JAMB Chemistry 1988

C: Sublimation

This laboratory process is conventionally described as sublimation. In detail, heated NH₄Cl dissociates reversibly into NH₃ and HCl gases, which recombine on the cooler surface to form solid ammonium chloride. Crystal formation is the return to the solid, rather than the solid-to-gas step.

C: M₄X₃

The combining capacities must balance. Four atoms of a trivalent element provide 4×3 = 12 valence units; three atoms of a tetravalent element provide 3×4 = 12. The simplest ratio is therefore M₄X₃.

B: 67.2 dm³

Complete combustion follows C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. At STP, water is condensed, so the gaseous product counted is 3 mol CO₂. Its volume is 3×22.4 = 67.2 dm³.

B: Charles’ law

The plot shows volume proportional to absolute temperature. For a fixed amount of gas at constant pressure, V/T is constant: Charles’ law. A pressure–temperature relationship at fixed volume describes a different gas law.

D: Van der Waals forces

Naphthalene is a molecular solid. Its non-polar molecules attract one another mainly through London dispersion forces, a type of van der Waals force. Melting overcomes these intermolecular attractions without breaking the covalent bonds within each molecule.

D: Atomic radius

Across a period, the number of protons increases while added electrons occupy the same main shell. The increasing effective nuclear attraction generally pulls electrons closer, decreasing atomic radius.

A: −1 and +7

Atomic number 17 identifies chlorine. It has oxidation state −1 in chlorides and +7 in perchlorates such as ClO₄⁻, where x + 4(−2) = −1 gives x = +7.

C: Electron affinity

Electron affinity describes the energy change when an electron is added to an isolated gaseous atom to form an anion. Ionisation energy instead concerns removing an electron; electronegativity concerns attraction for bonding electrons.

A: Na₂SO₄·7H₂O

Water lost is 1.34−0.71 = 0.63 g, or 0.63/18 = 0.035 mol. Anhydrous Na₂SO₄ has molar mass 142, so 0.71 g is 0.005 mol. The water-to-salt ratio is 0.035/0.005 = 7, giving Na₂SO₄·7H₂O.

A: Mg²⁺

Magnesium ions form insoluble salts with soap anions and prevent easy lather formation. Water that lathers readily therefore contains very little Mg²⁺ compared with hard water. Potassium and the listed carbonate species do not themselves cause this soap scum.

D: NaCl

NaCl is formed from a strong acid and a strong base. Its ions do not appreciably hydrolyse in water, so its solution is approximately neutral. NH₄Cl and FeCl₃ give acidic solutions, while Na₂CO₃ gives an alkaline solution.

D: 30.0 cm³

Complete neutralisation requires three moles of NaOH per mole H₃PO₄: H₃PO₄ + 3NaOH → Na₃PO₄ + 3H₂O. NaOH amount is 0.0450×0.2 = 0.0090 mol, so acid amount is 0.0030 mol. Its volume is 0.0030/0.1 = 0.0300 dm³ = 30.0 cm³.

B: Mg(OH)Cl

A basic salt retains hydroxide groups from incomplete neutralisation of a base. Mg(OH)Cl contains both OH⁻ and Cl⁻, balancing Mg²⁺. This structural meaning differs from simply giving an alkaline solution, as Na₂CO₃ does.

A: Cu²⁺(aq) + 2e⁻ → Cu(s)

Reduction occurs at the cathode. Copper(II) ions accept two electrons each to form copper metal: Cu²⁺ + 2e⁻ → Cu. Chloride oxidation occurs at the anode, and copper dissolution is oxidation rather than cathodic reduction.

D: The enthalpy of solution is negative

Dissolving solid NaOH in water releases heat to the surroundings. The process is exothermic, so its enthalpy of solution is negative. A positive enthalpy would instead mean heat absorption.

B: Activation energy

A catalyst provides a reaction pathway with lower activation energy. It speeds both directions of a reversible reaction without changing the enthalpy change, free-energy change or equilibrium position.

B: Carbon dioxide

Carbon dioxide is used in CO₂ fire extinguishers. It does not burn and can reduce the oxygen concentration around a suitable fire. The other listed gases are not the extinguishing agent in this type of extinguisher.

C: It is only sparingly soluble in water

Carbon monoxide dissolves only slightly in water, so it can displace the water and collect in the inverted jar. Density relative to air is not the controlling property for collection over water.

C: Nitrogen(IV) oxide (NO₂)

Copper(II) nitrate releases nitrogen dioxide on heating: 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂. The equation conserves two copper atoms, four nitrogen atoms and twelve oxygen atoms. In NO₂, nitrogen has oxidation number +4, so the gas is nitrogen(IV) oxide.

B: Electrolysis of brine

In the chlor-alkali process, concentrated sodium chloride solution (brine) is electrolysed. Chloride ions are oxidised at the anode: 2Cl⁻ → Cl₂ + 2e⁻. This produces chlorine gas.

B: By electrolysing aluminium oxide dissolved in molten cryolite

Aluminium is too reactive for its oxide to be reduced commercially with coke. In the Hall–Héroult process, alumina is dissolved in molten cryolite and electrolysed. Aluminium ions gain electrons at the cathode to form aluminium metal.

B: The crude copper is made the anode

In electrolytic refining, impure copper is the anode and a sheet of pure copper is the cathode. Copper atoms at the anode lose electrons and enter solution as Cu²⁺. Copper ions gain electrons at the cathode, depositing purer metal. Some impurities remain dissolved and others form anode sludge.

B: Alkanols

An alkanoic acid reacts with an alkanol in esterification to form an alkyl alkanoate ester and water. Reaction with sodium instead forms an ionic carboxylate salt, not an alkyl ester.

B: Alkanes, alkenes, alkynes and aromatic hydrocarbons

Alkanes, alkenes, alkynes and aromatic hydrocarbons are names of classes. Ethane, ethene, ethyne and benzene are individual members, while methane through butane are all members of the alkane class.

D: A catalyst

Diastase is an enzyme preparation that speeds the hydrolysis of starch into smaller sugars such as maltose. Enzymes act as catalysts: they provide a faster reaction pathway and are regenerated during the catalytic process.

A: CH₃CH₂CH₂CH₂OH

Butan-1-ol has an O–H group, so its molecules form hydrogen bonds with one another. The aldehyde and ether cannot donate hydrogen bonds to their own molecules, and butane is non-polar. For these similarly sized molecules, this makes butan-1-ol the highest-boiling compound.

B: Destructive distillation

Destructive distillation heats coal with air excluded. The coal decomposes into coke and volatile products. Cooling and separating the volatile products yields coal tar and ammoniacal liquor, while coal gas remains gaseous.