Physical Science Lesson 15 Chemical Reactions

Chemical Reactions Chemical Reactions - Catalysts: How Reaction Rate Is Affected

Objectives  At the end of the lesson, lesson, you should be able to define catalyst catalyst and describe describe how it affects affects the reaction rate. Reaction rate is rate is the change in the concentrat concentration ion of reactant or product per unit of time. In the previous lesson, you have learned that the concentration of reactants, temperature, and particle size are factors affecting the reaction rate. In this lesson, you will learn about the catalyst, which is another factor affecting the rate of a chemical reaction.  What is a catalyst? How does it affect reaction reaction rates?

Last Updated: 08.04.16

Learn about it!  A catalyst is catalyst is a substance that increases the reaction rate without being consumed by the reaction. It provides a new pathway for the reaction, one which has a lower activation energy. Consider the points A and B separated by a cliff.

The way to get from points A to B is to go around the cliff following the red path. It would take a lot of time and energy to reach point B.

What happens when a bridge is built between points A and B? 

The path from Point A to B is shorter so it would take less time to reach point B. In addition, there is less energy required to reach point B. Similarly, in chemical reactions, reactants (A) are combined to form products (B). For uncatalyzed reactions (red path), the time it takes for the reaction to be completed is relatively longer. However, for catalyzed reactions, the catalysts (bridge) speed up the reaction by providing an alternate path with less required energy for reactants to turn into products.

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Learn about it! Mechanism of Catalysis  A catalyzed reaction often involves a series of steps. 1. Bonding – The catalyst forms a bond with the reacting molecules. 2. Reaction – The reacting molecules combine or rearrange to form the product. 3. Separation – The product separates from the catalyst.  After separation, the catalyst is free again to form a bond with other reacting molecules.

Energy Diagram of a Catalyzed Reaction Below is the energy diagram for a chemical reaction. The red line represents the uncatalyzed reaction  while the blue one is the catalyzed reaction. Notice that the energies of the reactants and the products are the same for both catalyzed and uncatalyzed reactions. What is the difference between a catalyzed and an uncatalyzed reaction?  The catalyzed reaction has lower activation energy or energy barrier. When there is less energy required for a reaction to proceed, then the reacting molecules will form the products in less time.

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Learn about it! Types of Catalysts  A catalyst can be homogeneous or heterogeneous.  A homogeneous catalyst exists in the same phase as the reaction it catalyzes. It is often in gas or liquid phase. For example, the decomposition of ozone in the atmosphere is catalyzed by chlorine atoms. Overall reaction: The chlorine atom is a catalyst that is regenerated at the end of the reaction.  A heterogeneous catalyst exists in a different phase as the reaction it catalyzes. It often involves gaseous reactant molecules being adsorbed on a solid catalyst surface. For example, gaseous ethylene is hydrogenated to form ethane in the presence of a palladium catalyst.

Learn about it! Enzymes Enzymes are homogeneous, highly specific, and efficient biological catalysts. They speed u p  biological reactions even at relatively low temperatures within the body. They have a shapespecific active sitethat forms bonds with the reacting molecules called substrates. The substrates react and form the product, which then detaches from the catalyst.

Example  An example of an enzyme is sucrase, which catalyzes the hydrolysis of sucrose to glucose and fructose. The first step of the catalysis is the binding between the enzyme sucrase and the substrate sucrose. In the second step, the presence of water hydrolyzes or breaks sucrose into glucose and fructose. The third step involves the release of the products. In the fourth step, the active site of the sucrase is available for another molecule of sucrose.

Tips How do you identify a catalyst in a chemical reaction?  Recall that catalysts are substances that are not consumed in the reaction, or they are regenerated at the end of the reaction. For example, the esterification reaction of acetic acid () and methanol () is an acid-catalyzed reaction. The hydronium ion, , is present both in the reactant and product sides of the reaction. It is part of the chemical reaction, but it is regenerated after product formation.

What do you think? Some reactions are reversible, meaning that a reactant A, can be converted into product B. In the reverse reaction, B is the reactant that can be turned into a product A.  What will be the effect of adding a catalyst for this reaction? Will it increase the rate of the forward reaction? How about for the reverse reaction?

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 A catalyst is a substance that increases the reaction rate without being consumed by the reaction.  A catalyst increases the rate of the reaction by lowering the activation energy of a reaction.  A homogeneous catalyst exists in the same phase as the reaction it catalyzes.  A heterogeneous catalyst exists in a different phase as the reaction it catalyzes. Enzymes are homogeneous, highly specific, and efficient biological catalysts.

Question 1  Which of the following is true about catalysts? Incorrect!

3They increase the free energy of the reaction. Catalysts are substances that increase the reaction rate by lowering the activation energy of the reaction. Next question

1They lower the activation energy of the reaction. 2They are the reacting molecules that become products.4They are the products formed after a reaction. Question 2  What happens to a catalyst at the end of a reaction? Incorrect!

2It escapes as a gaseous product.  A catalyst is regenerated (not consumed) in a course of a reaction. Next question

1It is regenerated.3It is consumed.4It becomes an unrecyclable by-product.

Question 3

 Arrange the following steps of catalysis.

Question 4  Which of the following is true about homogeneous catalysts? Incorrect!

2They are solid catalysts that adsorb gaseous reactants. Homogeneous catalysts exist in the same phase as the reaction they catalyze. They are often in gas or liquid phase. Next question

1They become part of the products after the reaction. 3They are consumed in the reaction. 4They exist in the same phase as the reaction they catalyze. Question 5 Pepsin is a catalyst that aids in the digestion of proteins. It is active in acidic conditions of the stomach.

 Which of the following is the role of pepsin? Incorrect!

2heterogeneous catalyst Pepsin is a biological catalyst. It is an enzyme. Next question

1active site3enzyme4substrate

Question 10

 A certain reaction is carried out with an experimental set-up and a control set-up both at the same temperature. The experimental set-up uses a catalyst while the control set-up is carried out in standard conditions without a catalyst. The end of the reaction is marked by the appearance of a dark blue color.  What observations are expected? Incorrect!

1Both the control and experimental will have a dark blue color at the same time.

The only variable that is certain in the reaction is that the experimental is catalyzed, and the control is not. Therefore, we can only predict that the catalyzed reaction will happen faster compared to the uncatalyzed reaction. Finish quiz

2The control will not proceed with the reaction because it does not have a catalyst. 3The control will increase in temperature because a higher activation energy is required. 4The control will take a longer time for a dark blue color to appear.