(a) State the conditions necessary for the cracking of long-chain hydrocarbons to produce more gasoline.
(ii) State one drying agent for hydrogen chloride gas.
(d) Concentrated trioxonitrate (V) acid was added to a solution of iron (II) tetraoxosulphate (VI) and the mixture heated. The mixture turned from pale green to yellow with the evolution of a brown gas. Explain briefly these observations.
(f) Two isotopes of chlorine are \(^{35}_{17}\mathrm{Cl}\) and \(^{37}_{17}\mathrm{Cl}\). State one:
(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.
