Catalysts in Chemistry: Speeding Up Reactions and Transforming Industries
A catalyst is a chemical that speeds up the rate of a chemical reaction without being consumed in the process. Catalysts achieve this by lowering the activation energy required for the reaction to occur. They participate in the reaction mechanism but are regenerated, allowing them to be used repeatedly. This article explores the role of catalysts in chemistry, covering their mechanisms, types, and applications across various fields.
Understanding Catalysis: Lowering the Activation Energy
One way to increase the rate of a chemical reaction is to add a catalyst. Catalysts lower the energy of activation. The addition of a catalyst to a reaction lowers the activation energy, increasing the rate of the reaction. The activation energy of the uncatalyzed reaction is higher, while the catalyzed reaction is lower. The heat of reaction (ΔH) remains unchanged by the presence of the catalyst.
How Catalysts Work: Altering the Reaction Mechanism
A catalyst works by changing the specific way in which the reaction occurs, called its mechanism. After the reaction occurs, a catalyst returns to its original state. The important outcome from the use of a catalyst is that the overall activation energy of the reaction is lowered.
Biological Catalysts: Enzymes
Enzymes in your body act as nature's catalysts, allowing important biochemical reactions to occur at reasonable rates. Enzymes are biological catalysts. Living cells use catalysts to change hydrogen peroxide to water and oxygen because H2O2 is harmful, while H2O and O2 are not. Moreover, the spontaneous reaction is far too slow. Living cells can use the catalyst for other reactions too.
Types of Catalysts: Homogeneous and Heterogeneous
Catalysts can exist in the same phase as the reactants (homogeneous) or in a different phase (heterogeneous). Metals and metal salts are good catalysts because they have specific characteristics, or properties, that make them so suitable as catalysts. Manganese(IV) oxide, MnO2, is an insoluble black/brown powder.
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Applications of Catalysts
Environmental Catalysis: Catalytic Converters
Gasoline-powered vehicles emit various harmful materials. Nitrogen oxides are formed when atmospheric nitrogen reacts with oxygen at the high temperatures found in a car engine. Carbon monoxide is a by-product of the incomplete combustion of hydrocarbons. Evaporated and unused fuel releases volatile hydrocarbons into the atmosphere to help form smog.
A catalytic converter efficiently converts unwanted combustion products (e.g. CO, C, NOx) into CO2 and N2. CO, C and NOx are toxic to the environment, including humans. CO, C and NOx are climate pollutants. N2 came from the air anyway.
Industrial Catalysis: Haber-Bosch Process
Catalysts play a crucial role in the industrial manufacture of ammonia gas. The Haber-Bosch process uses a catalyst to react nitrogen from the inert nitrogen of the air with hydrogen under high pressure at moderate temperatures to produce ammonia. Ammonia is a vital ingredient in fertilizers, drugs, explosives and many other industries.
Industrial Catalysis: Production of Sulfuric Acid
Sulfuric acid production also relies on catalysts. One step involves the conversion of sulfur trioxide using vanadium(V) oxide.
Examples of Catalyzed Reactions
Decomposition of Hydrogen Peroxide
Hydrogen peroxide naturally decomposes to produce water and oxygen gas, but the reaction is very slow. Hydrogen peroxide is used as a disinfectant for scrapes and cuts, and is found in many medicine cabinets as a 3% aqueous solution. A bottle of hydrogen peroxide will last for several years before it needs to be replaced. However, the addition of just a small amount of manganese (IV) oxide to hydrogen peroxide will cause it to decompose completely in just a matter of minutes. The reaction proceeds rapidly and the manganese dioxide catalyst can be filtered off unchanged.
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Hydrogenation of Alkenes
Alkenes contain double bonds between carbon atoms. Catalysts are used in the margarine industry. Margarine is made from vegetable oils. The process involves breaking some of the double bonds in the long alkyl chain by ester linkages to produce a more solid product.
Transition Metals as Catalysts
Transition metals and their compounds often exhibit excellent catalytic activity. This is related to their electronic structure and ability to exist in multiple oxidation states.
Color and Electronic Transitions
Main group compounds are almost never colored. Salts form coloured solutions when dissolved in water because d-d transitions are possible. d orbitals absorbing specific wavelengths of light best explains the colour of a transition metal complex ion in solution.
Ligands and Complex Formation
The cyanide ion is replacing water molecules. Non-bonding pair availability is an essential feature of a ligand. Particles that can act as ligands in complex ion formation include molecules and ions with lone pairs of electrons. To act as ligands in complex ion formation, molecules must be able to coordinate to the transition metal ion.
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