BYJU'S Chemistry Worksheet: Questions 14-17 on balanced equations, endothermic reactions, and chemical reactions.

Problem Analysis and Solution:

The provided image contains several chemistry-related questions. Below, I will solve each question step by step with detailed explanations.

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#### Q-14: When steam is passed over heated iron, magnetic oxide of iron and hydrogen gas are formed. In the balanced equation, the coefficients of Fe and H₂ are respectively:

Given Reaction:
Iron (Fe) reacts with steam (H₂O) to form magnetic oxide of iron (Fe₃O₄) and hydrogen gas (H₂).

Unbalanced Equation:
\[ \text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2 \]

Balancing the Equation:
1. Start by balancing the iron (Fe) atoms:
- On the left side, there is 1 Fe atom.
- On the right side, there are 3 Fe atoms in Fe₃O₄.
- To balance Fe, multiply Fe on the left by 3:
\[ 3\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2 \]

2. Balance the oxygen (O) atoms:
- On the left side, there is 1 O atom in H₂O.
- On the right side, there are 4 O atoms in Fe₃O₄.
- To balance O, multiply H₂O on the left by 4:
\[ 3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2 \]

3. Balance the hydrogen (H) atoms:
- On the left side, there are \(4 \times 2 = 8\) H atoms in 4H₂O.
- On the right side, there are 2 H atoms in H₂.
- To balance H, multiply H₂ on the right by 4:
\[ 3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2 \]

Balanced Equation:
\[ 3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2 \]

Coefficients of Fe and H₂:
- Coefficient of Fe = 3
- Coefficient of H₂ = 4

Correct Answer:
\[ \boxed{a) 3, 4} \]

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#### Q-15: Which of the following is an example of an endothermic reaction?

Endothermic Reaction Definition:
An endothermic reaction absorbs heat from the surroundings, causing a decrease in temperature.

Options:
a) Respiration
b) Burning of coke
c) N₂ + O₂ → 2NO
d) Dissolution of sodium sulfate

Analysis:
- a) Respiration: This is an exothermic process because it releases energy in the form of heat.
- b) Burning of coke: Combustion reactions are generally exothermic as they release heat.
- c) N₂ + O₂ → 2NO: This is the formation of nitric oxide, which is an endothermic reaction because it requires heat to proceed.
- d) Dissolution of sodium sulfate: The dissolution of most salts is either slightly exothermic or endothermic, but sodium sulfate dissolves exothermically.

Correct Answer:
\[ \boxed{c) N_2 + O_2 \rightarrow 2NO} \]

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#### Q-16: Which of the following correctly represents a balanced chemical equation?

Options:
a) \((\text{CH}_3\text{COO})_2\text{Pb}(aq) + 2\text{HCl}(aq) \rightarrow \text{PbCl}_2(s) + \text{CH}_3\text{COOH}(aq)\)
b) \(\text{Fe}_2\text{O}_3(s) + 3\text{CO}(g) + \text{heat} \rightarrow 2\text{Fe}(s) + 3\text{CO}_2(g)\)
c) \(2\text{H}_2\text{S}(g) + \text{O}_2(g) \rightarrow \text{Si}(l) + \text{H}_2\text{O}(l)\)
d) \(\text{Mg}(s) + \text{Cl}_2(g) \rightarrow 2\text{MgCl}_2(s)\)

Analysis:
- a) The equation is balanced:
- Left side: 2 C, 4 H, 2 O, 1 Pb, 2 Cl
- Right side: 2 C, 4 H, 2 O, 1 Pb, 2 Cl
- Balanced.

- b) The equation is balanced:
- Left side: 2 Fe, 3 O, 3 C
- Right side: 2 Fe, 3 O, 3 C
- Balanced.

- c) The equation is not balanced:
- Left side: 4 H, 2 S, 1 O, 1 C
- Right side: 0 H, 0 S, 1 O, 1 Si
- Unbalanced.

