Simple Chemical Equations Questions

Correct Answer

📖 Governing Law / Core Concept

A decomposition reaction involves the breakdown of one compound into two or more substances, usually requiring energy input (heat, light, or electricity).

🧪 Step-by-Step Breakdown

In the reaction:

Silver chloride (AgCl) decomposes upon exposure to light, yielding solid silver (Ag) and chlorine gas (Cl₂). Observation includes color change from white to grayish black for silver.

🔍 Option Analysis

• A: Combustion of methane; does not decompose a single reactant.
• C: Redox reaction producing copper and water; not a decomposition.
• D: Single displacement reaction; iron displaces copper in copper(II) sulfate.

⚡ Mnemonic / Speed-Run

Remember: "Decompose means to break down!" to distinguish decomposition from other reaction types.

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

Correct Answer

📖 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

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

📖 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

📖 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

📖 Factual Verification & Reference

Correct Answer

📖 Governing Law / Core Concept

The governing chemical principle is photodecomposition.

The chemical reaction is represented as:

🧪 Step-by-Step Breakdown

When silver chloride (AgCl) is exposed to sunlight, it undergoes the following processes:

• Observation: A color change is noted, indicating silver precipitation.
• Decomposition Process: Chlorine gas is released and can be visually detected.
• Balanced Reaction:

🔍 Option Analysis

• Option A: in dark - No reaction occurs without light.
• Option C: on heating - Heating does not specifically trigger AgCl decomposition as sunlight does.
• Option D: under high pressure - Pressure does not influence the photosensitivity of AgCl.

⚡ Mnemonic / Speed-Run

To remember that AgCl decomposes in light: AGe Closer to the sun!

📖 Factual Verification & Reference