Chemistry
Chemistry
Problem
A mixture is prepared by dissolving 0.200 mol NaCl(s) in water (ΔH = +3.9 kJ mol⁻¹). Silver nitrate solution is then added, precipitating AgCl(s) according to Ag⁺(aq) + Cl⁻(aq) → AgCl(s) with ΔH = –65 kJ mol⁻¹. The total measured heat change for the combined sequence is –8.2 kJ. Which statement best explains whether the overall process should be classified primarily as a physical or a chemical change? a) It is primarily a physical change because both steps involve solids changing state or phase, and no oxidation numbers change. b) It is primarily a physical change because the net heat released, , is much smaller than typical chemical reaction energies. c) It is primarily a chemical change because most of the energy change comes from formation of Ag–Cl bonds during precipitation, which far outweighs the energy absorbed when dissolves. d) It is primarily a chemical change because dissolving breaks ionic bonds, making it the dominant chemical process in the sequence.Intuitive explanation
This problem tests whether you can recognize that a process involving both a physical change (dissolving) and a chemical change (precipitation) should be classified by its dominant character. The key insight is to use Hess's Law to combine the two enthalpy changes—the endothermic dissolution and the strongly exothermic precipitation—and then ask: which type of change drives the overall process? The answer lies in identifying which step is thermodynamically dominant and whether the net reaction involves breaking and forming chemical bonds (the hallmark of chemistry).
Intuitive explanation
This problem tests whether you can recognize that a process involving both a physical change (dissolving) and a chemical change (precipitation) should be classified by its dominant character. The key insight is to use Hess's Law to combine the two enthalpy changes—the endothermic dissolution and the strongly exothermic precipitation—and then ask: which type of change drives the overall process? The answer lies in identifying which step is thermodynamically dominant and whether the net reaction involves breaking and forming chemical bonds (the hallmark of chemistry).