Chemical Bonding 3

Question 1

Describe intramolecular forces and intermolecular forces.

Answer 1

Intramolecular forces are the strong electrostatic forces of attraction between nuclei and bonding electrons that hold atoms together in a molecule. Intermolecular forces are the attraction between molecules in simple molecular elements or compounds.

Question 2

Name 3 types of intermolecular forces.

Answer 2

1: Permanent dipole-permanent dipole interactions
2: hydrogen bonds
3: dispersion forces

Question 3

Define permanent dipole-permanent dipole interaction.

Answer 3

The weak electrostatic forces of attraction between the oppositely-charged ends of two polar molecules.

Question 4

State the relationship between the strength of the permanent dipole-permanent dipole interaction and the dipole moment.

Answer 4

The greater the dipole moment, the stronger the permanent dipole-permanent dipole attraction between the molecules.

Question 5

State 2 criteria for hydrogen bonding to exist in molecules.

Answer 5

1: One molecule must contain a H atom directly bonded to a highly electronegative atom (e.g. nitrogen, oxygen or fluorine)
2: The other molecule must contain an atom with a lone pair of electron (lone pair carries a negative charge) on the highly electronegative atom (e.g. nitrogen, oxygen or fluorine)

Question 6

Draw the intermolecular hydrogen bonding in HF.

Answer 6

(delta +) H —- (delta -) F : ||||| (delta +) H —- (delta -) F

Note:
- label ||||| as hydrogen bonding!
- draw the lone pair of electrons of F, O and N atom

Question 7

Name 3 examples of molecules that form intermolecular hydrogen bonding.

Answer 7

HF, H2O, NH3

Question 8

State the factors affecting the strength of hydrogen bonding, and describe each of their relationships with the strength of hydrogen bonding.

Answer 8

Factor 1: bond polarity
The hydrogen bonding formed between molecules is stronger if the molecule is more polar.
F-H (most polar)
O-H (secondmost polar)
N-H (thirdmost polar)
Cl-H (least polar)
Factor 2: extensiveness of hydrogen bonding
Hydrogen bonding formed between molecules is stronger when the hydrogen bonding between the molecules is more extensive.

Question 9

In terms of the strength and extent of the hydrogen bond in the compound, explain why the boiling point of HF is higher than that of NH3.

Answer 9

Both HF and NH3 are polar molecules with simple molecular structures. The intermolecular forces of attraction between HF molecules and NH3 molecules are both hydrogen bonds. However, since F is more electronegative than N, the N-F bond has a greater dipole moment than the N-H bond. Thus, more energy is required to overcome the stronger hydrogen bonds between HF molecules compared to those between NH3 molecules, hence the higher boiling point of HF.

Question 10

In terms of the strength and extent of the hydrogen bond in the compound, explain why the boiling point of H2O is higher than that of HF.

Answer 10

Both H2O and HF are polar molecules with simple molecular structures. The intermolecular forces of attraction between H2O molecules and HF molecules are both hydrogen bonds. However, H2O contains 2 H atoms and 2 lone pairs on the O atom and there is an average of 2 hydrogen bonds formed per H2O molecule. On the other hand, HF contains 1H atom and 3 lone pairs and there is an average of 1 hydrogen bond formed per HF molecule. Thus, more energy is required to overcome the more extensive hydrogen bonds between H2O molecules compared to the less extensive hydrogen bonds between HF molecules, hence the higher boiling point of H2O.

Question 11

In terms of the strength and extent of the hydrogen bond in the compound, explain why the boiling point of H2O is higher than that of NH3.

Answer 11

Both H2O and NH3 are polar molecules with simple molecular structures. The intermolecular forces of attraction between H2O molecules and NH3 are both hydrogen bonds. However, H2O contains 2 H atoms and 2 lone pairs on the O atom and there is an average of 2 hydrogen bonds formed per H2O molecule. On the other hand, NH3 contains 3 H atoms and 1 lone pair on the N atom and there is an average of 1 hydrogen bond formed per NH3 molecule. Moreover, since O is more electronegative than N, the O-H bond has a greater dipole moment than the N-H bond. Thus, more energy is required to overcome the stronger and more extensive hydrogen bonds between H2O molecules compared to the weaker and less extensive hydrogen bonds between NH3 molecules, hence the higher boiling point of H2O.

