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Answer.

(a) Concentration of reactant: The rate of a reaction generally increases with increase in concentration of the reactants. For a reaction to take place between two molecules, they must collide or come into contact. The probability of such collisions increase with the increase in the concentration of the reacting molecules. Different reactants can affect the rate of a particular reaction to different extent.

(b) Surface Area of the Reactants: The rate of the reaction increases with increase in surface area of the reactants.It is a common observation that small pieces of wood burn more rapidly than large logs of wood.
For example magnesium powder reacts with dilute sulphuric acid more rapidly than magnesium ribbons. Thus smaller the size of reacting particles, the greater is the total surface area exposed for reaction and consequently greater is the rate of the reaction.

(c) Effect of Temperature:Generally, the rates of chemical reactions increase with increase in temperature. It is a common observation that perishable foods like milk, cooked vegetables decay more quickly in summers than in winter.

(d) Catalyst:Catalysts are substances, which alter the rate of chemical reactions without undergoing any overall chemical change themselves.
In the manufacture of ammonia, iron is used as a catalyst and it enhances the rate of this reaction. Sometimes catalysts are used to retard or slow down the rates of reaction. For example., glycerol is used to slow down the decomposition of hydrogen peroxide. Such catalysts are known as negative catalysts.

Many industrial processes like the manufacture of polythene and polystyrene require catalysts. In the living bodies, a large number of complex chemical reactions occur which are catalysed by complex organic molecules called enzymes. For example., enzyme amylase present in saliva catalyses the decomposition of starch into maltose.

(e) Effect of Light: There are many reactions, which are influenced by light. For example., photosynthesis and photography. Such reactions are known as photochemical reaction. Light supplies the necessary energy for the reaction to take place. Photographic films which are normally coated with silver bromide (and Light supplies the necessary energy for the reaction to take place. Photographic films which are normally coated with silver bromide (and a very small amount of silver iodide) undergo chemical reaction when exposed to sunlight. This basic reaction is used in photography.

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