Chapter 7

Compare IR and UV spectroscopy in these ways: (a) energy of radiation (b) wavelength of radiation (c) frequency of radiation

Ultraviolet and infrared spectroscopies probe different aspects of molecules. (a) Ultraviolet radiation is energetic enough to cause transitions in the energies of which electrons in an atom? (b) What does infrared spectroscopy tell us about a given molecule?

In addition to \(\mathrm{CO}\) and \(\mathrm{CO}_{2}\), there are other carbon oxides. One is tricarbon dioxide, \(\mathrm{C}_{3} \mathrm{O}_{2},\) also called carbon suboxide, a foul-smelling gas. (a) Write the Lewis structure of this compound. (b) What is the value of the \(\mathrm{C}\) -to-C-to-O bond angle in carbon suboxide? (c) What is the value of the \(\mathrm{C}\) -to-C-to-C bond angle in tricarbon dioxide?

Use Lewis structures and VSEPR theory to predict the electron-region and molecular geometries of (a) \(\mathrm{PSCl}_{3}\). (b) \(\mathrm{SOF}_{6}\) (c) \(\left[\mathrm{S}_{2} \mathrm{O}_{4}\right]^{2-}\). (d) \(\left[\mathrm{TeF}_{4}\right]^{2-}\). Note any differences between these geometries.

Cyanidin chloride is an anthocyanin found in strawberries, apples, and cranberries. Would you expect this compound to absorb in the visible region of the spectrum? Explain your answer.

Carbon and oxygen form the squarate ion, a polyatomic ion with the formula \(\mathrm{C}_{4} \mathrm{O}_{4}^{2-}\). (a) Write the I.ewis structure for this ion. (b) Describe the hybridization of each carbon atom. (c) Based on your answers to parts (a) and (b), is there any inconsistency between the molecular shape and the predicted bond angles?

In addition to carbonate ion, carbon and oxygen form the croconate ion, a polyatomic ion with the formula \(\mathrm{C}_{5} \mathrm{O}_{5}^{2-}\). (a) Write the Lewis structure for this ion. (b) Describe the hybridization of each carbon atom. (c) Is the croconate ion planar? Explain your reasoning.

The dipole moment is \(3.57 \times 10^{-30} \mathrm{C} \mathrm{m}\) for the \(\mathrm{HCl}\) molecule, and the bond length is \(127.4 \mathrm{pm} ;\) the dipole moment of \(\mathrm{HF}\) is \(5.94 \times 10^{-30} \mathrm{C} \mathrm{m},\) with bond length of \(91.68 \mathrm{pm}\). Use the definition of dipole moment as a product of partial charge on each atom times the distance of separation (Section \(7-5)\) to calculate the quantity of charge in coulombs that is separated by the bond length in each dipolar molecule. Use your result to show that fluorine is more electronegative than chlorine.

In the gas phase, positive and negative ions form ion pairs that are like molecules. An example is \(\mathrm{KF}\), which is found to have a dipole moment of \(28.7 \times 10^{-30} \mathrm{C} \mathrm{m}\) and a distance of separation between the two ions of \(217.2 \mathrm{pm} .\) Use this information and the definition of dipole moment to calculate the partial charge on each atom. Compare your result with the expected charge, which is the charge on an electron, \(-1.602 \times 10^{-19} \mathrm{C}\). Based on your result, is KF really completely ionic?

How can a diatomic molecule be nonpolar? Polar?

Explain clearly in your own words how a molecule can have polar bonds yet have a dipole moment of zero.

The grid for Question 79 has nine lettered boxes, each of which contains an item that is used to answer the questions that follow. Items may be used more than once and there may be more than one correct item in response to a question. $$ \begin{aligned} &\text { Grid for Question } 79\\\ &\begin{array}{|l|l|l|} \hline \text { A } & \text { B } & \text { C } \\ \text { HCN } & \text { PO }_{4}^{3-} & \text { PH }_{3} \text { or } \mathrm{PF}_{3} \\ \hline \text { D } & \text { E } & \text { F } \\ \text { SiH }_{4} & \text { Cl }_{2} \mathrm{O} & \text { NH }_{2} \text { Cl } \\ \hline \text { G } & \text { H } & \text { I } \\ \text { HF or } \mathrm{F}_{2} & \text { CH }_{4} & \text { OF }_{2} \\ \hline \end{array} \end{aligned} $$ Place the letter(s) of the correct selection(s) on the appropriate line. (a) Electron-region geometry is the same as the molecular geometry_____ (b) Nonpolar molecule____ (c) Linear molecular geometry______ (d) Angular (bent) molecular geometry______ (e) Central atom is \(s p^{3}\) hybridized______ (f) Central atom is sp hybridized_____ (g) Which one in each pair of compounds has the lower boiling point?_____ (h) Which one in each pair of compounds has the higher vapor pressure?______ (i) Which one in each pair of compounds has the higher dipole moment?______ (j) Has dipole-dipole and hydrogen bonding intermolecular forces______

Explain why the boiling point \(\left(5.9^{\circ} \mathrm{C}\right)\) of methanethiol, \(\mathrm{CH}_{3} \mathrm{SH},\) is much lower than the boiling point \(\left(64.7{ }^{\circ} \mathrm{C}\right)\) of methanol, \(\mathrm{CH}_{3} \mathrm{OH}\).

