Chemical Reaction and Equations - Practice for Everyone

PYQs

One Marker Questions

1. In electrolysis of water, why is the volume of gas collected over one electrode double that of gas collected over the other electrode.

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Ans. Water contains hydrogen and oxygen in the ratio of 2 : 1, that is why the volume of hydrogen is double than that of oxygen.

2. What can be seen when a strip of copper metal is placed in a solution of silver nitrate?

View Answer

Ans. The solution will become blue. Shiny silver metal will get deposited.

Cu(s) + 2AgNO₃ (aq) — > Cu(NO₃)₂ (aq) + 2 Ag (s)

3. State one industrial application of reduction process.

View Answer

Ans. It is used in the extraction of metals e.g.

ZnO (s) + C (s) — > Zn (s) + CO₂ (g)

4. Which of the following is a chemical change? Give reason also.

(a) Burning of wax

View Answer

Ans. Burning of wax is a chemical change because new products with new properties will be formed on burning.

5. Which one is a chemical change:

Rusting of iron or melting of iron?

View Answer

Ans. Rusting of iron is a chemical change.

6. State one basic difference between physical change and a chemical change.

View Answer

Ans.

Physical Change	Chemical Change
1. In a physical change, no new substance is formed	1. In a chemical change, new substance(s) with new properties is/are formed.

7. Name the oxidizing and reducing agent in the following reaction:

View Answer

Ans. CuO + H₂ — > Cu + H₂O

CuO is oxidizing agent H₂ is reducing agent.

8. Write a complete balanced chemical equation for the following reaction:

View Answer

Sodium hydroxide + Sulphuric Acid — >

Ans. 2NaOH + H₂SO₄ — > Na₂SO₄ + 2H₂O

9. Name and state the law which is kept in mind when we balance chemical equations.

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Ans. Law of conservation of mass. It states matter can neither be created nor be destroyed. The total mass of reactants must be equal to total mass of products.

10. What is meant by a chemical reaction?

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Ans. Chemical reaction, a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products.

11. What does down arrow and up arrow indicate in a chemical equation.

View Answer

Ans. Up arrow shows evolution of gas, down arrow shows a precipitate.

12. Hydrogen being a highly inflammable gas and oxygen being a supporter of combustion, yet water, a compound made up of hydrogen and oxygen is used to extinguish fire. Why?

View Answer

Ans. H₂O is a compound constituted of hydrogen and oxygen elements and being a compound it has different properties as compared to its constituting elements.

13. N₂ (g) + 3H₂ (g) — > 2NH₃ (g), name the type of reaction.

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Ans. Combination reaction

14. If copper metal is heated over a flame it develops a coating. What is the colour and composition of coating?

View Answer

Ans. Black coloured coating is formed. It is due to formation of copper oxide.

15. Write a balanced chemical equation to represent the following reaction: carbon monoxide reacts with hydrogen gas at 340 atm to form methyl alcohol.

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Ans.

16. Which one is a chemical change : fermentation of fruit juice or diluting fruit juice?

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Ans. Fermentation of fruit juice is a chemical change.

17. Is burning of candle wax a physical or a chemical change?

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Ans. Burning of a candle wax is a chemical change.

18. Write a balanced equation for the chemical reaction that can be characterised as precipitation reaction.

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Ans. AgNO₃ (aq) + NaCl (aq) — > AgCl (s) + NaNO₃ (aq)

It is a precipitation reaction.

19. State the main difference between endothermic reaction and an exothermic reaction.

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Ans. In endothermic reaction, heat is absorbed. In exothermic reaction, heat is evolved.

20. What happens chemically when quick lime is added to water filled in bucket?

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Ans.

Calcium hydroxide (Slaked lime) is formed with evolution of heat and hissing sound

CaO (s) + H₂O (l) — > Ca(OH)₂ (aq)

21. Define oxidation and reduction.

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Ans. Oxidation is a process in which oxygen is added or loss of electrons take place. Reduction is a process in which hydrogen is added or gain of electrons takes place.

22. Give an example of double displacement reaction.

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Ans. BaCl2 (aq) + H2SO4 (dil) — > BaSO4 (s) + 2HCl (aq)

It is a double displacement reaction.

23. On what basis is a chemical reaction balanced?

View Answer

Ans. Chemical equation is balanced on the basis of law of conservation of mass.

24. What change in colour is observed when white silver chloride is left exposed to sunlight? State the type of chemical reaction in this change.

