Chemical Reactions and Equations 

CBSE extra question 

class 10





TWO MARKS QUESTIONS

1.Write the essential condition for the following reaction to take place: 
2AgBr $2Ag + Br2
Write application of this reaction.
Ans :The reaction will take place in presence of sunlight. 
This reaction is used in black and white photography.

2.(a) Complete the following equation for the chemical
reaction:
FeSO4 (s) Fe O + ___ +___
(b) What happens when water is added to quicklime (CaO)? Write the chemical equation.
Ans :(a) 2FeSO (s) Fe2 O3 (s) + SO2 (g) + SO3 (g)
Ans :(b) Slaked lime Ca(OH)2 is formed. Hissing sound and lot of heat is also produced:
CaO(s) + H2O(l) $Ca(OH)2 (aq)

3.Write balanced chemical equation for the following reactions:
(a) Hydrogen sulphide burns in air to give water and sulphur dioxide.
(b) Barium chloride reacts in aqueous solution with zinc sulphate to give zinc chloride and barium sulphate.
Ans :(g) + 3O2 (g) $2H2O(l) + 2SO2 (g)
Ans :(aq) + ZnSO(aq) $BaSO4 (s) + ZnCl2 (aq)


4. List two observations that are noticed when an iron nail is put inside copper sulphate solution. Write the chemical equation for the reaction that occurs.
Ans :The blue coloured solution will become pale green. Reddish brown metal will get deposited.
Fe (s) + CuSO 4 (aq) $FeSO4 (aq) + Cu (s)
(Blue) (Pale Green ) (Reddish brown)

5. Name the reducing agent in the following reaction: 
3MnO2 + 4A1 $3Mn + Al2 O3
State which is more reactive, Mn or Al and Why? 
Ans :Al is the reducing agent. Al is more reactive than Mn.
Reason: It is because Al is displacing Mn from MnO 

6.“We need to balance a skeleton chemical equation”.Give reason to justify the statement.
Ans :We must balance a skeletal equation so as to ensure that the reaction follows ‘Law of conservation of mass’. The total mass of reactants must be equal to the total mass of products, that is why all reactions should he balanced. 

7. A metal is treated with dilute H SO . The gas evolved is collected .
a. Answer the following questions:
b. Name the gas liberated.
c. Name the method used for collection of gas.
d. Is the gas soluble or insoluble in water? 
e. Is the gas lighter or heavier than air?



Ans :a. The gas liberated is H .
Ans :b. It is collected by downward displacement of water. 
Ans :c. The gas is insoluble in water.
Ans :d. Hydrogen gas is lighter than air.

8. Consider the following chemical equation:
X + Barium chloride $Y + Sodium chloride
(White ppt)
Identify (a) X and Y (b) The type of reaction

Ans :(a) X is silver nitrate, Y is silver chloride.
2AgNO 3 (aq) + BaCl 2 (aq) $2AgCl2 + Ba (3NO 3) 2 (aq)
(White ppt)
Ans :(b) The reaction is an example of double displacement (precipitation) reaction.

9. What is observed when a solution of potassium iodide is added to a solution of lead nitrate? Name the type of reaction. Write a balanced chemical equation to represent the above chemical reaction.
Ans : Yellow precipitate is formed due to formation of lead iodide.
It is a precipitation as well as double displacement reaction.
Pb (NO 3) (aq) + 2KI (ag) $PbI2(s) + 2KNO 3 (aq)
                                             (Yellow ppt)
  

10.Give one example of each:
a. Chemical reaction showing evolution of a gas.
b. Change in colour of a substance during chemical reaction.
Ans :a 2K(s) + 2H2O(l) $2KOH(aq) + H2 (g)
Ans :b. FeSO4 .7H2O(aq) $ FeSO4(s) + 7H2O(l)
         (Pale green)                 (Dirty white)

 11.What is a redox reaction? Identify the substances oxidised and the substance reduced in the following reactions:
a. MnO + 4HCl $MnCl + Cl + H O
b. CuO + H $Cu+ H O
Ans : reaction is a reaction in which oxidation and
reduction takes place simultaneously.
Ans :a.  is the substance oxidised, MnO is the substance getting reduced.
Ans :b. H is getting oxidised, CuO is getting reduced.

