Notes

Chapter 4 Chemical Reaction and Equations

NIOS Class 10 Science Chapter 4 Question Answers

INTEXT QUESTION 4.1

1. Write a chemical equation for each of the following reactions:

(i) Zinc metal reacts with aqueous hydrochloric acid to produce a solution of zinc chloride and hydrogen gas.

(ii) When solid mercury(II) oxide is heated, liquid mercury and oxygen gas are produced.

Ans:

(i) Zn(s) + 2HCl (aq) ⎯⎯→ ZnCl₂(aq) + H₂(g) + H₂

(ii) 2HgO (s) ⎯⎯→ 2Hg (l) + O₂

2. Balance the following chemical equations:

(i) H₂SO₄ (aq) + NaOH (aq) ⎯⎯→ Na₂SO₄ (aq) + H₂O (l)

(ii) Al (s) + HCl (aq) ⎯⎯→ AlCl₃ (aq) + H₂ (g)

Ans:

(i) H₂SO₄ (aq) + 2NaOH (aq) ⎯⎯→ Na₂SO₄ (aq) + 2H₂O (l)

(ii) 2Al (s) + 6HCl (aq) ⎯⎯→ 2AlCl₃ (aq) +3 H₂ (g)

3. What is a balanced chemical equation? Why should a chemical equation be balanced?

Ans: Volume of reactant and products in gaseous chemical reactions are related to each other by small integers, provided the volumes are measured at the same temperature and pressure. In a balanced gaseous chemical equation, we get a relation between volume and between the moles of the reactants and products.

INTEXT QUESTIONS 4.2

1. Examine the following reaction(s) and identify which of them are not example(s)

of a redox reaction?

(i) AgNO₃ (aq) + HCl (aq) ⎯⎯→ AgCl (s) + HNO₃ (aq)

(ii) MnO₂ (s) + 4HCl (aq) ⎯⎯→ MnCl₂ (aq) + 2H₂O (l) + Cl₂ (g)

(iii) 4Na (s) + O₂ (g) ⎯⎯→ 2Na₂O (s)

Ans: The Following equation is not an example of a redox reaction :

(i) AgNO3(aq) + HCl (aq) ⎯⎯→ AgCl(s) + HNO3(aq)

2. Identify the substances which are oxidised and the substances that are reduced in the following reactions:

(i) H₂ (g) + Cl₂ (g) ⎯⎯→ 2HCl (g)

(ii) H₂ (g) + CuO (s) ⎯⎯→ Cu (s) + H₂O (l)

(iii) Zn (s) + 2AgNO₃ (aq) ⎯⎯→ Zn(NO₃)₂ (aq) + 2Ag (s)

Ans:

(i) H₂ is oxidized and Cl₂ is reduced.
(ii) H₂ is oxidised and CuO is reduced.
(iii) Zn is oxidised and Ag₊ (in AgNO₃) is reduced.

TERMINAL EXERCISE

1. A. Write chemical equations of the following and balance them:

(a) Carbon + oxygen —→ Carbon dioxide

(b) Hydrogen + Chlorine –→ Hydrogen chloride

(c) Barium chloride + Sodium sulphate –→ Barium sulphate + sodium chloride

Ans:

(a) C + O₂ —→ CO₂

(b) H₂ + Cl₂ —→ 2HCl

B. Write balanced chemical equations with physical state symbols and necessary conditions, if any:

(a) Nitrogen reacts with hydrogen in the presence of iron as a catalyst at 200 atmospheric pressure and 600°C temperature, and the product obtained is ammonia.

(b) Aqueous solution of sodium hydroxide reacts with hydrochloric acid and produces sodium chloride and water.

(c) Phosphorus burns in chlorine gas to form phosphorous pentachloride.

Ans:

(a) N₂ (g) + 3H₂(g) 2NH₃(g)

(b) NaOH (aq) + HCl (aq) —→ NaCl (aq) + H₂O (l)

C. Balance the following chemical reactions:

(a) Ca(OH)₂ + HNO₃ —→ Ca(NO₃)₂ + H₂O

(b) BaCl₂ (aq) + H₂SO₄ (aq) —→ BaSO₄ (s) + HCl (aq)

(c) CuSO₄ (aq) + Zn (s) —→ ZnSO₄ (aq) + Cu (s)

(d) H₂S (g) + SO₂ (g) —→ S (s) + H₂O (l)

(e) BaCl₂ (aq) + Al₂(SO₄)₃ (aq) —→ AlCl₃ (aq) + BaSO₄ (s)

(f) Pb (NO₃)₂ (aq) + Fe₂(SO₄) (aq) —→ Fe(NO₃)₃ (aq) + PbSO₄ (s)

(g) Calcium hydroxide + carbon dioxide —→ Calcium carbonate + water

(h) Aluminium + Copper (II) chloride —→ Aluminium chloride + copper

(i) Calcium carbonate + hydrochloric acid —→ Calcium chloride + water + carbon dioxide

Ans:

(a) Ca(OH)₂ + 2HNO₃ —→ Ca(NO₃)₂ + 2H₂O
(b) BaCl₂ (aq) + H₂SO₄ (aq) —→ BaSO₄ (s) + 2HCl (aq)
(c) CuSO₄ (aq) + Zn (s) —→ ZnSO₄ (aq) + Cu (s)
(d) 2H₂S (g) + SO₂ (g) —→ 3S (s) + 2H₂O (l)
(e) 3BaCl₂ (aq) + Al₂(SO₄)₃ (aq) —→ 2AlCl₃ (aq) + 3BaSO₄ (s)
(f) 3Pb (NO₃)₂ (aq) + Fe₂(SO₄) (aq) —→ 2Fe(NO₃)₃ (aq) + 3PbSO₄ (s)
(g) Ca(OH)₂ + CO₂ —→ CaCO₃ + H₂O
(h) 2Al (s) + 3CuCl₂(aq) —→ 2AlCl₃ + 3Cu
(i) CaCO₃ + 2HCl —→ CaCl₂ + H₂O + CO₂