Explain why, even though \(\mathrm{CO}\) and \(\mathrm{N}_{2}\) each have a total of 14 electrons, the melting and boiling points of \(\mathrm{N}_{2}\) are slightly lower than those of \(\mathrm{CO}\).

Azidotrifluoromethane, \(\mathrm{CF}_{3} \mathrm{~N}_{3},\) is a colorless gas that is stable at room temperature. (a) Write a plausible Lewis structure for this compound and estimate the \(\mathrm{N}-\mathrm{N}-\mathrm{C}\) bond angle. (b) Estimate the \(\mathrm{N}-\mathrm{N}-\mathrm{N}\) bond angle. (c) Identify the hybridization of the central nitrogen atom and the hybridization of the carbon atom. (d) An intermediate compound with the molecular formula \(\mathrm{CF}_{3} \mathrm{~N}_{3} \mathrm{H}_{2}\) forms during the synthesis of azidotrifluoromethane. Draw the correct Lewis structures for two plausible resonance hybrids of this intermediate. Identify the hybridization of each nitrogen atom in each of the two structures.

What are the types of forces, in addition to London forces, that are overcome in these changes? Using structural formulas, make a sketch representing the major type of force in each case. (a) the evaporation of liquid methanol, \(\mathrm{CH}_{3} \mathrm{OH}\) (b) the decomposition of hydrogen peroxide, \(\mathrm{H}_{2} \mathrm{O}_{2}\), into water and oxygen (c) the melting of urea, \(\mathrm{H}_{2} \mathrm{NCONH}_{2}\) (d) the boiling of liquid \(\mathrm{HCl}\)

ACE solution was a mixture used as an anesthetic in the mid- to late-1800s. It contained ethanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right),\) chloroform ( \(\left.\mathrm{CHCl}_{3}\right),\) and diethyl ether, \(\mathrm{CH}_{3} \mathrm{CH}_{2}-\mathrm{O}-\mathrm{CH}_{2} \mathrm{CH}_{3} .\) Explain, on a molecular basis, why chloroform and diethyl ether are miscible, that is, they dissolve in each other.

Name a Group \(1 \mathrm{~A}\) to \(8 \mathrm{~A}\) element that could be the central atom (X) in these compounds. (a) \(\mathrm{XH}_{3}\) with one lone pair of electrons (b) \(\mathrm{XCl}_{3}\) (c) \(\mathrm{XF}_{5}\) (d) \(\mathrm{XCl}_{3}\) with two lone pairs of electrons

Name a Group \(1 \mathrm{~A}\) to \(8 \mathrm{~A}\) element that could be the central atom (X) in these compounds. (a) \(\mathrm{XCl}_{2}\) (b) \(\mathrm{XH}_{2}\) with two lone pairs of electrons (c) \(\mathrm{XF}_{4}\) with one lone pair of electrons (d) \(\mathrm{XF}_{4}\)

What is the maximum number of water molecules that could hydrogen-bond directly to an acetic acid molecule? Draw in the water molecules and use dotted lines to show the hydrogen bonds.

What is the maximum number of water molecules that could hydrogen-bond directly to an ethylamine molecule? Draw in the water molecules and use dotted lines to show the hydrogen bonds.

In another universe, elements try to achieve a nonet (nine valence electrons) instead of an octet when forming chemical bonds. As a result, covalent bonds form when a trio of electrons is shared between two atoms. Two compounds in this other universe are \(\mathrm{H}_{3} \mathrm{O}\) and \(\mathrm{H}_{2} \mathrm{~F}\). Draw their Lewis structures, then determine their electron-trio geometry and molecular geometry.

One of the three isomers of dichlorobenzene, \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2},\) has a dipole moment of zero. Draw the structural formula of this isomer and explain your choice.

There are three isomers with the formula \(\mathrm{C}_{6} \mathrm{H}_{6} \mathrm{O}_{2} .\) Each isomer contains a benzene ring to which two \(-\) OH groups are attached. (a) Write the Lewis structures for the three isomers. (b) Taking their molecular structure and the likelihood of hydrogen bonding into account, list them in order of increasing melting point.

