WAEC Chemistry 2019 Theory Past Questions

Model answer

There are two types of cracking: • Thermal cracking: large hydrocarbons are broken into smaller ones using a high temperature of about 450–750°C and a pressure of about 70 atmospheres. • Catalytic cracking: the hydrocarbon is brought into contact with a catalyst (commonly a zeolite) at about 500°C and a moderately low pressure.

Model answer

(i) To improve the appearance of the metal object. (ii) To protect the metal from corrosion.

Model answer

(i) Calcium oxide is alkaline (basic) while hydrogen chloride gas is acidic, so the two would react together instead of the CaO simply drying the gas. (A drying agent must not react chemically with the gas it is drying.) (ii) Concentrated tetraoxosulphate(VI) acid (concentrated H2SO4) is normally used to dry hydrogen chloride gas.

Model answer

The yellow colour obtained results from the oxidation of iron(II) ions (pale green, Fe²⁺) to iron(III) ions (yellow, Fe³⁺) by the concentrated HNO3. The brown gas evolved is nitrogen(IV) oxide (NO2), formed from the nitrogen(II) oxide produced in the reaction, upon exposure to air: 2NO + O2 → 2NO2.

Model answer

(i) I. ZnO + H2SO4 → ZnSO4 + H2O II. ZnO + 2NaOH + H2O → Na2Zn(OH)4 (ii) These equations demonstrate the amphoteric nature of ZnO — a substance is amphoteric if it can react with both an acid and a base.

Model answer

(i) Similarity: Both 35Cl and 37Cl have the same number of protons (17), i.e. the same atomic number. (ii) Difference: 35Cl contains 18 neutrons, while 37Cl contains 20 neutrons (they have different mass numbers/number of neutrons).

Model answer

When chlorine water is exposed to sunlight, it decomposes to give hydrochloric acid (HCl) and oxygen (O2): Cl2 + H2O → HCl + HOCl, then HOCl →(sunlight)→ HCl + ½O2. The two products are HCl and O2.

Model answer

(i) H2S undergoes oxidation — the oxidation number of sulphur increases from –2 in H2S to 0 in elemental sulphur, S. (ii) SO2 is the oxidizing agent — it undergoes reduction (its sulphur is reduced), and a species that is itself reduced acts as the oxidizing agent.

Model answer

The carbon-12 scale is the scale on which the relative atomic masses of different atoms are expressed. On this scale, the relative atomic mass of an element is defined as the number of times one atom of the element is heavier than one-twelfth (1/12) the mass of one atom of carbon-12.

Model answer

(i) A chemical equilibrium is dynamic in nature — both the forward and reverse reactions continue to occur, but at equal rates. (ii) The concentrations of all chemical species present in the system remain constant as long as the reaction conditions (temperature, pressure, etc.) are unchanged.

Model answer

(i) C10H22 → C6H14 + P. By balancing atoms, the molecular formula of P is C4H8 (but-1-ene or but-2-ene, an alkene). (ii) Two isomers of P (C4H8): • But-1-ene: CH2=CH–CH2–CH3 • But-2-ene: CH3–CH=CH–CH3 (iii) P can be polymerized because it is unsaturated — it contains a C=C double bond which can open up to link with other identical monomer units to form a polymer.

Model answer

• Aufbau principle: electrons fill orbitals starting from the lowest energy level before occupying higher-energy orbitals. • Pauli exclusion principle: no two electrons in an atom can have the same set of all four quantum numbers (i.e. an orbital can hold a maximum of 2 electrons with opposite spins). • Hund's rule of maximum multiplicity: when filling degenerate orbitals (of the same energy), electrons occupy each orbital singly before any orbital receives a second electron (pairing).

Model answer

(i) Intra-molecular forces (e.g. covalent/ionic bonds) hold atoms together within a molecule/compound, and are generally much stronger than inter-molecular forces, which hold separate molecules together in a liquid or solid and determine bulk properties such as melting and boiling points; they are electrostatic in nature. NaH is ionic (held by strong electrostatic ionic bonds); H2 and H2S are held together in the condensed state by weak van der Waals forces (H2S also has some dipole-dipole attraction); NH4Cl is ionic, held by strong electrostatic forces between NH4+ and Cl– ions. (ii) • NaH: reacts vigorously with water; an alkaline solution (NaOH) is formed and hydrogen gas is liberated. • H2: no reaction occurs when hydrogen gas is passed into water. • H2S: dissolves to give a weakly acidic solution which turns blue litmus red. • NH4Cl: dissolves appreciably in water; the container feels cold to the touch (the dissolution is endothermic). (iii) NaH + H2O → NaOH + H2 NH4Cl + H2O ⇌ H3O+ + NH3 + Cl– (or NH4Cl + H2O → NH4OH + HCl)

