WAEC SSCE - Chemistry - 2003 (Essay)

(a) Identifying the elements from configuration (not required, but shown for clarity)

• P: 1s\(^2\)2s\(^2\)2p\(^2\) (Z = 6, group IV)
• Q: 1s\(^2\)2s\(^2\)2p\(^4\) (Z = 8, group VI)
• R: 1s\(^2\)2s\(^2\)2p\(^6\) (Z = 10, group VIII, noble gas)
• S: 1s\(^2\)2s\(^2\)2p\(^6\)3s\(^2\) (Z = 12, group II)
• T: 1s\(^2\)2s\(^2\)2p\(^6\)3s\(^2\)3p\(^5\) (Z = 17, group VII)

(i) Belongs to group VI: Q (six outermost electrons, 2s\(^2\)2p\(^4\)).

(ii) Strongly metallic: S (a group II element that readily loses electrons).

(iii) Readily ionises by gaining one electron: T (needs one electron to complete its octet, forming a 1\(^-\) ion).

(iv) Two unpaired electrons in the ground-state atom: P and Q. In 2p\(^2\) the two electrons occupy separate orbitals (Hund's rule); in 2p\(^4\) one orbital is doubly filled leaving two singly-occupied orbitals - both give two unpaired electrons.

(v) Readily loses two electrons in bonding: S (3s\(^2\) outer electrons lost to form a 2\(^+\) ion).

(vi) Does not take part in chemical reactions: R (a stable, filled-shell noble gas).

(vii) An s-block element: S (its outermost electrons enter the 3s sub-shell).

(b)(i) Completed table

Neutrons = mass number - atomic number. For Al\(^{3+}\), three electrons are lost (13 - 3 = 10). For the oxide ion, two electrons are gained (8 + 2 = 10).

(b)(ii) Trend in atomic radius

Down a group the number of occupied electron shells increases and inner-shell shielding increases, so the outermost electrons lie farther from the nucleus and the radius increases. Across a period electrons are added to the same shell while the nuclear charge rises; the greater effective nuclear attraction pulls the shell inward, so the radius decreases.

(c)(i) Half-life

The half-life of a radioactive element is the time taken for half the atoms (nuclei) in a given sample to decay (disintegrate).

(c)(ii) Alpha decay of polonium-210

(d) Type of bonding

• (i) Chlorine as discrete molecules: covalent bonding (shared electron pair within each Cl\(_2\) molecule).
• (ii) Sodium chloride dissolving readily in water: ionic (electrovalent) bonding (the polar water molecules hydrate the Na\(^+\) and Cl\(^-\) ions).
• (iii) Deep-blue complex ion with excess ammonia: co-ordinate (dative) covalent bonding (lone pairs on N donate into the Cu\(^{2+}\) ion).

(a) Empirical and molecular formula of X (40% C, 6.67% H, O = 100 - 40 - 6.67 = 53.33%)

(i) Empirical formula = CH₂O (mass = 30).
(ii) \(\dfrac{60}{30}=2\), so molecular formula = C₂H₄O₂.

(b) Alkanoic acid Y, M_r = 74. For C_nH_2n+1COOH, propanoic acid C₂H₅COOH has M_r = 74.

(i) Functional group: carboxyl group, -COOH. (ii) Converting Y to an alkanoate (salt) is a neutralization reaction. (iii) Structural formula of Y: CH₃CH₂COOH. (iv) 2CH₃CH₂COOH + 2Na → 2CH₃CH₂COONa + H₂.

(v) X (C₂H₄O₂) is ethanoic acid; Y is propanoic acid, in the same homologous series. Y has a larger relative molecular mass (74 > 60) and longer chain, so stronger van der Waals forces; its boiling point will be higher than 118°C.

(c)(i) Fermentation is the slow chemical breakdown of a complex organic substance (such as sugar) into simpler substances (ethanol and carbon(IV) oxide) by the action of enzymes from micro-organisms such as yeast. (ii) C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. (iii) Yeast (which supplies the enzyme zymase) must be added. (iv) The palm wine keeps fermenting and produces carbon(IV) oxide gas; because the bottle is full to the brim and tightly corked, the gas cannot escape, so the pressure builds up until it exceeds the strength of the bottle and it shatters.

(a) (i) Used in bleaching: sulphur(IV) oxide, SO₂. (ii) Reddish-brown gas: nitrogen(IV) oxide, NO₂. (iii) Amphoteric (reacts with NaOH and HCl): aluminium oxide, Al₂O₃ (or ZnO). (iv) Dissolves in water to give pH greater than 7: sodium oxide, Na₂O (or CaO). (v) Oxidizes hot concentrated HCl to chlorine: manganese(IV) oxide, MnO₂.