Halothane, which had been used as an anesthetic, has the molecular formula \(\mathrm{CHBrClCF}_{3}\). (a) Write the Lewis structure for halothane. (b) Is halothane a polar molecule? Explain your answer. (c) Can hydrogen bonding occur in halothane? Explain.

Ketene, \(\mathrm{C}_{2} \mathrm{H}_{2} \mathrm{O},\) is a reactant for synthesizing cellulose acetate, which is used to make films, fibers, and fashionable clothing. (a) Write the Lewis structure of ketene. Ketene does not contain an \(-\mathrm{OH}\) bond. (b) Identify the electron-region geometry and the molecular geometry around each carbon atom and all the bond angles in the molecule. (c) Identify the hybridization of each carbon and oxygen atom. (d) Is the molecule polar or nonpolar? Use appropriate data to support your answer.

Gamma hydroxybutyric acid, GHB, infamous as a "date rape" drug, is used illicitly because of its effects on the nervous system. The condensed molecular formula for \(\mathrm{GHB}\) is \(\mathrm{HO}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{COOH} .\) (a) Write the Lewis structure for GHB. (b) Identify the hybridization of the carbon atom in the \(\mathrm{CH}_{2}\) groups and of the terminal carbon. (c) Is hydrogen bonding possible in GHB? If so, write Lewis structures to illustrate the hydrogen bonding. (d) Which carbon atoms are involved in sigma bonds? In pi bonds? (e) Which oxygen atom is involved in sigma bonds? In pi bonds?

There are two compounds with the molecular formula \(\mathrm{HN}_{3}\). One is called hydrogen azide; the other is cyclotriazene. (a) Write the Lewis structure for each compound. (b) Designate the hybridization of each nitrogen in hydrogen azide. (c) What is the hybridization of each nitrogen in cyclotriazene? (d) How many sigma bonds are in hydrogen azide? In cyclotriazene? (e) How many pi bonds are in hydrogen azide? In cyclotriazene? (f) Give approximate values for the \(\mathrm{N}\) -to-N-to-N bond angles in each molecule.

Nitrosyl azide, a yellow solid first synthesized in \(1993,\) has the molecular formula \(\mathrm{N}_{4} \mathrm{O}\). (a) Write its Lewis structure. (b) What is the hybridization on the terminal nitrogen? (c) What is the hybridization on the "central" nitrogen? (d) Which is the shortest nitrogen-nitrogen bond? (e) Give the approximate bond angle between the three nitrogens, beginning with the nitrogen that is bonded to oxygen. (f) Give the approximate bond angle between the last three nitrogens, those not involved in bonding to oxygen. (g) How many sigma bonds are there? How many pi bonds?

Three compounds have the molecular formula \(\mathrm{N}_{2} \mathrm{H}_{2}\). (a) Write the correct Lewis structure for each compound. (b) If there are geometric isomers, properly label each.

Two compounds have the molecular formula \(\mathrm{N}_{3} \mathrm{H}_{3}\). One of the compounds, triazene, contains an \(\mathrm{N}=\mathrm{N}\) bond; the other compound, triaziridene, does not. (a) Write the correct Lewis structures for each compound. (b) Approximate the bond angle between the three nitrogen atoms in each compound.

The bond angles around the central \(\mathrm{N}\) in this series\(\mathrm{NH}_{3}, \mathrm{~N}\left(\mathrm{CH}_{3}\right)_{3}, \mathrm{~N}\left(\mathrm{SiH}_{3}\right)_{3},\) and \(\mathrm{N}\left(\mathrm{GeH}_{3}\right)_{3}-\) are \(107.5^{\circ},\) \(110.9^{\circ}, 120^{\circ},\) and \(120^{\circ}\), respectively. Explain the trend in these bond angles.

There are two crystalline forms of oxalic acid, \(\mathrm{HOOC}-\mathrm{COOH},\) both involving hydrogen bonding. The alpha form consists of three-dimensional zig-zag chains of oxalic acid molecules; the beta form has long, linear chains of oxalic acid molecules. Draw Lewis structures that illustrate hydrogen bonding in each form.

Consider a hydrogen bond \(\mathrm{X}-\mathrm{H} \cdots \mathrm{Z}-\). (a) What kinds of vibrations can the \(\mathrm{H}\) atom in the hydrogen bond undergo? Draw a diagram showing how the \(\mathrm{H}\) atom would move in each vibration. (b) Suppose that a \(D\) atom participated in the hydrogen bond (D is the isotope of hydrogen with one neutron and one proton in the nucleus.). How would substitution of \(D\) for \(H\) affect the vibrations you described in part (a)?