2025 - JAMB Chemistry Past Questions and Answers - page 8

During the preparation of chlorine gas, hydrogen chloride gas is often formed along with chlorine as an impurity.

Hydrogen chloride gas is highly soluble in water. When the gas mixture passes through the wash bottle, the hydrogen chloride dissolves readily in the water.

Chlorine gas, however, is only slightly soluble in water compared to hydrogen chloride, so most of it passes through without dissolving.

This process removes hydrogen chloride from the mixture, leaving chlorine gas relatively pure.

Therefore, the purpose of passing the gas through water is to absorb HCl gas.

Basicity refers to the number of hydrogen ions (H\(^+\)) that one molecule of an acid can release in solution.

The structure of oxalic acid, C\(_2\)H\(_2\)O\(_4\), contains two hydrogen atoms that are attached to oxygen atoms in carboxyl groups. These hydrogen atoms can be released as hydrogen ions.

Since each molecule can release two hydrogen ions, it can react with two hydroxide ions during neutralization.

This means the acid has two replaceable hydrogen ions.

Therefore, the basicity of C\(_2\)H\(_2\)O\(_4\) is 2.

The compound consists of a complex ion and chloride ions. The complex ion is [Cu(NH\(_3\))\(_4\)]\(^{2+}\), which carries a positive charge.

The chloride ions, Cl\(^-\), carry negative charges.

The attraction between positively charged ions and negatively charged ions is due to electrostatic forces.

This type of attraction between oppositely charged ions is known as an ionic bond.

Therefore, the bond between the complex ion and the chloride ions is ionic.

Metals that occur in the free state are those that are very unreactive. Such metals do not easily combine with oxygen, water, or other elements.

Silver is a very unreactive metal and does not readily form compounds under normal environmental conditions.

Because of this low reactivity, silver can exist in nature in its pure metallic form.

More reactive metals such as zinc and iron usually occur as compounds in ores rather than as free metals.

Therefore, the metal found in the free state in nature is silver.

The fountain experiment requires a gas that is highly soluble in water.

When the gas dissolves rapidly in water, it reduces the pressure inside the container. This creates a pressure difference between the inside and outside.

The higher external atmospheric pressure then forces water into the container, producing a fountain effect.

Hydrogen chloride gas dissolves very easily in water, making it suitable for demonstrating this experiment.

Therefore, hydrogen chloride is the gas commonly used.

Nitric acid is a strong oxidizing agent and usually does not produce hydrogen gas when reacting with metals because it oxidizes hydrogen to form water.

However, very reactive metals such as magnesium can react with very dilute nitric acid.

Magnesium is high in the reactivity series and can displace hydrogen from dilute acid.

The reaction produces magnesium nitrate and hydrogen gas:

\[ Mg + 2HNO_3 → Mg(NO_3)_2 + H_2 \]

Therefore, magnesium can liberate hydrogen gas from dilute nitric acid.

Sublimation is the direct change of a substance from solid to gas without passing through the liquid state.

In a solid, particles are held together by strong intermolecular forces.

For the particles to escape into the gas phase, energy must be supplied to overcome these forces.

This energy is absorbed from the surroundings, allowing the particles to move freely as a gas.

Therefore, sublimation involves absorption of energy.

When propane reacts with chlorine in the presence of sunlight, ultraviolet light provides energy to break the chlorine molecule into reactive chlorine atoms.

These chlorine atoms replace hydrogen atoms in the propane molecule.

This process results in the formation of chlorinated products and hydrogen chloride.

Since one atom replaces another atom in the compound, the reaction is classified as a substitution reaction.

Therefore, the reaction is substitution.

Petrol must burn smoothly in an engine to ensure proper performance.

If petrol burns unevenly or too early, it produces knocking sounds known as pinking.

Pinking reduces engine efficiency and can damage engine components.

Testing petrol ensures it has the correct composition and octane rating to prevent this problem.

Therefore, petrol is evaluated to prevent pinking.

Nitrogen obtained from the air is widely used in industrial chemical processes. One major application is in the Haber process, where nitrogen reacts with hydrogen to produce ammonia, NH\(_3\). This ammonia is then oxidized in subsequent reactions to form nitric acid, HNO\(_3\). Nitric acid is an important industrial chemical used in fertilizer production, explosives, and other chemical syntheses.

Therefore, nitrogen from air is primarily used in the production of nitric acid.