Problem 49
From these data, $$\begin{array}{l}\mathrm{S} \text { (rhombic) }+\mathrm{O}_{2}(g) \longrightarrow \mathrm{SO}_{2}(g) \\\\\qquad \begin{aligned}\Delta H_{\mathrm{rxn}}^{\circ} &=-296.4 \mathrm{~kJ} / \mathrm{mol}\end{aligned} \\\\\mathrm{S} \text { (monoclinic) }+\mathrm{O}_{2}(g) \longrightarrow \mathrm{SO}_{2}(g) \\\\\Delta H_{\mathrm{rxn}}^{\circ}=-296.7 \mathrm{~kJ} / \mathrm{mol}\end{array}$$ calculate the enthalpy change for the transformation \(\mathrm{S}\) (rhombic) \(\longrightarrow \mathrm{S}\) (monoclinic) (Monoclinic and rhombic are different allotropic forms of elemental sulfur.)
Problem 51
From the following heats of combustion, \(\begin{aligned} \mathrm{CH}_{3} \mathrm{OH}(l)+\frac{3}{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(l) & \Delta H_{\mathrm{rxn}}^{\circ}=-726.4 \mathrm{~kJ} / \mathrm{mol} \\\ \mathrm{C}(\text { graphite })+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(g) & \\ \Delta H_{\mathrm{rxn}}^{\circ}=-393.5 \mathrm{~kJ} / \mathrm{mol} \\ \mathrm{H}_{2}(g)+\frac{1}{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l) & \\ \Delta H_{\mathrm{rxn}}^{\circ}=-285.8 \mathrm{~kJ} / \mathrm{mol} \end{aligned}\) calculate the enthalpy of formation of methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\) from its elements: $$ \mathrm{C}(\text { graphite })+2 \mathrm{H}_{2}(g)+\frac{1}{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{CH}_{3} \mathrm{OH}(l) $$
Problem 52
Calculate the standard enthalpy change for the reaction $$ 2 \mathrm{Al}(s)+\mathrm{Fe}_{2} \mathrm{O}_{3}(s) \longrightarrow 2 \mathrm{Fe}(s)+\mathrm{Al}_{2} \mathrm{O}_{3}(s) $$ given that $$\begin{aligned} 2 \mathrm{Al}(s)+\frac{3}{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{Al}_{2} \mathrm{O}_{3}(s) & \\ \Delta H_{\mathrm{rxn}}^{\circ}=&-1669.8 \mathrm{~kJ} / \mathrm{mol} \\ 2 \mathrm{Fe}(s)+\frac{3}{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{Fe}_{2} \mathrm{O}_{3}(s) & \\ \Delta H_{\mathrm{rxn}}^{\circ}=-822.2 \mathrm{~kJ} / \mathrm{mol} \end{aligned}$$
Problem 53
Determine the enthalpy change for the gaseous reaction of sulfur dioxide with ozone to form sulfur trioxide given the following thermochemical data: $$ \begin{aligned} 2 \mathrm{SO}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{SO}_{2}(g) & \Delta H^{\circ}=-602.8 \mathrm{~kJ} / \mathrm{mol} \\ 3 \mathrm{SO}(g)+2 \mathrm{O}_{3}(g) \longrightarrow 3 \mathrm{SO}_{3}(g) & \\\ \Delta H_{\mathrm{rxn}}^{\circ}=-1485.03 \mathrm{~kJ} / \mathrm{mol} \\ \frac{3}{2} \mathrm{O}_{2}(g) \longrightarrow \mathrm{O}_{3}(g) & \Delta H_{\mathrm{rxn}}^{\circ}=142.2 \mathrm{~kJ} / \mathrm{mol} \end{aligned} $$
Problem 60
How are the standard enthalpies of an element and of a compound determined?
Problem 61
What is meant by the standard enthalpy of a reaction?
Problem 62
Write the equation for calculating the enthalpy of a reaction. Define all the terms.
Problem 63
Which of the following standard enthalpy of formation values is not zero at \(25^{\circ} \mathrm{C}: \mathrm{Na}(\) monoclinic \(), \mathrm{Ne}(g)\) \(\mathrm{CH}_{4}(g), \mathrm{S}_{8}(\) monoclinic \(), \mathrm{Hg}(l), \mathrm{H}(g) ?\)
Problem 64
The \(\Delta H_{\mathrm{f}}^{\circ}\) values of the two allotropes of oxygen, \(\mathrm{O}_{2}\) and \(\mathrm{O}_{3}\), are 0 and \(142.2 \mathrm{~kJ} / \mathrm{mol}\), respectively, at \(25^{\circ} \mathrm{C}\). Which is the more stable form at this temperature?
Problem 66
The standard enthalpies of formation of ions in aqueous solutions are obtained by arbitrarily assigning a value of zero to \(\mathrm{H}^{+}\) ions; that is, \(\Delta H_{\mathrm{f}}^{\mathrm{o}}\left[\mathrm{H}^{+}(a q)\right]=0 .\) (a) For the following reaction \(\begin{aligned} \mathrm{HCl}(g) \stackrel{\mathrm{H}_{2} \mathrm{O}}{\longrightarrow} \mathrm{H}^{+}(a q)+\mathrm{Cl}^{-}(a q) & \Delta H^{\circ}=-74.9 \mathrm{~kJ} / \mathrm{mol} \end{aligned}\) calculate \(\Delta H_{\mathrm{f}}^{\circ}\) for the \(\mathrm{Cl}^{-}\) ions. \((\mathrm{b})\) Given that \(\Delta H_{\mathrm{f}}^{\circ}\) for \(\mathrm{OH}^{-}\) ions is \(-229.6 \mathrm{~kJ} / \mathrm{mol},\) calculate the enthalpy of neutralization when 1 mole of a strong monoprotic acid (such as \(\mathrm{HCl}\) ) is titrated by \(1 \mathrm{~mole}\) of a strong base \((\) such as \(\mathrm{KOH})\) at \(25^{\circ} \mathrm{C}\).
