WAEC SSCE - Chemistry - 2012 (Essay)

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(a)(i) I. Biotechnology: the use of living organisms (or their components, such as enzymes or micro-organisms) to make useful products or to carry out useful processes for man. II. Biogas: a mixture of gases (mainly methane and carbon(IV) oxide) produced by the anaerobic bacterial decomposition of organic matter, used as a fuel.

(a)(ii) Applications of biotechnology: production of alcoholic drinks/ethanol by fermentation; production of antibiotics and drugs; production of biogas; making of bread, yoghurt and cheese (any two).

(b)(i) Ethanol from sugar cane juice: the sugar cane juice, which contains sucrose, is diluted and yeast is added. The enzyme invertase (sucrase) in yeast hydrolyses sucrose to glucose and fructose, then zymase ferments these sugars to ethanol at about 30-40 °C in the absence of air:

\[ \text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{zymase}} 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2 \]

The ethanol is then concentrated by fractional distillation.

(b)(ii) By-product: carbon(IV) oxide (CO₂).

(b)(iii) Uses of the by-product: making fizzy (carbonated) drinks; used as a fire extinguisher; solid CO₂ (dry ice) as a refrigerant.

(b)(iv) Ethanol from cane sugar is renewable because sugar cane is a crop that can be replanted and regrown within a short time, so the source is continuously replaceable. Ethanol from petroleum is non-renewable because petroleum is a fossil fuel formed over millions of years and cannot be replaced once used up.

(c) Heavy vs fine chemicals: heavy chemicals are produced in very large quantities, are relatively cheap and impure, e.g. tetraoxosulphate(VI) acid (H₂SO₄) or sodium trioxocarbonate(IV). Fine chemicals are produced in small quantities, are of high purity and are expensive, e.g. drugs/pharmaceuticals or dyes.

(d) Increasing order of deviation from ideal behaviour: O₂ < Cl₂ < HCl. Reason: deviation from ideal behaviour increases with the strength of intermolecular forces and with molecular size/polarity. O₂ is small and non-polar (least deviation); Cl₂ is larger and more polarisable; HCl is polar with strong dipole-dipole attractions, so it deviates most.

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(a)(i) Nuclear fission

The splitting of a heavy atomic nucleus into two or more lighter nuclei of comparable mass, accompanied by the release of neutrons and a very large amount of energy, usually when the nucleus is struck by a neutron.

(a)(ii) Alpha and beta particles emitted from \(^{238}_{92}\)U to \(^{230}_{90}\)Th

Change in mass number: \(238 - 230 = 8\). Only alpha emission changes the mass number (by 4 each):

\[ \text{number of }\alpha = \frac{8}{4} = 2 \]

Change in atomic number: 2 alphas alone would give \(92 - (2\times 2) = 88\), but the product has Z = 90. Each beta emission raises Z by 1:

\[ 88 + b = 90 \Rightarrow b = 2 \]

So 2 alpha particles and 2 beta particles were emitted.

(b) Equilibrium: A\(_2\)(g) + 3B\(_2\)(g) \(\rightleftharpoons\) 2AB\(_3\)(g); \(\Delta H = +x\) kJ mol\(^{-1}\) (endothermic)

• (i) Increase in concentration of B: equilibrium shifts to the right (forward) to use up the added B, giving more AB\(_3\).
• (ii) Decrease in pressure: the system shifts toward the side with the greater number of gas moles. The left has 4 moles and the right 2 moles, so equilibrium shifts to the left (backward), forming more A\(_2\) and B\(_2\).
• (iii) Addition of a catalyst: no effect on the equilibrium composition; it only speeds up the forward and backward reactions equally, so equilibrium is reached sooner.

(c) Trend in lattice energy of the sodium halides

Compound	NaF	NaBr	NaI
Lattice energy/kJ mol\(^{-1}\)	890	719	670

From F\(^-\) to Br\(^-\) to I\(^-\) the ionic radius of the halide increases, so the distance between the Na\(^+\) and the halide ion grows. Lattice energy is inversely related to this interionic distance, so the electrostatic attraction weakens and the lattice energy falls: NaF > NaBr > NaI.

