Chapter 7

Describe the VSEPR model. How is the model used to predict molecular structure?

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

Designate the electron-region geometry for each case from two to six electron pairs around a central atom.

If you have three electron regions around a central atom, how can you have a triangular planar molecule? An angular molecule? What bond angles are predicted in each case?

Use VSEPR to explain why ethylene is a planar molecule.

How can a molecule with polar bonds be nonpolar? Give an example.

Explain the significance of Erwin Chargaff's research to the understanding of the structure of DNA.

(a) Describe how nucleotides are joined into a polynucleotide chain in DNA. (b) Explain the role of nitrogen bases in the DNA double helix.

Explain why the infrared spectrum of a molecule is referred to as its "fingerprint."

Use the various molecular modeling techniques (balland-stick, space-filling, two-dimensional pictures using wedges and dashed lines) to illustrate these simple molecules: (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{CO}_{2}\)

How is the frequency of infrared radiation absorbed by a molecule related to the motion of atoms in a molecule?

Draw the Lewis structure and identify the molecular shape of each molecule. (a) \(\mathrm{BeH}_{2}\) (b) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (c) \(\mathrm{BH}_{3}\) (d) \(\mathrm{SeCl}_{6}\) (e) \(\mathrm{PF}_{3}\)

Draw the Lewis structure for each molecule or ion. Describe the electron- region geometry and the molecular geometry. (a) \(\mathrm{NH}_{2} \mathrm{Cl}\) (b) \(\mathrm{OF}_{2}\) (c) \(\mathrm{SCN}^{-}\) (d) HOF

In each of these ions, three oxygen atoms are attached to a central atom. Draw the Lewis structure for each ion, and then describe the electron-region geometry and the molecular geometry. Comment on similarities and differences in the series. (a) \(\mathrm{BO}_{3}^{3-}\) (b) \(\mathrm{CO}_{3}^{2-}\) (c) \(\mathrm{SO}_{3}^{2-}\) (d) \(\mathrm{ClO}_{3}^{-}\)

In each of these molecules or ions, two oxygen atoms are attached to a central atom. Draw the Lewis structure for each, and then describe the electron-region geometry and the molecular geometry. Comment on similarities and differences in the series. (a) \(\mathrm{CO}_{2}\) (b) \(\mathrm{NO}_{2}^{-}\) (c) \(\mathrm{SO}_{2}\) (d) \(\mathrm{O}_{3}\) (e) \(\mathrm{ClO}_{2}^{-}\)

Write Lewis structures for \(\mathrm{XeOF}_{2}\) and \(\mathrm{ClOF}_{3}\). Use VSEPR theory to predict the electron-region and molecular geometries of these molecules, and note any differences between these geometries.

Write Lewis structures for \(\mathrm{HCP}\) and \(\left[\mathrm{IOF}_{4}\right]^{-} .\) Use VSEPR theory to predict the electron-region and molecular geometries of these species, and note any differences between these geometries.

These are examples of molecules and an ion that do not obey the octet rule. After drawing the Lewis structure, describe the electron-region geometry and the molecular geometry for each. (a) \(\mathrm{SiF}_{6}^{2-}\) (b) \(\mathrm{SF}_{4}\) (c) \(\mathrm{PF}_{5}\) (d) \(\mathrm{XeF}_{4}\)

These are examples of molecules and ions that do not obey the octet rule. After drawing the Lewis structure, describe the electron-region geometry and the molecular geometry for each. (a) \(\mathrm{ClF}_{2}^{-}\) (b) \(\mathrm{ClF}_{3}\) (c) \(\mathrm{ClF}_{4}^{-}\) (d) \(\mathrm{ClF}_{-}\)

Explain why \(\left(\mathrm{I}_{3}\right)^{+}\) is bent, but \(\left(\mathrm{I}_{3}\right)^{-}\) is linear.

Give approximate values for the indicated bond angles. (a) \(\mathrm{F}-\mathrm{Se}-\mathrm{F}\) angles in \(\mathrm{SeF}_{4}\) (b) angles in \(\mathrm{SOF}_{4}\) (The \(\mathrm{O}\) atom is in an equatorial position.) (c) angles in \(\mathrm{BrF}_{5}\)

Give approximate values for the indicated bond angles. (a) angles in \(\mathrm{SF}_{6}\) (b) angle in \(\mathrm{XeF}_{2}\) (c) angle in \(\mathrm{ClF}_{2}^{-}\)

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

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

Describe the geometry and hybridization of carbon in chloroform, \(\mathrm{CHCl}_{3}\).

Describe the geometry and hybridization for each \(\mathrm{C}\) and \(\mathrm{O}\) atom in ethylene glycol, \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH},\) the main component in antifreeze.

Describe the hybridization around the central atom and the bonding in \(\mathrm{SCl}_{2}\) and \(\mathrm{OCS}\).

The hybridization of the two carbon atoms differs in an acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH},\) molecule. (a) Designate the correct hybridization for each carbon atom in this molecule. (b) What is the approximate bond angle around each carbon?

