NCERT Class 10 very Important Frequently asked 3 mark questions of Science ch-1 (part-2)

Very Important 3 mark questions of Science Chemical reactions and equations (part-2)

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31. Explain the following terms with examples in each case: 

(i) Oxidation and (ii) Reduction

Ans: (i) Oxidation: It is a process in which Oxygen (or any electronegative element) is added up or Hydrogen ( or any electropositive element) is removed.

E.g., S + O2—–› SO2 (O2 is added to Sulphur)

         (ii) Reduction: It is a process in which Hydrogen (or any electropositive element) is added up or Oxygen (or electronegative element) is removed.

E.g., Cl2 + H2 –—› 2HCl ( H2 is added to CL)

32. State the electronic concept of Oxidation and Reduction.

Ans: (i) Oxidation: An Oxidation reaction is one in which electrons are released or lost. Magnesium atom is Oxidised to magnesium ion by loss of electrons.

   Mg —–› Mg2+   + 2e

        

        (ii) Reduction: A Reaction in which electrons are accepted is called a reduction. Oxygen atom is reduced to oxide ion by accepting two electrons. 

    O + 2e –—› O2-

33. (a) Consider the following Chemical equation: 

     Fe2O3 + 2Al —–› Al2O3 + 2Fe

Name two categories in which you can place this reaction.

(b) Define the term Oxidation of a substance and name the substance Oxidised and the Oxidising agent in the above Reaction.

Ans: (a) It is displacement as well as Oxidation Reaction.

         (b) A substance is said to be oxidised when it either gains oxygen or loses Hydrogen atom. The process is called Oxidation. In the given Reaction aluminium (Al) is Oxidised and ferric Oxide (Fe2O3) is the Oxidising agent.

34. Define the terms 

           (i) Oxidising agent

           (ii) Reducing agent

Ans: (i) Oxidising agent: It is that substance which in a Reaction gives up oxygen or accepts Hydrogen or accepts electrons.

         (ii) Reducing agent: It is that substance which in a Reaction gives up Hydrogen or accepts Oxygen or releases electrons.

35. What is a Oxidation- Reduction or Redox Reaction? Justify your answer by writing one such Chemical equation and name the substance Oxidised and the substance Reduced in it.

Ans: A Chemical reaction in which one substance is Oxidised and the other substance is reduced is called redox Reaction. All Oxidation-Reduction reactions are redox Reactions. In a chemical reaction, a substance gets oxidised only when another substance is present, Which gets reduced.

          CuO + H2 —–› Cu + H2O

Here, CuO is losing Oxygen, is being reduced. The Hydrogen is gaining Oxygen,is being Oxidised.

36. What are non-Redox Reactions? Explain with the help of examples.

Ans: The Reaction in which neither Oxidation nor Reduction takes or no species either gains or loses electrons are called non-redox Reactions.

E.g., HCl + NaOH —–› NaCl + H2O

        BaCl2 + Na2SO4 —–› BaSO4 + 2NaCl

37. Manganese dioxide when reacts with Hydrochloric acid forms Manganese chloride, water and chlorine.

(a) Express the above Reaction in the form of a balanced chemical equation.

(b) Identify

        (i) Reducing agent

       (ii) Oxidising agent.

Ans: (a) MnO2 + 4HCl —–› MnCl2 + Cl2 +  2H2O

          (b) (i) Reducing agent — HCl

                (ii) Oxidising agent— MnO2

38. Identify the components Oxidised in the following chemical equations:

(i) 2Cu + O2 —–› 2CuO

(ii) H2S + Cl2 —–› 2HCl + S

(iii) 2H2 + O2 —–› 2H2O

Ans: (i) Cu

         (ii) H2S

         (iii) H2

39. A blue colour salt becomes white on heating. Give reason for the above observation. What happens when we add water to the salt which was obtained after heating. Also write it's Formula.

Ans: The substance is copper sulphate (CuSO4.5H2O). On heating it loses water of crystallization and white CuSO4 is obtained. On adding water, it again becomes hydrated and blue colour is regained.

40. Select the Oxidising agent and reducing agent from the following Reactions:

   (i) H2S + I2 —–› 2HI + S

  (ii) CuO + H2 —–› Cu + H2O

 (iii) Zn + CuSO4 —–› ZnSO4 + Cu.

Ans: (i) H2S is reducing agent and I2 is Oxidising agent.

        (ii) CuO is Oxidising agent and H2 is reducing agent.

        (iii) Zn is reducing agent and CuSO4 is Oxidising agent. 

41. State some Reactions of Oxidation that you observe in your everyday life.

Ans: Some examples of Oxidation Reactions are: 

       (a) Shiny iron articles on exposure to air get coated with a brownish layer.

      (b) Copper articles on exposure to air get coated with greenish layer.

       (c) Silver articles on exposure becomes black.

       (d) Fats and oils in food left for long time get Oxidised.

