Chemistry 2022 Jamb Examination

Bromine water is used to test for unsaturation in organic compounds. Compounds containing double or triple bonds (alkenes or alkynes) decolourise bromine water by undergoing an addition reaction, forming a dibromo compound.
C₂H₄ (ethene) is an alkene with a double bond and therefore decolourises bromine water.
On the other hand, C₃H₈, C₂H₆, and C₄H₁₀ are alkanes and do not react with bromine water under normal conditions.

To find the empirical formula:
Assume 100 g of the compound, so: C = 69 g, H = 15.3 g, O = 30.7 g.
Convert to moles:
\(\text{C: } \dfrac{69}{12} = 5.75\), \(\text{H: } \dfrac{15.3}{1} = 15.3\), \(\text{O: } \dfrac{30.7}{16} = 1.92\)
Divide by the smallest value (1.92):
C = 3, H = 8, O = 1.
Thus, the empirical formula is C₃H₈O.

From the balanced equation 2H₂ + O₂ → 2H₂O, 2 volumes of hydrogen react with 1 volume of oxygen.
Given H₂ = 50 cm³, O₂ required = 50/2 = 25 cm³.
Available O₂ = 75 cm³.
Unreacted O₂ = 75 − 25 = 50 cm³.
Therefore, 50 cm³ of oxygen remains unreacted.

Hydrogen forms an ionic compound when it combines with a highly electropositive element such as sodium, forming a hydride ion (H⁻).
NaH (sodium hydride) is an ionic compound where Na⁺ and H⁻ ions are present.
In HCl, NH₃, and CH₄, hydrogen forms covalent bonds rather than ionic bonds.

Using Charles’ Law: V₁/T₁ = V₂/T₂ Given: V₁ = 29.5 cm³, T₁ = 0°C = 273 K, T₂ = 15°C = 288 K\[V_2 = V_1 \times \dfrac{T_2}{T_1} \]\[= 29.5 \times \dfrac{288}{273} = 31.1\,\text{cm}^3\]Hence, the volume of the gas at 15°C is 31.1 cm³.

The enthalpy of neutralisation is highest when a strong acid reacts with a strong base to form water and salt.
NaOH and HCl are both strong electrolytes, so their neutralisation produces the maximum heat energy, with ΔH ≈ −57 kJ/mol.
Other reactions involve weak acids or bases, resulting in less heat release.

Coal contains sulphur compounds that produce sulphur dioxide (SO₂) upon combustion.
This gas is a major air pollutant that contributes to acid rain and respiratory problems.
Hence, sulphur(IV) oxide is commonly found in cities relying on coal-fired electricity generation.

Dehydration of an alcohol involves the removal of a water molecule, forming an alkene.
CH₃CH₂CH₂CH₂OH (butan-1-ol) loses a molecule of water to form CH₃CH=CHCH₃ (but-2-ene).
This reaction is usually catalysed by concentrated H₂SO₄ and heat.

An endothermic reaction absorbs heat energy from its surroundings, leading to a decrease in external temperature.
Examples include photosynthesis and the thermal decomposition of calcium carbonate.
Exothermic reactions, by contrast, release heat energy.

The combination of hydrogen ions (H⁺) and hydroxide ions (OH⁻) to form water is the defining reaction of neutralisation.
This occurs when an acid reacts with a base, producing salt and water as products.