The Shapes of Molecules

State an ideal value for each of the bond angles in each molecule, and note where you expect deviations:

(a)  

(b)  

(c)  

Dinitrogen difluoride, ${\mathbf{N}}_2{\mathbf{F}}_2$, is the only stable, simple inorganic molecule with an N=N bond. The compound occurs in cis and trans forms.(a) Draw the molecular shapes of the two forms of ${\mathbf{N}}_2{\mathbf{F}}_2$.(b) Predict the polarity, if any, of each form.

Explain in general why the shape of a biomolecule is important to its function.

Consider the molecules ${\mathbf{SCl}}_2,{\mathbf{F}}_2,{\mathbf{CS}}_2,{\mathbf{CF}}_4,$ and BrCl.

1. Which has bonds that are the most polar?
2. Which have a molecular dipole moment?

Which molecule in each pair has the greater dipole moment? Give the reason for your choice.

1. ${\mathbf{SO}}_2\mathbf{or}{\mathbf{SO}}_3$
   
2.  $\mathbf{ICl}\mathbf{ }\mathbf{or}\mathbf{ }\mathbf{IF}$
3. ${\mathbf{SiF}}_4\mathbf{or}{\mathbf{SF}}_4$
   
4. ${\mathbf{H}}_2\mathbf{O}\mathbf{or}{\mathbf{H}}_2\mathbf{S}$
   

In addition to ammonia, nitrogen forms three other hydrides: hydrazine $\left({\mathbf{N}}_2{\mathbf{H}}_4\right)$, diazene $\left({\mathbf{N}}_2{\mathbf{H}}_2\right)$, and tetrazene $\left({\mathbf{N}}_4{\mathbf{H}}_4\right)$.

1. Use Lewis structures to compare the strength, length, and order of nitrogen-nitrogen bonds in hydrazine, diazene, and ${\mathbf{N}}_{\mathbf{2}}$.
2. Tetrazene (atom sequence ${\mathbf{H}}_2{\mathbf{NNNNH}}_2$) decomposes above ${\mathbf{0}}^{\mathbf{o}}$ to hydrazine and nitrogen gas. Draw a Lewis structure for tetrazene, and calculate ${\mathbf{\Delta H}}_{\mathrm{rxn}}^{\mathbf{o}}$ for this decomposition.

Give the molecular shape of each species in Problem 10.62.

$\begin{array}{l}\left(a\right){\mathbf{PF}}_5;\left(b\right){\mathbf{CCl}}_4;\left(c\right){\mathbf{H}}_3{\mathbf{O}}^{-};\left(d\right){\mathbf{ICl}}_3;\left(e\right){\mathbf{BeH}}_2;\left(f\right){\mathbf{PH}}_2^{-};\\ \left(g\right){\mathbf{GeBr}}_4;\left(h\right){\mathbf{CH}}_3^{-};\left(i\right){\mathbf{BCl}}_3;\left(j\right){\mathbf{BrF}}_4^{-};\left(k\right){\mathbf{XeO}}_3;\left(l\right){\mathbf{TeF}}_4.\end{array}$

Consider the following reaction of silicon tetrafluoride: ${\mathbf{SiF}}_4\text{ + }{\mathbf{F}}^{-}\stackrel{}{\to }{\mathbf{SiF}}_5^{-}$

1. Which depiction below best illustrates the change in molecular shape around Si?
2. Give the name and ${\mathbf{AX}}_m{\mathbf{E}}_n$ designation of each shape in the depiction chosen in part (a).

Draw a Lewis structure for each species:

$\begin{array}{l}\left(a\right){\mathbf{PF}}_5;\left(b\right){\mathbf{CCl}}_4;\left(c\right){\mathbf{H}}_3{\mathbf{O}}^{+};\left(d\right){\mathbf{ICl}}_3;\left(e\right){\mathbf{BeH}}_2;\left(f\right){\mathbf{PH}}_2^{-};\\ \left(g\right){\mathbf{GeBr}}_4;\left(h\right){\mathbf{CH}}_3^{-};\left(i\right){\mathbf{BCl}}_3;\left(j\right){\mathbf{BrF}}_4^{-};\left(k\right){\mathbf{XeO}}_3;\left(l\right){\mathbf{TeF}}_4.\end{array}$ 

Which molecule in each pair has the greater dipole moment? Give the reason for your choice.

