Chemistry 2019 Jamb Examination

In the ammonium ion (\(NH_4^+\)), a co-ordinate or dative covalent bond is formed when the nitrogen atom in ammonia (\(NH_3\)) donates a lone pair of electrons to bond with a hydrogen ion (\(H^+\)).
This bond is termed 'co-ordinate' because both shared electrons come from the nitrogen atom. The resulting ion is stable and carries a positive charge due to the added proton.

Ammonium chloride (\(NH_4Cl\)) contains both ionic and dative covalent bonds. The ammonium ion (\(NH_4^+\)) forms through a dative covalent bond between ammonia and hydrogen ion, while the bond between \(NH_4^+\) and \(Cl^-\) is ionic.
Thus, the compound exhibits both types of bonding, making option C correct.

For reactions involving gases, a decrease in pressure reduces the concentration of gaseous reactants and products, which leads to a decrease in the rate of reaction.
However, if the question refers to rate and not equilibrium, a decrease in pressure means gas molecules collide less frequently, lowering both forward and backward reaction rates.
But in this context, as the forward reaction involves gaseous reactants, the rate of the forward reaction will increase when pressure is increased and decrease when it is reduced. Hence, decreasing pressure increases the rate of the backward reaction relative to the forward one.

The equilibrium constant (\(K_c\)) for a general reaction \(aA + bB \rightleftharpoons cC + dD\) is given by:\[K_c = \dfrac{[C]^c [D]^d}{[A]^a [B]^b}\]Applying this to the given equation \(XP + yQ \rightleftharpoons mR + nS\):\[K_c = \dfrac{[R]^m [S]^n}{[P]^x [Q]^y}\]This shows that option D is correct.

Oxidation involves the loss of electrons. In the given options, the reaction \(2O^{2-} \rightarrow O_2 + 4e^-\) represents oxidation because the oxygen ions lose electrons to form molecular oxygen.
Hence, oxygen undergoes oxidation, making this process an oxidation reaction.

Acidified potassium dichromate(VI) is an oxidising agent. During reduction, the chromium(VI) ion (orange in colour) is reduced to chromium(III) ion (green in colour).
Thus, when it acts as an oxidising agent, the solution changes from orange to green, showing the reduction of dichromate ions.

The enthalpy change for the reaction is obtained by subtracting the sum of the heats of formation of reactants from that of the products:\[\Delta H = \Sigma \Delta H_f(\text{products}) - \Sigma \Delta H_f(\text{reactants})\]\[\Delta H = [−82] − [50 + 0]\]\[\Delta H = −132\,\text{kJ mol}^{−1}\]Hence, the heat evolved in the reaction is −132 kJ, indicating an exothermic process.

Catalysts alter the mechanism of a reaction by providing an alternative pathway with lower activation energy. Hence, the statement that 'catalysts do not alter the mechanism of the reaction and never appear in the rate law' is false.
This makes option D the incorrect statement.

Air is a mixture because its components can be separated physically and it lacks a definite chemical formula. However, the proportions of gases in air are not fixed by mass—they vary with environmental conditions.
Therefore, the statement that the constituents of air are in a fixed proportion by mass is incorrect and does not support the fact that air is a mixture.

Potassium chloride (KCl) and potassium trioxochlorate(V) (KClO₃) are both solids that differ in solubility. Fractional crystallisation separates solids dissolved in the same solvent based on their different solubilities.
Hence, this method is best suited for separating the two salts from their solution.