WAEC SSCE - Chemistry - 2021 (Essay)

Bayanin Amsa

(a)(i) Equation for the neutralization

\[H_2SO_{4(aq)} + 2KOH_{(aq)} \to K_2SO_{4(aq)} + 2H_2O_{(l)}\]

(a)(ii) Concentration of the acid

Moles of KOH \(= \dfrac{24.0}{1000} \times 0.150 = 3.6 \times 10^{-3}\,mol\).

From the equation, 2 mol KOH react with 1 mol \(H_2SO_4\), so moles of acid \(= \dfrac{3.6 \times 10^{-3}}{2} = 1.8 \times 10^{-3}\,mol\).

\[[H_2SO_4] = \frac{1.8 \times 10^{-3}}{25.0/1000} = 0.072\,mol\,dm^{-3}\]

(b) Magnesium burning in carbon(IV) oxide

Equation:

\[2Mg_{(s)} + CO_{2(g)} \to 2MgO_{(s)} + C_{(s)}\]

Explanation: magnesium is a very reactive metal and a powerful reducing agent. The heat of the burning ribbon is enough to decompose carbon(IV) oxide; magnesium removes the oxygen from the \(CO_2\), reducing it to carbon (seen as black specks) while itself being oxidized to white magnesium oxide. So the ribbon continues to burn even though \(CO_2\) does not normally support combustion.

Percentage mass of nitrogen in magnesium trioxonitrate(V), \(Mg(NO_3)_2\):

\[M = 24 + 2(14 + 48) = 148\]\[\%N = \frac{2 \times 14}{148} \times 100 = 18.92\%\]

(c) The compound \(CH_3CH_2CH=CHCOOH\)

This compound has both a carbon-carbon double bond (unsaturation) and a carboxylic acid (\(-COOH\)) group.

• (i) Reaction 1: add bromine water. Reaction 2: add sodium trioxocarbonate(IV) (\(Na_2CO_3\)) solution.
• (ii) With bromine water: the red-brown bromine water is decolourized (turns colourless), confirming the \(C=C\) double bond. With \(Na_2CO_3\): brisk effervescence occurs, giving off a colourless gas (\(CO_2\)) that turns limewater milky, confirming the acidic \(-COOH\) group.

(d) Manufacture of soap

Vegetable oil is boiled with concentrated sodium hydroxide (caustic soda) solution. The oil is hydrolyzed (saponified) to give soap (the sodium salt of the fatty acid) and glycerol. Common salt (sodium chloride) is then added to the mixture to salt out (precipitate) the soap, which floats to the top; it is filtered off, pressed and moulded.

(e) Electronegativity

Electronegativity is the relative tendency (power) of an atom to attract the shared pair of electrons in a covalent bond towards itself.

Bayanin Amsa

(a)(i) Why aluminium is not affected by air

When exposed to air, aluminium quickly forms a thin, tough, coherent and impervious layer of aluminium oxide on its surface. This oxide film sticks firmly to the metal and prevents air and moisture from reaching the metal beneath, so the metal is protected from further attack.

\[4Al_{(s)} + 3O_{2(g)} \to 2Al_2O_{3(s)}\]

(a)(ii) Extraction of aluminium from bauxite

• Substance used for purifying the ore: sodium hydroxide solution (used to dissolve the alumina away from insoluble impurities).
• Composition of the mixture electrolyzed: pure aluminium oxide (alumina, \(Al_2O_3\)) dissolved in molten cryolite (\(Na_3AlF_6\)).

(b) ZnO as an amphoteric oxide

Reacting as a base (with acid):

\[ZnO_{(s)} + 2HCl_{(aq)} \to ZnCl_{2(aq)} + H_2O_{(l)}\]

Reacting as an acid (with alkali):

\[ZnO_{(s)} + 2NaOH_{(aq)} \to Na_2ZnO_{2(aq)} + H_2O_{(l)}\]

(c)(i) Three uses of sodium trioxocarbonate(IV)

Manufacture of glass; manufacture of soap and detergents; softening of hard water (also used in paper manufacture).

(c)(ii) Why trioxonitrate(V) acid turns yellow on storage

On standing, especially in light and warmth, the acid partly decomposes, releasing brown nitrogen(IV) oxide which dissolves back in the acid and gives it a yellow colour:

\[4HNO_{3} \to 4NO_{2} + 2H_2O + O_{2}\]

(c)(iii) Test for trioxonitrate(V) ions

Brown-ring test: to the solution add freshly prepared iron(II) tetraoxosulphate(VI) solution, then carefully pour concentrated tetraoxosulphate(VI) acid down the side of the tilted test tube so it forms a layer below. A brown ring forms at the junction of the two liquids, confirming the presence of nitrate ions.

(d) Preparation of hydrogen chloride

\[\text{(i) } NaCl_{(s)} + H_2SO_{4(l)} \to NaHSO_{4(s)} + HCl_{(g)}\]\[\text{(ii) } H_{2(g)} + Cl_{2(g)} \to 2HCl_{(g)}\]

(iii) One use of hydrogen chloride: it is used to prepare hydrochloric acid and metal chlorides, and industrially in the manufacture of chlorine and in cleaning (pickling) metals.