 12.Write balanced equation for the reaction between Mg and hydrochloric acid. Name the product obtained, identify the type of reaction.
Ans :Mg(s) + 2HCl(dil) $MgCl (aq) + H (g) Magnesium chloride and hydrogen gas are formed in this reaction. It is a displacement reaction.

13.Reaction of compound X with aluminium is used to join railway tracks or cracked machine parts.
a. Identify the compound.
b. Name the reaction.
c. Write a balanced chemical equation for the reaction.
Ans :a. The compound X is Fe2O3 (Ferric oxide) or Iron (III) oxide.
Ans :b. It is called Thermite Reaction.
Ans :c. 2Al(s) + Fe2O3 (s) $Al2O3 (s) + 2Fe(l) 

 14.Using balanced chemical equation explain the difference between a displacement reaction and a double displacement reaction.
Ans :Displacement reaction: A reaction in which a more reactive element displaces a less reactive element from its salt solution e.g., 2KBr(aq) + Cl2 (g) $2KCl(aq) + Br2 (aq) Double displacement reaction: A reaction in which two compounds exchange their ions to form two new compounds e.g.,
KOH + HNO3 $KNO3 + H2O

15.Give an example each for thermal decomposition and photochemical decomposition reactions. Write balanced chemical equation also.
Ans :Thermal decomposition:
ZnCO3 (s)$ ZnO(s) + CO2 (g) 
Photochemical decomposition:
2AgI(s) $ Ag(s) + I2 (g)

16.When a metal X is added to salt solution of metal Y, the following chemical reaction takes place:
Metal X + Salt solution of Y $ Salt solution of X + Metal Y
Mention the inference you draw regarding the reactivity of metal X and Y and also the type of reaction. State the reason of your conclusions.
Ans :X is more reactive than Y because , X displaces Y from its salt solution. Hence, this reaction is an example of displacement reaction.


17.Identify the type of each of the following reactions: 
a. A reaction in which a single product is formed from two or more reactants.
a. The reaction mixture becomes warm.
b. An insoluble substance is formed.
c. External surface of the container in which reaction takes place becomes cold.         [CBSE 2012]
Ans :
Ans :a. Combination reaction
Ans :b. Exothermic reaction
Ans :c. Precipitation reaction (Double displacement reaction)
Ans :d. Endothermic reaction

18. Balance the following reactions:
a. BaCl + H SO $BaSO + HCl
b. Ca(OH) + HNO $Ca(NO ) + H O
c. Pb(NO ) $PbO + NO + O
d. MnO + HCl $MnCl + Cl + H O           [CBSE 2011, 2009, 2008, 2008C]
Ans :a. BaCl 2 (aq) + H 2 SO 4 (dil) $BaSO4 (s) + 2HCl (aq) 
Ans :b. Ca (OH) 2 (aq) + 2HNO 3 $Ca(NO3)2(aq) + 2H 2 O (l)
Ans :c.  2Pb (NO 3) 2 (s) $ 2PbO (s) + 4NO 2 (g) + O 2 (g)
Ans :d. MnO2+ 4HCl $MnCl2 + Cl2 + 2H2O


19.A zinc plate was put into solution of copper sulphate kept in a glass container. It was found that blue colour of the solution gets fader and fader with passage of time. After few days when zinc plate was taken out of the solution, a number of holes were observed on it.
 a. State the reason for the changes observed on zinc plate.
b. Write the chemical equation for the reaction involved.                [CBSE 2011] 
Ans :a. Zinc displaces copper from copper sulphate
solution to form colourless ZnSO4 and copper metal is deposited. Zinc gets consumed due to which holes are formed.
Ans :b. Zn(s) + CuSO4 (aq) $ZnSO4 (aq) + Cu(s) 

20.What is redox reaction? When a magnesium ribbon burns in air with a dazzling flame and forms a white ash, is magnesium oxidised or reduced. Why?           [CBSE 2010, 2009]
Ans :Redox reaction is a reaction in which oxidation and reduction takes place simultaneously.
Mg is getting oxidised because it is gaining oxygen to form magnesium oxide.