Chapter 9

Predict whether the following compounds are ionic or covalent: \(\mathrm{KI}, \mathrm{MgS}, \mathrm{CS}_{2}, \mathrm{P}_{4} \mathrm{O}_{10}\).

Define lattice energy. Which should have the more negative lattice energy, LiF or CsF? Explain.

Which compound is not likely to exist: \(\operatorname{CaCl}_{2}\) or \(\mathrm{CaCl}_{4} ?\) Explain.

In boron compounds the B atom often is not surrounded by four valence electron pairs. Illustrate this with \(\mathrm{BCl}_{3}\) Show how the molecule can achieve an octet configuration by forming a coordinate covalent bond with ammonia \(\left(\mathrm{NH}_{3}\right)\).

Which of the following compounds or ions do not have an octet of electrons surrounding the central atom: \(\mathbf{B F}_{4}^{-}\) \(\mathrm{SiF}_{4}, \mathrm{SeF}_{4}, \mathrm{BrF}_{4}^{-}, \mathrm{XeF}_{4} ?^{-}\)

In which of the following does the central atom obey the octet rule: \(\mathrm{NO}_{2}, \mathrm{SF}_{4}, \mathrm{NH}_{3}, \mathrm{SO}_{3}, \mathrm{O}_{2}^{-}\) ? Are any of these species odd-electron molecules or ions?

Give the bond order of each bond in acetylene, \(\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H},\) and phosgene, \(\mathrm{Cl}_{2} \mathrm{CO}\).

Draw resonance structures for the formate ion, \(\mathrm{HCO}_{2}^{-}\) and then determine the \(\mathrm{C}-\) O bond order in the ion.

Determine the \(\mathrm{N}-\) O bond order in the nitrate ion, \(\mathrm{NO}_{3}^{-}\).

Consider a series of molecules in which carbon is bonded by single bonds to atoms of second-period elements: \(\mathrm{C}-\mathrm{O}, \mathrm{C}-\mathrm{F}, \mathrm{C}-\mathrm{N}, \mathrm{C}-\mathrm{C},\) and \(\mathrm{C}-\mathrm{B} .\) Place these bonds in order of increasing bond length.

Draw Lewis structures (and resonance structures where appropriate) for the following molecules and ions. What similarities and differences are there in this series? (a) \(\mathrm{CO}_{2}\) (b) \(\mathrm{N}_{3}^{-}\) (c) OCN

Does SO \(_{2}\) have a dipole moment? If so, what is the direction of the net dipole in \(\mathrm{SO}_{2} ?\)

What are the orders of the \(\mathrm{N}-\) O bonds in \(\mathrm{NO}_{2}^{-}\) and \(\mathrm{NO}_{2}^{+} ?\) The nitrogen-oxygen bond length in one of these ions is \(110 \mathrm{pm}\) and \(124 \mathrm{pm}\) in the other. Which bond length corresponds to which ion? Explain briefly.

Which has the greater \(\mathbf{O}-\mathbf{N}-\mathbf{O}\) bond angle, \(\mathbf{N O}_{2}^{-}\) or \(\mathrm{NO}_{2}^{+} ?\) Explain briefly.

Compare the \(\mathrm{F}-\mathrm{Cl}-\mathrm{F}\) angles in \(\mathrm{ClF}_{2}^{+}\) and \(\mathrm{ClF}_{2}^{-} .\) Using Lewis structures, determine the approximate bond angle in each ion. Decide which ion has the greater bond angle and explain your reasoning.

Draw an electron dot structure for the cyanide ion, \(\mathrm{CN}^{-}\). In aqueous solution this ion interacts with \(\mathrm{H}^{+}\) to form the acid. Should the acid formula be written as HCN or CNH?

