Symbols and Formulae Questions

Correct Answer

📖 Core Concept

Redox reactions involve the transfer of electrons, leading to changes in oxidation states of reactants. The general equation governing redox reactions can be represented as:

Oxidation: \( \text{Zn} \rightarrow \text{Zn}^{2+} + 2e^- \)

Reduction: \( \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \)

🧪 Step-by-Step Breakdown

The evaluation of each option determines if they are redox reactions:

• Option A: AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl

No change in oxidation states:

Oxidation State of Al: +3 (both sides)

Oxidation State of Cl: -1 (both sides)

• Option B: 2NaH → 2Na + H₂

Redox Reaction:

Oxidation: \( \text{Na}^{+} \rightarrow \text{Na}^0 \)

Reduction: \( \text{H}^0 \rightarrow \text{H}_2 \)

• Option C: 4Fe + 3O₂ → 2Fe₂O₃

Redox Reaction:

Oxidation: \( \text{Fe}^0 \rightarrow \text{Fe}^{+3} \)

Reduction: \( \text{O}_2^{0} \rightarrow \text{O}^{-2} \)

• Option D: CuSO₄ + Zn → Cu + ZnSO₄

Redox Reaction:

Oxidation: \( \text{Zn}^0 \rightarrow \text{Zn}^{+2} \)

Reduction: \( \text{Cu}^{+2} \rightarrow \text{Cu}^0 \)

🔍 Option Analysis

Options B, C, and D are valid redox reactions as they involve changes in oxidation states, while Option A is an acid-base reaction (no change in oxidation states).

⚡ Mnemonic / Speed-Run

Mnemonic: "LEO says GER" — Lose Electrons Oxidation, Gain Electrons Reduction.

6. Visual Suggestion

📖 Factual Verification & Reference

Correct Answer

📖 Governing Law / Core Concept

The reaction between carbon dioxide and lime water (calcium hydroxide) illustrates the principle of acid-base neutralization and carbonate formation.

🧪 Step-by-Step Breakdown

The correct reaction is:

When carbon dioxide (CO₂) is bubbled through lime water (a saturated solution of calcium hydroxide, Ca(OH)₂), it reacts to form calcium carbonate (CaCO₃), which is insoluble in water and precipitates out as a white solid causing the solution to turn milky.

🔍 Option Analysis

• Option (a): Incorrect. This reaction represents the formation of bicarbonate, not the precipitation reaction.
• Option (c): Incorrect. This does occur but is not related to lime water.
• Option (d): Incorrect. Involves calcium chloride and hydrochloric acid, not the correct reaction.

⚡ Mnemonic / Speed-Run

Remember: "Lime Water with CO₂ makes milky bubbles" to recall this reaction effectively.

6. Visual Suggestion

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📖 Governing Law / Core Concept

The reaction follows the principles of redox chemistry, where reducing and oxidizing agents are defined based on electron transfer. Here, N2H4 (hydrazine) undergoes oxidation while H2O2 (hydrogen peroxide) serves as the oxidizing agent.

🧪 Step-by-Step Breakdown

The given reaction:

Analyzing oxidation states:

• Oxidation state of N in N2H4: -2
• Oxidation state of N in N2: 0
• Oxidation state of O in H2O2: -1
• Oxidation state of O in H2O: -2

Change in oxidation states signifies:

• From -2 (N2H4) to 0 (N2): N2H4 is oxidized.
• O in H2O2 goes from -1 to -2: H2O2 is reduced.

Conclusion: N2H4 is acting as the reducing agent.

🔍 Option Analysis

• H2O2: Acts as the oxidizing agent, gaining electrons.
• N2: Product of reduction, not an agent.
• H2O: Also a product, no redox role.

⚡ Mnemonic / Speed-Run

Remember: "OIL RIG" - Oxidation Is Loss, Reduction Is Gain. N2H4 loses electrons, hence it is oxidized.

6. Visual Suggestion

📖 Factual Verification & Reference

Correct Answer

📖 Governing Law / Core Concept

Types of chemical reactions can be classified as Single displacement, Double displacement, Combination, and Decomposition reactions.

🧪 Step-by-Step Breakdown

• A. CaO(s) + H₂O(l) → Ca(OH)₂(aq): Combination Reaction
• B. CaCO₃(s) → CaO(s) + CO₂(g): Decomposition Reaction
• C. Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s): Single Displacement Reaction
• D. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq): Double Displacement Reaction

🔍 Option Analysis

• Option A matches 2 (Single displacement) incorrectly.
• Option B matches 4 (Decomposition) incorrectly.
• Option D matches 3 (Combination) incorrectly.

⚡ Mnemonic / Speed-Run

Mnemonic for Reaction Types: CDSD - Combination, Decomposition, Single displacement, Double displacement.

6. Visual Suggestion

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📖 Governing Law / Core Concept

The decomposition reaction of some compounds, like metal nitrates, typically requires energy input (heat), indicating an endothermic process.

🧪 Step-by-Step Breakdown

The decomposition of lead(II) nitrate is represented as:

This reaction absorbs heat, illustrated through the thermal properties noticed in laboratory settings.

🔍 Option Analysis

• A: \(\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l)\) - This is an exothermic combustion reaction.
• B: \(\text{C}_6\text{H}_{12}\text{O}_6(aq) + 6\text{O}_2(g) \rightarrow 6\text{CO}_2(g) + 6\text{H}_2\text{O}(l)\) - Also an exothermic biological oxidation reaction.
• C: \(\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)\) - A synthesis reaction that releases energy.

