Lesson Video: Catalysts | Nagwa Lesson Video: Catalysts | Nagwa

Lesson Video: Catalysts Science • Third Year of Preparatory School

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

15:06

Video Transcript

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

Some chemical reactions may be very slow to get started. The reactant molecules do not combine easily with each other to directly form products. Catalysts allow the reaction to happen via an easier route. They do this by providing a special surface where the reactant molecules are concentrated together so that they can react with each other easily. Some catalysts involve metals or metal compounds, and the reactants become attached to the surface of the catalyst, forming an intermediate compound. The catalyst is chemically involved in the reaction at this point. After a short time, the reactants have reacted on the catalyst surface and product molecules are formed. Finally, the product molecules must be released from the catalyst surface for the reaction to be complete.

The pathway for the process involving the catalyst involves less energy to get started. Since the pathway for this process is much easier than the direct reaction without the catalyst, the reaction takes place in less time, so the rate is faster. There is something to note about the catalyst when its job is complete in this example. Firstly, the catalyst is chemically unchanged after the reaction. Although the catalyst became chemically involved in the reaction, its chemical structure and properties are the same before and after the reaction that it took part in. We can therefore define a catalyst as a substance that increases the rate of a reaction without undergoing a permanent chemical change.

As an example of how a catalyst behaves, we could take one gram of a catalyst substance and add it to a mixture of reactants. The reactants may be involved in a very slow reaction, so we might not see much happening there. As soon as the catalyst were added, the reaction would happen very rapidly and products would be formed. After the reaction, one gram of catalyst would remain chemically unchanged. The catalyst could be separated out by filtration. It could be washed and dried and reused.

Notice that the mass of catalyst at the start of the reaction is exactly the same as the mass of the catalyst at the end of the reaction. In reality, only a very small amount of catalyst is required to speed up a reaction by a large amount. A good example of this is the reaction for the decomposition of hydrogen peroxide. Hydrogen peroxide has the molecular formula H2O2. In this decomposition reaction, two molecules of hydrogen peroxide are decomposed to make two molecules of water. At the same time, one molecule of oxygen gas is also formed. The decomposition reaction of hydrogen peroxide to form water and oxygen gas is a very slow reaction without a catalyst under normal conditions. Hydrogen peroxide is a liquid under normal conditions. And if we left a beaker of it lying around, we wouldn’t see many bubbles of oxygen being formed at all.

We could use the decomposition of hydrogen peroxide to perform a simple experiment to see how a catalyst works. Two test tubes would be clamped into stands and filled with an equal amount of hydrogen peroxide. Under normal conditions, there will be no bubbles observed coming from the hydrogen peroxide in either test tube over a short period of time. When a small amount of manganese dioxide powder is added to one of the test tubes, the decomposition reaction would suddenly take place much faster in that test tube. Manganese dioxide takes the form of a black powder and it’s a catalyst for the decomposition of hydrogen peroxide. Oxygen gas bubbles are produced rapidly in this test tube and the heat from the reaction would create some water vapor.

We can tell from these observations that manganese dioxide is behaving as a catalyst. In the test tube, where no catalyst was added to the hydrogen peroxide, we wouldn’t see any oxygen gas bubbles over a short period of time. After the experiment, the catalyst could be separated out and used over and over again. In this example, the manganese dioxide is behaving as a positive catalyst as it made the reaction happen faster.

Some catalysts may have the opposite effect, and they will slow a reaction down. These are termed negative catalysts. A negative catalyst is a substance that slows down the rate of a chemical reaction without being used up or significantly changed as the reactants turn into products.

In the next section of this video, we’ll take a look at how enzymes work. Enzymes are biological catalysts made by certain types of cell. Enzymes are very large molecules, and they have complex shapes. Amylase is an example of an enzyme that can break down large starch molecules. Starch is found in the food that we eat. And because it’s such a large molecule, it can’t pass easily through the gut wall during digestion. The starch molecule needs to be broken down into smaller sugar molecules to be absorbed into the body properly. It attaches itself to the enzyme during this digestion process. At the end of the chemical reaction, the enzyme is unchanged and the smaller sugar molecules are released.

We’ll now take a look at a simple experiment to observe the effect of an enzyme. The experiment involves the decomposition of hydrogen peroxide that we saw earlier. Firstly, two glass beakers would be filled with a small amount of an identical hydrogen peroxide solution. A small piece of freshly cut sweet potato is placed into one of the beakers. The other beaker is left alone as a comparison. After a very short time, bubbles of oxygen gas are seen to form rapidly in the beaker containing the sweet potato and the hydrogen peroxide.

This observation provides evidence that the decomposition of the hydrogen peroxide is occurring rapidly. No bubbles of oxygen gas are observed in the other beaker, which does not contain the sweet potato. The sweet potato contains a naturally occurring enzyme called oxidase, which speeds up the decomposition of hydrogen peroxide compared to normal conditions.

In the next section, we’ll take a look at the uses of catalysts and why they’re used in certain situations. Catalyst can be used to save time and energy in chemical processes. We can understand this by looking at an energy level diagram. If we can save energy in a chemical process, we’ll be saving money too. The diagram shows that a minimum amount of energy must be added to the reactants to make them react and turn into products. If a catalyst is added, less energy is needed to make the reaction happen. We can see this in the energy profile diagram for the reaction with a catalyst. In industry, we can save time and also the cost of energy required to run a chemical process by using the catalyst.

Catalysts are used in catalytic converters located in the exhaust pipes of cars. Catalytic converters contain precious metal catalysts that speed up the removal of toxic gases from the exhaust stream created by the petrol or diesel engine. Toxic gases leaving the engine from the combustion process may include carbon monoxide, nitric oxide, nitrogen dioxide, and even unburned fuel. Since the catalyst metals used are valuable, they’re spread out over a large honeycombs structure of thin tubes to increase their surface area. This means only a small amount of catalyst is needed inside the catalytic converter. Inside the catalytic converter, the toxic gases produced by the engine are converted into harmless products such as nitrogen, carbon dioxide, water, and oxygen.

Now, it’s time to look at a question to test our understanding of catalysts.

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.

Now, it’s time to review the key points from this video. Catalysts change the rate of a chemical reaction without being used up or chemically changed as reactants turn into products. A catalyst that speeds up a chemical reaction is known as a positive catalyst, whilst a catalyst that slows down a reaction is known as a negative catalyst. Enzymes are biological catalysts. They’re produced by certain types of specialist cells. Catalysts change the minimum amount of energy that needs to be added to reactant molecules to make them react. Catalytic converters are used in cars to reduce emissions of harmful pollutant gases.

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