Chemical Reactions and Equations -NCERT Questions with Answers

Question 1. Which of the statements about the reaction below are incorrect?

2PbO_(s) + C_(s) 2Pb_(s) + CO_2(g)

(a) Lead is getting reduced,

(b) Carbon,dioxide is getting oxidised.

(c) Carbon is getting oxidised.

(d) Lead oxide is getting reduced

(i) (a) and (b)

(ii) (a) and (c)

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

(iv) all

Answer.

The statements (a) and (b) are incorrect because PbO is getting reduced, not Pb, similarly C is getting oxidized not CO₂.

Question 2. Fe₂O₃ + 2Al Al₂O₃ + 2Fe

The above reaction is an example of a

(a) combination reaction

(b) double displacement reaction

(c) decomposition reaction,

(d) displacement reaction.

 

Answer.

(d) The given reaction is an example of displacement reaction as in this reaction Al displaces, Fe from Fe₂O₃.

Question 3. What happens when dilute hydrochloric acid is added to iron fillings? tick the correct answer.

(a) Hydrogen gas and iron chloride are produced

(b) Chlorine gas and iron hydroxide are produced

(c) No reaction takes place

(d) Iron salt and water are produced

 

Answer.

(a) Fe_{(s) +} 2HCl_(aq) FeCI_2(aq) + H_2(g)

Question 4. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer.

Balanced chemical equation: – A chemical equation is said to be balanced if the number of atoms of each element participating in the reaction are equal on both side of the equation.

The chemical reaction should be balanced because law of conservation of mass holds good which states that “in a chemical reaction total mass of the reactant must be equal to the total mass of the product”.

Quiestion 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.

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

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

Answer.

(a) N_2(g) + 3H_2(g) 2NH_3(g)

(b) 2H₂S_(g) +3O_2(g) 2H₂O_(l) +2SO_2(g)

(c) 3BaCl_2(aq) + Al₂ (SO₄)_3(s) 3BaSO_{4(s) +} 2AlCl_3(aq)

(d) 2K_(s) + 2H₂O_(l) 2KOH_(aq)+H_2(g)

Question 6. Balance the following chemical equations.

(a) 2HNO₃ + Ca(OH)₂ Ca(NO₃)₂ + H₂O

(b) NaOH + H₂SO₄ Na₂SO₄ + H₂O

(c) AgNO₃ + NaCl AgCl + NaNO₃.

(d) BaCl₂ + H₂SO₄ BaSO₄ + HCl

Answer.

(a) 2HNO₃ + Ca(OH)₂ Ca(NO₃)₂ + 2H₂O

(b) 2NaOH + H₂SO₄ Na₂SO₄ + 2H₂O

(c) AgNO₃ + NaCl AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ BaSO₄ + 2HCl

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

(a) Calcium hydroxide + Carbon dioxide Calcium carbonate + water

(b) Zinc + Silver nitrate Zinc nitrate + Silver

(c) Aluminium + Copper chloride Aluminium chloride + Copper

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

Answer.

(a) Ca(OH)₂ + CO₂ CaCO₃ + H₂O

(b) Zn+2AgNO₃ Zn(NO₃)₂ + 2Ag

(c) 2Al + 3CuCl₂ 2AlCl₃ + 3Cu

(d) BaCl₂ + K₂SO₄ BaSO₄+2KCl

Question 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_(aq)

(b) Zinc carbonate_(s) Zinc oxide_(s) + Carbon dioxide_(g)

(c) hydrogen_(g) + Chlorine_(g) Hydrogen chloride_(g)

(d) Magnesium_(s) + Hydrochloric acid_(aq) Magnesium chloride_(aq) + Hydrogen_(g)

Answer.

(a) 2KBr_(aq) + Bal_2(aq) 2KI_(aq) + BaBr_2(aq)

(b) ZnC0_3(s) ZnO(s) + CO₂(g)

(c) H_2(g) + Cl_2(g) 2HCl_(g)

(d) Mg_(s) + 2HCl_(aq) MgCl_2(aq) + H_2(g)

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

Answer.

Exothermic reaction: – Those reactions which occur with evolution of heat are exothermic reactions. e.g.

2NaOH_(aq) + H₂SO_4(aq) Na₂SO_4(aq) + 2H₂O_(l) + Q

Enothermic reaction: – Those reactions which require heat to occur are endothermic reactions, e.g.

2Pb(NO₃)_2(s) + Q 2PbO_(s) + 4NO_2(g) + O_2(g)

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

Answer.

During respiration oxidation of glucose occur which produce heat energy.

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

Answer.

During decomposition reaction a single reactant decompose to form two or more products, whereas in combination two or more reactant react to form a single product, e.g.

N₂ + 3H₂ 2NH₃ (Combination reaction.)

2H₂O 2H₂ + O₂ (Decomposition reaction.)

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

Answer.

 

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

Answer.

Displacement reaction: In this reaction an element which is more active displaces the other element which is less active from its salt solution.

e.g. Fe_(s) + CuSO_4(aq) FeSO_4(aq) + Cu_(s)

Double displacement reaction: In this reaction exchange of ions between two reactants take place, e.g.

BaCl_2(aq) + Na₂SO_4(aq) BaSO_4(s) + 2NaCl_(aq)

Question 14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer.

Cu_(s) + 2AgNO_3(aq) Cu(NO₃)_2(aq) + 2Ag_(s)

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

Answer.

Those reactions which produce an isoluble product or precipitate are known as precipitation reaction. e.g.AgNO_3(aq) + NaCl_(aq) AgCl_(s) +NaNO_3(aq)

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

(a) Oxidation

(b) Reduction

Answer.

Oxidation: Oxidation involve addition of oxygen e.g.

C + O₂ CO₂, C is oxidised to CO₂.

N₂ + O₂ 2NO, N₂ is oxidised to NO.

Reduction: Reduction involve removal of oxygen, e.g.

C + H₂O CO + H₂, H₂O is reduced to H₂.

CuO + H₂ Cu + H₂O, CuO is reduced to Cu.

Question 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.

Answer.

Question 18. Why do we apply paint on iron articles?

Answer.

To prevent iron articles from rusting, we apply paint on it, by which iron does not come in contact with air and moisture which cause rusting.

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

Answer.

When the oil and fat containing food is surrounded by unreactive gas nitrogen, there is no oxygen to cause its oxidation and make it rancid.

Question 20. Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

 

Answer.

Corrosion: It is a process in which metals are eaten up gradually by the action of air, moisture or a chemical on their surface.

Rancidity: When fats and oil containing food are oxidised with air or oxygen, their smells and tastes changes. This process is called rancidity.