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Ans. Grey coloured silver metal is formed and pungent smelling chlorine gas is evolved.

2AgCl (s) —– > 2 Ag(s) + Cl₂ (g)

It is a photochemical decomposition reaction.

25. Write a balanced chemical equation for the reaction between sodium chloride and silver nitrate indicating the physical state of the reactants and the products.

View Answer

Ans. AgNO₃ (aq) + NaCl (aq) — > AgCl (s) + NaNO₃ (aq)

26. Complete and balance the following equation;

Fe₂O₃ + Al — >

View Answer

Ans. Fe₂O₃ + 2Al — > 2Fe + Al₂O₃

27. Balance the following chemical equation:

Pb(NO₃)₂ —- > PbO + NO₂ + O₂

View Answer

Ans. 2Pb(NO₃)₂ —- > 2PbO + 4NO₂ + O₂

28. Identify the type of reaction in the following example:

Na₂SO₄ (aq) + BaCl₂ (aq) — > BaSO₄ (s) + 2NaCl (aq)

View Answer

Ans. Double displacement reaction

29. Identify the type of reaction in the following example:

Fe (s) + CuSO₄ (aq) — > FeSO₄ (aq) + Cu (s)

View Answer

Ans. Displacement reaction

30. Identify the type of reaction in the following example:

2H₂ (g) + O₂ (g) — > 2H₂O (l)

View Answer

Ans. Combination reaction

31. Balance the given chemical equation:

Al (s) + CuCl₂ (aq) — > AlCl₃ (aq) + Cu (s)

View Answer

Ans. 2Al (s) + 3CuCl₂ (aq) — > 2AlCl₃ (aq) + 3Cu (s)

32. Balance the given chemical equation:

FeSO₄ (s) —- > Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)

View Answer

Ans. 2FeSO₄ (s) —- > Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)

33. Balance the given chemical equation:

Fe (s) + H₂O (g) —- > Fe₃O₄ (s) + H₂ (g)

View Answer

Ans. 3Fe (s) + 4H₂O (g) —- > Fe₃O₄ (s) + 4H₂ (g)

34. On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue green. Predict the new compound formed which imparts a blue green colour to the solution.

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Ans. CuO + 2HCl — > CuCl₂ + 2H₂O

Copper chloride solution imparts blue green colour to the solution.

35. Why is respiration considered as exothermic process?

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Ans. Respiration is an exothermic process because energy is given out in respiration.

Two Marks Questions

36. Write the essential condition for the following reaction to take place:

2AgBr — > 2Ag + Br₂

Write application of this reaction.

View Answer

Ans. The reaction will take place in presence of sunlight. This reaction is used in black and white photography.

37 (a) Complete the following equation for the chemical reaction.

FeSO₄ (s) — > Fe₂O₃ + _______ + ______

(b) What happens when water is added to quicklime (CaO)? Write the chemical equation.

View Answer

Ans. (a) FeSO₄ (s) — > Fe₂O₃ + SO₂ (g) + SO₃ (g)

(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)

38. 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.

View Answer

a) 2H₂S (g) + 3O₂ (g) — > 2H₂O (l) + 2SO₂ (g)

b) BaCl₂ (aq) + ZnSO₄ (aq) — > BaSO₄ (s) + ZnCl₂ (aq)

39. 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.

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Ans. The blue coloured solution will become pale green. Reddish brown metal will get deposited.

Fe (s) + CusO₄ (aq) —- > FeSO₄ (aq) + Cu (s)

(Blue) (Pale Green) (Reddish Brown)

40. Name the reducing agent in the following reaction:

3MnO₂ + 4Al —- > 3Mn + Al₂O₃

State which is more reactive, Mn or Al and why?

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Ans. Al is the reducing agent. Al is more reactive than Mn.

Reason: It is because Al is displacing Mn from MnO₂

41. “We need to balance a skeleton chemical equation”. Give reason to justify the statement.

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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 be balanced.

42. A metal is treated with dilute H₂SO₄. The gas evolved is collected by the method as shown in the figure.

Answer the following questions:

a) Name the gas liberated.

b) Name the method used for collection of gas

c) Is the gas soluble or insoluble in water?

d) Is the gas lighter or heavier than air?

View Answer

Ans. a) The gas liberated is H₂

b) It is collected by downward displacement of water.

c) The is insoluble in water

d) Hydrogen gas is lighter than air.

43. Giving an example list two important information which makes a chemical equation more useful (informative).

View Answer

Ans. a) It should include physical states of reactants and products.

