A heterogeneous catalyst is a catalyst that is present in a different phase usually a solid than the reactants. Such catalysts generally function by furnishing an active surface upon which a reaction can occur. Gas and liquid phase reactions catalyzed by heterogeneous catalysts occur on the surface of the catalyst rather than within the gas or liquid phase.
Any one of these steps may be slow and thus may serve as the rate determining step. In general, however, in the presence of the catalyst, the overall rate of the reaction is faster than it would be if the reactants were in the gas or liquid phase.
Figure 6 illustrates the steps that chemists believe to occur in the reaction of compounds containing a carbon—carbon double bond with hydrogen on a nickel catalyst.
Nickel is the catalyst used in the hydrogenation of polyunsaturated fats and oils which contain several carbon—carbon double bonds to produce saturated fats and oils which contain only carbon—carbon single bonds.
Other significant industrial processes that involve the use of heterogeneous catalysts include the preparation of sulfuric acid, the preparation of ammonia, the oxidation of ammonia to nitric acid, and the synthesis of methanol, CH 3 OH. Heterogeneous catalysts are also used in the catalytic converters found on most gasoline-powered automobiles Figure 7.
Scientists developed catalytic converters to reduce the amount of toxic emissions produced by burning gasoline in internal combustion engines. Catalytic converters take advantage of all five factors that affect the speed of chemical reactions to ensure that exhaust emissions are as safe as possible. By utilizing a carefully selected blend of catalytically active metals, it is possible to effect complete combustion of all carbon-containing compounds to carbon dioxide while also reducing the output of nitrogen oxides.
This is particularly impressive when we consider that one step involves adding more oxygen to the molecule and the other involves removing the oxygen Figure 7. Most modern, three-way catalytic converters possess a surface impregnated with a platinum-rhodium catalyst, which catalyzes the conversion nitric oxide into dinitrogen and oxygen as well as the conversion of carbon monoxide and hydrocarbons such as octane into carbon dioxide and water vapor:.
In order to be as efficient as possible, most catalytic converters are preheated by an electric heater. This ensures that the metals in the catalyst are fully active even before the automobile exhaust is hot enough to maintain appropriate reaction temperatures. The study of enzymes is an important interconnection between biology and chemistry. Enzymes are usually proteins polypeptides that help to control the rate of chemical reactions between biologically important compounds, particularly those that are involved in cellular metabolism.
Different classes of enzymes perform a variety of functions, as shown in Table Enzyme molecules possess an active site, a part of the molecule with a shape that allows it to bond to a specific substrate a reactant molecule , forming an enzyme-substrate complex as a reaction intermediate.
There are two models that attempt to explain how this active site works. The most simplistic model is referred to as the lock-and-key hypothesis, which suggests that the molecular shapes of the active site and substrate are complementary, fitting together like a key in a lock.
The induced fit hypothesis, on the other hand, suggests that the enzyme molecule is flexible and changes shape to accommodate a bond with the substrate. Both the lock-and-key model and the induced fit model account for the fact that enzymes can only bind with specific substrates, since in general a particular enzyme only catalyzes a particular reaction Figure 8.
The Royal Society of Chemistry provides an excellent introduction to enzymes for students and teachers. Catalysts affect the rate of a chemical reaction by altering its mechanism to provide a lower activation energy. Catalysts can be homogenous in the same phase as the reactants or heterogeneous a different phase than the reactants. How do reaction rates give information about mechanisms?
What is a reaction rate constant? How do the reaction rates change as the system approaches equilibrium? Why does the rate of reaction increase with concentration? Why do rates of reaction change with pH? See all questions in Rate of Reactions. Impact of this question views around the world. You can reuse this answer Creative Commons License. Enzymes are important for controlling reactions in cells. They are also important in industry. The use of enzymes allows some industrial reactions to happen at lower temperatures and pressures than traditionally needed.
Yeast is a single-celled fungus. The enzymes in yeast are used to produce wine, beer and other alcoholic drinks by fermentation of sugars.
Catalysts A catalyst is a substance that: increases the rate of a reaction does not alter the products of the reaction is unchanged chemically and in mass at the end of the reaction Only a very small mass of catalyst is needed to increase the rate of a reaction.
Catalyst Reaction catalysed Iron Haber process making ammonia Vanadium V oxide Contact process a stage in making sulfuric acid Manganese dioxide Decomposition of hydrogen peroxide produces water and oxygen Notice that these catalysts are transition metals or compounds of transition metals.
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