Study Material

1. Which of the statements about the reaction below are incorrect ?

    2PbO(s) + C(s) ® 2Pb(s) + CO2 (g)

          a) Lead is getting reduced.

          b) Carbon dioxide is getting oxidised.

          c) Carbon is getting oxidised.

          d) Lead oxide is getting reduced.

              i) (a) and (b)             ii) (a) and (c)         iii) (a), (b) and (c)       iv) all

Ans. i) (a) and (b)

   2.   Fe2O3 + 2Al ® Al2O3 + 2Fe

          The above reaction is an example of a

          a) combination reaction.

          b) double displacement reaction.

          c) decomposition reaction.

          d) displacement reaction.

Ans. d) displacement reaction.

   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.

Ans. a) Hydrogen gas and iron chloride are produced.

   4.   What is a balanced chemical equation? Why should chemical equations be balanced?

Ans. A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.

              The chemical equations should be balanced to satisfy the law of conservation of mass.

   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 sulphur 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.

Ans. a) 3H2 (g) + N2 (g) ® 2NH3 (g)

          b) 2H2S (g) + 3O2 (g) ® 2SO2 (g) + 2H2O(l)

          c)  3BaCl2 (aq) + Al2(SO4)3 (aq) ® 2AlCl3 (aq) + 3BaSO4 ¯(s)

          d) 2K (s) + 2H2O (l) ® 2KOH (aq) + H2 (g)

   6.   Balance the following chemical equations.

          a) HNO3 + Ca(OH)2 ® Ca(NO3)2 + H2O       b) NaOH + H2SO4 ® Na2SO4 + H2O

          c) NaCl + AgNO3 ® AgCl + NaNO3            d) BaCl2 + H2SO4 ® BaSO4 + HCl

Ans. a) 2HNO3 + Ca(OH)2 ® Ca(NO3)2 + 2H2O   b) 2NaOH + H2SO4 ® Na2SO4 + 2H2O

          c)  NaCl + AgNO3 ® AgCl + NaNO3             d) BaCl2 + H2SO4 ® BaSO4 + 2HCl

   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

Ans. a) Ca(OH)2 + CO2 ® CaCO3 + H2O        b)  Zn + 2AgNO3 ® Zn(NO3)2 + 2Ag

          c)  2Al + 3 CuCl2 ® 2AlCl3 + 3Cu            d)  BaCl2 + K2SO4 ® BaSO4 + 2KCl

   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)

Ans. a) 2KBr (aq) + BaI2(aq) ® 2KI(aq) + BaBr2(s)

              Type of reaction : Double displacement reaction

          b) ZnCO3 (s) ® ZnO (s) + CO2 (g)

              Type of reaction : Decomposition reaction

          c)  H2 (g) + Cl2 (g) ® 2HCl(g)

              Type of reaction : Combination reaction

          d) Mg (s) + 2HCl (aq) ® MgCl2 (aq) + H2 (g)

              Type of reaction : Displacement reaction

   9.   What does one mean by exothermic and endothermic reactions? Give examples.

Ans. 1) Exothermic reactions : Reactions in which energy is released are known as Exothermic reactions.

              Example :

              i)   C (s) + O2 (g) ® CO2 (g) + Heat

              ii)  N2 (g) + 3H2 (g) ® 2NH3 (g) + Heat

          2) Endothermic reactions : Reactions in which energy is absorbed are known as endothermic reactions.

              Examples :

              i)   C (s) + 2S (s) ® CS2 (l) – Heat

              ii)  N2 (g) + O2 (g) ® 2NO(g) – Heat

10.   Why is respiration considered an exothermic reaction? Explain.

Ans. 1) Respiration is considered an exothermic reaction because it releases energy in the form of heat.

          2) During respiration, glucose and oxygen are combined in cells to produce carbon dioxide, water and energy in the form of ATP.

