B.Sc. 2nd Year Chemistry – Detailed and Exam-Oriented Notes
Catalysis is the phenomenon in which the rate of a chemical reaction is altered by the presence of a substance called a catalyst, which is not consumed overall in the reaction.
A catalyst generally increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. In some cases, substances called negative catalysts or inhibitors decrease the rate of reaction.
Catalysis is extremely important in industrial chemistry, petroleum processing, environmental chemistry, biological systems and the manufacture of important chemicals.
A catalyst is a substance that changes the rate of a chemical reaction without undergoing permanent overall chemical change at the end of the reaction.
The catalyst may participate in individual elementary steps and may form intermediate compounds, but it is regenerated during the overall catalytic cycle.
The decomposition of hydrogen peroxide is slow in the absence of a suitable catalyst but is greatly accelerated by substances such as manganese dioxide.
MnO2 acts as a catalyst and is not consumed in the overall reaction.
When a substance increases the rate of a chemical reaction, the phenomenon is called positive catalysis.
Example: Manganese dioxide accelerates the decomposition of hydrogen peroxide.
When a substance decreases the rate of a reaction, the phenomenon is called negative catalysis.
Such substances are often called inhibitors or retarders.
In homogeneous catalysis, the catalyst and reactants are present in the same physical phase.
For example, in a liquid-phase reaction, both the reactants and catalyst may be present in the same solution.
Nitric oxide can participate in the oxidation of sulfur dioxide through a homogeneous gas-phase catalytic mechanism.
Nitric oxide participates in intermediate steps and is regenerated.
In heterogeneous catalysis, the catalyst and reactants are present in different physical phases.
A common example is a solid catalyst used in a gas-phase reaction.
Finely divided iron is used as the catalyst in the industrial manufacture of ammonia.
Nickel provides an active surface on which hydrogen and the organic molecules can adsorb and react.
| Homogeneous catalysis | Heterogeneous catalysis |
|---|---|
| Catalyst and reactants are in the same phase. | Catalyst and reactants are in different phases. |
| Reaction occurs throughout the phase. | Reaction commonly occurs at the catalyst surface. |
| Separation of catalyst may be difficult. | Solid catalysts can often be separated easily. |
| Mechanisms may involve soluble intermediates. | Adsorption and surface reactions are often important. |
Autocatalysis occurs when one of the products formed during a reaction acts as a catalyst for the same reaction.
In the oxidation of oxalate ions by permanganate ions in acidic medium, Mn2+, one of the products, accelerates the reaction.
The reaction initially proceeds slowly and becomes faster as Mn2+ accumulates.
Induced catalysis refers to a situation in which one reaction influences or facilitates another reaction that would otherwise occur very slowly under the same conditions.
The phenomenon can occur when a species generated in one reaction participates in or promotes another reaction.
Enzymes are biological catalysts produced by living organisms. Most enzymes are proteins, although some catalytic RNA molecules also exist.
Enzymes accelerate biochemical reactions under mild conditions and generally show high specificity toward their substrates.
The enzyme catalase accelerates the decomposition of hydrogen peroxide.
Enzyme catalysis is commonly explained by the formation of an enzyme-substrate complex.
Where:
The substrate binds to a specific region of the enzyme known as the active site.
The lock-and-key model, proposed by Emil Fischer, describes the enzyme active site as having a shape complementary to the substrate.
According to this model, the substrate fits into the active site in a manner similar to a key fitting into a lock.
The induced-fit model, associated with Daniel Koshland, proposes that the enzyme active site can undergo a conformational change when the substrate approaches.
Binding of the substrate induces a suitable structural arrangement that facilitates the reaction.
This model explains both enzyme specificity and the ability of enzymes to stabilize the transition state.
A chemical reaction requires an energy barrier known as activation energy.
A catalyst provides an alternative reaction pathway with a lower activation energy.
The catalyst stabilizes the transition state or provides a sequence of elementary steps that requires less activation energy than the uncatalysed pathway.
A catalyst does not change the position of equilibrium.
In a reversible reaction, the catalyst lowers the activation energy for both the forward and reverse reactions.
Therefore, equilibrium is reached more rapidly, but the equilibrium composition remains unchanged.
The equilibrium constant is not changed by the presence of a catalyst.
A promoter is a substance that increases the activity or efficiency of a catalyst without itself necessarily acting as the principal catalyst.
In the Haber process, iron is the main catalyst and substances such as potassium compounds and aluminium oxide can act as promoters in industrial catalyst formulations.
Catalyst poisons are substances that decrease the activity of a catalyst, often by strongly adsorbing onto active sites.
Poisoning is particularly important in heterogeneous catalysis because the poison may block active surface sites.