The hybridization of the two nitrogen atoms differs in \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (a) Designate the correct hybridization for each nitrogen atom. (b) What is the approximate bond angle around each nitrogen?

(a) Identify the type of hybridization and approximate bond angle for each carbon atom in \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CCH}\). (b) Which is the shortest carbon-to-carbon bond length in this molecule? (c) Which is the strongest carbon-to-carbon bond in this molecule?

Write the Lewis structure and designate which are sigma and pi bonds in each of these molecules. (a) \(\mathrm{OCS}\) (b) \(\mathrm{NH}_{2} \mathrm{OH}\) (c) \(\mathrm{CH}_{2} \mathrm{CHCHO}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}\)

Write the Lewis structure and designate which are sigma and pi bonds in each of these molecules. (a) \(\mathrm{HCN}\) (b) \(\mathrm{N}_{2} \mathrm{H}_{2}\) (c) \(\mathrm{HN}_{3}\)

Which of these molecules is (are) polar? For each polar molecule, what is the direction of polarity; that is, which is the partial negative end and which is the partial positive end of the molecule? (a) \(\mathrm{CO}_{2}\) (b) \(\mathrm{HBF}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{Cl}\) (d) \(\mathrm{SO}_{3}\)

Which of these molecules has a net dipole moment? For each of these polar molecules, indicate the direction of the dipole in the molecule. (a) \(\mathrm{XeF}_{2}\) (b) \(\mathrm{H}_{2} \mathrm{~S}\) (c) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (d) \(\mathrm{HCN}\)

Explain the differences in the dipole moments of (a) \(\operatorname{BrF}(1.29 \mathrm{D})\) and \(\operatorname{BrCl}(0.52 \mathrm{D})\). (b) \(\mathrm{H}_{2} \mathrm{O}(1.86 \mathrm{D})\) and \(\mathrm{H}_{2} \mathrm{~S}(0.95 \mathrm{D})\).

Which of these molecules is polar? For each of the polar molecules, indicate the direction of the dipole in the molecule. (a) hydroxylamine, \(\mathrm{NH}_{2} \mathrm{OH}\) (b) sulfur dichloride, \(\mathrm{SCl}_{2}\), an unstable, red liquid

Which of these molecules has a net dipole moment? For each of the polar molecules, indicate the direction of the dipole in the molecule. (a) nitrosyl fluoride, FNO (b) disulfur difluoride, \(\mathrm{S}_{2} \mathrm{~F}_{2}\)

Construct a table that includes all the types of noncovalent interactions and comment about the strength of each. Also include an example of a substance that exhibits each type of noncovalent interaction in the table.

Use molecular structures and noncovalent interactions to explain why dimethyl ether, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{O},\) is completely miscible in water, but dimethylsulfide, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{~S},\) is only slightly water soluble.

Explain in terms of noncovalent interactions why water and ethanol are miscible, but water and cyclohexane are not.

Explain why water "beads up" on a freshly waxed car, but not on a dirty, unwaxed car.

Explain why water will not remove tar from your shoe, but kerosene will.

Which of these form intermolecular hydrogen bonds? (a) \(\mathrm{CH}_{2} \mathrm{Br}_{2}\) (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}\) (d) \(\mathrm{H}_{2} \mathrm{SO}_{3}\) (e) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\)

Arrange these substances in order of increasing boiling point. Explain your reasoning. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (c) \(\mathrm{CBr}_{3} \mathrm{CBr}_{2} \mathrm{CBr}_{3}\) (d) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\)

Arrange the noble gases in order of increasing boiling point. Explain your reasoning.

The structural formula for vitamin \(\mathrm{C}\) is Give a molecular-level explanation why vitamin \(\mathrm{C}\) is a water-soluble rather than a fat-soluble vitamin.

Which is most soluble in cyclohexane, \(\mathrm{C}_{6} \mathrm{H}_{12}\) ? Least soluble? Explain your reasoning. (a) \(\mathrm{NaCl}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{C}_{3} \mathrm{H}_{8}\)

What types of forces must be overcome in each change? (a) sublimation of solid \(\mathrm{C}_{10} \mathrm{H}_{8}\) (b) melting of propane, \(\mathrm{C}_{3} \mathrm{H}_{8}\) (c) decomposition of water into \(\mathrm{H}_{2}\) and \(\mathrm{O}_{2}\) (d) evaporation of liquid \(\mathrm{PCl}_{3}\) (e) unzipping the DNA double helix during replication

One strand of DNA contains the base sequence \(\mathrm{T}-\mathrm{C}-\mathrm{G}\). Draw a structure of this section of DNA that shows the hydrogen bonding between the base pairs of this strand and its complementary strand.

Discuss the differences between the extent of hydrogen bonding for the pairs \(\mathrm{G}-\mathrm{C}\) and \(\mathrm{A}\) - \(\mathrm{T}\) in nucleic acids. If a strand of DNA has more G-C pairs than A-T pairs in the double helix, the melting point (unwinding point) increases. The melting point for strands with more A-T pairing will decrease in comparison. Explain.