Model answer

• Limestone (CaCO3): its function is to decompose on heating to produce calcium oxide (used to regenerate ammonia from NH4Cl) and carbon(IV) oxide (used in forming the sodium hydrogentrioxocarbonate(IV)): CaCO3 →(Δ)→ CaO + CO2 • Ammonia (NH3): it combines with water and carbon(IV) oxide to form ammonium hydrogentrioxocarbonate(IV), which then reacts with brine to precipitate sodium hydrogentrioxocarbonate(IV): NH3 + H2O + CO2 → NH4HCO3

Model answer

C6H12O6 →(zymase/yeast)→ 2C2H5OH + 2CO2

Model answer

As the glucose inside the palm-wine ferments, carbon(IV) oxide gas is continuously released. Since the bottle is tightly corked and filled to the brim, the gas cannot escape, so it exerts increasing pressure inside the bottle. As fermentation progresses, the pressure eventually becomes too great for the glass to withstand, and the bottle shatters.

Model answer

(i) Increasing reactivity: Cu < Fe < Na < K (ii) Only sodium (Na) and potassium (K) react with cold water. (iii) Copper (Cu) will form coloured salts, since it is a transition element.

Model answer

(i) A redox reaction is one in which a chemical species undergoes reduction while another species simultaneously undergoes oxidation (i.e. electron transfer occurs between reactants). (ii)/(iii): I. 2Na+Cl2→2NaCl — REDOX (Na is oxidized: 2Na→2Na⁺+2e–; Cl2 is reduced: Cl2+2e–→2Cl–) II. AgCl+2NH3→[Ag(NH3)2]Cl — NOT redox (it is a complex-formation/ligand-substitution reaction; oxidation states of all atoms remain unchanged throughout) III. C2H2+H2→C2H4 — REDOX (an addition/hydrogenation reaction in which C2H2 is reduced by the addition of hydrogen while H2 is oxidized) IV. 2FeCl3+2KI→2FeCl2+2KCl+I2 — REDOX (Fe is reduced from +3 to +2: 2Fe3++2e–→2Fe2+; I is oxidized from –1 to 0: 2I–→I2+2e–) (iv) Cl2+2e–→2Cl– (reduction) and 2Na→2Na⁺+2e– (oxidation); or 2Fe3++2e–→2Fe2+ (reduction) and 2I–→I2+2e– (oxidation).

Model answer

(i) X is oxygen while Y is sulphur. (ii) I. S(s)+O2(g)→SO2(g) II. 2SO2(g)+O2(g)⇌2SO3(g) III. SO3(g)+H2SO4(aq)→H2S2O7(l) (oleum) IV. H2S2O7(l)+H2O(l)→2H2SO4(aq) (iii) The catalyst used in process II is vanadium(V) oxide, V2O5. (iv) The forward reaction (II) is exothermic, so by Le Chatelier's principle, increasing the temperature would only favour the backward (endothermic) reaction, reducing the yield of SO3. (v) SO2 is used in the manufacture of H2SO4, and for bleaching wood pulp/wool/silk (also used for destroying bacteria and fungi/as a preservative).

Model answer

Mole ratio of H2:O2 in the equation is 2:1, so 40 cm³ of H2 requires only 20 cm³ of O2 to react completely. Volume of unused (excess) oxygen = 30 − 20 = 10 cm³.

Model answer

(i) CaCO3(s) →(Δ)→ CaO(s) + CO2(g) (ii) Moles of CaCO3 = 1.0/100 = 0.01 mol. Since mole ratio CaCO3:CaO = 1:1, moles of CaO (residue) = 0.01 mol. Molar mass CaO = 56 g/mol, so mass of residue = 0.01×56 = 0.56 g. (iii) Since mole ratio CaCO3:CO2 = 1:1, moles of CO2 = 0.01 mol. Volume at s.t.p. = moles × 22.4 dm³ = 0.01×22.4 = 0.224 dm³. (iv) Using Charles'/combined gas law with P constant: V1/T1 = V2/T2, where V1=0.224 dm³, T1=273K, T2=(15+273)=288K. V2 = V1×T2/T1 = 0.224×288/273 = 0.236 dm³.

Model answer

(i) Dry chlorine gas is prepared in the laboratory by heating concentrated hydrochloric acid with manganese(IV) oxide (MnO2) in a flask; the gas produced is passed through concentrated H2SO4 (to dry it) and collected by downward delivery (chlorine is denser than air), as shown in the labelled diagram. (ii) Chlorine is used for water purification/disinfection, and for the manufacture of important organic solvents such as tetrachloromethane and trichloromethane (it is also used in bleaching powder manufacture and aerosol propellants).

