Chemistry 2015 Jamb Examination

Calcium compounds produce a brick red flame when heated strongly. The colour results from the excitation of calcium ions, which emit visible light as they return to their ground state. This characteristic flame colour is used for qualitative identification in flame tests. Sodium produces a bright yellow flame, magnesium burns with a dazzling white flame, while lead does not exhibit a prominent flame colour.

During soap manufacture, concentrated sodium chloride solution is added to decrease the solubility of soap in the reaction mixture. This process is known as salting out. Sodium chloride reduces the solubility of soap molecules, causing them to precipitate from the solution as solid soap, which can then be separated. It does not contribute to saponification, as that occurs earlier in the process through the reaction of fats with an alkali.

Fats are non-polar substances and are soluble in non-polar organic solvents such as kerosene. Polar solvents like water cannot dissolve fats because of the difference in polarity. Kerosene, being non-polar, interacts effectively with the hydrocarbon chains in fats, leading to dissolution.

The relationship \(P_1V_1 = P_2V_2\) shows that for a fixed mass of gas at constant temperature, the pressure of the gas is inversely proportional to its volume. This is the mathematical expression of Boyle’s law, which states that \(PV = \text{constant}\) when temperature and quantity of gas remain constant.

Since pressure is constant, Charles’s law applies:\[\dfrac{V_1}{T_1} = \dfrac{V_2}{T_2}\]Given:
\(V_1 = 92\,\text{cm}^3\), \(T_1 = 3 + 273 = 276\,\text{K}\), \(T_2 = 18 + 273 = 291\,\text{K}\).
Substituting:\[\dfrac{92}{276} = \dfrac{V_2}{291}\]\[V_2 = \dfrac{92 \times 291}{276}\]\[V_2 = 97\,\text{cm}^3\]Therefore, the volume increases to 97 cm³ when temperature rises from 3°C to 18°C at constant pressure.

The quantity of electricity \(Q\) required for discharge is given by:\[Q = n \times z \times F\]where \(n\) is the number of moles, \(z\) is the charge on the ion, and \(F\) is the Faraday constant.
Calculating for each:
Q(A) = 2.0 × 3 = 6
Q(B) = 2.5 × 2 = 5
Q(C) = 3.0 × 1 = 3
Q(D) = 4.0 × 1 = 4
Hence, Q(A) = 6 has the highest value. Therefore, 2.0 moles of Q³⁺ require the largest amount of electricity for discharge.

Metals above hydrogen in the electrochemical series can displace hydrogen from water or alkalis. Calcium, being more reactive than hydrogen, reacts with hot alkalis to liberate hydrogen gas and form calcium hydroxide. Less reactive metals like copper, tin, and iron cannot perform this displacement under the same conditions.

Primary alkanols are oxidised first to aldehydes and then to alkanoic acids. Secondary alkanols are oxidised to ketones, which do not readily oxidise further under normal conditions. Tertiary alkanols, however, resist oxidation because the carbon atom bearing the hydroxyl group is not attached to a hydrogen atom, making oxidation difficult without breaking the carbon chain. Hence, only primary alkanols undergo oxidation to yield alkanoic acids.

Oxides of nitrogen (NO and NO₂) are primarily produced in internal combustion engines and during fuel combustion at high temperatures. The nitrogen and oxygen in the air combine due to the high heat generated during fuel burning. Hence, burning fuel is a major source of nitrogen oxides.

Uranium, particularly uranium-235, is widely used as a nuclear fuel in reactors because it undergoes fission easily, releasing a significant amount of energy. Although thorium can also be used, it is less common. Palladium and actinium are not used as primary nuclear fuels.