1. Describe in general how a catalyst increases the rate of a reaction without being consumed.
2. Describe the essentials of heterogeneous, homogeneous, and enzyme catalysis.

Mechanism of a Catalyzed Reaction. A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the reaction. For this reason a catalyst does not appear in the stoichiometric equation. Chromium(III) oxide, Cr₂O₃, is the catalyst present in catalytic converters in American automobiles. This compound accelerates the reaction of carbon monoxide with oxygen. By converting CO to CO₂ in the exhaust stream, CO emissions are reduced.

The lifetime of catalytic converters is quite long; the Cr₂O₃ and other catalysts do not need to be replaced.

Catalysts accelerate reaction rates by providing a new reaction pathway (mechanism) which has a lower activation energy. See Figure 13.20 (textbook). In the catalytic converter both CO and O₂ are chemically adsorbed on the catalyst's surface. Adsorption of O₂ by Cr₂O₃ weakens the O—O bond enough so that oxygen atoms can react with adsorbed CO. The reaction path involving a weakened O—O bond has a significantly lower activation energy than that of the reaction occurring purely in the gas phase. Recall that as the activation energy is lowered the rate constant for a reaction increases. After CO₂ is formed, it desorbs from the surface, leaving the Cr₂O₃ catalyst chemically unaltered. The catalyst is not consumed in the reaction. Catalysts are regenerated in one of the last steps of the mechanism. The textbook describes three types of catalysts, which we will review here.

Heterogeneous Catalysis. In heterogeneous catalysis the reactants are in one phase and the catalyst is in another phase. The catalyst is usually a solid, and the reactants are gases or liquids. Ordinarily the site of the reaction is the surface of the solid catalyst. Many industrially important reactions involve gases and are catalyzed by solid surfaces. For example, the Haber process for the synthesis of ammonia is catalyzed by iron plus a few percent of the oxides of potassium and aluminum. The famous Ziegler-Natta catalyst for polymerizing ethylene gas (C₂H₄) into polyethylene polymer contains triethylaluminum and titanium or vanadium salts.

Homogeneous Catalysis. In homogeneous catalysis the reactants, products, and catalysts are all in the same phase, which is usually the gas or the liquid phase. Many reactions are catalyzed by acids. The decomposition of formic acid (HCO₂H) is an example.

HCO₂H H₂O + CO

Formic acid is normally stable and lasts a long time on the shelf. However, when sulfuric acid is added, bubbles of carbon monoxide gas can be observed immediately. The hydrogen ions from the sulfuric acid initiate a new reaction path. The mechanism is

HCO₂H + H⁺ HCO₂H⁺
HCO₂H⁺ H₂O + HCO⁺
HCO⁺ CO + H⁺
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HCO₂H CO + H₂O

Note that the H⁺ ion is consumed in the first step, but, as for all catalysts, H⁺ is regenerated. The net reaction is the sum of the three steps. The intermediate cancels out as does the catalyst.

Enzyme Catalysis. Nearly all chemical reactions that occur in living organisms require catalysts. Enzymes are biological catalysts. All enzymes are protein molecules with molecular masses well over 10,000 amu. All enzyme molecules are highly specific. That is, they can only affect the reaction rates of a few specific reactant molecules. Typically, an enzyme catalyzes a single reaction, or a group of closely related reactions. The molecule on which an enzyme acts is called a substrate.

The simplest mechanism which explains enzyme activity, and is consistent with these trends is one in which the substrate S and the enzyme E form an enzyme-substrate complex, ES. This complex has a lower energy than the activated complex without the enzyme ( Figure 13.27 of the text). The enzyme-substrate complex can either dissociate back into E and S, or break apart into the products P and the regenerated enzyme E. The simplest mechanism has the two steps shown below.