Long Answer -Class 10 – Science -Chemistry -Chapter 1 – Chemical Reaction and Equation

Class 10 – Science -Chemistry -Long Answer -Chapter 1 – Chemical Reaction and Equation

Q1. (a) Define a balanced chemical equation. Why should an equation be balanced?

(b) Write the balanced chemical equation for the following reaction:

(i) Phosphorus burns in presence of chlorine to form phosphorus pentachloride.

(ii) Burning of natural gas.

(iii) The process of respiration.

Ans:

(a) Balanced chemical equation has an equal number of atoms of different elements in the reactants and products. According to law of conservation of mass, matter can neither be created nor be destroyed in a chemical reaction.

(b)(i) P4 (s) + 10Cl2 (g) ———> 4PCl5 (S)

(ii)CH4 (g) + 2O2 (g) ———> CO2 (g) + 2H2O(l) + heat energy

(iii) C6H12O6 (s) + 6O2 (g) + 6H2O ———> 6CO2 (aq) + 12H2O (l) + energy

Q2. (a) Explain two ways by which food industries prevent rancidity.

        (b) Discuss the importance of decomposition reaction in metal industry with three points.

Ans:

(a) (i) Rancidity can be prevented by adding antioxidants to food containing fat and oil, e.g. butylated hydroxy anisole is added to butter as antioxidant.

(ii) It can be prevented by packaging fat and oil containing foods in nitrogen gas.

(b) (i) Molten NaCl is electrolytically decomposed to form sodium metal.

(ii) Aluminium metal is obtained by electric decomposition of bauxite ore mixed with cryolite.

(iii) Carbonate ores are thermally decomposed to give metal oxide which on reduction give metal.

Q3. With the help of an activity show that iron is more reactive than copper?

Ans. First take some aqueous solution of Cuso4 (blue is colour) and dip iron nails into it leave for half an hour the blue colour of the solution changes into light green. At the same time brown deposit appears on the nails. This happens because iron is placed above copper in reactivity series hence it displaces copper from cuso4 solution. The brown deposit is of copper. $$Fe(s)+CuSO_{4}(aq)\rightarrow FeSO_{4}(aq)+Cu(s)$$

Q4. Identify the type of chemical reaction in the following statement and define each of them:

a. Digestion of food in our body.

b. Rusting of iron.

c. Heating of manganese dioxide with aluminium powder.

d. Blue colour of copper sulphate solution disappears when iron filings are added to it.

e. Dilute hydrochloric acid is added to sodium hydroxide solution to form sodium chloride and water.

Ans :  a. Decomposition reaction: It is a process in which a compound is broken down into simple substances.

b. Oxidation: The process in which oxygen is added or electrons are lost.

c. Displacement reaction: The reaction in which a more reactive element can displace a less reactive element from its salt solution. Oxidation and Reduction are taking place simultaneously in rusting of iron.

d. Displacement reaction: The reaction in which a more reactive element can displace a less reactive element.

e. Neutralisation reaction: The reaction in which acid reacts with base to form salt and water.

Q5. Identify the type of reactions taking place in each of the following: a. Barium chloride solution is mixed with copper sulphate solution and white precipitate is formed.

b. On heating copper powder in china dish, the surface of copper powder turns black.

c. On heating green coloured ferrous sulphate crystals, radish brown solid is left and smell of a gas having odour of burning sulphur is experienced.

d. Iron nails when left dipped in blue copper sulphate solution become reddish brown in colour and the blue colour of copper sulphate fades away.

e. Quick lime reacts vigorously with water releasing a large amount of heat.

 Ans :

a. Double displacement reaction,

b. Oxidation,

c. Decomposition reaction,

d. Displacement reaction,

e. Combination reaction.

Q6 . a. Define a balanced chemical equation. Why should an equation be balanced?

b. Write a balanced chemical equation for the following reactions:

(i) Phosphorus burns in the presence of chlorine to form phosphorus pentachloride.

(ii) Burning of natural gas.

(iii) The process of respiration.

Ans : 

a. Balanced chemical equation is a equation in which number of atoms of various elements are equal on both sides of the equation. The equation should be balanced due to law of conservation of mass.

b. $$(i) 2P(s) + 5Cl2 (g) \rightarrow  2PCl5 (S)$$

$$(ii) CH4 (g) + O2 (g) \rightarrow  CO2 (g) + 2H2 O(l)$$

$$(iii) C6 H12 O6 (s) + 6O2 (g) \rightarrow 6CO2 (g) + 6H2 O(l)$$