It should specify conditions under which reaction takes place e.g

44. Consider the following chemical equation:

X + Barium chloride — > Y + Sodium chloride

Identify a) X and Y

b) The type of reaction

View Answer

Ans. A) X is silver nitrate, Y is silver chloride

2AgNO₃ (ag) + BaCl₂ (aq) — > 2 AgCl + Ba(NO₃)₂ (aq)

b) The reaction is an example of double displacement (precipitation) reaction.

45. a) Write a balanced chemical equation for the process of photosynthesis.

b) When do desert plant take up carbon dioxide and perform photosynthesis.

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Ans. a) 6CO₂ (g) + 6H₂O (l) ——-> C₆H₁₂O₆ (aq) + 6O₂ (g)

b) Desert plant take up carbon dioxide and perform photosynthesis at night.

46. Two reactions are given below:

a) 2KI + Cl₂ — > 2KCl + I₂

b) 2K + Cl₂ — > 2KCl

Identify the type of reaction, giving justification in each case.

View Answer

Ans. a) Displacement reaction because Cl₂ is displacing I₂ from KI solution.

b) Combination reaction because K reacts with Cl₂ to form potassium chloride.

47. On heating copper powder in air, the surface of copper powder becomes coated with black CuO. How can this black coating be converted into brown copper? Write chemical equation for the reaction that occurs during the colour change.

View Answer

Ans. 2Cu + O₂ — > 2CuO

Copper oxide on heating with H₂ will change back to reddish brown copper metal.

CuO (s) + H2 (g) — > Cu (s) + H₂O (g)

48. 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.

View Answer

Ans. Yellow precipitate is formed due to formation of lead iodide.

It is a precipitation as well as double displacement reaction.

Pb(NO₃)₂ (aq) + 2KI (aq) —- > PbI₂ (s) + 2KNO₃ (aq)

Yellow precipitate

49. Write a balanced chemical equation for the process of photosynthesis and the conditions of reactions giving physical state of all substances.

View Answer

Ans.

50. Give one example of each:

a) Chemical reaction showing evolution of a gas.

b) Change in colour of a substance during chemical reaction.

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Ans. a) 2K (s) + 2H₂O (l) —> 2KOH (aq) + H₂ (g)

b) FeSO4.7H2O (aq) —– > FeSO4 (s) + 7 H2O (l)

(Pale green) (Dirty white)

51. Translate the following statement into chemical equation and then balance it. “A metal in the form of ribbon burns with a dazzling white flame and changes into white powder.”

View Answer

Ans. 2Mg (s) + O₂ (g) —- > 2MgO (s) + light + heat

52. It has been found that marble of Taj is getting corroded due to development of industrial areas around it. Explain this fact giving a chemical equation.

View Answer

Ans. Taj is made up of CaCO3 which reacts with acid formed by pollution of SO2 (g) and NO2 from Mathura refinery and other industries.

CaCO₃ + 2H₂SO₄ —- > CaSO₄ + CO₂ + 2H₂O

CaCO₃ + 2HNO₃ —- > Ca(NO₃)₂ + 2H₂O + CO₂

53. Consider the chemical equations given below and answer the questions which follow:

(i) CuO + H₂ —- > Cu + H₂O

(ii) ZnO + C — > Zn + CO

a) Name the substances that are oxidized and reduced respectively in each case.

b) Identify the reducing agent in each case.

View Answer

Ans. a) H₂ is getting oxidized to H₂O, CuO is getting reduced to Cu.

b) H₂ is reducing agent in (i) reaction whereas C is reducing agent in (ii) reaction.

54. What is a redox reaction? Identify the substances oxidized and the substances reduced in the following reactions:

a) MnO₂ + 4HCl — > MnCl₂ + Cl₂ + H₂O

b) CuO + H₂ —-> Cu + H₂O

View Answer

Ans. Redox reaction is a reaction in which oxidation and reduction takes place simultaneously.

a) HCl is the substance oxidised, MnO₂ is the substance getting reduced.

b) H₂ is getting oxidised, CuO is getting reduced.

55. Write balanced chemical equations for the following reactions:

a) Silver bromide on exposure to sunlight decomposes into silver and bromine.

b) Sodium metal reacts with water to form sodium hydroxide and hydrogen gas.