42. Can a Displacement Reaction be a redox Reaction? Explain with the help of an example.

Ans: Consider the following Displacement Reaction:

          Zn + CuSO4 —–› ZnSO4 + Cu

Writing this in ionic from, we get

         Zn + Cu2+ —–› Zn+2 + Cu

      Here, zinc atom donates two electrons (two copper atom) and forms a zinc ion, Zn2+. By definition, this is an Oxidation reaction.

      Also copper ion, Cu2+ accepts two electrons ( given by zinc atom) and forms, copper atom, Cu. By definition, this is reduction. 

     SO4 2- ions remain as such and hence above Reaction involves both Reduction as well as Oxidation processes and is a redox reaction.

    Thus, a displacement reaction can be a redox reaction.

43. What is the relationship between Oxidation and Oxidising agent in a redox reaction? Give an example of a redox reaction showing the relationship between Oxidation and Oxidising agent.  

Ans: An Oxidising agent is a substance which oxidises other substance but itself gets reduced. For example,  

         Zn +    Cu2+  —–› Zn2+   + Cu

In the above example, Cu2+ Oxidises Zn to Zn2+ and itself gets reduced to Cu.

Cu2+ is an oxidising agent. The reaction is called Oxidation reaction.

44. Name the substance Oxidised, reduced, oxidising agent and reducing agent in the following reactions: 

(i) Ca + Cl2 —–› CaCl2

(ii) 3MnO2 + 4Al —–› 3Mn + 2Al2O3

(iii) Fe + S —–› FeS

Ans:   

Substance   Oxidised	Substance  Reduced	Reducing  agent	Oxidising agent
Ca	Cl2	Ca	Cl2.

Al	MnO2	Al	MnO2
Fe	S	Fe	S

45. In the following chemical reaction

             ZnO + C —–› Zn + CO

(i) Identify the substance getting oxidised and the one getting reduced.

(ii) State the reason for choosing the substance in (i).

(iii) Name the type of Reaction and give another example of similar type of reaction.

Ans: (i) C — Oxidised       ZnO — reduced

        (ii) ZnO loses oxygen and C gains Oxygen.

       (iii) This is a redox reaction.

Reaction between Hydrogen sulphide and chlorine to give sulphur and Hydrochloric acid is another example of a redox reaction.

         H2S + Cl2 —–› S + 2HCl

46. A brown substance X on heating in air forms a substance Y. When Hydrogen gas is passed over heated Y it again changes back into X.

(i) Name the substance X and Y.

(ii) Name the type of chemical reactions occuring during both the changes.

(iii) Write the chemical equations of the reactions.

Ans: (i) X is Cu and Y is CuO.

        (ii) Oxidation, Reduction reactions.

       (iii) 2Cu + O2 —–› 2CuO

              CuO + H2 —–› Cu + H2O

47. What is corrosion? What are its effects?

Ans: Due to the effect of moisture and acids, metals get corroded. This effect is called Corrosion.

          Corrosion causes damage to metal articles like car bodies, bridges, iron railings, ships and other substances of daily use.  

48. How is Corrosion different from rusting?

Ans: Corrosion is seen in all metals, when exposed to air. A layer of metal compound is formed on the surface due to presence of moisture, acids or other gases pesent in air. Rusting is the process in which iron metal reacts with air and moisture to form brownish powder called rust.

49. What is rusting of iron? How is it prevented? 

Ans: The Corrosion of iron due to moisture is called rusting of iron. Shiny iron articles on exposure to moisture get coated with a brownish powder which peels off easily. In this way enormous amount of iron is damaged and lost.

Prevention of rusting:

(i) The iron articles should be painted.

(ii) The machine parts should be oiled and greased.

(iii) Galvanised iron pipes are used for water supply.

(iv) Iron can be coated with chromium.

50. What is Rancidity? How is it prevented?

Ans: Fats and Oils in food kept for long time get Oxidised and become rancid and taste of food changes and causes infection on eating. This is called rancidity.

         To prevent rancidity antioxidants (which prevent Oxidation) are added to foods containing fats and oils. Rancidity can also be prevented by flushing out oxygen with an inert gas like nitrogen. For example, packets of food items like chips are flushed with nitrogen so that these can be used even after long duration. Keeping food in air tight containers also help to slow down Oxidation and to preserve food for a longer time.

51. Give two examples from everyday life situations where Corrosion Reactions are taking place.

Ans: Two Common situations that we come across and involve corrosion Reactions are: 

(i) Rusting of iron: When iron objects are exposed to humid air, a red brown flaky mixture of iron oxide, Fe2O3 and iron hydroxide, Fe(OH)3 is formed. It is soft and porous. Here iron is oxidised to iron oxide and oxygen is reduced to oxide.

               3Fe + 2O2 —–› Fe2O3

(ii) Corrosion of metals: When copper vessel is left in humid atmosphere for a few days, a green coating is formed on the surface due to the formation of basic copper carbonate.

   2Cu + H2O + CO2 + O2 —–› CuCO3.Cu(OH)2

 Here, Copper is oxidised to Cu2+ ions and oxygen is reduced to oxide.

