[Chemistry Class Notes] How Does a Catalyst Affect The Rate of Chemical Reactions? Pdf for Exam

What is a Catalyst? What is Catalysis?

Catalysts are substances that modify the reaction rate and themselves remain chemically and quantitatively unchanged after the reaction. The mechanism by which a catalyst increases the reaction rate is referred to as catalysis.

In this article, we will study what is the meaning of catalyst, how does a catalyst affect the rate of chemical reactions, and also the type of catalysis.

What is a Catalyst in a Chemical Reaction? 

Let’s try to understand this with the help of an example- When potassium chlorate is heated, it readily decomposes to give dioxygen. This decomposition occurs at high temperatures- 653-873K.

2KClO3 → 2KCl + 3O2

When manganese dioxide is added, decomposition takes place at a lower temperature and a much faster rate. The catalyst, manganese dioxide thus accelerates the chemical reaction while itself remaining unchanged throughout the reaction. Here manganese oxide acts as a catalyst.

Hence, this reaction is known as a catalytic reaction.

The mechanism followed by the catalyst is catalysis. There are two types of catalysis- heterogeneous and homogeneous catalysis.

What is Homogeneous Catalysis?

If the reactant and catalysts are in the same phase, they are said to be homogeneous catalysis. 

2SO2(g) + O2(g) → 2SO3(g)

What is Heterogeneous Catalysis?

If the reactant and the catalysts are in different phases, they are said to be heterogeneous catalysis.

2SO2(g) → 2SO3

Here the catalyst is in a solid phase while the reactant in the gaseous phase.

Vegetable oils(l) + H2(g) → Vegetable ghee(s)

Here one reactant is in the liquid phase while the catalyst is in a solid phase.

How Does a Catalyst Affect the Rate of Chemical Reactions?

• The reactant molecules must have threshold energy for reactants to react and give a product, and the number of molecules with this energy should also be above the threshold value. Activation Energy is the name of this basic energy. Only those reactant molecules would be able to form products that have energy above the energy of activation.

Now the question is  – ”What do catalyst do?”

• Catalyst adjusts this activation energy or has a different mechanism of reaction that needs lower activation energy to form products. In intermediate-complex theory, the role of a catalyst in chemical reactions is best explained.

• It brings down the activation energy for a reaction, according to intermediate-complex theory, or offers a separate reaction pathway where activation energy is lower.

• To form an intermediate complex, it makes temporary bonds with the reactant molecules. To provide the components and the catalyst, this intermediate complex then decomposes. Prior to and after the reaction, the catalyst remains the same. No (chemical) changes are found in them.

• A catalyst can only catalyze spontaneous reactions, since it can not modify the Gibbs Free Energy, G, and can thus not catalyze a non-spontaneous reaction.

• It has been noted that for a reaction, a catalyst does not alter the equilibrium constant but rather accelerates backwards as well as the forward reaction to rapidly reach equilibrium. A catalyst catalyzes both the forward and the backward response to the same degree, so the point of equilibrium stays the same and is easily reached compared to the reaction without it.

Do you know?

• Even our body has different kinds of catalysts, which are called enzymes, which play an important role in chemical reactions that occur within our body. 

• Enzymes are complex organic nitrogenous compounds that are provided by plants and animals. They are the protein of high molecular mass molecules and form colloidal solutions in water. 

• They are very powerful catalysts; they catalyze various reactions, particularly numerous reactions. To continue the life process, the bodies of animals and plants are catalyzed by enzymes. The enzymes are thus called biochemical enzymes. The phenomenon and catalysts are known as biochemical catalysis.