NCERT Solutions for Class 10 Science Chapter 1 Chemical Reactions and Equations

NCERT Solutions for Class 10 Science Chapter 1 Chemical Reactions and Equations

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.

(d) Lead oxide is getting reduced.

(i) (a) and (b)

(ii) (a) and (c)

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

(iv) all

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

2. Fe2O3 + 2Al → Al2O3 + 2Fe

The above reaction is an example of a

(a) combination reaction.

(b) double displacement reaction.

(d) displacement reaction.

Ans: (d) displacement reaction.

3. What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.

(a) Hydrogen gas and iron chloride are produced.

(b) Chlorine gas and iron hydroxide are produced.

(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 reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation.Chemical reaction should be balanced to follow 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.

(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) → 2H2O (l) + 2SO2 (g)

(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

(d) BaCl2 + H2SO4 → BaSO4 + HCl

Ans: (a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O

(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O

(d) BaCl2 + H2SO4 → BaSO4 + 2HCl

7. Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide →nCalcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

(d) Barium chloride + Potassium sulphate →nBarium sulphate + Potassium chloride

Ans: (a) Ca(OH)2 + CO2 → CaCO3 + H2O

(b) Zn + 2AgNO3 → Zn(NO3)2 + 2Ag

(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)

(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride(aq) + Hydrogen(g)

Ans: (a) 2KBr (aq) + BaI2 (aq) → 2KI (aq) + BaBr2 (s): Double displacement reaction

(b) ZnCO3 (s) → ZnO (s) + CO2 (g): Decomposition reaction

(d) Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g) : Displacement Reaction

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

Ans: Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions.

Example:

C (g) + O2 (g) → CO2 + Heat Energy

Reactions that absorb energy or require energy in order to proceed are called endothermic reactions.

Example:

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

Ans: Respiration is considered as an exothermic reaction because in respiration oxidation of glucose takes place which produces large amount of heat energy.

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

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

Ans: Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.

For Example: Decomposition Reaction:

Combination Reaction:

CaO (s) + H2O (l) → Ca(OH)2 (aq)

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

Ans:

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

Ans: In a displacement reaction, a more reactive element replaces a less reactive element from a compound.

For Example:

CuSo4 (aq) + Zn (s) → ZnSO4 (aq) + Cu (s)

In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.

For Example:

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

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: 2AgNO3 (aq) + Cu (s) → Cu(NO3)2 (aq) + 2Ag (s)

Silver Nitrate + Copper → Copper Nitrate + Silver

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

Ans: A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.For Example:

Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2NaCl (aq)

Sodium Carbonate + Calcium Chloride → Calcium Carbonate + Sodium Chloride

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.

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

(a) Oxidation

(b) Reduction

Ans: Oxidation Reaction: It is a chemical reaction in which gain of oxygen or loss of hydrogen takes place.

Reduction Reaction: It is a chemical reaction in which loss of oxygen or gain of hydrogen takes place.

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: ‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below.

18. Why do we apply paint on iron articles?

Ans: Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles from moisture and air is cut off. Hence, rusting is prevented.

19. Oil and fat containing food items are flushed with nitrogen. Why?

Ans: Oil and fat containing food items flushed with nitrogen because nitrogen acts as an antioxidant and it prevent them from being oxidised.

20. Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Ans: Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.

For example, iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide.

4Fe + 3O2 + nH2O → 2Fe2O3.2H2O

Rancidity is the process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

For example, the taste and smell of butter changes when kept for long.