Chemical Reactions and Equations 

CBSE extra question 

class 10


THREE MARKS QUESTIONS
1.Decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Write an equation each for decomposition reactions where energy is supplied in the form of heat, light and electricity.                   [CBSE 2018, 2014]
Ans :
a. CaCO3(s)  $ CaO(s) + CO2(g)
b. 2AgBr(s)  $ 2Ag(s)  + Br2(g)
c. 2H2O(l)  $  2H2(g) + O2(g)

2.In the electrolysis of water:
a. Name the gas collected at the cathode and anode respectively.
b. Why is volume of gas collected at one electrode double than that at the other? Name this gas.
c. How will you test this gas?                 [CBSE 2012, CBSE Sample Paper 2018] 
Ans :
a. Hydrogen is collected at the cathode, oxygen is collected at the anode.
b. It is because H2O contains hydrogen and oxygen in the ratio 2 : 1.
c. Bring a burning matchstick near the gas, if the gas burns with ‘pop’sound, the gas is H2 .

3. State one example each characterised by following along with suitable chemical equation.
a. Change in state,
b. Evolution of gas,
c. Change in temperature.                         [CBSE 2016]
Ans :
a. Change in state:
 AgNO (aq) + HCl(aq) $AgCl(s) + HNO 

b. Evolution of gas:
       CaCO3(s)  + 2HCl(dil) $ CaCl2(aq) + H2O(l) + CO2(g)

c. Change in temperature: 
CH4(g) + 2O2(g) $  CO2(g) + 2H2O(l)  + Heat
          

 4.What is an oxidation reaction? Give an example of oxidation reaction. Is oxidation an exothermic or an endothermic reaction?              [CBSE 2009]
 Ans : 
Oxidation reaction: The reaction in which O2 is added or H2 is removed or loss of electrons takes place is called oxidation reaction. E.g.,
2Cu(s) + O2 (g) $ 2CuO(s)
It is an exothermic reaction.

5.Name the type of reactions represented by the following equations:
a. CaO + H2O $ Ca(OH)2
b. 3BaCl2 + Al2(SO4) $ 3BaSO4 + 2AlCl3
c.  2FeSO4 $ Fe2O3 + SO2 + SO3                                [CBSE 2015]
Ans :
a. Combination reaction,
b. Double displacement reaction,
c. Decomposition reaction

6.Write the balanced chemical equations for the following chemical reactions:
a. Hydrogen + Chlorine $ Hydrogen Chloride 
b. Lead + Copper Chloride $ Lead chloride + Copper
c. Zinc oxide + Carbon $  Zinc + Carbon Monoxide                    [CBSE 2013] 
Ans :
a. H2(g) + Cl2(g) $ 2HCl(g)
b. Pb(s) + CuCl2(aq) $ PbCl2(s) + Cu(s) 
c. ZnO(s) + C(s) $ Zn(s) + CO(g)

 7.(a) Why is it necessary to balance a chemical
equation?
(b) Write the balanced chemical equation for the following reactions:
(i)Natural gas burns in air to form carbon dioxide and water.
(ii)During respiration, glucose combines with oxygen and forms carbon dioxide and water along with the release of energy.
Ans :
a. Chemical equation must be balanced so as to follow the law of conservation of mass.
b. (i) CH4 (g) + 2O2(g) $ CO2(g) + 2H2O(l)
    (ii) C6H12O6 (s) + 6O2 (g) $ 6CO2(g) + 6H2O (l) + Heat

8.(a) Can combination reaction be an oxidation reaction?
   (b) How will you test whether the gas evolved in a reaction is hydrogen?
   (c) Why does copper not evolve hydrogen on reacting with dilute sulphuric acid?     [CBSE 2011]
Ans :
a. Yes, combination reaction can be called a oxidation reaction.
b. Bring a burning splinter near the gas, if it burns with pop sound, it is hydrogen gas.
c. It is because copper is less reactive than hydrogen.

9.(a) Why is respiration considered an exothermic reaction?
   (b)  Define the terms oxidation and reduction,
   (c)  Identify the substance that is oxidised and reduced in the following reaction:           
  CuO(s) + Zn(s) Cu(s) + ZnO(s)                                        [CBSE 2011]
Ans :
a. In respiration, energy is released, therefore, it is considered an exothermic reaction.
b. Oxidation involves addition of O2 or loss of electrons. Reduction involves addition of H or gain of electrons.
c. Zn is getting oxidised, CuO is getting reduced.    


10. State the type of chemical reactions and chemical equations that take place in the following:
a. Magnesium wire is burnt in air.
b. Electric current is passed through water.
c. Ammonia and hydrogen chloride gases are mixed.        [CBSE 2015]
Ans :
a. 2Mg(s) + O2 (g) $ 2MgO(s); 
Oxidation reaction
b. 2H2O(l) $ 2H2 (g) + O2 (g);
Decomposition reaction

c. NH3 (g) + HCl(g) $ NH4Cl(s);
Combination reaction