12.1.1 An Introduction to Intermolecular Forces and States of Matter

Question 1

An Introduction to Intermolecular Forces and States of Matter

Answer 1

• The state of a substance depends on the energy of the system and attractive forces between molecules.
• There is a distribution of energies in the molecules of a sample; some have high kinetic energies, allowing them to leave the liquid phase.
• The intermolecular forces exhibited by a molecule are influenced by its dipole moment.
• The direction of a phase change is reversible, depending on whether energy is added to or removed from the system.

Question 2

note

Answer 2

• The state of a substance depends on the energy of the system
  and the attractive forces between molecules.
• Solids are ordered, with molecules held tightly together by strong attractive forces.
• Liquids have greater molecular motion than solids, and have slightly weaker attractive forces between the molecules.
• Gases have still greater molecular motion than liquids, and have almost no attractive forces between the molecules.
• There is a distribution of energies in the molecules of a sample.
• At higher temperatures, more molecules have enough kinetic energy to leave the liquid phase and move into the gas phase.
• If the attractive forces between molecules are weak, more molecules will have sufficient energy to move out of the liquid phase.
• The intermolecular forces exhibited by a molecule are influenced by its dipole moment.
• The bond dipole moment ( ) is based on the difference in electronegativity of the atoms in the bond. The overall dipole moment of a molecule is the vector sum of its component bond dipole moments.
• In general, the stronger the dipole moment, the more energy required to change phases from liquid to gas.
• The direction of a phase change is reversible, depending on whether energy is added to or removed from the system.
• For example, if enough energy is added to a solid, it will melt. If enough energy is removed from a liquid, it will freeze.

Question 3

At a given temperature, a certain percentage of the molecules in a liquid have enough energy to escape into the gas phase. This percentage depends on what?

Answer 3

the temperature and the forces between molecules

Question 4

What characteristic of a water molecule causes it to have a dipole moment?

Answer 4

Its bent shape

Question 5

Which of the following molecules has the smallest dipole moment?

Answer 5

G

Question 6

Ammonia is a pyramidal molecule. If it were made flat, how would its dipole moment change?

Answer 6

The dipole moment would be zero.

Question 7

Why do all the water molecules not fly out of a cup of water and disappear?

Answer 7

The molecules do not have enough energy to overcome the intermolecular forces.

Question 8

Water molecules are bent as shown in the diagram below. Suppose, however, that water molecules were linear. How would this change in structure most likely affect the polarity of the water molecule?

Answer 8

The water molecule would be nonpolar.

Question 9

What are the forces that cause molecules to attract other molecules called?

Answer 9

Intermolecular forces

Question 10

Which of the following describes a dipole moment?

Answer 10

opposite charges separated by some distance

Question 11

Why can a neutral molecule with two atoms have a dipole moment?

Answer 11

Because the electron density is greater around one atom

Question 12

Which of the molecules shown below is most likely to have a dipole moment?

Answer 12

HCl