2. What is a balanced chemical equation? Write 3 characteristics of a balanced chemical equation?

Ans: A chemical equation where the number of atoms on either side of the equation is the same or balanced is called a balanced chemical equation. A balanced chemical equation tells us:

(i) The number of atoms and molecules taking part in the reaction and the corresponding masses in atomic mass units (amu or u).
(ii) The number of moles taking part in the reaction, with the corresponding masses in grams or in other convenient units.
(iii) Relationship between the volume of the reactants and the products if all of them are in the gaseous state.

3. In what way is a displacement reaction different from a double-displacement reaction? Explain with two suitable examples.

Ans: The displacement reaction is one in which one element displaces another element from its compounds.

Eg., Zn (s) + CuSO₄ (aq) —→ ZnSO₄ (aq) + Cu (s)

Here, Zinc replaces copper.

Reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

Eg. Na₂SO₄ (aq) + BaCl₂ (aq) —→ 2NaCl (aq) + BaSO₄ (s)

Here, both sodium and barium replace each other.

4. What happens when dilute hydrochloric acid is added to iron filings? Mark (√) at the correct answer from the following:

(a) Hydrogen gas and iron chloride are produced and is classified as a displacement reaction.

(b) Iron chloride and chlorine gas are produced and is classified as a decomposion reaction.

(c) Iron hydroxide and water are produced and is classified as a combination reaction.

(d) No reaction takes place but is classified as a double displacement reaction.

Ans: (a) Hydrogen gas and iron chloride are produced and is classified as a displacement reaction. (√)

7. What do you mean by an exothermic reaction? Give a suitable example.

Ans: Reactions in which heat is released along with the formation of the products are called exothermic reactions.

Burning of natural gas (CH₄) is used for cooking.

CH₄(g) + 2O₂ (g) —→ CO₂ (g) + 2H₂O (g)

8. Classify each of the following reactions as combination, decomposition, displacement or double displacement reactions:

(a) Zn (s) + 2AgNO₃ (aq) —→ Zn(NO₃)₂ + 2Ag (s)

(b) 2KNO₃ (s) heat —→ 2KNO₂ + O₂ (g)

(c) Ni (NO₃)₂ (aq) + 2NaOH (aq) –→ Ni(OH)₂ (s) + 2NaNO₃ (aq)

(d) 2KClO₃ (s) heat —→ 2KCl (s) + 3O₂ (g)

(e) MgO (s) + C (s) —→ CO (g) + Mg (s)

Ans:

(a) Displacement reaction
(b) Decomposition reaction
(c) Displacement reaction
(d) Decomposition reaction
(e) Displacement reaction

9. What is the difference between a combination and a decomposition reaction? Illustrate with suitable examples.

Ans: In combination reactions, two or more substances (elements or compounds) simply combine to form a new substance.

2Mg (s) + O2 (g) —→ 2MgO (s)

C (s) + O2 (g) —→ CO2 (g)

A decomposition reaction is one in which a compound decomposes into two or more substances (elements or compounds).

10. Is there any oxidation without reduction? Justify your answer.

Ans: No, there is no oxidation without reduction. Oxidation means to lose electrons, and reduction means to gain those electrons. Thus, the element or compound that loses its electron becomes oxidised and the one that gains it becomes reduced.

11. ‘Both combination reaction and displacement reaction fall in the category of redox reactions’. Do you agree? If so, discuss this aspect with suitable examples.

Ans: Yes, both combination and displacement reactions come under the category of redox reactions. In both cases, there is a loss and a gain of electrons.

In this combination reaction, 2Mg(s)+O₂(g)→2MgO(s), Mg is oxidised and oxygen is reduced.
In this single displacement reaction, Zn(s)+CuSO₄(aq)→ZnSO₄(aq)+Cu(s), Zn is oxidised and Cu is reduced.

12. Give two examples from everyday life situations where redox reaction takes place. How will you prove it?

Ans:

Rusting or iron:

4Fe+3O₂+6H₂O→4Fe(OH)₃→Fe₂O₃⋅nH₂O (rust)

Blue colour of CuSO₄ solution fades, and red-brown copper metal deposits on zinc:

Zn(s)+CuSO₄(aq)→ZnSO₄(aq)+Cu(s)

13. In the following reactions, name the substances which are oxidized and reduce,d and also mention the oxidizing and reducing agents:

(a) Ca (s) + Cl₂ (g) heat —→ CaCl₂ (s)

(b) 3MnO₂ (s) + 4Al (s) heat —→ 3Mn (l) + 2Al₂O₃ (s)

(c) Fe₂O₃ (s) + 3CO (g) heat —→ 2Fe (s) + 3CO₂ (g)

Ans:

(a) Ca is oxidised while Cl₂ is reduced
(b) Mn is reduced and Al is oxidised
(c) Fe is reduced and C is oxidised

14. Explain the following in terms of electron transfer:

(a) Oxidation (b) Reduction

Ans:

A reaction in which a species loses electrons is called an oxidation reaction.
A reaction in which a species gains electrons is called a reduction reaction.

17. What is the law of definite proportion by volume? Explain.

Ans: When gases react together at constant temperature and pressure, the volumes of the gaseous reactants and products are in simple whole-number ratios (when measured in the same units).