Questions
SECTION B: ESSAY QUESTIONS
Answer FOUR questions; ONE question from each course.
CHM 001: GENERAL CHEMISTRY
1. (a) What is a Limiting reagent? [1 mark]
(b) Hydrofluoric acid finds use in etching of glass by reacting with the trioxosilicate in glass according to the following stoichiometry:
CaSiO₃ + 6HF → CaF₂ + SiF₄ + 3H₂O
If 25.0g of hydrofluoric acid reacts with 50.0 g of the trioxosilicate to produce calcium fluoride (CaF₂),
(i) Identify the limiting reagent.
(ii) Calculate the theoretical yield of Calcium fluoride.
(Ca=40, Si=28, O=16, F=19, H=1) [4 marks]
(c) (i) State Hund's rule of maximum multiplicity
(ii) Write the electronic configuration of Cr³⁺ and Zn [3 marks]
(d) (i) Define the term electronegativity
(ii) Arrange C, F, H, N, O in order of increasing electronegativity [2 marks]
2. (a) (i) Define a standard solution? [1 mark]
(ii) A fluoride of Tungsten, WFx, contains 38.27% of Fluorine. What is the oxidation state of Tungsten in this compound? (W = 183.84; F = 19.00) [2 marks]
(b) Carbon burns completely in oxygen according to the equation:
C(s) + O₂(g) → CO₂(g)
Determine the volume of CO₂ that could be formed when 0.500 g of carbon is burnt under conditions of standard temperature and pressure? (Molar volume of a gas at stp = 22.4 dm³, C = 12.00) [2 marks]
(c) Write the balanced chemical equations for the following redox reactions:
(i) Fe(OH)₂(s) + Pb(OH)₃⁻(aq) → Fe(OH)₃(s) + Pb(s) [basic medium]
(ii) ClO₃⁻(aq) + Br⁻(aq) → Br₂(s) + Cl⁻(aq) [acidic medium] [3 marks]
(d) A sample of brass containing 70% of copper was analyzed by a chemist for five times and the results were 70.10%, 69.99%, 70.20%, 70.00% and 69.98%. Use the analyst data to determine:
(i) Absolute error
(ii) Percentage relative error [2 marks]
CHM 002: PHYSICAL CHEMISTRY
3. (a) Given the following half-cell reactions:
Cr³⁺(aq) + 3e⁻ → Cr(s) E° = −0.74 V
NO₃⁻(aq) + 4H⁺(aq) + 3e⁻ → NO(g) + 2H₂O(l) E° = +0.96 V
(i) Write the balanced chemical equation for the cell reaction
(ii) Determine the equilibrium constant at 300 K for the cell
(R = 8.314 J/mol.K, F = 96500 C) [4 marks]
(b) The reaction quotient Q for the reaction:
2NOCl(g) ⇌ 2NO(g) + Cl₂(g) is 0.034 at 27°C.
Given that 5 g of NOCl(g) gave 2.29 g of NO(g) and 2.70 g of Cl₂(g) in a 1.0 L flask at equilibrium, what direction will the system proceed to reach equilibrium?
(N = 14, O = 16, Cl = 35.5) [4 marks]
(c) State any two applications of radioactive nuclides [2 marks]
4. (a) Identify the acid, base, conjugate acid and conjugate base in the following aqueous reaction:
Al(H₂O)₆³⁺ + H₂O ⇌ H₃O⁺ + Al(H₂O)₅(OH)²⁺ [2 marks]
(b) Use the following data for three aqueous solutions of CaCl₂ to calculate the average value of the van't Hoff factor. (Kf = 1.86 °C/m)
| Solutions | Molality (m) | Freezing Point Depression (°C) |
|---|---|---|
| 1 | 0.0225 | 0.110 |
| 2 | 0.0910 | 0.440 |
| 3 | 0.278 | 1.330 |
[4 marks]
(c) (i) Predict the spontaneity of chemical reactions from a standard Gibb's free energy equation.