(d) Property of nitrogen(IV) oxide

• (i) 4Cu + 2NO\(_2\) \(\rightarrow\) 4CuO + N\(_2\): nitrogen is reduced from +4 to 0, so NO\(_2\) acts as an oxidising agent.
• (ii) H\(_2\)O + 2NO\(_2\) \(\rightarrow\) HNO\(_3\) + HNO\(_2\): nitrogen goes to +5 (in HNO\(_3\)) and +3 (in HNO\(_2\)), so NO\(_2\) undergoes disproportionation (it is simultaneously oxidised and reduced).

(e) Reactions in the blast furnace

• (i) Top (cooler zone) - reduction of the ore by carbon(II) oxide: \[ \text{Fe}_2\text{O}_3 + 3\text{CO} \rightarrow 2\text{Fe} + 3\text{CO}_2 \]
• (ii) Middle - decomposition of limestone and formation of carbon(II) oxide: \[ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \qquad \text{CO}_2 + \text{C} \rightarrow 2\text{CO} \]
• (iii) Bottom (hottest zone, at the tuyeres) - combustion of coke: \[ \text{C} + \text{O}_2 \rightarrow \text{CO}_2 \]

(Formation of slag, CaO + SiO\(_2\) \(\rightarrow\) CaSiO\(_3\), also occurs in the lower middle region.)

(f)(i) Two products of the destructive distillation of coal: coke and coal tar (coal gas and ammoniacal liquor are also produced).

(f)(ii) One use of each:

• Coke: used as a fuel and as the reducing agent in the extraction of iron in the blast furnace.
• Coal tar: used as a raw material for making dyes, drugs and antiseptics (and for surfacing roads).

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(a)(i) Rutherford's atomic structure: the atom has a very small, dense, positively charged nucleus at the centre where nearly all the mass is concentrated, with the negatively charged electrons moving in the mostly empty space around the nucleus.

(a)(ii) Atomic number vs mass number: the atomic number is the number of protons in the nucleus of an atom; the mass number is the total number of protons and neutrons in the nucleus.

(a)(iii) Chlorine exists as two isotopes, ³⁵Cl and ³⁷Cl, in the fixed ratio 3:1. The relative atomic mass is the weighted average of these isotopic masses, \(\frac{(35\times3)+(37\times1)}{4}=35.5\), which is not a whole number.

(b)(i) First ionization energy: the minimum energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous singly charged positive ions.

(b)(ii) Three properties of electrovalent compounds:

• They have high melting and boiling points.
• They are usually soluble in water but insoluble in organic solvents.
• They conduct electricity when molten or in aqueous solution.

(b)(iii) Each pair has the same number of electrons in the outermost shell (F and Cl each have 7; Mg and Ca each have 2). Since chemical properties depend on the number of valence electrons, members of the same group have similar chemical properties.

(c)(i) Homologous series: a family of organic compounds with the same general formula in which successive members differ by a CH₂ unit and show a gradual change in physical properties, e.g. the alkanes (CH₄, C₂H₆, C₃H₈...).

(c)(ii) Heterolytic fission: the breaking of a covalent bond in which both shared electrons go to one of the atoms, producing oppositely charged ions, e.g. \(\text{HCl} \to \text{H}^+ + \text{Cl}^-\).

(d) Indicators:

• (i) HCl vs NaOH (strong acid/strong base): methyl orange or phenolphthalein.
• (ii) HCl vs NH₄OH (strong acid/weak base): methyl orange.
• (iii) Ethanoic acid vs NaOH (weak acid/strong base): phenolphthalein.

(e) E = ammonium chloride (NH₄Cl, a solid chloride that sublimes); F = an alkali such as sodium hydroxide (NaOH); G = ammonia (NH₃), the choking gas that turns moist red litmus blue.

\[ \text{NH}_4\text{Cl} + \text{NaOH} \to \text{NaCl} + \text{H}_2\text{O} + \text{NH}_3 \]

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