Chemistry 2006 Jamb Examination

Copper(II) sulphide (CuS) is insoluble in dilute HCl. Most metal sulphides like FeS and ZnS dissolve in dilute HCl, releasing H₂S gas.

Examples:
FeS (s) + 2HCl (aq) → FeCl₂ (aq) + H₂S (g)
ZnS (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂S (g)

But CuS does not react because it is highly insoluble in dilute acids.

Photochemical reactions are driven by the absorption of light energy. A classic example is the decomposition of silver halides (used in photography):

Equation:
2AgCl (s) →(light)→ 2Ag (s) + Cl₂ (g)

This is why photographic films darken when exposed to light.

The main product of anaerobic decomposition of organic matter (biogas production) is methane (CH₄), along with some CO₂.

Equation (simplified):
Organic matter →(anaerobic bacteria)→ CH₄ + CO₂ + traces of H₂S

This methane is also called marsh gas because it is often found in swamps and buried organic matter.

Nigerian crude oil is called light crude because it has a low sulphur content (called "sweet crude").

Low sulphur makes refining easier, cheaper and reduces pollution compared to "sour crude" with high sulphur.

Propene (C₃H₆) is an aliphatic (open-chain) unsaturated hydrocarbon because it contains a C=C double bond.

- Pentane (C₅H₁₂) → aliphatic saturated (alkane)
- Butanol (C₄H₉OH) → alcohol
- Benzene (C₆H₆) → unsaturated, but aromatic (ring structure, not aliphatic)

Equation for addition reaction:
C₃H₆ + H₂ → C₃H₈ (in presence of Ni catalyst)

C₄H₁₀ has two structural isomers: n-butane and isobutane (2-methylpropane). Small molecules like C₂H₆ and C₂H₄ have only one possible structure. Although C₆H₆ has valence isomers in advanced contexts, in elementary organic questions the standard expected answer is butane.

Alkyne carbons (C≡C) are sp-hybridized; the saturated carbons in CH₃-CH₂- are sp³. So the molecule contains both sp³ (in CH₃, CH₂) and sp (in C≡C) hybridizations.

Key idea: single-bonded C (alkane) → sp³; double-bonded C (alkene) → sp²; triple-bonded C (alkyne) → sp.

Vapour density (VD) = 20 ⇒ molar mass M = 2×VD = 40 g mol⁻¹. If one mole contains 36 g C, then moles of C = 36/12 = 3 carbons, leaving 4 g H ⇒ 4 H atoms. Formula is C₃H₄.

Among the options, CH₃C≡CH (propyne) is C₃H₄ with M = 40. Others are C₃H₆ (42), C₄H₆ (54), and C₃H₈ (44).

An emulsion is a dispersion of one liquid in another immiscible liquid. Detergent (a surfactant) stabilizes droplets of oil in water by lowering interfacial tension and forming micelles. Shaking detergent solution with palm oil forms an oil-in-water emulsion.

Candle wax is mainly alkanes (≈ C_nH_2n+2). Complete combustion yields CO₂ and H₂O.

General equation:
C_nH_2n+2 + (3n+1)/2 O₂ → n CO₂ + (n+1) H₂O

For example, C₂₅H₅₂ + 38.5 O₂ → 25 CO₂ + 26 H₂O.