Chemical Reactions and Equations : NCERT Exercise Questions

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Q.1       Which of the statements about the reaction below are incorrect?
              2PbO\left( s \right) + C\left( s \right) \to 2Pb\left( s \right) + C{O_2}\left( g \right)
              (a) Lead is getting reduced.
              (b) Carbon dioxide is getting oxidized.
              (c) Carbon is getting oxidized.
              (d) Lead oxide is getting reduced.
              (i) (a) and (b)        (ii) (a) and (c)      (iii) (a), (b) and (c)         (iv) all
Sol.        (i) (a) and (b)

Q.2       2.F{e_2}{O_3} + 2Al \to A{l_2}{O_3} + 2Fe
              The above reaction is an example of a
              (a) combination reaction.
              (b) double displacement reaction.
              (c) decomposition reaction.
              (d) displacement reaction.
Sol.        (d) displacement reaction.

Q.3       What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.

             (b) Chlorine gas and iron hydroxide are produced.
             (c) No reaction takes place.
             (d) Iron salt and water are produced.
Sol.       (a) Hydrogen gas and iron chloride are produced.

Q.4      What is a balanced chemical equation? Why should chemical equations be balanced?
Sol.       When the number of atoms of reactants is equal to the number of atoms of the products, the reaction is called a balanced chemical equation. According to the Law of conservation of mass, total mass of the elements present in the reactants must be equal to the total mass of the elements present in products. That’s why a chemical equations should be balanced always.

Q.5      Translate the following statements into chemical equations and then balance them.
            (a) Hydrogen gas combines with nitrogen to form ammonia.
            (b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
            (c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Sol.      (a) 3{H_2} + {N_2} \to 2N{H_3}
            (b) 2{H_2}S + 3{O_2} \to 2{H_2}O + 2S{O_2}
            (c) 3BaC{l_2} + A{l_2}{\left( {S{O_4}} \right)_3} \to 2AlC{l_3} + 3BaS{O_4}
            (d) 2K + 2{H_2}O \to 2KOH + {H_2}

Q.6     Balance the following chemical equations:
           (a) HN{O_3} + Ca{\left( {OH} \right)_2} \to Ca{\left( {N{O_3}} \right)_2} + {H_2}O
           (b) NaOH + {H_2}S{O_4} \to N{a_2}S{O_4} + {H_2}O
           (c) NaCl + AgN{O_3} \to AgCl + NaN{O_3}
           (d) BaC{l_2} + {H_2}S{O_4} \to BaS{O_4} + HCl
Sol.      (a) 2HN{O_3} + Ca{\left( {OH} \right)_2} \to Ca{\left( {N{O_3}} \right)_2} + 2{H_2}O
            (b) 2NaOH + {H_2}S{O_4} \to N{a_2}S{O_4} + 2{H_2}O
            (c) NaCl + AgN{O_3} \to AgCl + NaN{O_3}
            (d) BaC{l_2} + {H_2}S{O_4} \to BaS{O_4} + 2HCl

Q.7     Write the balanced chemical equations for the following reactions.
           (a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
           (b) Zinc + Silver nitrate → Zinc nitrate + Silver
           (c) Aluminium + Copper chloride → Aluminium chloride + Copper
           (d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Sol.     (a) Ca{\left( {OH} \right)_2} + C{O_2} \to CaC{O_3} + {H_2}O
           (b) Zn + 2AgN{O_3} \to Zn{\left( {N{O_3}} \right)_2} + 2Ag
           (c) 2Al + 3CuC{l_2} \to 2AlC{l_3} + 3Cu
           (d) BaC{l_2} + {K_2}S{O_4} \to BaS{O_4} + 2KCl

Q.8     Write the balanced chemical equation for the following and identify the type of reaction in each case.
           (a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
           (b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
           (c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
           (d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Sol.     (a) 2KBr\left( {aq} \right) + BaC{l_2} \to 2Kl\left( {aq} \right) + BaB{r_3}\left( s \right)
           It is a double displacement reaction.
           (b) ZnC{O_3}\left( s \right) \to ZnO\left( s \right) + C{O_2}\left( g \right)
           It is a decomposition reaction.
           (c) {H_2}\left( g \right) + C{l_2} \to 2HCl\left( g \right)
           It is a combination reaction.
           (d) Mg\left( s \right) + 2HCl\left( {aq} \right) \to MgC{l_2} + {H_2}\left( g \right)
           It is a displacement reaction.

Q.9     What does one mean by exothermic and endothermic reactions? Give examples.
Sol.      Exothermic reaction – A reaction in which heat is released is called the exothermic reaction. Burning of fuel is an example of exothermic reaction. When methane is burnt it gives heat along with carbon dioxide and water.


