7. [intermolecular Forces] Practice

1. A.) What would the 3-D geometric configuration of the following Lewis structure be?

Bent (tetrahedral)

Trigonal planar

Linear

Octahedral

The nonbonding pairs of electrons on the oxygen atom cause repulsion from the hydrogen atoms, giving water its bent shape. Because there are four groups attached to the oxygen atom (two nonbonding electrons and two hydrogen atoms), the overall shape is said to be tetrahedral.

Explanation

The nonbonding pairs of electrons on the oxygen atom cause repulsion from the hydrogen atoms, giving water its bent shape. Because there are four groups attached to the oxygen atom (two nonbonding electrons and two hydrogen atoms), the overall shape is said to be tetrahedral.

2. D.) A drop of each of the following compounds was placed on a table and the amount of time required for the compound to evaporate was recorded. Which of the compounds likely had the weakest intermolecular forces?

Compound	Time to Evaporate
A	10 s
B	48 s
C	30 s
D	120 s

A

B

C

D

Intermolecular forces hold molecules together. When a substance evaporates, its molecules spread out to become a gas. The substance that evaporates the fastest is likely to have weak intermolecular forces.

Explanation

Intermolecular forces hold molecules together. When a substance evaporates, its molecules spread out to become a gas. The substance that evaporates the fastest is likely to have weak intermolecular forces.

3. E.) Which of the following solvents could be used to dissolve sodium chloride?

Water

Vegetable Oil

Methanol

Acetone

The charges of ionic compounds are attracted to the partial charges of polar molecules. Sodium chloride (an ionic compound) would therefore most likely dissolve in a polar solvent.

Explanation

The charges of ionic compounds are attracted to the partial charges of polar molecules. Sodium chloride (an ionic compound) would therefore most likely dissolve in a polar solvent.

4. C.) If the H-F bond in the following molecule is polar, which of the following statements is true?

The molecule is a dipole.

The molecule is not a dipole.

The molecule is nonpolar.

The molecule has no dipole moment.

If the bond between Hydrogen and Fluorine is polar, then the electrons will spend more time orbiting the fluorine nucleus. This means that the Fluorine atom will have a partial negative charge while the Hydrogen will be partially positive. If you were to split this molecule in half, there would be a positive end and a negative end, so this molecule is a dipole.

Explanation

If the bond between Hydrogen and Fluorine is polar, then the electrons will spend more time orbiting the fluorine nucleus. This means that the Fluorine atom will have a partial negative charge while the Hydrogen will be partially positive. If you were to split this molecule in half, there would be a positive end and a negative end, so this molecule is a dipole.

5. A.) What geometric configuration would a molecule of

B F subscript 3

have?

Trigonal planar

Tetrahedral

Linear

Octahedral

Boron only has three valence electrons, which means that it can only form three covalent bonds. When it does, there are no nonbonding pairs to cause repulsion, so the 3-D shape is simply triangular and flat.

Explanation

Boron only has three valence electrons, which means that it can only form three covalent bonds. When it does, there are no nonbonding pairs to cause repulsion, so the 3-D shape is simply triangular and flat.

6. E.) Solvent X is a highly polar solvent. Which of the following would NOT dissolve in (or mix with) solvent X?

Vegetable oil

Table Salt

Water

HCl

"Like dissolves like", which means that nonpolar substances only dissolve in nonpolar solvents. Vegetable oil is a nonpolar substance, so it would no likely dissolve in solvent X. It is also worth mentioning that ionic compounds dissolve in polar solvents because the partial positive and negative charges of the polar molecules are attracted to the charged ions of ionic compounds.

Explanation

"Like dissolves like", which means that nonpolar substances only dissolve in nonpolar solvents. Vegetable oil is a nonpolar substance, so it would no likely dissolve in solvent X. It is also worth mentioning that ionic compounds dissolve in polar solvents because the partial positive and negative charges of the polar molecules are attracted to the charged ions of ionic compounds.

7. B.) Which of the following pairs of elements will likely form a polar covalent bond?

H + Cl

Na + Na

Na + Cl

H + H

Polar covalent bonds are formed between two nonmetal elements that do not share electrons evenly. In the case of Hydrogen and Chlorine, neither element is electronegative enough to remove electrons from the other. However, Chlorine is more electronegative than Hydrogen, so the electrons in the covalent bond that the two atoms share will spend more time orbiting Chlorine's nucleus.

Explanation

Polar covalent bonds are formed between two nonmetal elements that do not share electrons evenly. In the case of Hydrogen and Chlorine, neither element is electronegative enough to remove electrons from the other. However, Chlorine is more electronegative than Hydrogen, so the electrons in the covalent bond that the two atoms share will spend more time orbiting Chlorine's nucleus.

8. D.) Which of the following compounds will likely have the highest melting point?

NaCl (ionic)

CH4 (nonpolar)

CH3OH (slightly polar)

HCl (polar)

Intermolecular forces between dipoles (like HCl) are stronger than they are between nonpolar compounds. However, intermolecular forces are, at best, between two partially charged atoms (that is, they have charges with values

Explanation

Intermolecular forces between dipoles (like HCl) are stronger than they are between nonpolar compounds. However, intermolecular forces are, at best, between two partially charged atoms (that is, they have charges with values

9. B.) Which of the following pairs of elements will likely form a nonpolar covalent bond?

H + Cl

Na + Na

Na + Cl

H + H

Nonpolar covalent bonds are formed when two atoms share electrons evenly. In the case of two Hydrogen atoms (a diatomic element), the electrons are shared due to Hydrogen being a nonmetal with a relatively high electronegativity. Because the two atoms are likely identical, the electrons spend an equal amount of time orbiting both nuclei.

Explanation

Nonpolar covalent bonds are formed when two atoms share electrons evenly. In the case of two Hydrogen atoms (a diatomic element), the electrons are shared due to Hydrogen being a nonmetal with a relatively high electronegativity. Because the two atoms are likely identical, the electrons spend an equal amount of time orbiting both nuclei.

10. F.) Hemoglobin is a large molecule with no repeating structures. Which of the following terms can be used to describe hemoglobin?

Macromolecule

Network Solid

Allotrope

Polymer

A macromolecule is just a big molecule that does not have any repeating units.

Explanation

A macromolecule is just a big molecule that does not have any repeating units.

11. F.) Polystyrene (styrofoam) is made up of a chain of identical molecules (shown below). Which of the following words could be used to describe polystyrene?

Both polymer and network solid

Both allotrope and macromolecule

Both polymer and allotrope

Both allotrope and element

Polystyrene is made up of a chain of repeating structures, making it a network solid (not a macromolecule). Because the repeating units are compounds (not elements), polystyrene is a polymer.

Explanation

Polystyrene is made up of a chain of repeating structures, making it a network solid (not a macromolecule). Because the repeating units are compounds (not elements), polystyrene is a polymer.

12. C.) If all of the bonds in the following molecules are polar, which compound will NOT be a dipole?

A

B

C

D

Carbon dioxide molecules are symmetrical. If you were to cut the molecule in half, both sides would be partially negatively charged because the Oxygen atoms on both sides pull electrons to themselves. In order to be a dipole, a molecule has to have a positive and a negative side.

Explanation

Carbon dioxide molecules are symmetrical. If you were to cut the molecule in half, both sides would be partially negatively charged because the Oxygen atoms on both sides pull electrons to themselves. In order to be a dipole, a molecule has to have a positive and a negative side.