Bayanin Amsa

(a) Molecular formula and structural formula

A molecular formula shows the actual number of atoms of each element present in one molecule of a compound (for example ethanol \( C_2H_6O \)). A structural formula shows how those atoms are arranged and joined together by bonds (for example \( CH_3CH_2OH \)).

(b) Three factors that determine ionization energy

• The atomic radius (distance of the outermost electron from the nucleus).
• The nuclear charge (number of protons).
• The shielding (screening) effect of the inner electrons.

(c) Two conditions for chemical equilibrium

• The reaction must be reversible.
• The system must be closed (no reactant or product enters or leaves) and kept at constant temperature.

(d) Ionization energies: A = 619, B = 518, C = 594 \( \text{kJ mol}^{-1} \).

Element	A	B	C
Ionization energy / kJ mol\(^{-1}\)	619	518	594

(i) The strongest reducing agent is B.

(ii) B has the lowest ionization energy, so it loses (donates) its outermost electron most easily; a good reducing agent is one that readily gives up electrons.

(e) Graham's law of diffusion

At constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density (or relative molecular mass): \( \text{rate} \propto \dfrac{1}{\sqrt{M}} \).

(f) (i) Readily soluble in water: \( Mg(NO_3)_2 \) and \( Na_2SO_4 \) (all nitrates and all sodium salts are soluble). (ii) Insoluble in water: \( CaCO_3 \).

(g) (i) Protein: condensation polymer. (ii) Perspex: addition polymer. (iii) Nylon: condensation polymer.

(h) Atomic radius

Atomic radius is half the distance between the nuclei of two identical atoms joined by a single covalent bond (half the internuclear distance).

(i) Ethanol versus propane

Ethanol molecules contain an \( -OH \) group and form hydrogen bonds between neighbouring molecules. These hydrogen bonds are much stronger than the weak van der Waals (dispersion) forces between propane molecules, so more energy is needed to separate ethanol molecules, giving it the higher boiling point even though the molar masses are comparable.

(j) Three significances of pH in everyday life

• Control of soil pH so that crops grow well (agriculture).
• Maintaining the correct pH of blood and other body fluids for good health (medicine).
• Quality control in the manufacture of foods, drinks, drugs and cosmetics, and in the treatment of drinking water.

Bayanin Amsa

(a)(i) Structures

• Graphite: a giant covalent structure made of flat layers of carbon atoms. Within each layer every carbon is covalently bonded to three others in a hexagonal (honeycomb) arrangement, leaving one delocalized electron per atom. The layers are held together only by weak van der Waals forces.
• Diamond: a giant covalent structure in which every carbon atom is covalently bonded tetrahedrally to four other carbon atoms, forming a rigid three-dimensional network.

(a)(iii) Hardness and conductivity

Diamond is hard because its carbon atoms are held in a rigid three-dimensional network of strong covalent bonds, and it is a non-conductor because all four valence electrons of each carbon are used in bonding, leaving no free electrons to carry charge. Graphite is soft because its layers are held by weak forces and can slide over one another, and it conducts electricity because each carbon uses only three electrons in bonding, leaving one delocalized electron per atom free to move and carry charge.

(b)(i) Water purification stages

• Aeration: water is sprayed into air to add dissolved oxygen and drive off dissolved gases and unpleasant odours; it also oxidizes dissolved iron and manganese.
• Screening: large floating and suspended solids (leaves, sticks, debris) are removed by passing water through screens/gratings.
• Sedimentation: the water is left to stand (often with a coagulant such as alum) so that fine suspended particles settle out to the bottom.

(b)(ii) Substances causing hardness

Dissolved calcium and magnesium salts, that is, the hydrogencarbonates and sulphates of calcium and magnesium (\(Ca^{2+}\) and \(Mg^{2+}\) ions).

(b)(iii) Methods of removing hardness

Boiling (removes temporary hardness), adding washing soda (\(Na_2CO_3\)), ion exchange, or distillation.

(b)(iv) One disadvantage of hard water

It wastes soap by forming an insoluble scum, and it deposits scale (fur) in kettles, boilers and pipes.

(c)(i) Extraction of tin

Tin is obtained from its ore cassiterite, \(SnO_2\). The ore is concentrated by washing and froth flotation to remove earthy impurities, then roasted in air to remove sulphur and arsenic. The concentrated oxide is then reduced with carbon (coke) in a reverberatory or blast furnace, and the molten tin is run off and refined.

(c)(ii) Equation for the reduction

\[SnO_{2(s)} + 2C_{(s)} \to Sn_{(l)} + 2CO_{(g)}\]

(c)(iii) Reactions of tin

\[\text{With oxygen: } Sn_{(s)} + O_{2(g)} \to SnO_{2(s)}\]\[\text{With chlorine: } Sn_{(s)} + 2Cl_{2(g)} \to SnCl_{4(l)}\]

Bayanin Amsa