Endothermic reaction – A reaction in which heat is absorbed is called endothermic reaction.
Example – When silver chloride is left in the sunlight, it absorbs heat and turns grey because of formation of silver metal.


Q.10     Why is respiration considered an exothermic reaction? Explain.
Sol.         In the course of respiration glucose is broken into carbon dioxide and energy is released. That’s why it is considered as an exothermic reaction.
              {C_6}{H_{12}}{O_6}\left( {aq} \right) + 6{O_2} \to 6C{O_2}\left( {aq} \right) + 6{H_2}O\left( I \right) + Energy

Q.11      Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Sol.         In a combination reaction two or more substances are combined to form new substance while in the decomposition reaction one substance decomposes into two or more substances. Hence, the decomposition reactions are called opposite of combination reactions.


Q.12      Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

In this reaction energy is supplied in the form of heat.

In this reaction energy is supplied in the form of light,i.e.sunlight

In this reaction energy is supplied in the form of electricity.

Q.13      What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Sol.        When a more reactive element displaces a less reactive element from its salt solution, then it is called displacement reaction. While there is an exchange of ions between the reactants to produce new substances, it is called double displacement reaction. In displacement reaction one displacement takes place. While in double displacement reaction two displacements takes place.
              Displacement reaction

Mg\left( s \right) + 2HCl\left( {aq} \right) \to \,MgC{l_2} + {H_2}\left( g \right)

              Double displacement reaction

2KBr\left( {aq} \right) + BaC{l_2} \to 2Kl\left( {aq} \right) + BaB{r_3}\left( s \right)

Q.14     In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

2AgN{O_3}\left( {aq} \right) + Cu\left( s \right) \to Cu{\left( {N{O_3}} \right)_2}\left( l \right) + 2Ag\left( s \right)

Q.15     What do you mean by a precipitation reaction? Explain by giving examples.
Sol.        Reactions in which precipitate is formed are called precipitation reactions. Generally in a precipitation reaction insoluble salts are settled down as precipitate.

N{a_2}S{O_4}\left( {aq} \right) + BaC{l_2}\left( {aq} \right) \to BaS{O_4}s + 2NaCl\left( {aq} \right)

              In the above reaction, when sodium sulphate reacts with barium chloride, the white precipitate of barium sulphate is formed along with the sodium chloride.

Q.16     Explain the following in terms of gain or loss of oxygen with two examples each.
              (a) Oxidation                  (b) Reduction
Sol.        Oxidation - When a substance gains oxygen or loses hydrogen during a reaction, it is called that the substance is oxidized. This phenomenon is called oxidation.                 

         In this reaction copper gains oxygen and hence is oxidized.

In the above reaction hydrogen gains oxygen, and hence oxidized.

             Reduction - If a substance loses oxygen or gains hydrogen during a reaction, it is that the                        substance is reduced. This phenomenon is called reduction.
             ZnO + C 


Zn + CO
             In this reaction zinc loses oxygen, and hence called is reduced.

Mn{O_2} + 4HCl \to MnC{l_2} + 2{H_2}O + C{l_2}

             In this reaction Mangnese dioxide loses oxygen and hence is called reduced.

Q.17     A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Sol.       Copper is a brown coloured shiny element, when heated in air, it becomes black in colour.

2Cu + {O_2} \to 2CuO

             Hence, the element ‘X’ is copper and black coloured compound formed is copper oxide.

Q.18    Why do we apply paint on iron articles?
Sol.       When articles made of iron come in contact with the moisture present in air, it forms iron oxide, which is known as rust. Hence, to prevent an iron article to come in contact with moisture present in air paint is applied. Paint prevents the iron to get rusted.

Q.19    Oil and fat containing food items are flushed with nitrogen. Why?
Sol.       When fats and oils are oxidised, they become rancid and their smell and taste change. Hence, food items containing oil and fat are flushed with nitrogen which prevents them to get oxidized and becoming rancid.

Q.20     Explain the following terms with one example each.
              (a) Corrosion            (b) Rancidity
Sol.        (a) Corrosion - Metals react with oxygen which is present in the atmospheric moisture. This leads to the formation of metal oxides. In due course of time, the metal keeps on changing into its oxide and finally the whole metal is lost due to oxidation. This process is called corrosion.
             Example: Iron articles; like iron gates or bridges tend to rust because of oxidation by atmospheric moisture. We know that rust is nothing but iron oxide. Conversion of iron into rust leads to corrosion of the iron articles. Due to this, the iron articles weaken and finally wither away.
             (b) Rancidity - When fats and oils are oxidised, their smell and taste change. This process is called rancidity. Oily food often become rancid and start giving out obnoxious smell. The taste also becomes bad. Such oily food is not fit for eating.
             Example: When packets of potato chips or other oily snacks are kept open for a long time; their taste and smell become bad. The oily food is no longer safe to eat.


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