subject: Types Of Chemical Reactions [print this page] Various changes can be observed around usVarious changes can be observed around us. Chemical reactions take place in our everyday life. e.g. digestion of food, burning of L.P.G., souring of milk, rusting of iron etc. The study of rate of chemical reactions and steps by which they occur is called chemical kinetics.
There are different types of chemical reactions which are as follows:
1. Slow and Fast Reactions
2. Reversible And Irreversible Reactions
3. Exothermic and Endothermic Reactions
Let us discuss small experiments related to each type:
Types of Chemical Reactions Experiments:
1. Slow And Fast Reactions:
In daily life we come across various types of reactions. Some of these ae very fats, while some are very slow. E.g. Burning of L.P.G. is a fast reaction, on the other hand, rusting of iron is a slow reaction.
Some reactions are very slow. They do not occur even at normal temperature.
E.g. Ammonia can only be obtained in the presence of a catalyst i.e. iron and at a pressure of 250 atmosphere and at a temperature of 450C.
N2 + 3H2 ----------- 2NH3
2. Exothermic and Endothermic Reactions:
Chemical reaction in which amount of energy is released in the form of heat and light is called an exothermic reaction. All combustion reactions are exothermic.
E.g. When methane burns, lot of heat is produced.
CH4 + 2O2 ----------- CO2 + 2H2O + Heat
Chemical reaction in which energy is absorbed in the form of heat or light is called endothermic process or reaction.
E.g. evaporation of water is an endothermic process.
Experiment:
1. Light the gas stove.
2. Place a vessel full of water on the gas stove.
3. After some time, you see that the water starts boiling and small water vapours are formed that move upward.
Thus, water absorbs the heat energy and gets converted to water vapours. So, it is an Endothermic Reaction.
The reaction which cannot be reversed in the opposite direction is called irreversible reaction. In such reaction, we cannot get back the reactants by reacting product.
e.g. Burning of L.P.G. In this reaction, butane reacts with oxygen to form carbon dioxide and water and huge amount of heat is produced.
2C4H10 + 13O2 ------------ 8CO2 + 10H2O + Heat
But we cannot get butane and oxygen from carbon dioxide and water.
In reversible reactions, we get back the reactants from the products after changing the conditions.
e.g. Decomposition of ammonium chloride is a reversible reaction. Ammonium chloride, on decomposition, produces ammonia and hydrogen chloride gas.
NH4Cl ----------- NH3 + HCl
On cooling, whte solid of ammonium chloride is formed.
NH3 (g) + HCl (g) ------------ NH4Cl (s)
Reaction of CO2 with water is also an example of a reversible reaction.
Experiment:
1. Take some tap water in the conical flask.
2. Add some drops of the phenol red indicator to water.
3. You can add 2-3 drops of NaOH solution to get the red colour.
4. Blow gently into the solution to observe the colour change.
The following reactions take place:
CO2(aq) + H2O(l) H+(aq) + HCO3(aq)
CO2 also reacts with NaOH. This reaction produces the less alkaline Na2CO3:
2NaOH(aq) + CO2(g) Na2CO3(aq) + H2O(l)
The reverse reaction is also possible.
Reversible Chemical Reactions Examples:
Reversible Chemical Reaction Example 1:
Our life depends on the reversible attachment of oxygen to a protein called haemoglobin. This protein is found in our blood. Oxygen is picked up by haemoglobin as it passes through the blood vessels of the lungs. It is then carried to tissues in other parts of the body. In our organs like muscles and brain, the oxygen is detached and is used by the cells of these organs. This happens due to change in conditions in these organs. This is how the attachment of oxygen with haemoglobin is a reversible process.
Reversible Chemical Reaction Example 2:
Thermal decomposition of salts such as hydrates copper-sulphate results in the dehydration of the salt.
CuSO4.5H2O ----------- CuSO4 (s) + 5H2O (g)
In this reaction, colour changes from blue to white. Water is removed from the copper sulphate. This dehydration reaction can be reversed by slowly adding water back to the powder. This reaction is exothermic and the blue colour of the powder is retained.
CuSO4 (s) + 5H2O ------------ CuSO4.5H2O
Thus, the overall reversible reaction can be shown as follows:
CuSO4.5H2O ---------- CuSO4 (s) + 5H2O
Reversible Chemical Reaction Example 3:
Decomposition of ammonium chloride is also a reversible reaction. Ammonium chloride decomposes on heating to ammonia and hydrogen chloride gas.
NH4Cl ------------- NH3 + HCl
However, on cooling, white solid of ammonium chloride is formed.
NH3 (g) + HCl (g) ------------ NH4Cl (s)
Thus, this reversible reaction can be given as:
NH4Cl ---------- NH3 + HCl
When ammonium chloride is heated in open vessel, reverse reaction does not occur. On the other hand, when ammonium chloride is heated in a closed vessel, it gets decomposed into ammonia and hydrogen chloride. Reversible reactions occur only in closed system. In open vessel, reversible reaction is impossible.
Reversible Chemical Reaction Example 4:
Evaporation of water in a closed vessel is also an example of the reversible reaction. When water is heated in a closed vessel, it is converted into water vapours. On cooloing, these water vapours are again converted into water. The reversible reaction is shown as follows:
H2O(l) ------- H2O(g)
Other Reversible Reactions Examples:
(i) Hydrolysis of salt
FeCl3 + 3H2O -------- Fe(OH)3 + 3HCl
(ii) Thermal decomposition of phosphorus pentachloride