(ii) A 1.0 L gas jar contains 0.050 mole of oxygen gas and 0.015 mole of Nitrogen gas at 30 °C and 1 atm. Calculate the partial pressure of each gas in mmHg? [4 marks]
CHM 003: INORGANIC CHEMISTRY
5. (a) (i) State the Fajan's rule of bond types
(ii) Name ONE oxide anion for each of the following transition metals: Vanadium, Chromium, and Manganese that occurs without the M²⁺ cation state
(iii) Mention THREE catalytic applications of transition metals in industries [4 marks]
(b) Use pertinent equations ONLY to describe the role of tetraoxosulphate (VI) acid as
(i) an oxidizing agent
(ii) a dehydrating agent
(iii) a sulphonating agent [3 marks]
(c) (i) Outline the processes involved in the manufacture of sodium trioxocarbonate (IV) by the Solvay process.
(ii) Compare transition elements with Group II elements. [3 marks]
6. (a) Highlight FOUR ways in which beryllium resembles aluminium [2 marks]
(b) Explain the following observations:
(i) Ionization potential of group II elements decreases down the group
(ii) Beryllium is capable of forming covalent compounds
(iii) PbO is more stable than PbO₂, but SiO₂ is more stable than SiO [3 marks]
(c) (i) Give TWO reasons why alkali metals are often difficult to prepare by chemical reduction of their ores
(ii) Name of the following complexes:
i. [Cr(en)₃]Cl₃
ii. K₃[Fe(CN)₅NO]
iii. [Co(NH₃)₃Cl(NO₂)CN] [5 marks]
CHM 004: ORGANIC CHEMISTRY
7. (a) In the preparation of Ester (Alkylalkanoate), Ethanal can be used as the starting material according to the following steps:
CH₃CHO —Step 1→ CH₃COOH —Step 2→ CH₃COOCH₂CH₃
(i) State the reagents needed for Step 1 and identify the type of reaction occurring.
(ii) Name other organic compound needed in Step 2.
(iii) Give the name of the ester formed. [3 marks]
(b) Penicillin G (whose structure is given below) was the first antibiotic used to fight infections.
[Structure of Penicillin G shown: a benzyl group attached to a carbonyl-amine chain fused to a β-lactam-thiazolidine ring system with a methyl branch and a COOH group]
(i) Determine the molecular formula of penicillin G.
(ii) How many chiral carbons are present in the structure of penicillin G?
(iii) Which functional group in the structure of penicillin G above can undergo Nucleophilic addition reaction with cyanide ion? [3 marks]
(c) Cyclohexene (as shown below) behaves as a typical alkene.
[Structure of cyclohexene shown]
(i) Name the type of polymerization that cyclohexene undergoes and, hence, state the IUPAC nomenclature of the organic products formed when one molecule of cyclohexene reacts with:
ii) one molecule of H₂
iii) one molecule of acidified KMnO₄ [3 marks]
8. (a) Name the following organic compounds and determine the number of sigma and pie bonds present in each case:
[Structure 1: H₂C=CH–CH=CH₂ type chain, drawn as H–C(H)(H)–C(H)=C(H)–C(H)(H)–H — but-2-ene skeleton with terminal CH₃ groups]
[Structure 2: CH₃COOH — acetic acid, drawn with H₃C–C(=O)–OH]
[Structure 3: CH₃COCH₃ — acetone, drawn as H₃C–C(=O)–CH₃] [4½ Marks]
(b) Arrange the following compounds in order of increasing susceptibility to SN1 reactions: CH₃Br, CH₃CH₂Br, C(CH₃)₃Br, CH₃CH(CH₃)Br [1 mark]
(c) (i) Define the term Polymers.
(ii) List two types of polymerization and give two examples of polymers formed in each case. [3 marks]
(d) Use the structure of Tylenol given below to answer the following:
[Structure of Tylenol (acetaminophen) shown: a benzene ring with an –OH group at one end and an –NH–C(=O)–CH₃ (acetamide) group at the other]
(i) Identify TWO (2) functional groups in Tylenol
(ii) Write the molecular formula of the compound [1½ marks]
solutions
CHM 001: General Chemistry
1(a) A limiting reagent is the reactant that is completely used up in a reaction, thereby determining (limiting) the maximum amount of product that can form.