          3) C6H12O6 (aq) + 6O2 (aq)  ® 6CO2 (aq) + 6H2O(l) + energy

              (Glucose)

11.   Why are decomposition reactions called the opposite of combination reactions ? Write equations for these reactions.

Ans. 1) Decomposition reactions are called the opposite of combination reactions because they involve the breakdown of a single compound into two or more simpler substances, while combination reactions involve the formation of a single compound from two or more simpler substances.

          2) For example, Combination Reaction : When hydrogen (H2) combines with oxygen (O2) to form water (H2O) : 2H2 + O2 ® 2H2O

          3) Decomposition Reaction : When water (H2O) decomposes into hydrogen (H2) and oxygen (O2) : 2H2O ® 2H2 + O2

              This is the opposite of a combination reaction.

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

Ans. CaCO3 (s)       CaO(s)          +    CO2 (g)

          (Limestone)                 (Quick lime)

          2AgCl(s)  2Ag(s) + Cl2 (g)

          2H2O (l)  2H2 (g) + O2 (g)

13.   What is the difference between displacement and double displacement reactions? Write equations for these reactions.

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

Ans. 1) In the refining of silver, the recovery of silver from a silver nitrate solution involves displacement by copper metal.

          2) The chemical reaction for this process is as follows :

                   2AgNO3 (aq) + Cu (s) ® Cu(NO3)2 (aq) + 2Ag (s)

15.   What do you mean by a precipitation reaction ? Explain by giving examples.

Ans. 1) Any reaction that produces a precipitate can be called a precipitation reaction.

          2) Example : Add barium chloride solution to sodium sulphate solution in a test tube.

                   Na2SO4 (aq) + BaCl2(aq)  ® BaSO4 (s) + 2NaCl(aq)

                   (Sodium          (Barium         (Barium       (Sodium

                   sulphate)         chloride)      sulphate)     chloride)

          3) The white precipitate of BaSO4 is formed by the reaction of  SO42– and Ba2+.

          4) The other product formed is sodium chloride which remains in the solution.

          5) Here are other examples of precipitation reactions

              AgNO3 (aq) + NaCl (aq) ® AgCl (s) + NaNO3 (aq)

              Pb(NO3)2 (aq) + 2KI (aq) ® PbI2 (s) + 2KNO3 (aq)

16.   Explain the following in terms of gain or loss of oxygen with two examples each.

          a) Oxidation                               b) Reduction

Ans. a) Oxidation Reactions : In oxidation reactions, a substance gains oxygen or loses hydrogen.

              Combustion of Hydrogen : 2H2 (g) + O2 (g) ® 2H2O (g)

              Oxidation of Iron : 4Fe (s) + 3O2 (g) ®2Fe2O3 (s)

          b) Reduction Reactions : In reduction reactions, a substance loses oxygen or gains hydrogen.

              Reduction of Copper (II) Oxide by Hydrogen :

CuO (s) + H2 (g) ® Cu (s) + H2O (g)

              Reduction of Zinc by Carbon : ZnO + C ® Zn + CO

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.

Ans. 1) Element ‘X’ is copper (Cu).

          2) The black coloured compound is copper oxide (CuO).

          3) The reaction involved is

                              2Cu            +          O2   ¾¾¾®        2CuO

                   Copper (Brown)     Oxygen                   Copper oxide

18.   Why do we apply paint on iron articles ?

Ans. We apply paint on iron articles to protect them from corrosion (rusting) by creating a barrier that prevents moisture and oxygen from coming into contact with the iron surface.

 

Ans. 1) Oil and fat-containing food items are flushed with nitrogen to prevent oxidation and rancidity.

          2) Nitrogen displaces oxygen from the food packaging, creating an oxygen-free environment.

          3) This helps extend the shelf life of the products by reducing the oxidation of fats and oils.

20.   Explain the following terms with one example each.

          a) Corrosion                               b) Rancidity

Ans. a) Corrosion :

              1) Definition : Corrosion is the process by which metals deteriorate and weaken due to a chemical reaction with their environment, typically with oxygen and moisture.

