2025 - JAMB Chemistry Past Questions and Answers - page 9

Oxidation refers to a process in which a substance loses electrons. Electrons carry negative charge, so when a substance loses electrons, its overall oxidation number increases. This loss of electrons is the defining feature of oxidation. For example, when a metal atom forms a positive ion, it does so by releasing electrons.

Therefore, in oxidation reactions, electrons are removed.

The given compound has the structure CH\(_3\)CH\(_2\)COOCH\(_2\)CH\(_3\), which belongs to the ester family. Esters are named by first identifying the alkyl group attached to the oxygen atom and then naming the acid-derived part as an alkanoate. The portion CH\(_2\)CH\(_3\) attached to oxygen corresponds to an ethyl group. The remaining part, CH\(_3\)CH\(_2\)COO−, comes from propanoic acid, which gives the name propanoate. Combining these two parts gives the name ethyl propanoate.

Benzene has the molecular formula C\(_6\)H\(_6\) and contains three double bonds arranged in a ring. During catalytic hydrogenation, hydrogen molecules are added across these double bonds in the presence of a catalyst such as nickel or platinum. Each double bond reacts with one molecule of hydrogen, so three hydrogen molecules are added in total. This converts the unsaturated benzene into a fully saturated six-carbon ring called cyclohexane, which has the formula C\(_6\)H\(_{12}\).

Hard water contains dissolved calcium and magnesium salts. When hard water flows through pipes, especially lead pipes, the calcium compounds can form a solid coating on the inner surface of the pipe. This coating acts as a protective layer that prevents the lead from dissolving into the water. Because of this protective effect, hard water can safely pass through lead pipes.

Therefore, the statement that hard water cannot be supplied in lead pipes is incorrect.

Aluminium ions are reduced according to the equation Al\(^{3+}\) + 3e\(^-\) → Al. This shows that 3 moles of electrons are needed to produce 1 mole of aluminium. One mole of aluminium has a mass of 27 g and requires 3 faradays of charge. Since 1 faraday equals 96500 C, the total charge required to produce 27 g is 3 × 96500 = 289500 C. To produce 9 g, which is one-third of 27 g, the charge required is 289500 ÷ 3 = 96500 C. Using the formula Q = It, where Q is charge, I is current, and t is time, we substitute the values to get 96500 = 18 × t. Solving for t gives t = 96500 ÷ 18 = 5361.11 seconds. Converting to minutes by dividing by 60 gives t = 5361.11 ÷ 60 = 89.35 minutes.

Therefore, the required time is approximately 89.34 minutes.

Compound V has the general formula of an ester, RCOOR'. Esters are produced when a carboxylic acid reacts with an alcohol in the presence of an acid catalyst. During this reaction, the hydroxyl group from the acid and a hydrogen atom from the alcohol combine to form water, while the remaining parts join to form the ester. This reaction is called esterification.

Therefore, the two compounds that react must be an alcohol and a carboxylic acid, which correspond to compounds I and IV.

Tristearin is a fat composed of glycerol joined to three fatty acid molecules through ester bonds. When it undergoes alkaline hydrolysis, also known as saponification, the ester bonds are broken by the alkali. This produces soap, which is the salt of the fatty acid, and glycerol. Glycerol contains three hydroxyl (−OH) groups attached to its carbon atoms. Because it has three hydroxyl groups, it is classified as a trihydric alkanol.

Oxygen dissolved in water is necessary for aquatic organisms such as fish to carry out respiration. It supports life and helps maintain a healthy aquatic environment. Pollutants, on the other hand, cause harm to living organisms or disrupt the natural balance. Since oxygen helps sustain aquatic life instead of harming it, it is not classified as a water pollutant.

An alkaline gas is one that produces a basic solution when dissolved in water. Ammonia, NH\(_3\), reacts with water to form ammonium ions and hydroxide ions. The hydroxide ions increase the basicity of the solution, making it alkaline. Because ammonia produces hydroxide ions in water, it is classified as an alkaline gas.