Chemical Kinetics


Catalysis is the phenomena of altering the velocity of a chemical reaction by the presence of a catalyst. The temperature at which the catalyst activity is maximum is the optimum temp richer. The excess energy that the reactant molecules must acquire to change into the product is the activation energy. It is denoted by Ea. It is given by the energy required to break the bond minus the current energy of the reactants.



Properties of Catalyst

  • A catalyst remains unchanged in mass and composition at the end of the reaction: This means the catalyst which we have taken at the starting of the reaction and the catalyst which we get at the end of the reaction its mass and composition is the same. It does not mean that the catalyst will remain the same in its physical state also.
  • Catalyst is specific in action: This means that a particular substance can act as a catalyst only for a specific reaction, not for others. For example, HNO2 decomposes KMnO3 But not KMnO4. The notable exception being the transition metals which can catalyse a variety of reactions.
  • Nature of products remains unchanged by the presence of a catalyst: That is the reaction reactants to product cycle it’s same but the catalyst only increases the weight of the reaction. There are some exceptions for the catalyst which make different products have different conditions.
  • Catalyst does not alter the position of equilibrium: Catalyst only helps in the attending of the equilibrium more quickly because it increases the rate of forwarding reaction as well as backward reaction. It does not affect the concentration so equilibrium constant remains the same.
  • Catalyst does not initiate the reaction: This is a controversial topic as there are certain reactions that do not occur in absence of the catalyst.

Types of Catalysis

There are 6 types of catalysis these are homogeneous catalysis, heterogeneous catalysis, autocatalysis, enzyme catalysis, positive catalysis, negative catalysis. The broad categories are homogeneous catalysis and heterogeneous catalysis.

1. Homogenous Catalysis

They refer to the reaction where reactants and catalyst, are in the same face may it be the solid, liquid, or gaseous physical state. The entities that don’t have definite separation boundaries are called in the same face, for example, chlorine and water mixture will stop in chlorine add water mixture we can’t separate chlorine and water. In the physical state, the entities may have a definite boundary for example water and oil are in the same physical state but we have a definite boundary, we can separate oil from water. On this basis we can divide homogeneous catalysis into type one is in the liquid phase and the second is in the gaseous phase.

  • The liquid phase – As the name suggests all the reactants and the catalyst involved are in the liquid face. Most of the reaction comes in this category used acid add base as a catalyst. For example, the hydrolysis of Ester which takes Easter and water in the presence of at acid or base as a catalyst it gives acid and alcohol taken example is the inversion of sucrose that is sucrose plus water in the presence of acid it gives fructose and glucose, 3rd is the conversion of acetone to diacetone alcohol in which we take base as the catalyst.
  • The gaseous phase – As indicated from the heading, all the reactants and catalyst involved hair are in gases face will stop the first example is the oxidation of carbon monoxide which take place in the presence of nitrogen oxide second example is the lead chamber process for the manufacture of sulphuric acid.

Heterogeneous Catalysis

Hair catalyst and reactants are in different phases. In most of the cases, the catalyst is in the solid face while reactants are either in liquid or gas face. This is also called as contact catalysis because the reaction occurs when the reactants come into contact with the catalyst surface.

The first example is the oxidation of sulfur dioxide in which we use platinum as a catalyst in the solid-state but the reactance is in the gaseous state take an example is the hydrogenation of oil, 3rd example is the manufacture of ammonia by Haber process in which we take nitrogen and hydrogen in the presence of a solid catalyst which makes ammonia. For example, is the decomposition of hydrogen peroxide but here it is in the presence of platinum as the catalyst which is solid.

Auto Catalysis

This includes the reaction where one of the products form to act as a catalyst. In the starting the rate of the reaction is low and increases very slowly but as soon as the products are formed after certain intervals of time the rate increases rapidly. What’s ample titration of warm oxalic acid weather km oh 4. In this, the first few drops of potassium per magnet take appreciable time to decolourise, change the reaction is initially slow. But after some time, decolourization becomes rapid as the Mn+2 ions Formed during the reaction catalyses the reaction.

Positive Catalysis

When the catalyst speeds up the reaction in a forwarding direction, the direction is set to be positive.

Negative Catalysis

When the catalyst closes down the rate of reaction, the catalysis is said to be negative. For example, chloroform, when in contact with air, gets oxidized to carbon allyl chloride which is poisonous. To prevent this, ethanol is added which acts as a negative catalyst so that the wastage of chloroform is prevented and the poisonous substance is decreased. This is an advantage of negative catalysis.

FAQs about Catalysis

Q.1. What is the difference between homogenous and heterogeneous catalysis?

Answer – The differences between homogenous and heterogeneous catalysis are as follows –

  1. In heterogeneous catalysis reactants and catalyst are in the same face while in heterogeneous catalysis the reactant and catalysis are in a different phase.
  2. Homogeneous catalysis is mostly observed in liquid and gases catalysis while the heterogeneous catalysis is observed when the catalyst is in solid form.
  3. In homogeneous catalysis, products cannot be separated while in heterogeneous catalysis the final product can be separated.
  4. Homogeneous catalysis has low selectivity while heterogeneous catalysis has high selectivity.

Q.2. Write 2 examples when the nature of the product changed by the presence of a catalyst.

Answer- First exception is carbon monoxide and hydrogen makes methane and water in the presence of nickel as a catalyst. Carbon monoxide and hydrogen in the presence of copper as a catalyst form formaldehyde. Carbon monoxide and hydrogen in the presence of zinc oxide and chromium trioxide as the catalyst give methanol.  

The second exception is the chlorination of tolerance in the presence of iron chloride as a catalyst in the absence of sunlight on the other hand in the presence of sunlight and without the use of catalyst the products are different.

Q.3. Write 2 examples of catalysis when the catalyst remains unchanged in the product of the reaction.

Answer- A) Platinum gauze Catalyst is useful in ammonium ammonia oxidation. When we use it in the starting it is very smooth but after using it for 5 to 6 weeks it becomes harsh in its appearance.

B) Manganese dioxide is in use in the decomposition of potassium chlorate. At the start of the reaction we take crystals off manganese dioxide but as the reaction proceeds, it is converted into a fine powder which means it is the part of the reaction which is used up in the reaction but it is generated later.

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