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
