RATE OF CHEMICAL REACTIONS
SLOW AND FAST CHEMICAL REACTIONS
All of us come across different types of chemical reactions in our daily life. For example iron rusts if exposed to humid atmosphere, during summer milk left at room temperature for a few hours becomes sour and is converted into a curd, a freshly cut apple when exposed to air becomes brown after some time. When some dilute sulphuric acid is added to barium chloride solution, a thick white precipitate of barium sulphate is formed. Out of these reactions some reactions are very fast while others take place at a very slow rate. So, the branch of science which deals with the rates of chemical reactions and the factors which deals with the rates of chemical reactions is called chemical kinetics.
SLOW AND FAST CHEMICAL REACTIONS
As we have discussed above that there are so many types of chemical reaction which are taking place around us. Some of these reactions are very fast while others are slow. So we can categories the chemical reactions broadly into two types depending upon their rate of reaction
1. Slow chemical reactions and
2. Fast chemical reactions.
1. Slow chemical reactions
These are those chemical reactions which take place at very slow rate. These reactions can take days, months or even years to complete. In general the reactions between covalent compounds are slow. For example rusting of iron or fermentation of sugar into ethyl alcohol and carbon dioxide can take place in several hours or even in several days. In the same way weathering of rocks takes place in millions of years.
Usually these reactions do not take place at normal temperature and pressure i.e. they need high temperature, high pressure or catalysts to complete. For example nitrogen can react with hydrogen at the temperature of 450°C and atmospheric pressure of 250 in the presence of catalyst iron to form ammonia (NH3).
This reaction is called Haber’s process.
Enzymes are complex chemical compounds made up of proteins which act as catalysts in our body i.e. the enzymes increase the rate of chemical reactions taking place in our body. For example, the digestion of food in our takes place in 4-5 hours due to action of enzymes. In the absence of these enzymes it is estimated that the digestion of food will take place in approximately 50 years.
2. Fast chemical reactions
These are those chemical reactions which take place at a very fast rate. These reactions can take place in seconds or in minutes. In general the reactions between ionic compounds are fast. For example, combustion of LPG gas in kitchen takes place in a few seconds so it is a fast reaction. In the same way reaction of an acid like HCl with a base such as NaOH takes place in seconds to produce a salt NaCl and water is very fast reaction. This reaction is also called neutralization reaction.
Also the reaction between aqueous solution of barium chloride (BaCl2) and sulphuric acid (H2SO4) to produce barium sulphate (BaSO4) and hydrochloric acid (HCl) is fast chemical reaction.
And the reactions that lead to the contraction of muscles and the transmission of nerve impulses in our body are still more rapid.
Femto chemical kinetics
Some reactions take place at a very fast rate. For example, combustion of LPG in kitchens is a femto chemical reaction because it takes place at a very fast rate. These chemical reactions can take place in 10-15 seconds. So, the study of such chemical reactions which take place in 10-15 seconds is called femto chemical kinetics.