1. ${\mathbf{ClO}}_2\mathbf{or}{\mathbf{SO}}_2$
   
2. HBr or HCl
3. ${\mathbf{BeCl}}_2\mathbf{or}{\mathbf{SCl}}_2$
   
4. ${\mathbf{AsF}}_3\mathbf{or}{\mathbf{AsF}}_5$
   

Because both tin and carbon are members of Group 4A(14), they form structurally similar compounds. But tin exhibits a greater variety of structures because it forms several ionic species. Predict the shapes and ideal bond angles, including any deviations:

(a)  $\mathbf{Sn}\mathbf{(}\mathbf{C}{\mathbf{H}}_3{\mathbf{)}}_2^{}$

(b) $\mathbf{Sn}\mathbf{(}\mathbf{Cl}{\mathbf{)}}_3^{-}$

(c)  $\mathbf{Sn}\mathbf{(}\mathbf{C}{\mathbf{H}}_3{\mathbf{)}}_4^{}$

(d)  ${\mathbf{SnF}}_5^{-}$

(e)  ${\mathbf{SnF}}_6^{-}$

For molecules of general formula ${\mathbf{AX}}_n$  (where n > 2), how do you determine if a particular molecule is polar?

How can a molecule with polar covalent bonds not be polar? Give an example.

Consider the molecules  and ${\mathbf{BF}}_3,{\mathbf{PF}}_3,{\mathbf{BrF}}_3,{\mathbf{SF}}_4$.

a.Which has bonds that are the most polar?

b.Which have a molecular dipole moment?

Both aluminium and iodine form chlorides, ${\mathbf{Al}}_2{\mathbf{Cl}}_6$ and ${\mathbf{l}}_2{\mathbf{Cl}}_6$, with “bridging” Cl atoms. The Lewis structures are

1. What is the formal charge on each atom?
2. Which of these molecules has a planar shape? Explain.

The VSEPR model was developed before any xenon compounds had been prepared. Thus, these compounds provided an excellent test of the model’s predictive power. What would you have predicted for the shapes of ${\mathbf{XeF}}_2,{\mathbf{XeF}}_4,$and ${\mathbf{XeF}}_6$?

When ${\mathbf{SO}}_{\mathbf{3}}$ gains two electrons, ${\mathbf{SO}}_{\mathbf{3}}^{\mathbf{2}\mathbf{-}}$ forms. 

The actual bond angle in ${\mathbf{NO}}_{\mathbf{2}}$ is $\mathbf{134}.{\mathbf{3}}^o$, and in ${\mathbf{NO}}_{\mathbf{2}}^{\mathbf{-}}$ it is $\mathbf{115}{\mathbf{.4}}^o$, although the ideal bond angle is ${\mathbf{120}}^o$ in both. Explain.

A gaseous compound has a composition by mass of 24.8% carbon, 2.08% hydrogen, and 73.1% chlorine. At STP, the gas has a density of 4.3 g/L. Draw a Lewis structure that satisfies these facts. Would another structure also satisfy them? Explain

“Inert” xenon actually forms several compounds, especially with the highly electronegative elements oxygen and fluorine. The simple fluorides  ${\mathbf{XeF}}_2$, ${\mathbf{XeF}}_4$, ${\mathbf{XeF}}_6$  and   are all formed by the direct reaction of the elements. As you might expect from the size of the xenon atom, the Xe-F bond is not a strong one. Calculate the Xe-F bond energy in  ${\mathbf{XeF}}_6$, given that the heat of formation is - 402 kJ/mol.

Propylene oxide is used to make many products, including plastics such as polyurethane. One method for synthesizing it involves oxidizing propene with hydrogen peroxide:

${\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{=}{\mathbf{CH}}_2\mathbf{+}{\mathbf{H}}_2{\mathbf{O}}_2\mathbf{\to }{\mathbf{CH}}_3{\mathbf{CHCH}}_2\mathbf{O}\mathbf{+}{\mathbf{H}}_2\mathbf{O}$

(a) What is the molecular shape and ideal bond angle around each carbon atom in propylene oxide? 

