Catalysts ; How a Catalyst Works ??

Catalyst:

A catalyst is a substance that increases the rate of a chemical reaction without being consumed by the reaction. Catalysts are of tremendous importance in all facets of chemistry, from the laboratory to industry.

How a Catalyst Works ?? 

A catalyst works by lowering the activation energy of a reaction so that a larger fraction of the reactants have sufficient energy to react. It lowers the activation energy by providing an alternative mechanism for the reaction.

The potential energy diagram in Figure below shows the activation energy for an uncatalyzed reaction and the activation energy for the same reaction with the addition of a catalyst.

In Figure above, the catalyzed reaction consists of a two-step mechanism. The uncatalyzed reaction consists of a one-step mechanism. To see how a catalyst works, in general, consider a simple, one-step, bimolecular reaction:

A + B → AB

A catalyst can increase the rate of this reaction by providing an alternative mechanism with lower activation energy. A possible mechanism for the catalyzed reaction is shown below.

Step 1: A + catalyst → A-catalyst

Step 2: A-catalyst + B → AB + catalyst

Overall reaction: A + B → AB


Both steps are faster than the original, uncatalyzed reaction. Therefore,although the overall reaction of the catalyzed mechanism has the same reactants and products as the uncatalyzed reaction, the catalyzed mechanism is faster. The chemical species A-catalyst is a reaction intermediate. It is produced in step: 1 but consumed in step: 2. By contrast, the catalyst is regenerated in the reaction. It appears as a reactant in step 1 and as a product in step 2. Although the catalyst changes during the overall reaction, it is regenerated unchanged at the end of the overall reaction.

Catalysts are divided into two categories, depending on whether or not they are in the same phase as the reactants:


1.) Homogeneous Catalysts:

A homogeneous catalyst exists in the same phase as the reactants.
Homogeneous catalysts most often catalyze gaseous and aqueous
reactions. For example, aqueous zinc chloride, ZnCl2, is used to catalyze the following reaction.

The reaction takes place in aqueous solution, and the catalyst is soluble in water. Therefore, ZnCl2 is a homogeneous catalyst when it is used with this reaction.

2.) Heterogeneous catalyst:

A heterogeneous catalyst exists in a phase that is different from the phase of the reaction it catalyzes. An important industrial use of heterogeneous catalysts is the addition of hydrogen to an organic compound that contains C=C double bonds. This process is called hydrogenation. Consider the hydrogenation of ethylene, shown below.

Without a catalyst, the reaction is very slow. When the reaction is catalyzed by a metal such as palladium or platinum, however, the rate increases dramatically. The ethylene and hydrogen molecules form bonds with the metal surface. This weakens the bonds of the hydrogen and ethylene. The H-H bonds of the hydrogen molecules break, and the hydrogen atoms are somewhat stabilized because of their attraction to the metal. The hydrogen atoms react with the ethylene, forming ethane.

Biological Catalysts:

Our body depends on reactions that are catalyzed by amazingly efficient and specific biological catalysts. Biological catalysts are enormous protein molecules called enzymes. Their molecular masses range from 15,000 to 1,000,000 g/mol.

Only a small portion of the enzyme, called the active site, is actually involved in the catalysis reaction. In terms of the enzyme’s overall shape, the active site is like a nook or a fold in its surface. The reactant molecule, called the substrate in an enzyme reaction, binds to the active site. The enzyme works by stabilizing the reaction’s transition state.

Two models currently exist to explain how an enzyme and its substrate interact. One model, called the lock and key model, suggests that an enzyme is like a lock, and its substrate is like a key. The shape of the active site on the enzyme exactly fits the shape of the substrate. A second model, called the induced fit model, suggests that the active site of an enzyme changes its shape to fit its substrate. The figure below shows both models.

Diagram A shows the lock and key model of enzyme function. Diagram B shows the induced-fit model of enzyme function.

Enzymes are involved in many functions of the human body, including digestion and metabolism. For example, the enzyme lactose is responsible for catalyzing the breakdown of lactose, a sugar found in milk. People who are lactose-intolerant are usually missing lactose, or they have insufficient amounts. If we are lactose-intolerant, we can take commercially produced supplements that contain lactose.



By:-
Anjani Kumar Singh