View Answer

Ans. a) 2AgBr (s) —- > 2Ag (s) + Br₂ (g) {in presence of sunlight}

b) 2Na (s) + 2H₂O (l) —- > 2NaOH (l) + H₂ (g)

56. Identify the type of reaction from the following equations:

a) CH₄ + 2O₂ —- > CO₂ + 2H₂O

b) Pb(NO₃)₂ + 2KI —- > PbI₂ + 2KNO₃

c) CaO + H₂O —–> Ca(OH)₂

d) CuSO₄ + Zn —- > ZnSO₄ + Cu

View Answer

Ans. a) Oxidation reaction

b) Double displacement reaction

c) Combination reaction

d) Displacement reaction

57. Write balanced equation for the reaction between Mg and hydrochloric acid. Name the product obtained, identify the type of reaction.

View Answer

Ans. Mg (s) + 2HCl (dil) —- > MgCl₂ (aq) + H₂ (g)

Magnesium chloride and hydrogen gas are formed in this reaction. It is a displacement reaction.

58. a) What is colour of ferrous sulphate crystals? How does this colour change after heating?

b) Name the products formed on strongly heating ferrous sulphate crystals. What type of chemical reaction occurs in this change?

View Answer

Ans. a) FeSO4.7H2O crystals are pale green in colour. They become dirty white on heating.

b) Ferric oxide, Sulphur dioxide and Sulphur trioxide are formed.

2FeSO₄ (s) —– > Fe₂O₃ (s) + SO₂ (g) + SO₃ (g)

It is a decomposition reaction.

59. 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.

View Answer

Ans. a) The compound X is Fe₂O₃ (Ferric Oxide) or Iron (III) oxide

b) It is called Thermite Reaction.

c) 2Al (s) + Fe₂O₃ (s) —- > Al₂O₃ (s) + 2Fe (l)

60. Using balanced chemical equation explain the difference between a displacement reaction and a double displacement reaction.

View Answer

Ans. Displacement reaction: A reaction in which a more reactive element displaces a less reactive element from its salt. 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

61. Give an example each for thermal decomposition and photochemical decomposition reactions. Write balanced chemical equation also.

View Answer

Ans. Thermal decomposition:

ZnCO₃ (s) —— > ZnO (s) + CO₂ (g)

Photochemical decomposition:

2AgI (s) —— > 2 Ag (s) + I₂ (g)

62. Why are decomposition reactions called the opposite of combination reactions? Give chemical equations for these reactions.

View Answer

Ans. In decomposition reactions, a compound is broken down into one or more elements or compounds e.g., CaCO₃ (s) —– > CaO (s) + CO₂ (g)

In combination reactions, two or more elements or compounds combine to form a new compound. Therefore, decomposition reactions are opposite to combination reactions e.g.,

3Mg (s) + N₂ (g) —– > Mg₃N₂ (s)

63. Why does the colour of copper sulphate solution changes when an iron is dipped in it?

View Answer

Ans. Iron being more reactive displaces copper from copper sulphate (blue) solution to form iron (II) sulphate (pale green) solution and reddish brown copper metal gets deposited.

Fe (s) + CuSO₄ (aq) —- > FeSO₄ (aq) + Cu (s)

64. 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. State the two types in which this reaction can be classified.

d) Potassium reacts with water to give potassium hydroxide and hydrogen gas.

View Answer

Ans. a) 3H₂ (g) + N₂ (g) — > 2NH₃ (g)

b) 2H₂S (g) + 3O₂ (g) —- > 2H₂O (l) + 2SO₂ (g)

c) 3BaCl₂ (aq) + Al₂(SO₄)₃ (aq) —- > 3BaSO₄ (s) + 2AlCl₃ (aq)

This reaction can be classified as a double displacement reaction as the two reacting species (BaSO₄, Al₂SO₄)₃ ) undergoes mutual exchange of ions. It can also be classified as precipitation reaction, since a white ppt of BaSO₄ is obtained.

d) 2K (s) + 2H₂O (l) —- > 2KOH (aq) + H₂ (g)

65. 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.

View Answer

Ans. X is more reactive than Y because, X displaces Y from its salt solution. Hence, this reaction is an example of displace reaction.

66. Identify the type of each of the following reactions:

a) A reaction in which a single product is formed from two or more reactants

b) The reaction mixture becomes warm.

c) An insoluble substance is formed.

d) External surface of the container in which reaction takes place becomes cold.

View Answer

Ans. a) Combination reaction

b) Exothermic reaction

c) Precipitation reaction (Double displacement reaction)

d) Endothermic reaction.

67. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of reaction.

View Answer

Ans. AgNO₃ (aq) + KCl (aq) —- > AgCl (s) + KNO₃ (aq)

It is a double displacement reaction.

