---
title: "A student mixes equal volumes of \\(0.10\\text{ M }\\text{Pb(NO}_3\\text{)}_2\\text{(aq)}\\) and \\(0.20\\text{ M }\\text{KI(aq)}\\) at \\(298\\text{ K}\\), immediately observing the formation of a bright yellow precipitate of \\(\\text{PbI}_2\\text{(s)}\\). The net ionic equation for the reaction is shown below.  \\[\\text{Pb}^{2+}\\text{(aq)} + 2\\,\\text{I}^-\\text{(aq)} \\rightarrow \\text{PbI}_2\\text{(s)}\\]  Which of the following correctly predicts and justifies the sign of the standard entropy change of the reaction, \\(\\Delta S^\\circ\\)?"
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date_modified: "2026-08-23T04:23:12+00:00"
---

# A student mixes equal volumes of \(0.10\text{ M }\text{Pb(NO}_3\text{)}_2\text{(aq)}\) and \(0.20\text{ M }\text{KI(aq)}\) at \(298\text{ K}\), immediately observing the formation of a bright yellow precipitate of \(\text{PbI}_2\text{(s)}\). The net ionic equation for the reaction is shown below.

\[\text{Pb}^{2+}\text{(aq)} + 2\,\text{I}^-\text{(aq)} \rightarrow \text{PbI}_2\text{(s)}\]

Which of the following correctly predicts and justifies the sign of the standard entropy change of the reaction, \(\Delta S^\circ\)?

A student mixes equal volumes of \(0.10\text{ M }\text{Pb(NO}_3\text{)}_2\text{(aq)}\) and \(0.20\text{ M }\text{KI(aq)}\) at \(298\text{ K}\), immediately observing the formation of a bright yellow precipitate of \(\text{PbI}_2\text{(s)}\). The net ionic equation for the reaction is shown below.

\[\text{Pb}^{2+}\text{(aq)} + 2\,\text{I}^-\text{(aq)} \rightarrow \text{PbI}_2\text{(s)}\]

Which of the following correctly predicts and justifies the sign of the standard entropy change of the reaction, \(\Delta S^\circ\)?

- **A.** \(\Delta S^\circ > 0\), because the precipitation process is thermodynamically favorable and all favorable reactions must have a positive entropy change for the system.
- **B.** \(\Delta S^\circ > 0\), because the formation of a solid from aqueous solution releases energy, which increases the dispersal of matter within the system.
- **C.** \(\Delta S^\circ < 0\), because matter is consumed as the precipitate forms, resulting in a decrease in the total mass of the system.
- **D.** \(\Delta S^\circ < 0\), because mobile, dispersed aqueous ions are arranged into a rigid, highly ordered crystal lattice, decreasing the number of accessible microstates.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120337/*