              2) Example : Rusting of iron is a common example of corrosion.

          b) Rancidity :

              1) Definition : Rancidity is the development of an unpleasant odor or taste in fats, oils, or food products due to oxidation.

              2) Example : The development of a stale taste in old, exposed cooking oil is an example of rancidity.

Questions given in the Lesson

   1.   Why should a magnesium ribbon be cleaned before burning in air ?

Ans. A magnesium ribbon should be cleaned before burning in air because it may have a layer of oxide or other impurities on its surface. These impurities can interfere with the combustion process and affect the accuracy of the reaction.

   2.   Write the balanced equation for the following chemical reactions.

          i)    Hydrogen + Chlorine ® Hydrogen chloride

          ii)   Barium chloride + Aluminium sulphate ® Barium sulphate + Aluminium                   chloride

          iii)  Sodium + Water ® Sodium hydroxide + Hydrogen

Ans. i)    H2 + Cl2  ® 2HCl

          ii)   3BaCl2 + Al2(SO4)3 ® 3BaSO4 + 2AlCl3

          iii)  2Na + 2H2O ® 2NaOH + H2

   3.   Write a balanced chemical equation with state symbols for the following reactions.

          i)    Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

          ii)   Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Ans. i)    BaCl2(aq) + Na2SO4(aq) ® BaSO4(s) + 2NaCl(aq)

          ii)   NaOH(aq) + HCl(aq) ® NaCl(aq) + H2O(l)

   4.   A solution of a substance ‘X’ is used for white washing.

          i)   Name the substance ‘X’ and write its formula.

          ii)  Write the reaction of the substance ‘X’ named in (i) above with water.

Ans. i)   The substance used for white washing is calcium oxide, which is also known as quick lime. Its chemical formula is CaO.

          ii)  When calcium oxide (CaO) reacts with water (H2O), it undergoes a chemical reaction to produce calcium hydroxide (Ca(OH)2) : CaO(s) + H2O(l) ®                                                                                                     Ca(OH)2(aq)

   5.   Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Ans. 1) Water has the chemical formula H2O, meaning it consists of two hydrogen atoms (H2) for every one oxygen atom (O2).

          2) As a result, when water is electrolyzed, it breaks down into hydrogen gas and oxygen gas in a 2 : 1 ratio by volume.

          3) The gas is hydrogen.

   6.   Why does the colour of copper sulphate solution change when an iron nail is dipped in it ?

Ans. 1) When an iron nail is placed in a copper sulphate (CuSO4) solution, iron displaces copper from the copper sulphate solution because iron is more reactive.

          2) The iron atoms go into solution as iron ions (Fe²+), while copper ions (Cu²+) from the solution get deposited on the iron nail.

          3) The change in colour is because the copper ions give the solution its characteristic blue colour and when they are reduced and deposited onto the iron nail, the solution loses its blue colour.

   7.   Give an example of a double displacement reaction other than the one given in Activity 1.10.

Ans. AgNO3(aq) + NaCl(aq) ® AgCl(s) + NaNO3(aq)

   8.   Identify the substances that are oxidised and the substances that are reduced in the following reactions.

          i)   4Na(s) + O2(g) ® 2Na2O(s)

          ii)  CuO(s) + H2(g) ® Cu(s) + H2O(l)

Ans. i)   4Na(s) + O2(g) ® 2Na2O(s)

              Oxidation : In this reaction, sodium (Na) is oxidized as it gains oxygen (O2) to  form sodium oxide (Na2O).

              Reduction : Oxygen (O2) is reduced as it loses electrons.

          ii)  CuO(s) + H2(g) ® Cu(s) + H2O(l)

              Oxidation : In this reaction, copper oxide (CuO) is reduced to copper (Cu) as it gains electrons and forms solid copper.

            Reduction : Hydrogen gas (H2) is oxidized as it loses electrons to form water (H2O).