68. Using a suitable chemical equation justify that some chemical reactions are determined by

a) Change in colour

b) Change in temperature

View Answer

Ans. a) Change in colour: Blue coloured crystalline copper sulphate crystals changes to white CuSO4 (anhydrous) on heating

CuSO₄.5H₂O —— > CuSO4 + 5H₂O

(Blue) (Dirty white)

b) Change in temperature:

CaO (s) + H₂O(l) —- > Ca(OH)₂ (aq) + heat

69. a) A solution of substance X is used for white washing. What is substance X? Write the chemical reaction of X with water.

b) Why does the colour of copper sulphate solution changes when iron nail is dipped in it?

View Answer

Ans. a) X is CaO, Calcium oxide.

CaO (s) + H₂O (l) —- > Ca(OH)₂ (aq)

b) It is because Fe displaces Cu from CuSO₄ (blue) solution to form FeSO₄ (pale green) and reddish brown Cu metal gets deposited.

70. Balance the following reactions:

a) BaCl₂ + H₂SO₄ —- > BaSO₄ + HCl

b) Ca(OH)₂ + HNO₃ —- > Ca(NO₃)₂ + H₂O

c) Pb(NO₃)₂ —– > PbO + NO₂ + O₂

d) MnO₂ + HCl —- > MnCl₂ + Cl₂ + H₂O

View Answer

Ans. a) BaCl₂ + H₂SO₄ —- > BaSO₄ + 2HCl

b) Ca(OH)₂ + 2HNO₃ —- > Ca(NO₃)₂ + 2H₂O

c) 2Pb(NO₃)₂ —– > 2PbO + 4NO₂ + O₂

d) MnO₂ + 4HCl —- > MnCl₂ + Cl₂ + 2H₂O

71. Write the balanced equation for the following reactions and identify the type of reaction in each case:

a) Potassium bromide + Barium Iodide —- > Barium bromide + Potassium Iodide

b) Hydrogen (g) + Chlorine (g) —– > Hydrogen chloride (g)

View Answer

Ans. a) 2KBr (aq) + BaI₂ —– > BaBr₂ (aq) + 2KI (aq)

It is double displacement reaction

b) H₂ (g) + Cl₂ (g) —- > 2HCl (g)

It is combination reaction.

72. A zinc place 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.

View Answer

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.

b) Zn (s) + CuSO4 (aq) —- > ZnSO4 (aq) + Cu (s)

73. A white salt on heating decomposes to give brown fumes and a white residue is left behind.

a) Name the salt.

b) Write the equation for the decomposition reaction

View Answer

Ans. a) Lead nitrate

b) 2Pb(NO₃)₂ (s) —– > 2PbO (s) + 4NO₂ (g) + O₂ (g)

74. When a solution of potassium iodide is added to a solution of lead nitrate in a test tube, a reaction takes place.

a) What type of reaction is this?

b) Write the balanced chemical equation to represent the above reaction.

View Answer

Ans. a) It is precipitation as well as double displacement reaction.

b) Pb(NO₃)₂ (aq) + 2KI (aq) —– > PbI₂ (s) + 2KNO₃ (aq)

75. Define combination reaction. Give one example of a combination reaction which is also exothermic.

View Answer

Ans. Combination reaction: The reaction in which two or more elements or compounds combine to form compounds(s) e.g.,

C (s) + O₂ (g) —– > CO₂ (g) + heat

It is an exothermic, combination reaction.

76. What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State the physical conditions of reactants in which the reaction between them will not take place. Write the balanced chemical equation for the reaction and name the type of reaction.

View Answer

Ans. White precipitate of BaSO₄ is formed.

BaCl₂ (aq) + Na₂SO₄ (aq) —- > BaSO₄ (s) + 2NaCl (aq)

It is a double displacement reaction. If reactants are taken in solid state, products will not be formed.

77. What is redox reaction. When a magnesium ribbon burns in air with dazzling flame and forms a white ash, is magnesium oxidised or reduced. Why?

View Answer

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.

79. When the powder of common metal is heated in open china dish its colour turns black. However, when hydrogen is passed over the hot black substance so formed, it regains its original colour. Based on the above information answer the following questions:

a) What type of chemical reaction takes place in each of the two given steps?

b) Name the metal initially taken in powdered form. Write balanced equations for both the reactions.

