2024 - JAMB Chemistry Past Questions and Answers - page 6

Here's the breakdown of the electrolysis process:

Anode (Positive Electrode): Oxidation occurs at the anode. Chloride ions (Cl⁻) from the sodium chloride solution are oxidized to chlorine gas (Cl₂). The reaction is: 2Cl⁻ → Cl₂ + 2e⁻

Cathode (Negative Electrode): Reduction occurs at the cathode. Sodium ions (Na⁺) and water (H₂O) are present in the solution. Since the overpotential for sodium ion reduction is very high on a mercury cathode, sodium metal is formed, which then forms an amalgam with mercury. The reaction is: Na⁺ + e⁻ → Na

Therefore, the products at the anode and cathode are chlorine and sodium, respectively.

Calcium carbonate (CaCO₃) does not sublime (change directly from solid to gas). It decomposes upon heating. NH₄Cl, I₂, and S₈ all undergo sublimation, changing directly from solid to gas upon heating.

The reactivity series is a list of metals arranged in order of decreasing reactivity. The order of increasing reactivity for the given metals is: Gold (Au) < Copper (Cu) < Tin (Sn) < Iron (Fe) < Calcium (Ca).

Here's how to solve this problem using Faraday's laws of electrolysis:

1. Write the half-reactions:

Ag⁺ + e⁻ → Ag
Mg²⁺ + 2e⁻ → Mg

2. Determine the number of moles of silver liberated:
Moles of Ag = 16.0 g / 108 g/mol ≈ 0.148 mol

3. Determine the number of moles of electrons involved:
1 mole of Ag requires 1 mole of electrons → 0.148 mol of electrons

4. Determine the mass of magnesium liberated:
1 mole of Mg requires 2 moles of electrons → 0.148 / 2 = 0.074 mol of Mg
Mass of Mg = 0.074 × 24 g/mol ≈ 1.78 g

The isotopes of hydrogen are protium (hydrogen-1), deuterium (hydrogen-2), and tritium (hydrogen-3). Tritium has the highest mass number (3) and is thus the highest isotope of hydrogen.

When n = 3, this refers to the principal quantum number, which describes the energy level.

The number of orbitals in an energy level is n². Therefore, in the third energy level (n=3), there are 3² = 9 orbitals.

Each orbital can hold a maximum of 2 electrons, so the total number of electrons in the third energy level is 9 × 2 = 18 electrons.

Strong acids and weak acids can be differentiated by their properties:

A. Conductivity measurement: Strong acids ionize completely in solution, producing more ions and hence exhibiting higher conductivity compared to weak acids.

B. The use of litmus paper: Both strong and weak acids turn blue litmus paper red. Litmus paper only indicates the presence of an acid, not its strength.

C. Measurement of pH: Strong acids have a lower pH (closer to 0) than weak acids at the same concentration.

D. Measurement of heat of reaction: Strong acids release more heat during a reaction (e.g., with a base) because the reaction goes to completion more readily.

Here's how to determine the molecular formula:

1. Assume a 100 g sample: Percentages equal grams: Carbon 53.1 g, Hydrogen 6.2 g, Nitrogen 12.4 g, Oxygen 28.3 g.

2. Calculate moles: Carbon 53.1/12 = 4.425 mol, Hydrogen 6.2/1 = 6.2 mol, Nitrogen 12.4/14 ≈ 0.886 mol, Oxygen 28.3/16 ≈ 1.769 mol.

3. Determine the empirical formula: Divide all by smallest number (0.886): C₅H₇NO₂.

4. Check with vapour density: Matches molecular formula C₅H₇NO₂.

The half-life (t₁/₂) of a first-order reaction is calculated using the formula:

t₁/₂ = 0.693 / k

Given k = 4.5 x 10⁻³ s⁻¹,

t₁/₂ = 0.693 / 0.0045 ≈ 154 s

The reaction between hydrogen and chlorine to produce hydrogen chloride gas is a photochemical reaction. It is explosive in the presence of sunlight because sunlight provides the energy needed to initiate the reaction by breaking chlorine molecules into highly reactive chlorine radicals.