⚡ Mnemonic / Speed-Run

• Endothermic AIS: Absorb, Inputs heat, Skills temperature.

6. Visual Suggestion

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📖 Governing Law / Core Concept

Hypobromous acid (HOBr) is a weak acid where bromine has an oxidation state of +1. This acid is used in various applications including water treatment and as an antimicrobial agent.

🧪 Step-by-Step Breakdown

The chemical formula for hypobromous acid is derived from its structure:

Here, H represents hydrogen, O represents oxygen, and Br represents bromine. The oxidation state of bromine in hypobromous acid can be determined by the formula: \[ \text{Oxidation state of Br} = +1 \]

🔍 Option Analysis

• A (HBrO₄): This is the formula for perbromic acid, where Br is in the +7 oxidation state.
• C (HBr): This represents hydrobromic acid, a strong acid with Br in the -1 oxidation state.
• D (HBrO₃): This is the formula for bromic acid, with Br in the +5 oxidation state.

⚡ Mnemonic / Speed-Run

To remember the formula of hypobromous acid, "Hypo" contains "Ho" - which can help recall HOBr. The order of the acids can also assist: HBr, HOBr, HBrO₃, HBrO₄ from least to most oxygen.

6. Visual Suggestion

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📖 Governing Law / Core Concept

Electrolytic refining is based on the principle of electrolysis where an electric current is passed through a solution to separate and purify metal ions.

🧪 Step-by-Step Breakdown

In the electrolytic refining of copper:

• Anode: Impure copper (Cu_imp) dissolves during the electrolysis, releasing copper ions (Cu²⁺). This is represented by the reaction:
\[ \text{Cu}_{\text{imp}} \rightarrow \text{Cu}^{2+} + 2 \text{e}^- \]
• Cathode: Pure copper is deposited from the copper ions in the electrolyte, shown as:
\[ \text{Cu}^{2+} + 2 \text{e}^- \rightarrow \text{Cu}_{\text{pure}} \]
• Electrolyte: Acidified copper sulfate (\( \text{CuSO}_4 \)) facilitates the movement of copper ions. The addition of sulfuric acid (H₂SO₄) enhances conductivity.
• Color Observation: The blue color of the copper sulfate solution fades as copper ions deposit at the cathode.

🔍 Option Analysis

• Option A: Acidified copper chloride does not serve as a suitable electrolyte in copper refining.
• Option C: Potassium chloride is not relevant for copper purification.
• Option D: Sodium sulfate is also ineffective as an electrolyte for the process.

⚡ Mnemonic / Speed-Run

Remember: "Copper Soaks Easily" refers to the use of Copper Sulfate as the electrolyte in the refining process.

📊 Visual Suggestion

📖 Factual Verification & Reference

Correct Answer

📖 Governing Law / Core Concept

In organic chemistry, an addition reaction involves the addition of one or more atoms or groups to a molecule without the loss of any atom from it.

🧪 Step-by-Step Breakdown

Vegetable oils, composed mainly of unsaturated fatty acids, undergo hydrogenation to form saturated fats. This process is typically represented by the equation: Here, \( \text{C}_n\text{H}_{2n-2} \) represents unsaturated fatty acids and \( \text{C}_n\text{H}_{2n} \) is the resultant saturated fatty acid (vegetable ghee). The nickel catalyst facilitates this addition by breaking the double bonds in the unsaturated hydrocarbons.

🔍 Option Analysis

- **A: Substitution reaction:** Involves the replacement of an atom in a molecule. - **B: Elimination reaction:** Involves the loss of atoms to form a double bond. - **D: Free-radical polymerization:** A process involving the reaction of monomers that form polymers.

⚡ Mnemonic / Speed-Run

Remember: "Addition is for hydrogenation!" to quickly recall that hydrogenation leads to addition reactions.

6. Visual Suggestion

📖 Factual Verification & Reference

Correct Answer

📖 Governing Law / Core Concept

The reaction of certain metals and acids results in the evolution of hydrogen gas, which is governed by the reactivity of metals with acids and the nature of chemical reactions.

🧪 Step-by-Step Breakdown

Analyzing each option:

• A: Zinc (Zn) reacts with dilute sulphuric acid (H₂SO₄) to produce hydrogen gas.
\[ Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow \]
• C: Heating zinc metal with sodium hydroxide (NaOH) releases hydrogen gas.
\[ Zn + 2NaOH + 2H_2O \rightarrow Na_2[Zn(OH)_4] + H_2\uparrow \]
• D: Potassium (K) reacts vigorously with water to produce hydrogen gas.
\[ 2K + 2H_2O \rightarrow 2KOH + H_2\uparrow \]
• B: Mixing water with Plaster of Paris (Calcium Sulphate Hemihydrate) does not involve any metal displacement or hydrogen evolution.
\[ CaSO_4 \cdot \frac{1}{2}H_2O + H_2O \rightarrow CaSO_4 \cdot 2H_2O \

🔍 Option Analysis

Options A, C, and D involve reactions that result in hydrogen gas production, whereas option B merely hydrates the Plaster of Paris without any gas evolution.

⚡ Mnemonic / Speed-Run

Remember: "Zinc and Acid, Potassium and Water, Sodium too - all give hydrogen, but Plaster's just glue!"

📖 Factual Verification & Reference