View Answer

Ans. a) Oxidation reaction, Redox reaction

b) Copper metal was present initially

2Cu (s) + O₂ (g) —– > CuO (s)

CuO (s) + H₂ (g) —– > Cu (s) + H₂O (l)

80. Why do we store silver chloride in dark coloured bottle? Explain in brief.

View Answer

Ans. It is done so as to cut off the exposure to sunlight. AgCl is photosensitive, it will decompose to Ag and Cl2 in the presence of sunlight.

2AgCl (s) —– > 2Ag (s) + Cl₂ (g)

81. “Oxidation and reduction processes occur simultaneously”. Justify this statement with the help of example.

View Answer

Ans. Oxidation involves loss of electrons or additions of oxygen. Whereas reduction involves gain of electrons or addition of hydrogen e.g.,

H₂ is getting oxidised to H₂O, CuO is getting reduced to Cu.

It shows oxidation and reduction occur simultaneously.

82. Distinguish between a displacement reaction and a double displacement reaction. Identify the displacement and the double displacement reaction from the following reactions.

a) HCl (aq) + NaOH (aq) — > NaCl (aq) + H₂O (l)

b) Fe (s) + CuSO₄ (aq) —- > FeSO₄ (aq) + Cu (s)

View Answer

Ans. Displacement Reaction: When a more reactive metal displaces a less reactive metal from its salt solution.

Double displacement reaction: When two compounds exchange their ions to form two new compounds.

a) Double Displacement reaction

b) Displacement reaction

84. What is an oxidation reaction? Give an example of oxidation reaction. Is oxidation an exothermic or an endothermic reaction?

View Answer

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) —- > 2 CuO (s)

It is an exothermic reaction.

Three Marks Questions

85. 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.

View Answer

Ans.

86. In the electrolysis of water:

a) Name the gas collected at the cathode and anode respectively.

b) Why is volume of gas collected a t one electrode double than that at the other? Name this gas.

c) How will you test this gas?

View Answer

Ans. 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.

87. Define the term decomposition reaction. Give one example each of thermal decomposition and electrolytic decomposition reactions.

View Answer

Ans. Decomposition reaction: The reaction in which a compound is broken down into simpler elements or compounds.

88. Name two salts that are used in black and white photography. Give equations for the reactions when these are exposed to sunlight.

View Answer

Ans. AgBr (Silver bromide) and AgI (Silver iodide) are used in black and white photography.

89. State one example each characterized by following along with suitable chemical equation.

a) Change in state

b) Evolution of gas

c) Change in temperature

View Answer

Ans. a) Change in state:

AgNO₃ (aq) + HCl (aq) —- > AgCl (s) + HNO₃ (aq)

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

90. Name the type of reactions represented by the following equations:

a) CaO + H2O —- > Ca(OH)₂

b) 3BaCl₂ + Al₂(SO₄)₃ —- > 3BaSO₄ + 2AlCl₃

c) 2FeSO₄ ——-> Fe₂O₃ + SO₂ + SO₃

View Answer

Ans. a) Combination reaction

b) Double Displacement reaction

c) Decomposition reaction

91. Write the chemical equation of the reaction in which the following changes take place with an example of each.

a) Change in colour

b) Change in temperature

c) Formation of precipitate

View Answer

Ans. Change in colour:

Zn (s) + FeSO₄ (aq) —– > ZnSO₄ (aq) + Fe (s)

Pale green Colourless

b) Change in temperature:

2Mg + O₂ (g) —– > 2MgO (s) + heat

c) Formation of precipitate:

Pb(NO₃)₂ (aq) + 2HCl (dil) —- > PbCl₂ (s) + 2HNO₃ (aq)

White ppt

92. 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.

View Answer

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

93. 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.

View Answer

Ans. H₂ (g) + Cl₂ (g) —- > 2HCl (g)

b) Pb (s) + CuCl₂ (aq) —- > PbCl₂ (s) + Cu (s)

c) ZnO (s) + C (s) —- > Zn (s) + CO (g)

94. Write chemical equations for the reactions taking place when

a) Iron reacts with steam

b) Magnesium reacts with dil HCl

c) Copper is heated in air

View Answer

Ans. a) 3Fe (s) + 4H₂O (g) —- > Fe₂O₃ (s) + 4H₂ (g)

b) Mg (s) + 2HCl (dil) —– > MgCl₂ (aq)

c) 2Cu (s) + O₂ (g) —– > 2CuO (s)

95. In a schematic diagram for the preparation of hydrogen gas as shown in the figure. What would happen if the following changes are made?

a) In place of zinc granules, same amount of zinc dust is take in the test tube?

b) Instead of dilute sulphuric, dilute hydrochloric acid is taken?

c) Sodium hydroxide is taken in place of dilute sulphuric acid and the flask is heated?

