CHAPTER 4 TYPES OF CHEMICAL REACTIONS

 

Let us answer these: Page 70

(1) A solution of an oxide “X” is used for white washing.

(a) What could be “X”? Write its formula.

(b) Write the reaction of substance “X” named in (a) above with water. 

Ans:

(a) The substance ‘X” is calcium oxide. Its formula is CaO.

(b) CaO reacts with water to form calcium hydroxide, Ca(OH)₂.

CaO +  H₂O →   Ca(OH)₂

(2) Why is the amount of gas collected in one of the test tubes in activity 4.5 double of the amount collected in the other? Name this gas.

Ans: The gas which is collected in double of the amount in the electrolysis of water is hydrogen.

Electric current

2H2O(l) → 2H2(g) + O2(g)

This is because in water, H₂O the two constituent elements, hydrogen and oxygen are always combined in the fixed ratio of 2:1 by volume that is it contains two parts of hydrogen element as compared to one part of oxygen element.

Let us answer these: Page 80

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

Ans: The blue colour of copper sulphate discharges and changes to light green colouration when an iron nail is dipped in it. This is due to the displacement of copper from copper sulphate CuSO₄ solution by iron thereby forming ferrous sulphate, FeSO₄

Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

             (Blue) (Light green)

b) Give an example of double displacement reaction other than the one given in activity 4.8. Ans:    AgNO₃  (aq)    +  NaCl  (aq) →  AgCl↓ + NaNO₃(aq)

Silver nitrate Silver chloride (ppt)

c) Identify the substances that are oxidised and the substances that are reduced in the following reaction.

(i) 4 Na (s)  + O₂  (g) → 2 Na₂O(s)

Ans: Sodium, Na is oxidised and oxygen, O₂ is reduced.

(ii) CuO(s) +  H₂   (g) → Cu  (s)  +  H₂O(l) Ans: H₂ is oxidised to H₂O and CuO is reduced to Cu.

Exercises: Page 82

1. Which of the statements about the reaction given below are NOT correct? 

        2 PbO(s) + C(s) → 2 Pb(s) + CO₂(g)

(i) Lead is reduced

(ii) Carbon dioxide is oxidised (iii)Carbon is oxidised (iv)Lead oxide is reduced

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (c)

(iv) all

Ans: (i) (a) and (b)

2. Fe₂O₃ +  2 Al → Al₂O₃ +2Fe The above reaction is an example of

a) Combination reaction

b) Double displacement reaction

c) decomposition reaction

d) displacement reaction

Ans: (d) displacement reaction

3. What are combination reactions? Give one example.

Ans: Reactions in which a single product is formed from two or more reactants are called combination reactions.

Eg:  C  (s)   +  O (g) → CO₂ (g)

4. What are decomposition reactions? Give one experiment to demonstrate a decomposition reaction.

Ans: The reactions in which a compound is broken down into two or more simpler substances by the action of heat or by supplying energy are called decomposition reactions.

Experiment: Let us take about 4g of calcium carbonate, CaCO₃ in a hard glass test tube fitted with a delivery tube containing lime water.

The tube is heated over the flame of a spirit lamp or burner.

It is observed that lime water turns milky. When calcium carbonate is heated, it decomposes to calcium oxide and carbon dioxide gas.

CaCO₃ (s) ⮕ heat CaO (s) + CO₂ (g)

Fig. Heating of calcium carbonate and giving out carbon dioxide gas which turns lime

 5. Why are decomposition reactions called the opposites of combination reactions ? 

Ans: In decomposition reactions a substance is decomposed into two or more substances. 

Eg : CaCO₃ →  CaO + CO₂

On the other hand in combination reactions two or more substances combine together to give a single substance.

Eg : N₂ + 3H₂ → 2NH₃

Therefore decomposition reactions are opposite of combination reactions.

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

Ans:

(i) Calcium carbonate decomposes into CaO and CO₂ on heating.

CaCO₃ →  CaO + CO₂

(ii) Silver chloride decomposes into silver and chlorine in presence of light 

2 AgCl →  2Ag + Cl₂

(iii) On passing electric current, water decomposes into hydrogen and oxygen 

2H₂O  (l) →  2 H₂ (g)  + O₂ (g)

7. What is the difference between displacement and double displacement reactions? Write

equations for these reactions.

Ans: In displacement reaction a more reactive element displaces another less reactive element from its solution

CuSO₄  (aq)   +  Fe  (s) → FeSO₄ (aq) + Cu (s)

On the other hand in double displacement reaction there is mutual exchange of ions between the compounds in their aqueous state.

BaCl₂  (aq)   +   H₂SO₄ (aq) → BaSO₄ (s) + 2 HCl (aq)

8. What is a displacement reaction? Give one example. 

Ans : (Please see above)

9. In the refining of silver, silver can be recovered from silver nitrate solution by adding zinc metal. Write down the reaction involved.

Ans: Zn (s)+ AgNO₃(aq)→ Zn(NO₃)₂(aq) + Ag(s)

10. What is a precipitation reaction ? Explain it by giving one example.

 Ans: A reaction that produces a precipitate is called a precipitation reaction. Eg:

BaCl₂ (aq) + H₂SO₄ (aq) → BaSO₄ (s) + 2HCl (aq)

In the above reaction a white substance (BaSO₄) is formed as precipitate . Thus the reaction is a precipitation reaction.

11. What are exothermic and endothermic reactions? Give two examples each.

Ans: Reactions which take place with the evolution of energy in any form are called exothermic reactions.

Eg :

(a) CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2 H₂O (g)  + Energy Methane

(b) C₆H₁₂O₆ +  6O₂  (g) → 6CO₂ (g) + H₂O (g) + Energy

The reactions in which energy is absorbed are called endothermic reactions.

Eg:

(i) N₂  (g)  +  O₂  (g) +  heat → 2NO (g)

(ii) H₂  (g)  + Cl₂  (g) sunlight →  2 HCl (g)

12. Why is respiration considered an exothermic reaction?

Ans: Exothermic reaction: During respiration oxygen present in the blood combines with glucose in the body cells and converted into 𝐶𝑂2 and 𝐻2𝑂 with the liberation of energy.

Respiration

C6H12O6(aq)  +  6O2 (g) →  6CO2 (g) + H2O(g) + Energy

13. Explain the terms (a) oxidation and (b) reduction with two examples each. 

Ans:

(a) Oxidation is a process which involves gain of oxygen or loss of hydrogen.

Eg: Cu + O₂ → CuO (addition of oxygen)

H₂S   + Cl₂   → S   +   2HCl (removal of hydrogen from H₂S) 

Reduction is a process which involves loss of oxygen or gain of hydrogen.

Eg:

(a) 2Na + H₂ → 2 NaH (addition of hydrogen)

(b) ZnO + C → Zn + CO (removal of oxygen)

14. Why does painted iron not rust?

Ans: Painting prevents the iron from coming in contact with the damp air thereby reducing oxidation. Thus painting prevents iron from rusting.

15. Fat and oil containing food items are flushed with nitrogen, why?

Ans: It is because flushing the food items with nitrogen prevents them from oxidation thus preserving the food item

 

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