Q.1. What is the method of balancing chemical equations?
Ans. Hit and trial method is used for balancing simple chemical equations. In this method, coefficients before the symbols/formulae of the reactants and products are adjusted in such a way that the total number of atoms of each element on both sides becomes equal.
Q.2. Why is it necessary to balance a chemical equation?
Ans. We know that, according to the law of conservation of mass, the total mass of all the atoms present in the reactants should be equal to that of all the atoms present in the products. This law will hold good only when the number of atoms of all types of elements on both sides is equal. Thus balancing of chemical equation is necessary so that the law of conservation of mass may be obeyed.
Q.3. In the equations given below, state giving reasons, whether substances have been oxidised or reduced.
(i) PbO + CO → Pb + CO2
(ii) H2S + Cl2 → 2HCl + S
Ans. (i) Carbon monoxide is oxidised as it gains oxygen.
(ii) Chlorine is reduced as it gains hydrogen.
Q.4. A sample of water weed was placed in water and exposed to sunlight. Bubbles of gas are seen on the surface of the leaves.
(i) Name the gas evolved.
(ii) Name the process taking place.
(iii) Write a balanced equation of reaction taking place.
(iii) 6CO2 + 6H2O → C6H12O6 + 6O2
Q.5. Give balanced equations, wherever possible, or where this is not possible, explain the following by means of examples:
(i) A reaction which gives out heat.
(ii) A reaction which takes place with the help of sunlight.
(iii) A reaction which is brought about by the electric current.
(iv) A reversible reactions.
(v) A reaction with a solid and gas which produces heat.
(i) Coke on heating in air, catches fire and liberates large amount of heat.
C + O2 → CO2 + ΔT
(ii) Carbon dioxide and water react in the presence of chlorophyll and sunlight to form glucose and oxygen.
6CO2 + 6H2O → C6H12O6 + 6O2
(iii) Molten lead bromide decomposes into lead metal and bromide on the passage of electric current.
PbBr2 → Pb + Br2
(iv) The reaction between red hot iron and steam is reversible.
3Fe + 4H2O ⇌ Fe3O4 + 4H2
(v) When magnesium burns in air or oxygen it liberates large amount of heat.
2Mg + O2 → 2MgO + ΔT.
Q.6. Balance the following chemical equations:
(i) Mg + N2 → Mg3N2
(ii) KClO3 → KCL + O2
(i) 3Mg + N2 → Mg3N2
(ii) 2KClO3 → 2KCL + 3O2.
Ques 7. Write the uses of decomposition reactions.
Ans. The decomposition reactions carried out by electricity are used to extract several metals from their naturally occurring compounds like chlorides and oxides.
When passing electricity decomposes the fused metal chloride or metal oxide, then metal is produced at the cathode.
Q.8. What are the characteristics of chemical reactions?
Ans. Characteristics of chemical reactions are as follows:
(i) Evolution of a gas
(ii) Formation of a precipitate
(iii) Change in color
(iv) Change in temperature
(v) Change in the state.
Q.9. Can rancidity retard by storing foods away from the light?
Ans. In the absence of light, the oxidation of fats and oils present in food is slowed down and hence the development of rancidity is retarded.
Q.10. What are the different ways can make more informative about the chemical equation?
(i) By indicating the “physical states” of reactants and products.
(ii) By indicating the “heat changes” taking place in the reaction.
(iii) By indicating the “conditions” under which the reaction takes place.