Model answer

To prepare hydrogen from water gas, the water gas (a mixture of CO and H2) is mixed with excess steam and passed over an iron oxide catalyst at 400–500°C. This produces more hydrogen while the carbon(II) oxide in the water gas is oxidized to carbon(IV) oxide: CO + H2 + H2O →(catalyst)→ CO2 + 2H2 The carbon(IV) oxide formed is removed by dissolving it in water under pressure (about 30 atmospheres); any unreacted CO is absorbed in an ammoniacal solution of copper(I) methanoate, leaving pure hydrogen gas.

Model answer

(i) The chief ore of aluminium is bauxite. (ii) The ore is purified because it contains impurities such as iron(III) oxide and trioxosilicates, which must be removed before electrolysis. (iii) In the electrolysis of purified bauxite, the anode is made of graphite (carbon) while the cathode is iron. (iv) Cryolite is added because it lowers the melting point of bauxite (alumina), reducing the energy cost of electrolysis; it also serves as a solvent for the alumina.

Model answer

The products obtained directly from the destructive distillation of coal are: coal gas, coal tar, ammoniacal liquor and coke (any three of these).

Model answer

Sample titration results: Rough=21.20, I=17.60, II=18.40, III=16.50 (final readings); average volume of A used (from concordant titres I–III) = (16.60+16.40+16.50)/3 = 16.50 cm³. (b)(i) Molar mass Na2S2O3 = (23×2)+(32×2)+(16×3)=158 g/mol. Concentration in gdm⁻³ of Na2S2O3 = 15.8 gdm⁻³, so Concentration in moldm⁻³ = 15.8/158 = 0.1 moldm⁻³. (ii) Using CaVa/Na = CbVb/Nb with Ca=0.1 moldm⁻³, Va=16.50 cm³, Na=2 (S2O3²⁻), Vb=25 cm³, Nb=1 (I2): Cb = (Ca×Va×Nb)/(Vb×Na) = (0.1×16.50×1)/(25×2) = 0.033 moldm⁻³. (iii) Molar mass I2 = 254. Concentration of I2 in gdm⁻³ = 0.033×254 = 8.38 gdm⁻³ (mass of I2 in the sample). Given mass of impure I2 sample = 9.0 gdm⁻³. Percentage by mass of I2 = (8.38/9.0)×100 = 93.1%. (c) The starch indicator is added only near the end-point because if added at the beginning, the iodine is strongly adsorbed onto the starch, making it difficult to desorb, which reduces the accuracy/sharpness of the final colour-change reading.

Model answer

(a) Part of the 10cm³ of distilled water sample dissolves the soluble part of C, leaving an insoluble residue; on shaking thoroughly and filtering, a colourless filtrate is obtained and an insoluble residue is retained — C is a mixture of a soluble and an insoluble part. (b)(i) A white precipitate is formed with the filtrate and AgNO3, which is insoluble in dilute HNO3 — this indicates the presence of chloride (Cl⁻) ions. (ii) On adding excess NH3, the white precipitate dissolves in excess NH3(aq), consistent with AgCl formation and its dissolution to form a soluble complex. (c)(i) The residue partly dissolves in dilute HCl, giving a colourless (and odourless) gas with rapid effervescence, which turns lime water milky (identified as CO2, i.e. carbonate/CO3²⁻ present) — indicating the residue contains a carbonate. (ii) A blue gelatinous precipitate is formed with NH3 in drops, which dissolves in excess NH3(aq) to form a deep blue solution — indicating the presence of Cu²⁺ ions.

Model answer

(a)(i) A white precipitate (BaCO3) is formed with BaCl2; on adding excess dilute HCl, the white precipitate dissolves with the evolution of a colourless, odourless gas that turns lime water milky (CO2), i.e. Ba2++CO3²⁻→BaCO3, then BaCO3+2HCl→BaCl2+H2O+CO2. (ii) Q is likely sulphur(IV) oxide, SO2 (a reducing gas that decolorizes acidified KMnO4 by reducing it). (b)(i) Concentrated tetraoxosulphate(VI) acid is normally used to dry hydrogen chloride gas; ammonia gas itself is dried using calcium oxide (quicklime), since it is basic and would react with acidic drying agents. (ii) Carbon(IV) oxide is dried using concentrated H2SO4. (c) Sodium hydroxide pellets are deliquescent — they absorb moisture from the atmosphere, so their mass keeps changing/varying, meaning an accurate, fixed concentration (a true standard solution) cannot be obtained from them.