NEET Chemistry Notes Surface Chemistry – Promoters and Inhibitors

NEET Chemistry Notes Surface Chemistry – Promoters and Inhibitors

Promoters and Inhibitors

Promoters and Inhibitors

Promoters are the substances which can increase the efficiency of a catalyst. These are also known as coenzymes or activators. Inhibitors are the substknces which can make the catalyst inactive, e.g. Mg2+ acts as activators for many enzymes while  act as inhibitors as they form complex with Mg⁺².

Characteristics of Catalysts

Some important characteristics of catalysts are discussed below:

• They become temporarily involved in a reaction, providing an alternative reaction path of lower activation energy than that for the uncatalysed reaction.
• They catalyse both forward and backward reactions to the same extent in a reversible reaction and thus have no effect on the equilibrium constant.
• The catalyst remains unchanged in amount and chemical composition at the end of the reaction. It may undergo some physical change.
• Only a small quantity of the catalyst is generally needed. One mole of colloidal platinum in 10⁸ L can catalyse decomposition of H₂0₂.
• In certain reactions, the rate of the reaction is dependent on the concentration of the catalyst, e.g. Rate of inversion of cane sugar is dependent on the concentration of H⁺ used as catalyst.
• Rate of the reaction in certain heterogeneous reaction varies with surface area of the catalyst. Hence, finely
divided metals are preferred in the form of catalyst
The catalyst does not initiate the reaction and are specific in their action, e.g. starting from H₂and CO, 3 different products are possible using different catalysts as

Activity and Selectivity of Catalyst

Activity of catalyst depends upon the strength of chemisorption to a large extent.
Selectivity means a given substance can act as catalyst only in a particular reaction and not for all reactions.

Enzyme Catalysis

Enzymes are biochemical catalysts. They are proteins and extremely specific in nature.

Concept of Colloidal State

A substance is said to be in the colloidal state, when it is dispersed in another medium in form of very small particular having diameter between 10^-4 to 10^-7 cm.

Types of Solutions

Depending upon the size of dispersed particles, there are three different types of solutions viz, true solutions, suspensions and colloidal solutions.
In true solutions, the dispersed particles are present as single molecules or ions. The size of the dispersed particles is less than 1 nm. True solutions are homogeneous.
In colloidal solution, the size of dispersed particles is 1 to 100 nm. Colloidal solutions are heterogeneous. In suspension, the size of dispersed particles is more than 100 nm. Suspensions are also heterogeneous.

Dispersed Phase afid Dispersion Medium

The colloidal system is made of two phases :

•  The substance distributed as the colloidal particles, called the dispersed phase.
•  The phase in which the colloidal particles are scattered, called dispersion medium.
  The dispersed phase or dispersion medium can be a gas, liquid or solid. There are eight types of colloidal system.
  

Classification of Colloids

Colloids can be classified in a number of ways based upon some of their important characteristics as given below:

Based upon Appearance

• The colloidal systems, in which the dispersion medium is a liquid or gas, are called sols. They are called hydrosols or aqua sols, if the dispersion medium is water. When the dispersion medium is alcohol or benzene, they are accordingly called alcosols or benzosol.
•  The colloidal systems, in which the dispersion medium is gas are called aerosols.

Based upon Charge

The colloids can be classified into positive or negative colloids according to the charge present on the dispersed phase particles.

 Based on Affinity of Phase

• Lyophilic colloids represent such colloidal systems in which the particles of dispersed phase have great affinity for the dispersion medium.
  These are reversible colloids, e.g. gum, gelatin, rubber, proteins etc.
  Lyophobic colloids are less stable due to the presence of electric charge on their particles.
• Lyophobic colloids represent such colloidal systems in which particles of the dispersed phase have no affinity for the dispersion medium.
  These are irreversible, e.g. soils of metals and their insoluble compounds like sulphides and oxides.
  If water is the dispersion medium, the terms used are hydrophilic and hydrophobic colloids.
  On the otherhand, lyophilic colloids are stable due to their presencfe of charge as well as their extensive solvation.

 Based on Molecular Size

•  Multimolecular colloids are the colloids in which colloidal particles consist of aggregate of atoms or small molecules with diameter less than 10^-9 m or 1 nm, e.g. a sol of gold, a sol of sulphur.
• Macromolecular colloids are the colloids in which colloidal particles themselves are large molecules.
  Associated colloids or micelles are the substances which behave as normal electrolytes at low concentration but as colloids at higher concentration.
  This is because at higher concentration, they form associated particles called micelles, e.g. soap and synthetic detergents. The concentration above which micelle formation occurs, is called CMC (critical micelle concentration) and the temperature above which micelle formation occurs is called Kraft temperature.
  The colligative properties of colloidal systems are low due to aggregation. Hence, all colloidal solutions exhibit very low osmotic pressure, very small elevation in boiling point and depression in freezing point

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