Chapter 1: Chemical Reactions and Equations

1. Balance the following chemical equation: (2008)

Fe(s) + H²O(g) Fe³O4(s) + H²(g)

Answer: Balanced Equation: 3Fe(s) + 4H²O(g) Fe³O4(s) + 4H²(g)

Explanation: To balance the equation, the number of Fe atoms, H²O molecules, and H² molecules must be equal on both sides. By balancing the equation, it becomes clear that three Fe atoms react with four water molecules to form Fe³O4 and four hydrogen molecules. Other options may not have the correct stoichiometric coefficients.

2. Why is respiration considered an exothermic process? (2008)

Answer: Respiration is considered an exothermic process because it releases energy. During respiration, glucose (C6H¹²O6) reacts with oxygen to form carbon dioxide (CO²) and water (H²O), releasing energy that is used by the body for various functions. The reaction releases more energy than it absorbs, making it exothermic. It is the opposite of an endothermic process, where energy is absorbed.

3. Balance the following chemical equation: (2008)

Pb(NO³)² (s) -heat PbO(s) + NO²(g) + O²(g)

Answer: Balanced Equation: 2Pb(NO³)² (s) -heat 2PbO(s) + 4NO²(g) + O²(g)

Explanation: The decomposition of lead nitrate requires balancing the lead, nitrogen, and oxygen atoms. By balancing the coefficients, we get the correct equation where two molecules of Pb(NO³)² decompose to form two molecules of PbO, four molecules of NO², and one molecule of O².

4. Name a reducing agent that may be used to obtain manganese from manganese dioxide. (2009)

Answer: A reducing agent that can be used to obtain manganese from manganese dioxide (MnO²) is carbon (C). The carbon reduces the MnO² to manganese metal (Mn) by donating electrons to it. The reaction is: MnO² + C Mn + CO².

5. What change in the colour of iron nails and copper sulphate solution you observe after keeping the iron nails dipped in copper sulphate solution for about 30 minutes? (2010)

Answer: When iron nails are dipped in copper sulphate solution, the iron reacts with copper ions in the solution, causing the copper ions to be displaced by the iron. The copper metal forms a reddish-brown coating on the iron nails, and the blue color of the copper sulphate solution fades, turning green. This is an example of a single displacement reaction.

6. In a double displacement reaction such as the reaction between sodium sulphate solution and barium chloride solution: (2020)

(A) Exchange of atoms takes place
(B) Exchange of ions takes place
(C) A precipitate is produced
(D) An insoluble salt is produced

Answer: The correct option is:

(d) (B), (C) and (D)

Explanation: In a double displacement reaction, ions from two compounds exchange places to form new compounds. In this case, barium sulphate (BaSO4) is formed as an insoluble salt, which precipitates out of the solution. The exchange of ions (B), formation of a precipitate (C), and the production of an insoluble salt (D) are all correct, while option (A) is incorrect because atoms are not exchanged in this reaction.

7. A student took sodium sulphate solution in a test tube and added barium chloride solution to it. He observed that an insoluble substance was formed. The colour and molecular formula of the insoluble substance is: (2021)

(a) Grey BaSO4

(b) Yellow BaSO4

(c) White BaSO4

(d) Pink BaSO4

Answer: (c) White, BaSO4

Explanation: The reaction between sodium sulphate (Na²SO4) and barium chloride (BaCl²) forms barium sulphate (BaSO4), which is a white precipitate. The molecular formula of the insoluble substance is BaSO4. The other options are incorrect because barium sulphate is white, not grey, yellow, or pink.

8. Sodium reacts with water to form sodium hydroxide and hydrogen gas. The balanced equation which represents the above reaction is: (2021)

Answer: (b) 2Na(s) + H²O(l) 2NaOH(aq) + H²(g)

Explanation: The reaction between sodium and water involves the formation of sodium hydroxide (NaOH) and hydrogen gas (H²). The correct balanced equation is option (b), where two sodium atoms react with two molecules of water to produce two molecules of sodium hydroxide and one molecule of hydrogen gas. Other options are incorrect because they do not follow the correct stoichiometric coefficients.

9. C6H¹²O6(aq) + 6O²(aq) 6CO²(aq) + 6H²O(l) (2021)

The above reaction is a/an:

Answer: (c) Exothermic reaction

Explanation: The reaction represents cellular respiration, where glucose and oxygen are consumed to produce carbon dioxide and water, releasing energy. This is an exothermic process because it releases energy in the form of heat. It is not an endothermic reaction, where energy is absorbed.