- d) The equation is not balanced:
- Left side: 1 Mg, 2 Cl
- Right side: 2 Mg, 4 Cl
- Unbalanced.

Correct Answer:
\[ \boxed{a) (\text{CH}_3\text{COO})_2\text{Pb}(aq) + 2\text{HCl}(aq) \rightarrow \text{PbCl}_2(s) + \text{CH}_3\text{COOH}(aq)} \]

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#### Q-17: How many grams of sodium iodide, NaI, must be used to produce 22.4 g of iodine, I₂?

Given:
- Mass of I₂ produced = 22.4 g
- Molar mass of I₂ = 2 × 127 g/mol = 254 g/mol
- Molar mass of NaI = 23 + 127 = 150 g/mol

Reaction:
\[ 2\text{NaI} \rightarrow \text{I}_2 + 2\text{Na} \]

Step 1: Calculate moles of I₂ produced.
\[ \text{Moles of I}_2 = \frac{\text{Mass of I}_2}{\text{Molar mass of I}_2} = \frac{22.4 \, \text{g}}{254 \, \text{g/mol}} \approx 0.0882 \, \text{mol} \]

Step 2: Use stoichiometry to find moles of NaI required.
From the balanced equation, 2 moles of NaI produce 1 mole of I₂.
\[ \text{Moles of NaI} = 2 \times \text{Moles of I}_2 = 2 \times 0.0882 \approx 0.1764 \, \text{mol} \]

Step 3: Calculate the mass of NaI required.
\[ \text{Mass of NaI} = \text{Moles of NaI} \times \text{Molar mass of NaI} = 0.1764 \, \text{mol} \times 150 \, \text{g/mol} \approx 26.46 \, \text{g} \]

Correct Answer:
\[ \boxed{26.46 \, \text{g}} \]

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#### Q-18: Assertion: When copper powder is heated in air, it turns black. Reason: Copper reacts with the H₂S gas of the air forming black CuS.

Assertion:
When copper powder is heated in air, it turns black.

Reason:
Copper reacts with the H₂S gas of the air forming black CuS.

Analysis:
- Assertion: True. When copper is heated in air, it reacts with oxygen to form black copper(II) oxide (CuO):
\[ 2\text{Cu} + \text{O}_2 \rightarrow 2\text{CuO} \]
This is why the copper turns black.

- Reason: False. The reason given is incorrect. Copper does not react with H₂S gas in air. Instead, it reacts with oxygen to form CuO.

Conclusion:
Both A and R are true, but R is not the correct explanation of the assertion.

Correct Answer:
\[ \boxed{b) Both A and R are true but R is not the correct explanation of the assertion.} \]

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#### Q-19: Why is hydrogen peroxide stored in colored bottles?

Explanation:
Hydrogen peroxide (H₂O₂) is stored in colored bottles because it is light-sensitive and decomposes when exposed to light. The decomposition reaction is:
\[ 2\text{H}_2\text{O}_2 \xrightarrow{\text{light}} 2\text{H}_2\text{O} + \text{O}_2 \]
Colored bottles (usually brown or amber) block ultraviolet and visible light, preventing the decomposition of H₂O₂ and ensuring its stability.

Answer:
Hydrogen peroxide is stored in colored bottles to protect it from light, which can cause it to decompose.

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Final Answers:

1. Q-14: \(\boxed{a) 3, 4}\)
2. Q-15: \(\boxed{c) N_2 + O_2 \rightarrow 2NO}\)
3. Q-16: \(\boxed{a) (\text{CH}_3\text{COO})_2\text{Pb}(aq) + 2\text{HCl}(aq) \rightarrow \text{PbCl}_2(s) + \text{CH}_3\text{COOH}(aq)}\)
4. Q-17: \(\boxed{26.46 \, \text{g}}\)
5. Q-18: \(\boxed{b) Both A and R are true but R is not the correct explanation of the assertion.}\)
6. Q-19: Hydrogen peroxide is stored in colored bottles to protect it from light, which can cause it to decompose.