Chapter 9

Draw four reasonable resonance structures for the \(\mathrm{PO}_{3} \mathrm{~F}^{2-}\) ion. The central \(\mathrm{P}\) atom is bonded to the three \(\mathrm{O}\) atoms and to the \(\mathrm{F}\) atom. Show formal charges.

Draw reasonable resonance structures for these (a) \(\mathrm{HSO}_{4}^{-},(\mathrm{b}) \mathrm{SO}_{4}^{2-},(\mathrm{c}) \mathrm{HSO}_{3}^{-}\), sulfur-containing ions: (d) \(\mathrm{SO}_{3}^{2-}\).

True or false: (a) Formal charges represent actual separation of charges; (b) \(\Delta H_{\mathrm{rxn}}^{\circ}\) can be estimated from bond enthalpies of reactants and products; (c) all second-period elements obey the octet rule in their compounds; (d) the resonance structures of a molecule can be separated from one another.

A rule for drawing plausible Lewis structures is that the central atom is invariably less electronegative than the surrounding atoms. Explain why this is so.

Using this information: $$ \begin{array}{rr} \mathrm{C}(s) \longrightarrow \mathrm{C}(g) & \Delta H_{\mathrm{rxn}}^{\circ}=716 \mathrm{~kJ} / \mathrm{mol} \\ 2 \mathrm{H}_{2}(g) \longrightarrow 4 \mathrm{H}(g) & \Delta H_{\mathrm{rxn}}^{\circ}=872.8 \mathrm{~kJ} / \mathrm{mol} \end{array} $$ and the fact that the average \(\mathrm{C}-\mathrm{H}\) bond enthalpy is \(414 \mathrm{~kJ} / \mathrm{mol}\), estimate the standard enthalpy of formation of methane \(\left(\mathrm{CH}_{4}\right)\).

Which of these molecules has the shortest nitrogento-nitrogen bond? Explain. $$ \begin{array}{llll} \mathrm{N}_{2} \mathrm{H}_{4} & \mathrm{~N}_{2} \mathrm{O} & \mathrm{N}_{2} & \mathrm{~N}_{2} \mathrm{O}_{4} \end{array} $$

Most organic acids can be represented as \(\mathrm{RCOOH}\), in which \(\mathrm{COOH}\) is the carboxyl group and \(\mathrm{R}\) is the rest of the molecule. (For example, \(\mathrm{R}\) is \(\mathrm{CH}_{3}\) in acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\).) (a) Draw a Lewis structure of the carboxyl group. (b) Upon ionization, the carboxyl group is converted to the carboxylate group, \(\mathrm{COO}^{-}\). Draw resonance structures of the carboxylate group.

Which of these molecules or ions are isoelectronic: \(\mathrm{NH}_{4}^{+}, \mathrm{C}_{6} \mathrm{H}_{6}, \mathrm{CO}, \mathrm{CH}_{4}, \mathrm{~N}_{2}, \mathrm{~B}_{3} \mathrm{~N}_{3} \mathrm{H}_{6} ?\)

These species have been detected in interstellar space: (a) \(\mathrm{CH},\) (b) \(\mathrm{OH},\) (c) \(\mathrm{C}_{2},\) (d) \(\mathrm{HNC}\), (e) HCO. Draw Lewis structures of these species and indicate whether they are diamagnetic or paramagnetic.

The amide ion, \(\mathrm{NH}_{2}^{-}\), is a Bronsted base. Represent the reaction between the amide ion and water in terms of Lewis structures.