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Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

Section 5: Catalysts

Catalysts and Activation Energy

Figure 12-7. Catalysts and Activation Energy

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Catalysts and Activation Energy

Figure 12-7. Catalysts and Activation Energy

In the presence of a catalyst, the activation energy for the reaction is lower. Note how the two new steps both have activation barriers that are much lower than the original uncatalyzed activation barrier.

Another way to increase the reaction rate is to add a catalyst to the reaction. A catalyst works by providing an alternate mechanism for the reaction to occur. The alternate pathway has a lower activation energy, allowing the reaction to proceed faster. (Figure 12-7) In actuality, what happens is that the catalyst will allow for a new mechanism with new steps, each of which has a new activation barrier; but if the catalyst really speeds up the reaction, the tallest "hill" on the new path will be shorter than the original "hill" or activation barrier.

With a lower net activation energy, more reactants can make it over the hump, and the reaction goes faster.

Catalytic Converter

Figure 12-8. Catalytic Converter

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Catalytic Converter

Figure 12-8. Catalytic Converter

Catalytic converters in the exhaust system of a car convert toxic substances such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) into gases that are safe to breathe.

The catalytic converters in the exhaust system of cars use various metal catalysts to reduce pollution. Car exhaust contains toxic substances such as carbon monoxide, nitrogen oxides, and hydrocarbons. In the catalytic converter, these gases pass through the pores of a catalyst-coated mesh; the toxins react to form gases that are safe to breathe: CO2, H2O, and N2. (Figure 12-8)

The thousands of chemical reactions that occur in living things could not occur without catalysts. In living things, catalysts called "enzymes" regulate everything from the construction of DNA to the digestion of nutrients. To digest the sugar in milk (lactose), the enzyme lactase must first break a bond to create the simpler sugars, galactose and glucose. (Figure 12-9) Human infants produce lactase to digest their mothers' milk; as most people grow older and switch to other foods, lactase production stops. The inability to digest lactose is called "lactose intolerance"; eating milk products can then cause bloating and gastrointestinal upset.

The Lactase Enzyme

Figure 12-9. The Lactase Enzyme

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The Lactase Enzyme

Figure 12-9. The Lactase Enzyme

The lactase enzyme breaks down lactose into simpler sugars. If the body stops producing lactase as we get older, we either have to cut back on the eating of dairy products or take supplements that contain the lactase enzyme that the body no longer makes. Without the enzyme, the reaction to digest lactose is too slow to occur in the body.

Glossary

Catalyst

A substance that speeds up a chemical reaction, yet remains unchanged by the chemical reaction.

Enzyme

A biological catalyst.

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