1(b) CaSiO₃ + 6HF → CaF₂ + SiF₄ + 3H₂O
- M(CaSiO₃) = 40+28+48 = 116 g/mol; M(HF) = 20 g/mol
- mol HF = 25.0/20 = 1.25 mol; mol CaSiO₃ = 50.0/116 = 0.431 mol
- Required HF for all the CaSiO₃ = 6 × 0.431 = 2.59 mol — but only 1.25 mol HF available → HF is limiting
- mol CaF₂ formed = 1.25/6 = 0.208 mol; M(CaF₂) = 78 g/mol
- Theoretical yield = 0.208 × 78 = 16.25 g
1(c)(i) Hund's rule: electrons occupy degenerate orbitals singly (with parallel spins) before any orbital is doubly occupied.
(ii) Cr³⁺: [Ar]3d³; Zn: [Ar]3d¹⁰4s²
1(d)(i) Electronegativity is the tendency of an atom in a molecule to attract the shared (bonding) pair of electrons toward itself.
(ii) Increasing order: H < C < N < O < F
2(a)(i) A standard solution is a solution of accurately known concentration.
(ii) WFₓ, 38.27% F → mol F = 38.27/19 = 2.014; mol W = 61.73/183.84 = 0.336; ratio = 6.0 → WF₆, W is +6
2(b) mol C = 0.500/12 = 0.0417 mol = mol CO₂ → V = 0.0417 × 22.4 = 0.933 dm³
2(c)(i) Basic medium:
2Fe(OH)₂(s) + Pb(OH)₃⁻(aq) → 2Fe(OH)₃(s) + Pb(s) + OH⁻(aq)
(ii) Acidic medium:
ClO₃⁻(aq) + 6H⁺(aq) + 6Br⁻(aq) → Cl⁻(aq) + 3H₂O(l) + 3Br₂(s)
2(d) Mean = (70.10+69.99+70.20+70.00+69.98)/5 = 70.054%
- Absolute error = 70.054 − 70.00 = 0.054%
- % relative error = (0.054/70.00) × 100 = 0.077%
CHM 002: Physical Chemistry
3(a)(i) Cathode (reduction, higher E°): NO₃⁻; anode (oxidation): Cr
Cr(s) + NO₃⁻(aq) + 4H⁺(aq) → Cr³⁺(aq) + NO(g) + 2H₂O(l), E°cell = 0.96−(−0.74) = 1.70 V
(ii) ln K = nFE°/RT = (3×96500×1.70)/(8.314×300) = 197.3
log K = 197.3/2.303 = 85.7 → K ≈ 10⁸⁵·⁷ (reaction essentially goes to completion)
3(b) mol NOCl(initial) = 5/65.5 = 0.0763; mol NO = 2.29/30 = 0.0763; mol Cl₂ = 2.70/71 = 0.0380
NOCl remaining ≈ 0.0763 − 0.0763 ≈ 1×10⁻⁵ mol
Q = [NO]²[Cl₂]/[NOCl]² ≈ (0.0763)²(0.0380)/(1×10⁻⁵)² ≈ 2.2×10⁶
Since Q >> K (0.034), the system will shift in reverse (toward NOCl) to reach equilibrium.
3(c) (i) Radiotherapy/treatment of cancer (e.g., Co-60); (ii) Radiocarbon (C-14) dating of archaeological samples.
4(a) Acid: Al(H₂O)₆³⁺; Base: H₂O; Conjugate base: Al(H₂O)₅(OH)²⁺; Conjugate acid: H₃O⁺
4(b) i = ΔTf/(Kf·m)
- i₁ = 0.110/(1.86×0.0225) = 2.63
- i₂ = 0.440/(1.86×0.0910) = 2.60
- i₃ = 1.330/(1.86×0.278) = 2.57
Average i ≈ 2.60
4(c)(i) ΔG° = ΔH° − TΔS°; if ΔG°<0 → spontaneous, ΔG°>0 → non‑spontaneous, ΔG°=0 → equilibrium.
(ii) T = 303 K; RT-factor: 0.0821×303 = 24.88
P(O₂) = 0.050×24.88 = 1.244 atm = 945 mmHg
P(N₂) = 0.015×24.88 = 0.373 atm = 284 mmHg
CHM 003: Inorganic Chemistry
5(a)(i) Fajans' rules: covalent character increases with small, highly charged cation; large, polarizable anion; and non-noble-gas (pseudo-noble-gas) cation configuration.
