Another method of increasing the reac-tion rate is by employing a catalyst. A cata-lyst is a substance that speeds up the reaction but is not consumed in the reaction.
A substance that slows down or stops a reac-tion in known as an inhibitor. To under-stand how catalysts work and their role in reaction kinetics requires knowledge of reaction mechanisms. A reaction mecha-nism is the series of reactions or steps involved in the conversion of reactant to Figure 12.3
Dividing a substance into smaller pieces increases the amount of exposed surface area. The volume of the large cube and the total volume of the eight small cubes are the same, but the small cubes have twice the total surface area.
products. When a chemical reaction is writ-ten, the equation often represents the sum of a series of reactions. For example, the deple-tion of ozone in the atmosphere can be rep-resented with the reaction
O3 O p 2O2
According to this reaction, ozone, O3, com-bines with atomic oxygen to produce molec-ular oxygen. One mechanism to explain this reaction is
The reaction mechanism shows that while the sum of reactions 1 and 2 results in the original reaction given for ozone depletion, the ozone does not react directly with atomic oxygen but with chlorine. Steps 1 and 2 are called the elementary steps in the mechanism. Summing the elementary steps in a reaction mechanism gives the overall or net reaction. Chemical reactions are gener-ally presented as the net reaction, and the elementary steps are typically omitted. It is important to remember that while the net reaction gives the reactants and products, the mechanisms show how the reactants became products. We can think of the reac-tants and products as the start and destina-tion of a trip. We may start a trip in New York City and end in Los Angeles, but there are numerous paths we could use to make our trip. We might go directly from New York to L.A., but we just as easily could have stopped in St. Louis and Denver on the way. Similarly, reactants may go directly to products, but there may also be intermedi-ate reactions along the way.
The reaction mechanism shown for ozone depletion includes chorine. Chlorine in this reaction acts as a catalyst. A principal source of this chlorine is from the ultravio-let breakdown of CFC
(chlorofluorocar-bons) in the upper atmosphere. A catalyst works by altering the reaction mechanism so that the activation energy of the reaction is lowered. If the reaction mechanism is con-sidered as a path, a catalyst can be viewed as providing a shortcut around the activation energy barrier. As an analogy, consider a group of bicycle riders who need to climb a steep mountain. One path might be a road straight to the top, while another might be a tunnel bored through the mountain. Only the heartiest cyclists may be able to make it over the mountain by going up the road, but many more will be able to get to the other side by using the tunnel. To see how a cata-lyst works in a chemical reaction, reconsider the decomposition of hydrogen peroxide. As pointed out at the start of this chapter, hydrogen peroxide slowly decomposes into water and oxygen. Iodide serves as a cata-lyst for this reaction according to the fol-lowing mechanism:
In the decomposition of hydrogen peroxide, the iodide catalyst reacts in the first ele-mentary step, and it is regenerated in the second step.
In a reaction mechanism with two or more steps, the slowest step will control the rate of the net reaction. This step is referred to as the rate determining step. The rate determining step in a reaction mechanism can be compared to the slowest step in a series of activities. For example, say we were mailing out letters and set up an assembly line of several people that included the following tasks: 1) take enve-lope out of box, 2) place stamp on enveenve-lope, 3) put letter in envelope, 4) address enve-lope, and 5) seal envelope. All the steps except step 4 could be done in a matter of seconds. It might take a minute or two to ClⳭ O3r ClO Ⳮ O2 1
address the envelope, and this step essen-tially controls how fast the letters can be prepared. In a similar fashion, the slowest reaction in a series of reactions making up the mechanism controls the overall rate of the reaction.
The importance of catalysts in chemical reactions cannot be overestimated. In the destruction of ozone previously mentioned, chlorine serves as a catalyst. Because of its detrimental effect to the environment, CFCs and other chlorine compounds have been banned internationally. Nearly every indus-trial chemical process is associated with numerous catalysts. These catalysts make the reactions commercially feasible, and chemists are continually searching for new catalysts. Some examples of important cat-alysts include iron, potassium oxide, and aluminum oxide in the Haber process to manufacture ammonia; platinum and rhodium in the Ostwald synthesis of nitric
acid; and nickel when vegetable oil is hydro-genated to produce saturated fats such as margarine. Catalytic converters in automo-biles rely on platinum, rhodium, and palla-dium as catalysts. The catalytic converters aid in the complete combustion of emissions from engines by converting carbon monox-ide to carbon dioxmonox-ide, nitric oxmonox-ides into nitrogen and oxygen, and hydrocarbons into carbon dioxide and water. Because catalytic converters must perform both oxidation and reduction (see Chapter 14 on electrochem-istry), catalytic converters generally work in two stages. The hot exhaust first passes through a bed of rhodium catalyst to reduce nitric oxides and then through a platinum catalyst to oxidize CO and hydrocarbons (Figure 12.4).
Enzymes are catalysts that accelerate metabolic processes in organisms. The word
“enzyme” comes originally from the Greek term “enzymos” for leavening. Enzymes are
Figure 12.4
Catalytic Converter (Rae Déjur)
a critical ingredient in bread, wine, yogurt, and beer. Without enzymes, the biochemi-cal processes would be much too slow.
Enzymes have the ability to speed chemical reactions tremendously. It is not unusual for enzymes to increase the reaction rate by a factor of a million. Like any other catalyst, enzymes work by lowering the activation energy. Enzymes, though, are very specific with respect to the compound they interact with and the reactions they catalyze. The compound associated with a particular enzyme is known as the substrate. A gen-eral model to explain how enzymes work is the lock-and-key model (Figure 12.5). In this model, the enzyme, which is typically a large protein molecule, has a “keyhole” or region known as an active site. The active site has the specific shape and chemical characteristics to act on a particular sub-strate. The substrate enters the active site just as a key would enter a keyhole. The enzyme-substrate forms a complex where the chemical reaction takes place. The enzyme catalyzes the substrate into the product and then the product separates from
the enzyme. The human body contains thou-sands of enzymes involved in all biochemi-cal processes such as digestion, respiration, and reproduction.
Enzymes in humans work best at tem-peratures of 37°C. When temtem-peratures climb too high, the efficiency of an enzyme can be greatly reduced. This is one example where an increase in temperature can retard the reaction rate. Another problem is that certain substances can disrupt enzymes by blocking active sites and preventing the sub-strate from bonding with the enzyme. Sub-stances that disrupt enzymes are known as inhibitors. Many poisons and drugs fit in this category.