the reactant hydrogen peroxide molecules break down more rapidly.

Hydrogen peroxide can also be mixed with sweet potato to make the hydrogen peroxide molecules decompose more rapidly. The following image shows how a student can use an altogether different set of scientific equipment to show how naturally occurring enzymes speed up the decomposition of hydrogen peroxide molecules.

How To: Determining How a Piece of Sweet Potato Affects the Decomposition of Hydrogen Peroxide Molecules

The setup for this reaction is shown below.

The steps to perform this experiment are as follows:

1. Fill two identical beakers with 300 cm3 of hydrogen peroxide liquid.
2. Cut a small piece of sweet potato and add it to one of the beakers.
3. Observe the beakers to determine which sample releases oxygen bubbles at a faster rate.

Students should find that lots of (oxygen) gas bubbles are produced at the edge of the sweet potato sample. The sweet potato acts as a positive catalyst in this experiment because it makes the hydrogen peroxide molecules turn into product gas molecules more rapidly. The sweet potato can speed up the decomposition of hydrogen peroxide molecules because it contains a relatively high concentration of the oxidase enzyme.

Chemists have been using metal catalysts for the last few decades to speed up the decomposition of pollutant gases and turn them into less toxic products. Most cars that were produced during and after the 1980s were built with catalytic converters. The following image shows how catalytic converters can be used to speed up the decomposition of pollutant gases and turn them into less harmful products like water and nitrogen molecules.

Catalytic converters contain large honeycomb-like structures that are made of catalytic metals such as platinum. These honeycomb-like structures have very high surface areas, and the catalytic converters can react with lots of harmful gases before they are expelled from the car engine through the car exhaust.

Catalysts are used by humans everyday to make all sorts of different chemical substances and to enhance all sorts of important chemical reactions. Most people have a plentiful supply of the lactase enzyme, which speeds up the decomposition of lactose sugars, but some people are lactose intolerant and they usually add lactase enzymes to milk to make it easier to digest.

Enzymes are also added to both laundry and dishwasher detergents to make them more effective at breaking down dirt and oily residues. Catalysts are also used by chefs to enhance the Maillard reaction and make pretzel crust a more appetizing brown color. New catalysts are even being added to objects like windows to give them the ability to break down dirt and clean themselves.

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