(b) Predict any deviation from the ideal for the actual C-C-C bond angles (assume the three atoms in the ring form an equilateral triangle).

Chloral, ${\mathbf{Cl}}_3\mathbf{CCH}\mathbf{=}\mathbf{O}$ , reacts with water to form the sedative and hypnotic agent chloral hydrate,  ${\mathbf{Cl}}_3\mathbf{CCH}\mathbf{(}\mathbf{OH}{\mathbf{)}}_2$  . Draw Lewis structures for these substances, and describe the change in molecular shape, if any, that occurs around each of the carbon atoms during the reaction.

Dichlorine heptaoxide, ${\mathbf{Cl}}_2{\mathbf{O}}_7$ , can be viewed as two ${\mathbf{ClO}}_4$  groups sharing an O atom. Draw a Lewis structure for ${\mathbf{Cl}}_2{\mathbf{O}}_7$  with the lowest formal charges, and predict any deviation from the ideal for the Cl-O-Cl bond angle.

Like several other bonds, carbon-oxygen bonds have lengths and strengths that depend on the bond order. Draw Lewis structures for the following species, and arrange them in order of increasing carbon-oxygen bond length and then by increasing carbon-oxygen bond strength:

1. $\mathbf{CO}$
   
2. ${\mathbf{CO}}_3^{2-}$
   
3. ${\mathbf{H}}_2\mathbf{CO}$
   
4. ${\mathbf{H}}_4{\mathbf{CO}}_{}^{}$
   
5.  ${\mathbf{HCO}}_3^{-}$(H attached to O).

In the 1980s, there was an international agreement to destroy all stockpiles of mustard gas, ${\mathbf{ClCH}}_2{\mathbf{CH}}_2{\mathbf{SCH}}_2{\mathbf{CH}}_2\mathbf{Cl}$ . When this substance contacts the moisture in eyes, nasal passages, and skin, the -OH groups of water replace the Cl atoms and create high local concentrations of hydrochloric acid, which cause severe blistering and tissue destruction. Write a balanced equation for this reaction, and calculate  $\mathbf{\Delta H}{\mathbf{°}}_{\mathrm{rxn}}$

The four bonds of carbon tetrachloride ( ${\mathbf{CCl}}_4$ ) are polar, but the molecule is non-polar because the bond polarity is canceled by the symmetric tetrahedral shape. When other atoms substitute for some of the Cl atoms, the symmetry is broken and the molecule becomes polar. Use Figure 9.20 (p. 364) to rank the following molecules from the least polar to the most polar: ${\mathbf{CH}}_2{\mathbf{Br}}_2$ ,  ${\mathbf{CF}}_2{\mathbf{Cl}}_2$ , ${\mathbf{CH}}_2{\mathbf{F}}_2$ , ${\mathbf{CH}}_2{\mathbf{Cl}}_2$ , ${\mathbf{CBr}}_4$ ,  ${\mathbf{CF}}_2{\mathbf{Br}}_2$

Ethanol ( ${\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{OH}$ ) is being used as a gasoline additive or alternative in many parts of the world.

1. Use bond energies to find  $\mathbf{\Delta H}{\mathbf{°}}_{\mathrm{rxn}}$ for the combustion of gaseous ethanol. (Assume  ${\mathbf{H}}_2\mathbf{O}$ forms as a gas.)
2. In its standard state at  $\mathbf{25}\mathbf{°}\mathbf{C}$ , ethanol is a liquid. Its vaporization requires 40.5 kJ/mol. Correct the value from part (a) to find the heat of reaction for the combustion of liquid ethanol.
3. How does the value from part (b) compare with the value you calculate from standard heats of formation (Appendix B)?
4. “Greener” methods produce ethanol from corn and other plant material, but the main industrial method involves hydrating ethylene from petroleum. Use Lewis structures and bond energies to calculate $\mathbf{\Delta H}{\mathbf{°}}_{\mathrm{rxn}}$  for the formation of gaseous ethanol from ethylene gas with water vapor. 