(b)(i) Three methods to remove hardness due to Ca(HCO₃)₂ (temporary hardness): boiling; adding slaked lime (Clark's method); adding sodium trioxocarbonate(IV) (washing soda).

(ii) Sulphur(IV) oxide pollution forms acid with rain water (SO₂ + H₂O → H₂SO₃, and this is oxidized to H₂SO₄). This acid attacks limestone (CaCO₃), wearing the building away: CaCO₃ + H₂SO₃ → CaSO₃ + H₂O + CO₂. The limestone is eroded, so it is unwise to build with it there.

(c)(i) Two potassium compounds that yield oxygen on strong heating: potassium trioxochlorate(V), KClO₃, and potassium trioxonitrate(V), KNO₃ (or KMnO₄).

(ii) Amount of gas in 250 cm³ at s.t.p.:

(iii) 250 cm³ at s.t.p. (273 K) heated to 27°C (300 K) at constant pressure:

(iv) On heating at constant pressure the molecules gain kinetic energy and move faster, colliding more frequently and more forcefully; the gas expands (volume increases) so that the number of collisions per unit area of the walls, and hence the pressure, stays constant.

(a) (i) Sand bed: filtration, removes suspended solid particles. (ii) Alum: coagulant, makes fine suspended/colloidal particles clump together and settle. (iii) Chlorine: sterilization, kills harmful bacteria and micro-organisms.

(b)(i) Three uses of H₂SO₄: manufacture of fertilizers (e.g. ammonium sulphate); manufacture of detergents; as the electrolyte in lead-acid accumulators (car batteries).

(ii) Concentrated H₂SO₄ is a powerful dehydrating agent: it removes hydrogen and oxygen (as water) from the litmus (a carbohydrate), leaving black carbon, so the paper is charred. Dilute H₂SO₄ is not a dehydrating agent; it is simply acidic, ionizing to give H⁺ ions that turn blue litmus red without charring.

(iii) Zn + 2H₂SO_4(conc) → ZnSO₄ + SO₂ + 2H₂O (hot, concentrated).

(c)(i) Two gaseous pollutants from burning coal: sulphur(IV) oxide (SO₂) and carbon(II) oxide (CO). (ii) Broken coal has a much larger surface area exposed to oxygen than a lump, so more coal is in contact with air and the rate of burning (frequency of effective collisions) increases, making it burn faster. (iii) Methane (firedamp) is responsible for most explosions in coal mines. (iv) The non-volatile residue from destructive distillation of coal is coke.

(d)(i) Allotropy is the existence of an element in two or more different forms in the same physical state. (ii) Diamond and graphite. (iii) Two other elements showing allotropy: sulphur and phosphorus. (iv) Mass of one mole of C₆₀ = 60 × 12 = 720 g.

(a)(i) Two reasons aluminium is preferred to copper for overhead cables: aluminium is much lighter (lower density), so the cables sag less and need fewer supports; aluminium is cheaper and more abundant than copper (while still being a good conductor).

(ii) Electrolytic extraction of aluminium: Purified alumina (Al₂O₃) is dissolved in molten cryolite (Na₃AlF₆) to lower the melting point and improve conductivity. The molten mixture is electrolyzed in a steel tank lined with graphite (the cathode), with carbon anodes.
Cathode: Al³⁺ + 3e^- → Al (molten aluminium collects at the bottom and is tapped off).
Anode: 2O^2- → O₂ + 4e^-; the oxygen burns the carbon anodes to CO₂, so they are replaced from time to time.

(b) In copper purification the impure copper is the anode and pure copper is the cathode, in copper(II) tetraoxosulphate(VI) solution.
(i) The cathode (pure copper) increases in mass, because Cu²⁺ ions are discharged and deposited on it (Cu²⁺ + 2e^- → Cu).
(ii) Oxidation occurs at the anode, because there copper atoms lose electrons to form ions (Cu → Cu²⁺ + 2e^-).
(iii) III is the electrolyte, copper(II) sulphate solution. Its blue colour does not fade because the rate at which Cu²⁺ ions leave the solution at the cathode equals the rate at which Cu²⁺ ions enter the solution from the dissolving anode, so [Cu²⁺] stays constant.

(c) Deposit 8.00 g Ca in 1 h 15 min = 4500 s, Ca²⁺ + 2e^- → Ca.