View Answer

Ans. a) Zinc dust will react faster, H₂ (g) will be liberated at a faster rate.

b) Same volume of H₂ (g) will be formed.

c) On heating Zn with NaOH, hydrogen gas will be formed at a faster rate.

96. 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.

View Answer

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

97. (i) Solid calcium oxide was taken in a container and water was added slowly to it

(a) Write the observations

b) Write the chemical formula of the product formed.

ii) What happens when carbon dioxide is bubbled through lime water (a) in small amount (b) in excess?

View Answer

Ans. (i) a) The container becomes hot and hissing solid is produced.

b) Ca(OH)₂ is the formula of the product formed.

(ii) a) Lime water turns milky when CO₂ (g) is passed through it.

Ca(OH)₂ (aq) + CO₂ (g) —- > CaCO₃ (s) + H₂O (l)

b) If excess of CO2 is passed milkiness disappears

CaCO₃ (s) + CO₂ (g) + H₂O (l) —– > Ca(HCO₃)₂ (aq)

98. Write the balanced chemical equation for the following reaction and identify the type of reaction.

Thermite reaction, iron (III) oxide reacts with aluminium and give molten iron and aluminium oxide.

View Answer

Ans. Fe₃O₄ (s) + 2Al(s) —- > Al₂O₃ (s) + 2Fe (l)

It is displacement as well as redox reaction.

99. An aqueous solution of metal nitrate ‘P’ reacts with sodium bromide solution to form yellow precipitate ‘Q’ which is used in photography. ‘Q’ on exposure to sunlight undergoes decomposition to form metal present along with a reddish brown gas. Identify ‘P’ and ‘Q’ write the balanced chemical equation for the chemical reaction. List the two categories in which reaction can be placed.

View Answer

Ans. ‘P’ is silver nitrate (AgNO₃)

AgNO₃ (aq) + NaBr (aq) —- > AgBr (s) + NaNO₃ (aq)

Yellow ppt

This reaction is categorized as double displacement as well as precipitation reaction.

2AgBr (s) ——– > 2Ag (s) + Br₂ (g)

The above reaction is photochemical decomposition reaction.

100. A green coloured hydrated metallic salt in heating loses water of crystallization molecules and gives a gas with suffocating smell. Identify the salt and write the chemical equation for the reaction.

View Answer

Ans. The salt is FeSO4.7H2O (Hydrated ferrous sulphate)

FeSO₄.7H₂O ——– > FeSO₄ (s) + 7 H₂O (l)

2FeSO₄ (s) ——- > Fe₂O₃ (s) + SO₂ (g) SO₃ (g)

101. 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 hydrochloric acid?

View Answer

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.

102. Write balanced equations for the following reactions:

a) Aluminium + Bromine —- > Aluminium bromide

b) Calcium carbonate ——- > Calcium oxide + Carbon dioxide (in presence of heat)

c) Silver chloride ——- > Silver + chlorine (in presence of sunlight)

View Answer

Ans. a) 2Al (s) + 3Br₂ (g) —- > 2AlBr₃ (s)

b) CaCO₃ (s) —— > CaO (s) + CO₂ (g)

c) 2AgCl (s) —— > 2Ag (s) + Cl₂ (g)

103. 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)

View Answer

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 H₂ or gain of electrons.

c) Zn is getting oxidised, CuO is getting reduced.

104. Write balanced chemical equations for the following reactions:

a) Hydrogen sulphide gas burns in air to give water and Sulphur dioxide.

b) Barium chloride reacts with zinc sulphate to give zinc chloride and barium sulphate.

c) Natural gas burns in air to form carbon dioxide and water.

View Answer

Ans. a) 2H₂S (g) + 3O₂ (g) —- > 2H₂O (l) + 2SO₂ (g)

b) BaCl₂ (aq) + ZnSO₄ (aq) —– > BaSO₄ (aq) + Zncl₂ (aq

c) CH₄ (g) + 2O₂ (g) —– > CO₂ (g) + 2H₂O (l)

Five Marks Questions

105. Write balanced chemical equation for the following statements:

a) NaOH solution is heated with zinc granules.

b) Excess of carbon dioxide is passed through lime water.

c) Dilute sulphuric acid is added to sodium carbonate

d) Egg shell is dropped in hydrochloric acid

e) Copper (II) oxide reacts with dilute hydrochloric acid.

