WAEC SSCE - Chemistry - 2015 (Essay)

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(a)(i) Two gases for the fountain experiment: ammonia (NH₃) and hydrogen chloride (HCl).
(ii) The property is their very high solubility in water.

(b)(i) I. Solubility is the maximum amount of a solute that dissolves in a given amount of solvent at a stated temperature to form a saturated solution.
II. A saturated solution is one that contains the maximum amount of solute it can dissolve at a given temperature, in contact with undissolved solute.

(ii) Two factors affecting solubility of a solid in a liquid: temperature; the nature of the solute and solvent.

(iii) Mass per dm³ \(= 10.2 \times \dfrac{1000}{15.4} = 662.3\ \text{g dm}^{-3}\).
Solubility \(= \dfrac{662.3}{331} = \mathbf{2.0\ mol\ dm^{-3}}\).

(c)(i) I. P = Fe²⁺ (iron(II)), Q = Fe³⁺ (iron(III)).
II. \(4Fe^{2+} + 4H^+ + O_2 \rightarrow 4Fe^{3+} + 2H_2O\)

(ii) Test for purity of water: pure water boils at exactly 100 °C (and freezes at 0 °C) at 1 atmosphere; impurities raise the boiling point.

(iii) I. Boiling temporary hard water removes the hardness (it decomposes the hydrogentrioxocarbonate(IV) to insoluble trioxocarbonate(IV), softening the water). II. Adding sodium trioxocarbonate(IV) crystals to permanent hard water softens it by precipitating the Ca²⁺/Mg²⁺ ions as insoluble trioxocarbonate(IV).

(iv) \(Ca(HCO_3)_2 \xrightarrow{heat} CaCO_3 + H_2O + CO_2\)

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(a) Esterification is the reaction between a carboxylic (alkanoic) acid and an alcohol (alkanol), in the presence of a little concentrated H₂SO₄, to form an ester (alkanoate) and water.

(b) Two properties of plastic: it is light (low density) and a poor conductor of heat and electricity (also resistant to corrosion, mouldable, non-biodegradable).

(c) Duralumin contains aluminium, copper, magnesium and manganese.

(d)(i) A raw material is a naturally occurring substance from which useful products are manufactured.
(ii) A primary product is the product obtained directly (first) from processing a raw material.

(e) Charles' law: the volume of a fixed mass of gas is directly proportional to its absolute (Kelvin) temperature at constant pressure. \(\dfrac{V}{T} = \text{constant}\).

(f) Four protective items: safety goggles; laboratory coat (overall); hand gloves; nose/face mask (also fume cupboard, safety boots).

(g) Two uses of ammonia: manufacture of fertilizers; manufacture of trioxonitrate(V) acid (also as a refrigerant).

(h)(i) The process is cracking.
(ii) Ethanol + sodium gives sodium ethoxide and hydrogen gas. \(2C_2H_5OH + 2Na \rightarrow 2C_2H_5ONa + H_2\)

(i) Sulphur in H₂SO₄: \(2(+1) + S + 4(-2) = 0 \Rightarrow S = +6\).

(j)(i) NaClO₃: sodium trioxochlorate(V).
(ii) CuSO₄·5H₂O: copper(II) tetraoxosulphate(VI) pentahydrate.

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Identify each element from its atomic number and mass number.

Element	Atomic number	Mass number	Identity
J	9	19	Fluorine, F
Q	13	27	Aluminium, Al
R	16	32	Sulfur, S
X	19	39	Potassium, K
Y	24	52	Chromium, Cr

(a)(i)

• I. Halogen: J (fluorine), a Group 7 element.
• II. Most likely attracted by a magnet: Y (chromium), a transition metal.
• III. Belongs to Group 1: X (potassium), configuration 2,8,8,1.
• IV. Readily forms an ion with a double negative charge: R (sulfur), which gains 2 electrons to form S^2-.

(a)(ii) J is a Group 7 non-metal and X is a Group 1 metal, so they form an ionic (electrovalent) bond (potassium transfers one electron to fluorine, giving K⁺ and F^-).

(a)(iii) Neutrons in Q:

\[\text{neutrons} = \text{mass number} - \text{atomic number} = 27 - 13 = \mathbf{14}\]

(a)(iv) R (S^2-) combines with X (K⁺). Balancing charges gives K₂S.

(a)(v) The element that exists as a diatomic molecule is J (fluorine, F₂).

(a)(vi) The d-block element is Y (chromium).

(b)(i) An atomic orbital is a region in space around the nucleus of an atom where the probability of finding an electron is highest.

(b)(ii) I. Three postulates of Dalton's atomic theory:

1. All matter is made up of tiny, indivisible particles called atoms.
2. Atoms of the same element are identical in mass and chemical properties, while atoms of different elements differ.
3. Atoms combine in simple whole-number ratios to form compounds, and atoms are neither created nor destroyed in a chemical reaction.

II. Two limitations of the theory:

1. The atom is not indivisible; it is made up of smaller particles (protons, neutrons and electrons).
2. Atoms of the same element are not always identical in mass, because of the existence of isotopes.

(b)(iii) In the solid state, sodium chloride has a giant ionic lattice. Na⁺ and Cl^- ions are arranged alternately in a face-centred cubic structure, each Na⁺ surrounded by 6 Cl^- ions and each Cl^- surrounded by 6 Na⁺ ions (6:6 coordination), held together by strong electrostatic forces of attraction.

(c) Carbon burnt at 0.50 g per second for 30 minutes.

Total mass of carbon burnt:

\[m = 0.50\ \text{g s}^{-1} \times (30 \times 60)\ \text{s} = 0.50 \times 1800 = 900\ \text{g}\]

Moles of carbon:

\[n_C = \frac{900}{12.0} = 75\ \text{mol}\]

(i) Balanced equation:

\[\text{C}(s) + \text{O}_2(g) \rightarrow \text{CO}_2(g)\]

(ii) I. From the equation, 1 mol C gives 1 mol CO₂, so 75 mol C gives 75 mol CO₂.

\[V_{CO_2} = 75\ \text{mol} \times 22.4\ \text{dm}^3\,\text{mol}^{-1} = \mathbf{1680\ dm^3}\]

II. From the equation, 1 mol C uses 1 mol O₂, so:

\[n_{O_2} = 75\ \text{mol} \times \frac{1}{1} = \mathbf{75\ mol}\]

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