52. Give two differences between rusting and burning.

Ans: (i) Rusting is a slow process while burning is a fast process.

        (ii) Rusting takes place in the presence of oxygen and moisture while burning requires only oxygen.

53. Rancidity is a process which takes place for spoiling of cooked food materials like vegetables etc. When kept for long time in open. Write the chemical reaction taking place. How can you prevent such a process to proceed? Give an example.

Ans: Rancidity is an Oxidation Reaction. Rancidity can be prevented by keeping the food material away from contact of air. It is easily done by flushing out oxygen with neutral gas like nitrogen. For example, potato chips can be preserved for a much longer time if oxygen is flushed out with the help of nitrogen and then packed.

54. A metal X available in the form of ribbons burns in air with a dazzling white light once heated to its ignition temperature. It leaves a white powder Y which dissolves in water. The solution so formed turns red litmus blue. Identify X and Y and write balanced chemical equation of the reaction.

Ans: Magnesium ribbon burns with dazzling white light once heated to its ignition temperature. The white powder formed is magnesium oxide which dissolves in water to give a basic solution which turns red litmus blue. Thus X is Mg and Y is MgO.

        2Mg + O2 —–› 2Mg

55. A student was given 5g of a pale green solid and was asked to observe what happens when it is heated. He found that on heating yellow coloured gas evolved giving smell of burning sulphur and a reddish brown solid was left-behind. When Zn is added to a solution of given compound, pale green colour fades away. Explain the above Reaction.

Ans: A yellow coloured gas with smell of burning sulphur is given by oxides of Sulphur. Further a pale green solid of Sulphur compound is Ferrous sulphate. This is Further confirmed because pale green colour of ferrous sulphate fades away due to displacement Reaction by adding Zn. Thus the above Reactions are:

          Zn + FeSO4 —–› ZnSO4 + Fe

          2FeSO4 —–› Fe2O3 + SO2 + SO3

56. Given the following reaction:

         2Al + Fe2O3 —–› 2Fe + Al2O3 + heat

Answer the following:

(a) Name the Oxidising agent.

(b) Name the reducing agent.

(c) Name the substance Oxidised.

(d) Name the substance Reduced.

(e) What is the commercial application of this process?   [ a+b=1;  c+d=1;  e=1m]

Ans: (a) Fe2O3                      (b) Al

         (c)  Al                             (d) Fe2O3

         (e) Heat produced melts iron which is used to fill up the gap of machine broken parts and railway lines.

57. A grey colour metal Z(atomic weight=65) is used in making dry cell. It reacts with dil. HCl to liberate a gas. What is the gas evolved? Calculate the minimum amount of Z required to produce 100 litres of the gas at NTP.

Ans: All active metals displaces H2 gas from dilute acids. Thus a grey coloured metal of atomic weight 65 is Zinc. The following Reaction takes place: 

        Zn + 2HCl —–› ZnCl2 + H2

According to the given Reaction, 22.4 litres of H2 will be given out if we use 65g Zn. For 100 litres H2, Zn to be used 

= 100 / 22.4 × 65 =  290.18g.

58. A brown coloured metal X is a good conductor of electricity and is mixed with gold to make ornaments. Metal Y is also used for making Jewellery but turns black (Z) in air.   Identify X, Y, Z. What type of reaction will take place between X and a salt of Z?

Ans: Brown coloured metal X which is a good conductor of electricity and is mixed with gold to make ornaments is copper. Metal Y which is used for jewellery is Silver because silver in the presence of atmosphere becomes black due to the formation of Ag2S. So Z is Ag2S. Since X or copper is more reactive than the metal in Z, so displacement reaction will take place.

For example: 

         Cu + 2AgNO3 —–› Cu(NO3)2 + 2Ag

59. An alloy of M is used for Soldering. When a nitrate of M is heated, it gives a yellowish brown coloured metal oxide together with a brown gas, G1 and a colourless gas G2. Identify G1 and G2 and write the balanced chemical equation.

Ans: Metal M used for Soldering is lead. When lead nitrate is heated, it gives yellowish brown lead oxide and brown vapours of nitrogen dioxide and colourless Oxygen are evolved. The following reaction takes place: 

           2Pb(NO3)2 —–› 2PbO + 4NO2 + O2

Thus, G1 is NO2 and G2 is O2.

60. A yellowish coloured compound X is a photo sensitive material. On exposure to sunlight, it gives a greyish substance Y and brown fumes of a gas Z. Identify X,Y and Z. How will you obtain X from the nitrate salt of Y? 

Ans: A yellowish coloured compound which is photo sensitive also is silver bromide (AgBr). When AgBr is exposed to sunlight, the following reaction takes place: 

          2AgBr —–› 2Ag + Br2

Thus X is AgBr;  Y is Ag and Z is Br2. 

Nitrate of Y is AgNO3. 

 X is precipitated when Y is heated with NaBr.

          AgNO3 + NaBr —–› AgBr + NaNO3.