View Answer

Ans. a) Zn (s) + 2NaOH —– > Na₂ZnO₂ + H₂

b) Ca(OH)₂ + 2CO₂ —– > Ca(HCO₃)₂

c) Na₂CO₃ + H₂SO₄ —- > Na₂SO₄ + H₂O + CO₂

d) CaCO₃ (s) + 2HCl (dil) —– > CaCl₂ + H₂O + CO₂

e) CuO (s) + 2HCl —- > CuCl₂ + H₂O

106. Identify the type of chemical reaction in the following statement and define each of them:

a) Digestion of food in our body.

b) Rusting of iron

c) Heating of manganese dioxide with aluminum powder.

d) Blue colour of copper sulphate solution disappears when iron fillings are added to it.

e) Dilute hydrochloric acid is added to sodium hydroxide solution to form sodium chloride and water.

View Answer

Ans. a) Decomposition reaction: It is a process in which a compound is broken down into simple substance.

b) Oxidation: The process in which oxygen is added or electrons are lost.

c) Displacement reaction: The reaction in which a more reactive element can displace a less reactive element from its salt solution. Oxidation and Reduction are taking place simultaneously in rusting of iron.

d) Displacement reaction: The reaction in which a more reactive element can displace a less reactive element.

e) Neutralisation reaction: The reaction in which acid reacts with base to form salt and water.

107. Define chemical reaction. State four observations which helps to determine whether a chemical reaction has taken place or not. Write one example of each observation with a balanced chemical equation.

View Answer

Ans. Chemical reaction is a reaction which represents a chemical change.

a) Change in colour:

Cu(s) + 2AgNO3 (aq) —- > Cu(NO3)2 (aq) + 2Ag (s)

b) Evolution of gas:

CaCO₃ (s) —– > CaO (s) + CO₂ (g) {in presence of heat}

c) Formation of precipitate:

Pb(NO₃)₂ (aq) + 2KI —– > PbI₂ (s) + KNO₃ (aq)

d) Change in temperature:

CaO (s) + H₂O (l) —— > Ca(OH)₂ + heat

108 a) Define a balanced chemical equation. Why should an equation be balanced?

b) Write a balanced chemical equation for the following reactions:

(i) Phosphorous burns in the presence of chlorine to form phosphorous pentachloride.

(ii) Burning of natural gas

(iii) The process of respiration.

View Answer

Ans. a) Balanced chemical equation is a equation in which number of atoms of various elements are equal on both sides of the equation.

The equation should be balanced due to law of conservation of mass.

b) (i) 2P (s) + 5Cl₂ (g) —– > 2 PCl₅ (s)

(ii) CH₄ (g) + O₂ (g) —– > CO₂ (g) + 2H₂O (l)

(iii) C₆H₁₂O₆ (s) + 6O₂ (g) —- > 6CO₂ (g) + 6H₂O (l)

109 (a) Write one example for each of decomposition reaction carried out with the help of (i) Electricity, (ii) Heat, (iii) Light.

(b) Which of the following statement is correct and why?

Copper can displace silver from silver nitrate solution and silver can displace copper from copper sulphate solution.

View Answer

Ans. a) (i) CaCO3 (s) —– > CaO (s) + CO₂ (g) {in presence of heat}

(ii) 2AgBr (s) —— > 2Ag (s) + Br₂ (g) {in presence of sunlight}

(iii) 2H₂O (l) —— > 2H₂ (g) + O₂ (g) {in presence of electricity}

b) Copper can displace Ag from AgNO3 (aq) solution because Cu is more reactive than ag

Cu (s) + 2AgNO₃ (aq) —- > Cu(NO₃)₂ (aq) + 2Ag (s)

110. Identify the type of reactions taking place in each of the following:

a) Barium chloride solution is mixed with copper sulphate solution and white precipitate is formed.

b) On heating copper powder in china dish, the surface of copper powder turns black.

c) On heating green coloured ferrous sulphate crystals, reddish brown solid is left and smell of a gas having odour of burning Sulphur is experienced.

d) Iron nails when left dipped in blue copper sulphate solution become reddish brown in colour and the blue colour of copper sulphate fades away.

e) Quick lime reacts vigorously with water releasing a large amount of heat.

View Answer

Ans. a) Double displacement reaction

b) Oxidation

c) Decomposition reaction

d) Displacement reaction

e) Combination reaction