---
title: "A student conducts a calorimetry experiment to determine the standard molar enthalpy of reaction, \\(\\Delta H_{\\text{rxn}}^\\circ\\), for the exothermic dissolution of calcium chloride:  \\[ \\text{CaCl}_2\\text{(s)} \\rightarrow \\text{Ca}^{2+}\\text{(aq)} + 2\\,\\text{Cl}^-\\text{(aq)} \\]  The student dissolves \\(5.55\\text{ g}\\) of \\(\\text{CaCl}_2\\text{(s)}\\) (\\(0.0500\\text{ mol}\\)) in \\(100.0\\text{ g}\\) of distilled water in a coffee-cup calorimeter. The student delays placing the lid on the calorimeter, allowing a significant amount of heat to escape to the surroundings before the maximum temperature of the solution is recorded. Which of the following correctly describes the effect of this error on the calculated \\(\\Delta H_{\\text{rxn}}^\\circ\\) and identifies an appropriate procedural modification to minimize this error in future trials?"
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url: "https://nerd-notes.com/ubq/123853/"
date_modified: "2026-09-28T12:30:34+00:00"
---

# A student conducts a calorimetry experiment to determine the standard molar enthalpy of reaction, \(\Delta H_{\text{rxn}}^\circ\), for the exothermic dissolution of calcium chloride:

\[ \text{CaCl}_2\text{(s)} \rightarrow \text{Ca}^{2+}\text{(aq)} + 2\,\text{Cl}^-\text{(aq)} \]

The student dissolves \(5.55\text{ g}\) of \(\text{CaCl}_2\text{(s)}\) (\(0.0500\text{ mol}\)) in \(100.0\text{ g}\) of distilled water in a coffee-cup calorimeter. The student delays placing the lid on the calorimeter, allowing a significant amount of heat to escape to the surroundings before the maximum temperature of the solution is recorded. Which of the following correctly describes the effect of this error on the calculated \(\Delta H_{\text{rxn}}^\circ\) and identifies an appropriate procedural modification to minimize this error in future trials?

A student conducts a calorimetry experiment to determine the standard molar enthalpy of reaction, \(\Delta H_{\text{rxn}}^\circ\), for the exothermic dissolution of calcium chloride:

\[ \text{CaCl}_2\text{(s)} \rightarrow \text{Ca}^{2+}\text{(aq)} + 2\,\text{Cl}^-\text{(aq)} \]

The student dissolves \(5.55\text{ g}\) of \(\text{CaCl}_2\text{(s)}\) (\(0.0500\text{ mol}\)) in \(100.0\text{ g}\) of distilled water in a coffee-cup calorimeter. The student delays placing the lid on the calorimeter, allowing a significant amount of heat to escape to the surroundings before the maximum temperature of the solution is recorded. Which of the following correctly describes the effect of this error on the calculated \(\Delta H_{\text{rxn}}^\circ\) and identifies an appropriate procedural modification to minimize this error in future trials?

- **A.** The calculated \(\Delta H_{\text{rxn}}^\circ\) is more negative than the true value; using a glass beaker instead of polystyrene cups will minimize heat exchange with the surroundings.
- **B.** The calculated \(\Delta H_{\text{rxn}}^\circ\) is less negative than the true value; immediately placing the lid on the calorimeter and stirring continuously will minimize heat exchange with the surroundings.
- **C.** The calculated \(\Delta H_{\text{rxn}}^\circ\) is more negative than the true value; doubling the volume of water while keeping the mass of \(\text{CaCl}_2\text{(s)}\) constant will minimize heat exchange with the surroundings.
- **D.** The calculated \(\Delta H_{\text{rxn}}^\circ\) is less negative than the true value; leaving the calorimeter uncovered throughout the trial will allow pressure to equilibrate and minimize heat exchange with the surroundings.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123853/*