2018 H2 Chem P1 Q16 — Thermodynamics Solution

The answer is A (1, 2 and 4).
• Statement 1 is correct: \((\Delta G_1 + \Delta G_2)\) is the free energy change for dissolving solid AgCl in \(\text{NH}_3\)(aq), and \((\Delta G_3 + \Delta G_4)\) is that for solid AgBr. AgCl dissolves even in dilute \(\text{NH}_3\) while AgBr needs concentrated \(\text{NH}_3\), so dissolving AgCl is more spontaneous and has the more negative \(\Delta G\): \((\Delta G_1 + \Delta G_2) < (\Delta G_3 + \Delta G_4)\).
• Statement 2 is correct: in solution both reactions 2 and 4 are the same change, \(\text{Ag}^+(\text{aq}) + 2\text{NH}_3(\text{aq}) \rightarrow [\text{Ag}(\text{NH}_3)_2]^+(\text{aq})\). \(\text{Cl}^-\) and \(\text{Br}^-\) are only spectator ions, so \(\Delta G_2 = \Delta G_4\).
• Statement 3 is false: it contradicts statement 2, since \(\Delta G_2 = \Delta G_4\).
• Statement 4 is correct: solid AgCl is more soluble in water than solid AgBr, so its dissolving is more exergonic and \(\Delta G_1 < \Delta G_3\). This also follows from statements 1 and 2: subtracting \(\Delta G_2 = \Delta G_4\) from both sides of statement 1 leaves \(\Delta G_1 < \Delta G_3\).
• B (1 and 3) is wrong: statement 3 is false, and it leaves out the correct statements 2 and 4.
• C (2 and 4 only) is wrong: it leaves out statement 1, which is correct.
• D (3 only) is wrong: statement 3 is the only false statement.