Question 12

Explain why small insects such as pond skater are able to float on water.

Answer 12

The strongly hydrogen-bonded molecules form a lattice across the surface of water, thus water has a high surface tension.

Question 13

State where do dispersion forces exist.

Answer 13

Dispersion forces exist in all molecules and individual atoms such as noble gases.

Question 14

Describe how dispersion forces are formed.

Answer 14

Dispersion forces are formed by temporary fluctuations in the electron density of an atom or a non-polar molecule.

Question 15

State 2 properties of dispersion forces.

Answer 15

• weak
• short-lived (the dipoles vanish as quickly as they are formed)

Question 16

Explain the relationship between the number of electrons in the molecules or atoms and the strength of dispersion forces.

Answer 16

The greater the number of electrons in the molecules or atoms, the greater the electron cloud size and the greater the polarisability of the electron clouds, thus the stronger the dispersion forces.

Question 17

Explain the relationship between the molecular shape and the strength of dispersion forces.

Answer 17

Shapes that have larger surface area allow for more points of contact and more polarisability, resulting in stronger dispersion forces.

Question 18

Explain why Cl2 is a gas at room temperature, but I2 is a solid at room temperature.

Answer 18

I2 is a larger molecule than Cl2, and the magnitude of dispersion forces in I2 is strong enough for a whole lattice of iodine molecules to be held together as a solid at room temperature.

Question 19

Which of the following is/are broken during melting or boiling?
1: covalent bonds
2: intermolecular forces

Answer 19

2
Weak intermolecular forces are overcame, but the strong covalent bonds are not broken.

Question 20

State the physical properties of simple covalent molecules, and explain the properties.

Answer 20

1: The melting and boiling points of simple covalent molecules are lower than those of metals, ionic compounds and giant molecular compounds.
This is because less energy is needed to overcome the weaker intermolecular attractions between the molecules, compared to the stronger metallic bonding, ionic bonding or covalent bonds.
2: Simple covalent molecules are unable to conduct electricity in the solid or liquid state.
This is because all the electrons in the molecules are either held in the covalent bonds or held by the nuclei, thus not free to move. There are thus no mobile ions to act as mobile charge carriers.
3: Many simple covalent molecules are soluble in organic solvents like propanone and benzene. Some are soluble in water (e.g. ethanol, ethanoic acid)

Question 21

Name 2 simple covalent substances that react to form a resulting solution that conducts electricity. Write the balanced chemical equation, with state symbols.

Answer 21

HCl (g) + H2O (l) -> Cl^- (aq) + H3O^+ (aq)

Question 22

Compound (boiling point):

methane, CH4 (-161)
ethane, C2H6 (-89)
propane, C3H8 (-42)

With reference to structure and bonding, explain the difference in boiling point between these 3 compounds.

Answer 22

All 3 compounds have simple molecular structures. As the number of carbon and hydrogen atoms increases from CH4 to C2H6 to C3H8, the number of electrons increases. Hence, the size of the electron cloud increases. The strength of dispersion forces between the molecules increases which requires more energy to overcome. Hence, the boiling point increases from CH4 to C2H6 and to C3H8.

Question 23

Magnesium fluoride, MgF2, is a solid at room temperature and pressure but chlorine monofluoride, ClF, is a gas. Explain the difference in their physical states in terms of structure and bonding.

Answer 23

MgF2 is an ionic compound with a giant ionic lattice structure while ClF is a covalent molecule with a simple molecular structure. More energy is required to overcome the stronger electrostatic forces of attraction between the oppositely charged ions of Mg2+ and F- ions, compared to the weaker dispersion forces between ClF molecules. Thus, MgF2 is a high melting point solid at room temperature, while ClF is a low boiling point gas at room temperature.

Question 24

1: Name the interaction between chlorine monofluoride, ClF, and water.
2: Draw a diagram to illustrate the interaction, showing only 1 particle of chlorine monofluoride and water each.

Answer 24

1: Hydrogen bond
2:
(delta +) Cl — (delta -) F : ||||| (delta +) H — (delta -) O – H

Note:
- indicate 2 lone pairs on O
- label hydrogen bond