Sulfur-containing compounds can poison several metal catalysts.
In heterogeneous catalysis, the catalytic reaction commonly occurs on the surface of a solid catalyst.
The availability of active surface sites is therefore important for catalytic efficiency.
Adsorption brings reactant molecules onto the catalyst surface and can weaken existing chemical bonds, making the molecules more reactive.
Two broad types of adsorption are commonly discussed:
Physical adsorption, or physisorption, involves relatively weak intermolecular forces.
Chemical adsorption, or chemisorption, involves stronger interaction between the adsorbate and catalyst surface and is particularly important in many catalytic reactions.
According to the contact theory, reactant molecules become adsorbed on the catalyst surface and are brought into close contact.
The adsorbed molecules become activated and react to form products. The products then leave the surface, making the active sites available for another catalytic cycle.
According to the intermediate compound theory, a catalyst may react temporarily with one or more reactants to form an unstable intermediate.
The intermediate subsequently reacts with another reactant to form the products and regenerate the catalyst.
Here C represents the catalyst. It participates in the individual steps but is regenerated in the overall reaction.
Catalysts are often selective or specific. A particular catalyst may strongly favor one reaction pathway over competing pathways.
This property is particularly important in organic synthesis and industrial chemistry because it can improve product yield and reduce unwanted by-products.
Ammonia is manufactured by catalytic reaction of nitrogen and hydrogen.
Finely divided iron-based catalysts are used in industrial practice.
Sulfur dioxide is oxidized to sulfur trioxide using a catalyst.
Vanadium(V) oxide is widely used as the catalyst in the industrial Contact process.
Nickel, palladium or platinum can catalyze hydrogenation reactions.
Ammonia is oxidized catalytically to nitric oxide in the industrial production of nitric acid.
Catalytic converters are used in automobile exhaust systems to reduce harmful gaseous pollutants.
Precious-metal catalysts such as platinum, palladium and rhodium may be used in catalytic converter systems.
Catalytic converters therefore help reduce emissions of carbon monoxide, unburned hydrocarbons and nitrogen oxides.
Temperature influences both the reaction rate and the adsorption of reactants on a catalyst surface. An optimum temperature is often required for maximum catalytic efficiency.
For many solid catalysts, increased surface area provides more active sites and can increase catalytic activity.
In homogeneous systems, increasing catalyst concentration can increase the reaction rate when catalyst concentration is a controlling factor.
Promoters may enhance catalyst activity or stability.
Poisons reduce catalytic activity by blocking active sites or otherwise interfering with the catalytic mechanism.
The concentration or partial pressure of reactants can influence the rate of catalytic reactions.
Catalytic activity refers to the ability of a catalyst to increase the rate of a reaction.
Selectivity refers to the ability of a catalyst to favor formation of a desired product over competing products.
A catalyst support is a material on which an active catalytic substance is dispersed.
Supports can increase surface area, improve dispersion of the active component and enhance mechanical stability.
Common support materials include alumina, silica and activated carbon.
Almost every living cell depends on enzyme-catalyzed reactions. Without enzymes, many biochemical reactions would proceed too slowly to support life.
Examples include:
Catalysis is one of the most important principles used in modern chemical industries.
| Type | Meaning | Example |
|---|---|---|
| Positive catalysis | Increases reaction rate. | MnO₂ in H₂O₂ decomposition. |
| Negative catalysis | Decreases reaction rate. | Inhibitors in oxidation reactions. |
| Homogeneous catalysis | Catalyst and reactants are in the same phase. | Soluble acid/base catalysis. |
| Heterogeneous catalysis | Catalyst and reactants are in different phases. | Fe in Haber process. |
| Autocatalysis | One product accelerates the reaction. | Mn²⁺ in permanganate-oxalate reaction. |
| Enzyme catalysis | Biological molecules catalyze biochemical reactions. | Catalase decomposition of H₂O₂. |
Catalysis is the phenomenon in which the rate of a chemical reaction is changed by the presence of a catalyst. Catalysts generally increase reaction rate by providing an alternative pathway with lower activation energy.
Catalysis may be homogeneous or heterogeneous. Other important forms include positive catalysis, negative catalysis, autocatalysis and enzyme catalysis.
In heterogeneous catalysis, adsorption of reactants on the catalyst surface is particularly important. The major steps include adsorption, activation, surface reaction and desorption.
Promoters can improve catalyst performance, whereas catalyst poisons reduce activity. Enzymes are highly specific biological catalysts that operate under relatively mild conditions.
Catalysis has enormous industrial importance in ammonia production, sulfuric acid production, nitric acid production, hydrogenation, petroleum processing and automobile emission control.