RATE OF CHEMICAL REACTIONS
FACTORS WHICH AFFECT THE RATE OF REACTION
There are a number of factors which affect the rate of reaction such as-
1. concentration of reactants
5. Surface area of reactants
Let us briefly discuss these factors
1. Concentration of reactants
It is has been observed that the rate of chemical reaction is directly proportional to the concentration of the reactants. In the starting of a reaction when the concentration of reactants is maximum, the rate of reaction is also maximum. But as the reaction precedes to completion the concentration of reactants decreases as some amount of reactants is converted into products. So, the rate of reaction also decreases. This can be understood with the help of following example
when a piece of wood if burnt in air having less amount of oxygen say 30% , it will burn slowly but if it is burnt is air having large amount of oxygen say 100% then it will burn rapidly because the concentration of oxygen is more.
In general, an increase in the temperature increases the rate of reaction or a decrease in temperature decreases the rate of reaction. This can be understood by taking following examples.
We generally observe that milk is spoilt earlier in summer than in winter. It is due to the reason that in summer the temperature is high as compared to winters. So the microorganisms responsible for spoiling the milk are more active in summer as compared to the winter. In the same way we keep fruits and vegetables in refrigerators so that these can remain fresh for a long time because inside the refrigerator the temperature is low so the food spoiling microorganisms cannot easily spoil the food because their activity is reduced. But if we keep these fruits and vegetable in open outside the refrigerator then it is spoilt very soon due to high activity of microorganism because the temperature is high.
When we react calcium carbonate with hydrochloric acid to produce carbon dioxide at normal temperature then this reaction takes place very slowly and a very few bubbles of carbon dioxide are produced. However, when we raise the temperature slightly more, then this reaction takes place very fast and a large no. of bubbles of carbon dioxide is produced.
In general for every 100C raise in the temperature the rate of most chemical reactions becomes double.
Catalysts are the substances which change the rate of chemical reactions without undergoing any change in them. It means that catalysts are those substances which help in changing the rate of chemical reactions but they don’t themselves undergo any change in the end of chemical reaction. Due to this reason these are also known as marriage broker in Chinese language. For example,
In the manufacture of ammonia, iron is used as a catalyst to increase the rate of reaction.
N2 (g) +3H2 (g) 2NH3 (g)
Similarly, reaction of Sulphur dioxide and oxygen to produce Sulphur trioxide takes place in the presence of catalyst nitrogen monoxide (NO).
2SO2 (g) + O2 (g) 2SO3 (g)
Types of catalysts:
Normally catalysts are used to increase the rate of a particular reaction. The catalysts which are used to increase the rate of a chemical reaction are known as Positive catalysts. But sometimes catalysts can also be used to slow down the rate of reaction. For example, glycerin is sometimes added to hydrogen peroxide to slow down it decomposition. The catalysts which slow down the rate of a particular reaction are known as Negative catalysts.
How catalysts work?
For a reaction to take place, the molecules must have a certain energy called activation energy. If the activation energy is higher, then the reaction will be slow. A catalyst works by lowering the activation energy of the chemical reaction.
Imagine the activation energy as a hill that you have to ride over. And the use of a catalyst is like taking another path, which avoids the hill. So a catalyst increases the rate of a chemical reaction by providing an alternate path having less activation energy.
The rate of some chemical reactions increases in the presence of light. For example,
1. The process of photosynthesis for preparation of food in plants takes place only in the presence of light of sun. In this process plants convert the carbon dioxide and water into starch and oxygen in the presence of sunlight with the help of green colouring pigment chlorophyll
2. Reaction of hydrogen with chlorine also takes place in the presence of sunlight.
3. The process of photography is also a photochemical reaction. Actually the photographic film is coated with silver bromide (AgBr) which undergoes chemical reaction when exposed to sunlight.
5. Surface area of reactants
The rate of chemical reactions also increases by increasing the surface area of reactants. For example, a log of wood burns slowly but if it is cut into small wooden chips, the burning takes place rapidly. This is due the reason that the total surface area of wooden chips is more as compared to the surface area of wooden log.