(ii) V: VO₃⁻/VO₄³⁻ (V⁵⁺); Cr: CrO₄²⁻/Cr₂O₇²⁻ (Cr⁶⁺); Mn: MnO₄⁻ (Mn⁷⁺)
(iii) Fe in the Haber process; V₂O₅ in the Contact process; Ni in hydrogenation of oils.
5(b) H₂SO₄(conc.):
- Oxidizing: Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O
- Dehydrating: C₁₂H₂₂O₁₁ → 12C + 11H₂O
- Sulphonating: C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O
5(c)(i) Solvay process: brine saturated with NH₃ → CO₂ passed through → NaHCO₃ precipitates (NaCl+NH₃+CO₂+H₂O→NaHCO₃+NH₄Cl) → NaHCO₃ filtered and heated (2NaHCO₃→Na₂CO₃+H₂O+CO₂) → CO₂ and NH₃ recycled.
(ii) Transition metals: variable oxidation states, colored ions, form complexes, good catalysts, partially filled d-orbitals. Group II: fixed +2 state, mostly colorless, few complexes, poor catalysts, no d-electrons involved.
6(a) Beryllium–aluminium similarities: both form amphoteric oxides; both form covalent, dimeric halides that hydrolyze in water; both have high polarizing power/small ionic size; both have carbides that yield methane on hydrolysis.
6(b)(i) Ionization potential decreases down Group II because atomic radius increases, so outer electrons are farther from the nucleus and more shielded → easier to remove.
(ii) Be forms covalent compounds due to its very small size and high charge density, giving it strong polarizing power (Fajans' rules).
(iii) PbO more stable than PbO₂ due to the inert-pair effect (heavier Group IV elements favor +2); SiO₂ more stable than SiO because Si favors its higher (+4) oxidation state and forms strong Si–O bonds.
6(c)(i) Alkali metals are strong reducing agents themselves (most electropositive), so no ordinary chemical reagent can reduce their ions; they're also too reactive to survive contact with reducing agents/media — hence electrolysis is used instead.
(ii)
i. [Cr(en)₃]Cl₃ → Tris(ethylenediamine)chromium(III) chloride
ii. K₃[Fe(CN)₅NO] → Potassium pentacyanonitrosylferrate(I)
iii. [Co(NH₃)₃Cl(NO₂)CN] → Triamminechlorocyanonitritocobalt(III)
CHM 004: Organic Chemistry
7(a)(i) Step 1 reagent: acidified KMnO₄ or acidified K₂Cr₂O₇ (with heat) — this is an oxidation reaction.
(ii) Step 2 needs ethanol (with conc. H₂SO₄ catalyst).
(iii) Ester formed: ethyl ethanoate (ethyl acetate).
7(b)(i) Molecular formula of penicillin G: C₁₆H₁₈N₂O₄S
(ii) 3 chiral carbons
(iii) The carbonyl group of the strained β-lactam ring undergoes nucleophilic addition with CN⁻.
7(c)(i) Cyclohexene, like other alkenes, undergoes addition reactions.
- with H₂ → cyclohexane
- with acidified (hot) KMnO₄ → oxidative ring cleavage giving hexanedioic acid (adipic acid)
8(a)
- CH₃–CH=CH–CH₃ → but-2-ene: 11 sigma bonds, 1 pi bond
- CH₃COOH → ethanoic acid: 7 sigma bonds, 1 pi bond
- CH₃COCH₃ → propanone: 9 sigma bonds, 1 pi bond
8(b) Increasing SN1 susceptibility (tracks carbocation stability):
CH₃Br < CH₃CH₂Br < CH₃CH(CH₃)Br < C(CH₃)₃Br
8(c)(i) A polymer is a large molecule (macromolecule) built from many repeating small units (monomers) joined by covalent bonds.
(ii)
- Addition polymerization: polyethene (from ethene), PVC (from chloroethene)
- Condensation polymerization: nylon (diamine + dicarboxylic acid), terylene/polyester (diol + dicarboxylic acid)
8(d)(i) Two functional groups in Tylenol: phenolic –OH and the amide group (–NH–CO–CH₃)
(ii) Molecular formula: C₈H₉NO₂