In the following compounds, the C atoms form a single ring. Draw a Lewis structure for each compound, identify cases for which resonance exists, and determine the carbon-carbon bond order(s):

1.  ${\mathbf{C}}_3{\mathbf{H}}_4$
2. ${\mathbf{C}}_3{\mathbf{H}}_6$
3. ${\mathbf{C}}_4{\mathbf{H}}_6$
4. ${\mathbf{C}}_4{\mathbf{H}}_4$
5. ${\mathbf{C}}_6{\mathbf{H}}_6$
   

An oxide of nitrogen is $\mathbf{\text{25.9\%}}$  N by mass, has a molar mass of 108 g/mol, and contains no nitrogen-nitrogen or oxygen- oxygen bonds. Draw its Lewis structure, and name it.

An experiment requires 50.0 mL of 0.040 M NaOH for the titration of 1.00 mmol of acid. Mass analysis of the acid shows 2.24% hydrogen, 26.7% carbon, and 71.1% oxygen. Draw the Lewis structure of the acid.

Methane burns in oxygen to form carbon dioxide and water vapor. Hydrogen sulfide burns in oxygen to form sulfur dioxide and water vapor. Use bond energies (Table 9.2, p. 353) to determine the heat of each reaction per mole of ${\mathbf{\text{O}}}_{\mathbf{\text{2}}}$  (assume Lewis structures with zero formal charges; BE of  $\mathbf{\text{S=O}}$  is 552 kJ/mol).

Perchlorates are powerful oxidizing agents used in fireworks, flares, and the booster rockets of space shuttles. Lewis structures for the perchlorate ion  $\left({\mathbf{ClO}}_4\right)$ can be drawn with all single bonds or with one, two, or three double bonds. Draw each of these possible resonance forms, use formal charges to determine the most important, and calculate its average bond order.

Hydrazine ( ${\mathbf{N}}_2{\mathbf{H}}_4$ ) is used as a rocket fuel because it reacts very exothermically with oxygen to form nitrogen gas and water vapour. The heat released and the increase in the number of moles of gas provide thrust. Calculate the heat of the reaction.

When gaseous sulfur trioxide is dissolved in concentrated sulfuric acid, disulfuric acid forms:   ${\mathbf{SO}}_3\mathbf{+}{\mathbf{H}}_2{\mathbf{SO}}_4\to {\mathbf{H}}_2{\mathbf{SO}}_7$

Use bond energies (Table 9.2, p. 353) to determine $\mathbf{\Delta H}{\mathbf{°}}_{\mathrm{rxn}}$. (The S atoms in disulfuric acid are bonded through an O atom. Assume Lewis structures with zero formal charges; BE of S-N-O is 552 kJ/mol.)

Hydrogen cyanide can be catalytically reduced with hydrogen to form methylamine. Use Lewis structures and bond energies to determine Heat of reaction.

A molecule of formula AY3 is found experimentally to be polar. Which molecular shapes are possible and which are impossible for AY3?

Except for nitrogen, the elements of Group 5A(15) all form pentafluorides, and most form pentachlorides. The chlorine atoms of PCl5 can be replaced with fluorine atoms one at a time to give, successively, ${\mathbf{\text{PCl}}}_{\mathbf{\text{4}}}{\mathbf{\text{F,PCl}}}_{\mathbf{3}}{\mathbf{\text{F}}}_{\mathbf{2}}$ , …, ${\mathbf{\text{PF}}}_{\mathbf{\text{5}}}$ .

Dinitrogen monoxide (${\mathbf{N}}_{\mathbf{2}}\mathbf{O}$ ) supports combustion in a manner similar to oxygen, with the nitrogen atoms forming  ${\mathbf{N}}_{\mathbf{2}}$. Draw three resonance structures for ${\mathbf{N}}_{\mathbf{2}}\mathbf{O}$  (one N is central), and use formal charges to decide the relative importance of each. What correlation can you suggest between the most important structure and the observation that ${\mathbf{N}}_{\mathbf{2}}\mathbf{O}$ supports combustion?