Draw the electron dot structure for the sulfite ion, \(\mathrm{SO}_{3}^{2-}\) In aqueous solution the ion interacts with \(\mathrm{H}^{+}\). Does \(\mathrm{H}^{+}\) attach itself to the S atom or the O atom of \(\mathrm{SO}_{3}^{2-} ?\)

The cyanate ion, NCO", has the least electronegative atom, \(\mathbf{C},\) in the center. The very unstable fulminate ion, CNO \(^{-}\), has the same formula, but the \(\mathrm{N}\) atom is in the center. (a) Draw the three possible resonance structures of \(\mathrm{CNO}^{-}\) (b) On the basis of formal charges, decide on the resonance structure with the most reasonable distribution of charge. (c) Mercury fulminate is so unstable it is used in blasting caps. Can you offer an explanation for this instability? (Hint: Are the formal charges in any resonance structure reasonable in view of the relative electron negativities of the atoms?)

Given that the spatial requirement of a lone pair is much greater than that of a bond pair, explain why (a) \(\mathrm{XeF}_{2}\) has a linear molecular structure and not a bent one. (b) CIF \(_{3}\) has a T-shaped structure and not a trigonal-planar one.

Amides are an important class of organic molecules. They are usually drawn as sketched here, but another resonance structure is possible. (EQUATION CAN'T COPY) (a) Draw that structure, and then suggest why it is usually not pictured. (b) Suggest a reason for the fact that the \(\mathrm{H}-\mathrm{N}-\mathrm{H}\) angle is close to \(120^{\circ}\)

Nitric acid, HNO \(_{3},\) has three resonance structures. One of them, however, contributes much less to the resonance hybrid than the other two. Sketch the three resonance structures and assign a formal charge to each atom. Which one of your structures is the least important?

Acrolein is used to make plastics. Suppose this compound can be prepared by inserting a carbon monoxide molecule into the \(\mathrm{C}-\mathrm{H}\) bond of ethylene. (EQUATION CAN'T COPY) (a) Which is the stronger carbon-carbon bond in acrolein? (b) Which is the longer carbon-carbon bond in acrolein? (c) Is ethylene or acrolein polar? (d) Is the reaction of CO with \(\mathrm{C}_{2} \mathrm{H}_{4}\) to give acrolein endothermic or exothermic?

(a) Glycolaldehyde was featured in the story "Molecules in Space" (page 372 ). Indicate the unique bond angles in this molecule. (b) One molecule found in the 1995 Hale-Bopp comet is HC \(_{3}\) N. Suggest a structure for this molecule. (Hint: it is based on a chain of atoms.)

The following molecules or ions have fluorine atoms attached to a central atom from Groups \(3 \mathrm{A}\) through \(7 \mathrm{A}\). Draw the Lewis structure for each one and then describe the electron-pair geometry and the molecular geometry. Comment on similarities and differences in the series. (a) \(\mathrm{BF}_{3}\) (b) \(\mathrm{CF}_{4}\) (c) \(\mathrm{PF}_{3}\) (d) \(\mathrm{OF}_{2}\) (e) \(\mathrm{HF}\)

Define "bond dissociation energy." Does the enthalpy change for a bond- breaking reaction [e.g., \(\mathrm{C}-\mathrm{H}(\mathrm{g}) \longrightarrow \mathrm{C}(\mathrm{g})+\mathrm{H}(\mathrm{g})]\) always have a positive sign, always have a negative sign, or vary? Explain briefly.

A molecule has four electron pairs around a central atom. Explain how the molecule can have a pyramidal structure. How can the molecule have a bent structure? What bond angles are predicted in each case?

What is the difference between the electron-pair geometry and the molecular geometry of a molecule? Use the water molecule as an example in your discussion.

The simple molecule acrylamide, \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CHC}(=\) O) \(\mathrm{NH}_{2}\), is a known neurotoxin and possible carcinogen. It was a shock to all consumers of potato chips and french fries a few years ago when was found to occur in those products. (Acrylamide arises during the cooking process from a reaction of the sugar glucose and the amino acid asparagine, both naturally found in many foods, \()\). (a) Draw an electron dot structure for acrylamide, showing any possible resonance structures. (b) Sketch the molecular structure of acrylamide, showing all unique bond angles. (c) Indicate which carbon-carbon bond is the stronger of the two. (d) Is the molecule polar or nonpolar? (e) The amount of acrylamide found in potato chips is \(1.7 \mathrm{mg} / \mathrm{kg} .\) If a serving of potato chips is \(28 \mathrm{g},\) how many moles of acrylamide are you consuming?