10. Which of the following statements about the reaction given below are correct?
MnO² + 4HCl MnCl² + 2H²O + Cl² (2021)

(a) HCl is oxidized to Cl²

(b) MnO² is reduced to MnCl²

(c) MnCl² acts as an oxidizing agent

(d) HCl acts as an oxidizing agent

Answer: (a) (i), (ii), and (iv)

Explanation: In this reaction, HCl is oxidized to chlorine gas (Cl²), and MnO² is reduced to MnCl². Therefore, statement (i) and (ii) are true. MnCl² acts as a reducing agent, not an oxidizing agent, so statement (iii) is incorrect. HCl acts as an oxidizing agent, making statement (iv) correct.

11. It is important to balance the chemical equations to satisfy the law of conservation of mass. Which of the following statements of the law is incorrect? (2021)

Answer: (c) The chemical composition of the reactants is the same before and after the reaction.
Explanation: According to the law of conservation of mass, the total mass of the reactants is equal to the total mass of the products. Statement (c) is incorrect because the chemical composition of the reactants may change as atoms rearrange during a chemical reaction.

12. Give example of thermal decomposition reaction? (2021)

Answer: CaCO³(s) CaO(s) + CO²(g)

Explanation: Thermal decomposition reactions involve the breakdown of a compound into two or more simpler substances due to heat. In this case, calcium carbonate (CaCO³) decomposes into calcium oxide (CaO) and carbon dioxide (CO²) upon heating. The other options do not involve thermal decomposition.

13. Assertion (A): Burning of natural gas is an endothermic process.
Reason (R): Methane gas combines with oxygen to produce carbon dioxide and water. (2021)

Answer: (d) (A) is false, but (R) is true.

Explanation: Burning of natural gas (methane) is an exothermic process, not endothermic, as it releases energy. However, the reaction between methane and oxygen does produce carbon dioxide and water, which makes the reason true.

14. When aqueous solutions of potassium iodide and lead nitrate are mixed, an insoluble substance separates out. The chemical equation for the reaction involved is: (2023)

Answer: (b) 2KI + Pb(NO³)² PbI² + 2KNO³

Explanation: The reaction between potassium iodide (KI) and lead nitrate (Pb(NO³)²) forms lead iodide (PbI²), which is a yellow precipitate. The correct balanced equation is option (b), where two molecules of KI react with one molecule of Pb(NO³)² to form one molecule of PbI² and two molecules of KNO³.

15. A metal ribbon 'X' burns in oxygen with a dazzling white flame forming a white ash 'Y'. The correct description of X, Y and the type of reaction is: (2023)

Answer: (b) X = Mg; Y = MgO; Type of reaction = Combination

Explanation: Magnesium (Mg) burns with a dazzling white flame in oxygen to form magnesium oxide (MgO), a white ash. This is a combination reaction because two reactants (Mg and O²) combine to form a single product (MgO). The other options are incorrect as they refer to different metals or reaction types.

16. A: Reaction of Quicklime with water is an exothermic reaction.

R: Quicklime reacts vigorously with water releasing a large amount of heat. (2023)

Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A)
Explanation: When quicklime (CaO) reacts with water, it forms slaked lime (Ca(OH)²) and releases a large amount of heat, making it an exothermic reaction. Statement (R) correctly explains the exothermic nature of the reaction.

17. Give an example of a decomposition reaction. Describe an activity to illustrate such a reaction by heating. (2008)

Answer: An example of a decomposition reaction is the heating of calcium carbonate (CaCO³) to produce calcium oxide (CaO) and carbon dioxide (CO²):

CaCO³(s) CaO(s) + CO²(g).

To illustrate this, heat calcium carbonate in a test tube and observe the release of carbon dioxide gas and the formation of calcium oxide.

18. (i) What is the colour of ferrous sulphate crystals? How does this colour change after heating?
(ii) Name the products formed on strongly heating ferrous sulphate crystals. What type of chemical reaction occurs in this change? (2008)

Answer: (i) Ferrous sulphate crystals are green in colour. Upon heating, they turn brown due to the formation of ferric oxide (Fe²O³). The heating also decomposes ferrous sulphate.
(ii) The products formed are ferric oxide (Fe²O³), sulfur dioxide (SO²), and oxygen gas (O²). The type of chemical reaction is thermal decomposition.

19. What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State...