Chemistry 2012 Jamb Examination

Isomers are compounds that have the same molecular formula but different structural arrangements. The molecular formula C\( _3 \)H\( _8 \)O can represent two alcohols: 1-propanol (CH\( _3 \)CH\( _2 \)CH\( _2 \)OH) and 2-propanol (CH\( _3 \)CHOHCH\( _3 \)). Both have the same molecular formula but differ in the position of the hydroxyl group on the carbon chain, making them position isomers. 2-propanone (acetone) has the same number of atoms but is a ketone, not an isomeric alcohol of this formula.

Ammonia (NH\( _3 \)) is a colourless gas with a characteristic pungent odour. It is basic, turning red litmus paper blue. When exposed to hydrogen chloride gas, it forms dense white fumes of ammonium chloride (NH\( _4 \)Cl). The reaction is:\[ \text{NH}_3(g) + \text{HCl}(g) \rightarrow \text{NH}_4 \text{Cl}(s) \] This reaction and the observable properties make ammonia distinct among the options.

Chlorine (Cl\( _2 \)) and sulphur(IV) oxide (SO\( _2 \)) are both effective commercial bleaching agents. Chlorine acts through oxidation, converting coloured compounds to colourless ones by destroying chromophores, while sulphur(IV) oxide acts through reduction, removing oxygen from coloured compounds. Both are widely used industrially—chlorine for paper and textile bleaching, and SO\( _2 \) for delicate materials like wool and silk.

Hydrogen peroxide (H\( _2 \)O\( _2 \)) is unstable and tends to decompose into water and oxygen, especially in alkaline conditions or in the presence of light and metal ions. The addition of small amounts of mineral acid helps stabilise the solution by maintaining an acidic medium, which suppresses decomposition and prolongs shelf life. Thus, mineral acids are used to minimise its decomposition, not to accelerate it.

Calcium compounds, such as calcium chloride, produce a brick-red colour in a flame test due to the excitation of calcium ions. When these ions return to their ground state, they emit light at a characteristic wavelength that appears brick-red. Lithium gives a crimson flame, sodium a bright yellow flame, and magnesium emits no visible colour in a flame test.

Wrought iron is the purest form of commercial iron, containing about 99.9% iron and only about 0.1% carbon. It is tough, malleable, and resistant to corrosion. Pig iron has a very high carbon content (about 4%) and is brittle, while cast iron contains about 2–3% carbon. Iron pyrite (FeS\( _2 \)) is not a form of metallic iron but a compound of iron and sulphur.

When a metal displaces another from its salt solution, it indicates that the displacing metal is more reactive or electropositive. Since M displaces zinc from ZnCl\( _2 \), it means M loses electrons more readily than zinc and therefore lies above zinc in the electrochemical series. This shows M is more electropositive than zinc.

Using Hess’s law:
ΔH_rxn = ΣΔH_f(products) − ΣΔH_f(reactants)
Products: CO₂(g) and H₂(g)
Reactants: CO(g) and H₂O(g)
ΔH_rxn = [(-394) + 0] − [(-110) + (-242)]
ΔH_rxn = -394 − (-352)
ΔH_rxn = -394 + 352 = -42 kJ mol^-1
The negative sign indicates an exothermic reaction.

Entropy measures the degree of disorder or randomness in a system. Mixing of gases leads to greater randomness because gas molecules spread out and occupy the available space uniformly, resulting in an increase in entropy. Processes like freezing, condensation, or solidification decrease randomness and therefore reduce entropy.

The rate of production of CO\( _2 \) depends on both the concentration of the acid and the surface area of the calcium carbonate. A higher acid concentration increases the frequency of effective collisions, while powdered CaCO\( _3 \) offers a larger surface area than lumps. Therefore, using 0.10 mol dm\( ^{-3} \) HCl (highest concentration) and powdered CaCO\( _3 \) (largest surface area) will produce CO\( _2 \) at the fastest rate.