Absorption and Catalysis: Negative Catalysis and General Characteristics of Catalyst (For CBSE, ICSE, IAS, NET, NRA 2022)

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Negative Catalysis

The catalysts retard a reaction rather than speed it up. They are known as negative catalysts.

For example:

  • Glycerol retards the decomposition of hydrogen peroxide.
  • Phenol retards the oxidation of sulphurous acid.

Promoters

  • The substances which increase the activity of a catalyst are called promoters.
  • For example, In Haber՚s process for the manufacture of ammonia, the catalytic activity of iron is enhanced by molybdenum which acts as promoter.

Poisons

  • The substance which decrease the activity of a catalyst are called poisons.
  • For example, In Haber՚s process the catalyst iron is poisoned by hydrogen Sulphide .
  • In contact process for the manufacture of sulphuric acid, the catalyst platinum is poisoned by even the traces of arsenious oxide .

General Characteristics of Catalyst

A Catalyst Remains Unchanged at the End of the Reaction

  • The amount and the chemical composition of a catalyst remain unchanged in a catalytic reaction.
  • The catalyst may undergo a physical change.
  • For example, manganese dioxide, which is used as a catalyst in thermal decomposition of potassium chlorate becomes powder during the course of the reaction.

A Small Quantity of the Catalyst is Generally Enough

  • Most of the reactions require only a minute amount of catalyst.
  • One gram of ions is sufficient to catalyse the oxidation of litres of sodium sulphite solution.
  • Catalysis by acids or bases are the type in which the rate of reaction is directly proportional to the concentration of catalyst present.

A Catalyst Does Not Alter the Position of Equilibrium State of a Reversible

  • A catalyst allows the equilibrium to be reached faster.
  • It does not alter the equilibrium composition of the reaction mixture because catalyst increases the rates of forward and backward reaction equally.

Catalysts Are Generally Specific in Their Action

  • One catalyst will change the rate of only one reaction.
  • For example, manganese dioxide catalysis the decomposition of potassium chlorate but not of potassium perchlorate.

A Catalyst Cannot Initiate a Reaction

  • A catalyst can change the rate of a reaction which occurs even in the absence of catalyst.
  • It cannot start a reaction.

The Activity of a Catalyst Can be Increased by the Presence of Promoters and Decreased by the Presence of Poisons

  • Presence of a promoter increases the activity of a catalyst.
  • The presence of a poison decreases it.

Homogeneous Catalysis

When the catalyst is present in the same phase as the reactants, the phenomenon is called homogeneous catalysis.

For example:

Nitric oxide catalysis the oxidation of Sulphur dioxide to Sulphur trioxide in the lead chamber process.

Hydrogen ions catalyse the inversion of cane sugar

Fructose Heterogeneous Catalysis

When the catalyst is present in a phase other than that of reactants the phenomenon is called heterogeneous catalysis.

For example:

Iron (s) catalysis the formation of gas

In contact process for the manufacture of sulphuric acid, platinized asbestos is used as the catalyst.

Catalysis and Activation Energy

  • A catalyst increases the rate of a reaction.
  • It can be explained by the following fig.
Catalysis and Activation Energy
  • is the activation energy of uncatalyzed reaction and is the activation energy of the catalysed reaction.
  • A catalyst lower the activation energy as you can see in the figure .
  • The reduction in activation energy is achieved by providing an alternative pathway of lower energy for the reaction.
  • The relative energies of reactants and products are not changed.
  • The enthalpy change is the same for the catalysed and uncatalyzed reactions.

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