Phosgene, COCl2(g), is a toxic gas used as an agent of...

Phosgene, $\operatorname{COCl}_{2}(g),$ is a toxic gas used as an agent of warfare in World War I. (a) Draw an electron-dot structure for phosgene. (b) Using the table of bond dissociation energies (Table 7.1$)$ and the value $\Delta H^{\circ} \mathrm{f}=716.7 \mathrm{kJ} / \mathrm{mol}$ for $\mathrm{C}(g),$ estimate $\Delta H_{\mathrm{f}}^{\circ}$ for $\mathrm{COCl}_{2}(g)$ at $25^{\circ} \mathrm{C}$ . Compare your answer to the actual $\Delta H_{f}^{\circ}$ given in Appendix $B,$ and explain why your calculation is only an estimate.

Phosgene, $\operatorname{COCl}_{2}(g),$ is a toxic gas used as an agent of warfare in World War I.
(a) Draw an electron-dot structure for phosgene.
(b) Using the table of bond dissociation energies (Table 7.1$)$ and the value $\Delta H^{\circ} \mathrm{f}=716.7 \mathrm{kJ} / \mathrm{mol}$ for $\mathrm{C}(g),$ estimate $\Delta H_{\mathrm{f}}^{\circ}$ for $\mathrm{COCl}_{2}(g)$ at $25^{\circ} \mathrm{C}$ . Compare your answer to the actual $\Delta H_{f}^{\circ}$ given in Appendix $B,$ and explain why your calculation is only an estimate.

Key Concepts

Estimation Approximations in Thermochemistry

Using bond dissociation energies to estimate the enthalpy of formation involves approximations because it assumes that the energy required to break bonds is simply additive, without accounting for factors like electronic delocalization, molecular geometry, and other subtle stabilizing or destabilizing effects. Consequently, such calculations yield approximate values that may differ from precise experimental measurements.

Electron-dot Structures

Electron-dot or Lewis structures are diagrams that represent the valence electrons in a molecule, showing how atoms are bonded together and where lone pairs are located. These diagrams are essential for predicting the shape, reactivity, and physical properties of molecules, as they visually communicate how electrons are arranged in chemical bonds.

Bond Dissociation Energies

Bond dissociation energies are the amounts of energy required to break a specific bond in a molecule, separating it into individual atoms or fragments in the gas phase. They play a critical role in estimating the overall energy changes in chemical reactions by allowing the summing of energies needed to break bonds, which provides insight into bond strength and stability.

Standard Enthalpy of Formation

The standard enthalpy of formation is defined as the heat change that occurs when one mole of a compound is formed from its constituent elements in their standard states under standard conditions. This thermodynamic quantity is used to evaluate and compare the energy content of different substances and to predict reaction energetics.

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