Oxalic acid (${\mathbf{H}}_{\mathbf{2}}{\mathbf{C}}_{\mathbf{2}}{\mathbf{O}}_{\mathbf{4}}$ ) is found in toxic concentrations in rhubarb leaves. The acid forms two ions, ${\mathbf{HC}}_{\mathbf{2}}{{\mathbf{O}}_{\mathbf{4}}}^{\mathbf{-}}$ and ${\mathit{C}}_{\mathbf{2}}{{\mathit{O}}_{\mathbf{4}}}^{\mathbf{2}\mathbf{-}}$ , by the sequential loss of H ions. Draw Lewis structures for the three species, and comment on the relative lengths and strengths of their carbon-oxygen bonds. The connections among the atoms are shown below with single bonds only.

A student isolates a product with the molecular shape shown at right (F is orange).

1. If the species is a neutral compound, can the black sphere represent selenium (Se)?
2. If the species is an anion, can the black sphere represent N?
3. If the black sphere represents Br, what is the charge of the species?

A major short-lived, neutral species in flames is OH.

1. What is unusual about the electronic structure of OH?
2. Use the standard heat of formation of OH(g) (39.0 kJ/mol) and bond energies to calculate the O-H bond energy in OH(g).
3. From the average value for the O-H bond energy in Table 9.2 (p. 353) and your value for the O-H bond energy in OH(g), find the energy needed to break the first O-H bond in water

Pure ${\mathbf{\text{HN}}}_{\mathbf{3}}$  (atom sequence HNNN) is explosive. In aqueous solution, it is a weak acid that yields the azide ion, N. Draw resonance structures to explain why the nitrogen-nitrogen bond lengths are equal in ${\mathbf{\text{N}}}_{\mathbf{3}}^{\mathbf{-}}$  but unequal in ${\mathbf{\text{HN}}}_{\mathbf{3}}$ .

Ethylene, ${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{4}}$ , and tetrafluoroethylene, ${\mathbf{C}}_{\mathbf{2}}{\mathbf{F}}_{\mathbf{4}}$, are used to make the polymers polyethylene and polytetrafluoroethylene (Teflon), respectively.

1. Draw the Lewis structures for ${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{4}}$ ,   and  ${\mathbf{C}}_{\mathbf{2}}{\mathbf{F}}_{\mathbf{4}}$, and give the ideal H-C-H and F-C-F bond angles.
2. The actual H-C-H and F-C-F bond angles are${\mathbf{117}\mathbf{.}\mathbf{4}}^{\mathbf{\circ }}$and $\mathbf{112}\mathbf{.}{\mathbf{4}}^{\mathbf{\circ }}$, respectively. Explain these deviations. 

Lewis structures of mescaline, a hallucinogenic compound in peyote cactus, and dopamine, a neurotransmitter in the mammalian brain, appear below. Suggest a reason for mescaline’s ability to disrupt nerve impulses.

Mescaline

Dopamine

Using bond lengths in Table 9.3 (p. 353) and assuming ideal geometry, calculate each of the following distances:

1. Between H atoms in  ${\mathbf{C}}_{\mathbf{2}}{\mathbf{H}}_{\mathbf{2}}$
2. Between F atoms in  ${\mathbf{SF}}_{\mathbf{6}}$ (two answers)
3. Between equatorial F atoms in  ${\mathbf{PF}}_{\mathbf{5}}$

Phosphorus pentachloride, a key industrial compound with annual world production of about $\mathbf{2}\mathbf{\times }{\mathbf{10}}^{\mathbf{7}}\mathbf{kg}$ , is used to make other compounds. It reacts with sulfur dioxide to produce phosphorus oxychloride ( ${\mathbf{POCl}}_{\mathbf{3}}$) and thionyl chloride  $\left({\mathbf{SOCl}}_{\mathbf{2}}\right)$. Draw a Lewis structure, and name the molecular shape of each product

The Murchison meteorite that landed in Australia in 1969 contained 92 different amino acids, including 21 found in Earth organisms. A skeleton structure (single bonds only) of one of these extraterrestrial amino acids is shown below. Draw a Lewis structure, and identify any atoms with a nonzero formal charge