(a) State the conditions necessary for the cracking of long-chain hydrocarbons to produce more gasoline. (b) State two reasons why metallic objects are elec...

(a) Conditions for cracking long-chain hydrocarbons to give more gasoline

A high temperature (about 400 - 600 °C) together with a catalyst (aluminosilicate / zeolite, i.e. catalytic cracking). Purely thermal cracking uses high temperature and high pressure.

(b) Two reasons metallic objects are electroplated

• To protect the metal from corrosion / rusting.
• To improve the appearance (make it bright and decorative). (Also: to resist wear and increase durability.)

(c)(i) Why calcium oxide cannot dry hydrogen chloride gas

Calcium oxide is a basic oxide, and hydrogen chloride is an acidic gas; the two react chemically (CaO + 2HCl → CaCl2 + H2O) instead of the CaO merely removing moisture, so the gas would be absorbed and lost.

(c)(ii) One drying agent for HCl gas: concentrated tetraoxosulphate(VI) acid (conc. H₂SO₄). (Anhydrous calcium chloride is also acceptable.)

(d) Conc. HNO₃ added to iron(II) tetraoxosulphate(VI) and heated

Concentrated trioxonitrate(V) acid is a strong oxidizing agent. It oxidizes the pale-green Fe2+ ion to the yellow (brown-yellow) Fe3+ ion, which accounts for the colour change from pale green to yellow. In being reduced, the acid liberates nitrogen(IV) oxide, the brown gas observed.

\[3Fe^{2+} + 4H^{+} + NO_3^{-} \longrightarrow 3Fe^{3+} + NO\uparrow + 2H_2O\]

(with concentrated acid the brown gas evolved is NO₂).

(e)(i) Zinc oxide with dilute acid and with alkali

\[ZnO + H_2SO_4 \longrightarrow ZnSO_4 + H_2O\]

\[ZnO + 2NaOH \longrightarrow Na_2ZnO_2 + H_2O\]

(sodium zincate is formed with the alkali).

(e)(ii) Property shown: zinc oxide is amphoteric (it reacts with both acids and bases).

(f) Isotopes 3517Cl and 3717Cl

• Similarity: both have the same atomic (proton) number, 17, hence the same number of electrons and the same chemical properties.
• Difference: they have different mass numbers (35 and 37), i.e. different numbers of neutrons (18 and 20).

(g) Two products when chlorine water is exposed to sunlight

Chlorine water (HOCl + HCl) decomposes in sunlight, 2HOCl → 2HCl + O2, giving hydrogen chloride (hydrochloric acid) and oxygen.

(h) For the reaction 2H2S + SO2 → 3S + 2H2O

Assigning oxidation numbers to sulphur: it is −2 in H₂S, +4 in SO₂, and 0 in the free sulphur produced.

• Species undergoing oxidation: H₂S - the sulphur in it rises from −2 to 0 (loses electrons).
• Oxidizing agent: SO₂ - the sulphur in it falls from +4 to 0 (it is reduced, so it oxidizes the H₂S).

(i) Carbon-12 scale

It is the scale of relative atomic masses on which one atom of the carbon-12 isotope is assigned a mass of exactly 12.000 units, and the mass of every other atom is measured relative to one-twelfth of the mass of a carbon-12 atom.

(j) Two properties of a chemical system in equilibrium

• It is dynamic: the forward and backward reactions continue at equal rates.
• Its macroscopic properties (concentrations, colour, pressure) remain constant, and it is attained only in a closed system.