2022 H2 Chem P1 Q5 — Periodic table Solution

The answer is A (1 only).

Vigorous effervescence (\(\text{CO}_2\)) needs a solution acidic enough to react quickly with carbonate: \(\text{CO}_3^{2-} + 2\text{H}^+ \rightarrow \text{CO}_2 + \text{H}_2\text{O}\).

• Statement 1 is correct: \(\text{Al}^{3+}\) is small and highly charged, with a high charge density, so \([\text{Al(H}_2\text{O)}_6]^{3+}\) is appreciably hydrolysed and gives a solution of about pH 3.
\(\text{AlCl}_3\text{(s)} + 6\text{H}_2\text{O(l)} \rightarrow [\text{Al(H}_2\text{O)}_6]^{3+}\text{(aq)} + 3\text{Cl}^-\text{(aq)}\)
\([\text{Al(H}_2\text{O)}_6]^{3+}\text{(aq)} \rightleftharpoons [\text{Al(H}_2\text{O)}_5\text{(OH)}]^{2+}\text{(aq)} + \text{H}^+\text{(aq)}\)
• Statement 2 is incorrect: \(\text{Mg}^{2+}\) has a lower charge density, so \([\text{Mg(H}_2\text{O)}_6]^{2+}\) is only very slightly hydrolysed (about pH 6.5). \([\text{H}^+]\) is too low to give vigorous effervescence.
• Statement 3 is incorrect: \(\text{Na}^+\) does not hydrolyse, so NaCl gives a neutral solution (pH 7) and no \(\text{CO}_2\).

• B is wrong: magnesium chloride is not acidic enough, and it leaves out aluminium chloride.
• C is wrong: sodium chloride gives a neutral solution.
• D is wrong: neither magnesium chloride nor sodium chloride gives vigorous effervescence.