Intermolecular forces

Question 1

Define intermolecular force.

Answer 1

Intermolecular force is a weak attraction between molecules. These forces are the key determinants of physical properties such as boiling point, vapour pressure, viscosity etc.

Question 2

List the three intermolecular forces.

Answer 2

The three intermolecular forces are London dispersion forces, dipole-dipole forces, and hydrogen bonding.

Question 3

Describe London dispersion forces.

Answer 3

London dispersion forces are the attraction between molecules as a result of temporary dipoles.The strength of such forces is related to how polarizable the electrons are, which in turn is related to how big the molecule is. We usually use molar mass as an indication of molecular size. Therefore, we say London dispersion forces are related to a molecule’s molar mass. The greater the molar mass, the stronger the London dispersion force.

Question 4

Describe diplole-dipole forces.

Answer 4

Dipole-dipole forces are the attraction between molecules as a result of permanent dipoles. That is, they are the attraction between polar molecules (or the attraction between the polar parts of the molecule). The more polar the molecule is, the stronger the dipole-dipole forces. Molecular polarity is determined by considering bond polarity and how these bonds are oriented as related to the molecular geometry.

Question 5

Describe hydrogen bonding.

Answer 5

Hydrogen bonding refers to forces of attraction between the nucleus of a hydrogen atom (which is covalently bonded to a nitrogen, oxygen, or fluorine) and a lone pair of electrons on a nitrogen, oxygen, or fluorine. Hydrogen bonding is an intermolecular force (weaker than a typical chemical bond) and can be considered as a very specific kind of dipole-dipole force. Because the hydrogen is covalently bonded to the highly electronegative nitrogen, oxygen, or fluorine, the hydrogen has large partial positive charge. As such, it is strongly attracted to the lone pair of electrons of nitrogen, oxygen, or fluorine on a different molecule.

Question 6

Which intermolecular force is common to all molecules?

Answer 6

London dispersion forces exist for all molecules. Some molecules (e.g. H₂) only experience London dispersion forces, whereas others (e.g. H₂O), in addition to London dispersion forces, have also dipole-dipole forces and hydrogen bonding.

Question 7

Is hydrogen bonding always the most important (i.e. strongest) intermolecular force for a molecule?

Answer 7

No- although it usually is. While hydrogen bonding is considered to be a “strong” intermolecular force, it may not always be the most important. For a small molecule, like H₂O, London dispersion forces are considered to be relatively weak, but the hydrogen bonding is quite significant. Whereas for a very large molecule like octan-1-ol, (CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₂OH), while hydrogen bonding (and dipole-dipole forces) are present, the most important intermolecular force would be London dispersion forces due to the large molar mass of this molecule.

Question 8

Define vapour pressure.

Answer 8

Vapour pressure is the pressure of a vapour in equilibrium with its liquid phase (at a particular temperature).

Question 9

Define boiling point.

Answer 9

Boiling point is where a phase transition occurs between liquid and gas. It is the temperature at which the vapour pressure is equal to the atmospheric pressure. Therefore a boiling point should specify both a temperature and an atmospheric pressure. Normal boiling point is the boiling point when atmospheric pressure equals exactly 1 atm. At the precise boiling point both the liquid and gas phases of a chemical are in equilibrium.

Question 10

Why is the boiling point related to the strength of intermolecular forces?

Answer 10

The boiling point indicates at which temperature molecules have sufficient energy to break from the forces of attraction between molecules. The stronger the intermolecular forces, the more energy that is required to break these forces of attraction, and the higher will be the boiling point.

Question 11

Why is the vapour pressure related to the strength of intermolecular forces?

Answer 11

Vapour pressure is the pressure of a vapour in equilibrium with its liquid phase (at a particular temperature). The stronger the inttermolecular forces, the more energy required to break these forces of attraction and (at a particular temperature) the fewer molecules that will be in the gas phase. Fewer molecules in the gas phase indicates a lower vapour pressure.

Question 12

Explain an important outcome of the fact that boiling point depends on atmospheric pressure with reference to cooking.

Answer 12

An important outcome of the fact that boiling point depends on atmospheric pressure is that the boiling point of chemical can be changed by changing the pressure. For instance, by using a pressure cooker, we can artificially increase the atmospheric pressure inside the cooker and has a result have water boil at a temperature much higher than 100 °C. This raised temperature allows food to cook more quickly. Alternatively, under conditions of low atmospheric pressure (at high elevations for example), water boils at temperature less than 100 °C, so longer cooking times may be required due to the lower temperature.

Question 13

Explain what information is available from a phase diagram.

Answer 13

A phase diagram for a chemical is a plot of atmospheric pressure versus temperature. This plot shows under which conditions of pressure and temperature the chemical is in the solid, liquid and gas phase. Further, it shows under which conditions the these phases are in equilibrium. Therefore at 1 atmospheric pressure, the graph shows the normal melting point and the normal boiling point. The slope of the line between the solid and liquid phases also indicates which phase is more dense. For water, this line has a negative slope because the liquid phase for water is more dense. For most other chemicals, this line has a positive slope, which indicates the solid phase is more dense.

Question 14

Define triple point.

Answer 14

The triple point in a phase diagram represents the conditions of pressure and temperature where all three phases of matter (solid, liquid and gas) are in equilibrium - that is, they coexist.

Question 15

Define sublimation.

Answer 15

Sublimation is the phase transition from solid to gas.

Question 16

Define critical point.

Answer 16

The critical point (for a particular chemical) indicates the temperature above which the gaseous phase cannot condensed to a liquid through the application of pressure. Under such conditions (i.e. high pressure above the critical point) the chemical has both gas and liquid-like properties and is referred to as a “supercritical fluid”.

Question 17

Explain from a molecular viewpoint why the solid phase for water is less dense than its liquid phase.

Answer 17

The solid phase for water is less dense than the liquid phase because in the solid phase the water molecules are fixed in space (in the liquid phase the water molecules flow over one another). The most energetically favourable arrangement for the water molecules in the solid phase is for the water molecules to align in a way to maximize hydrogen bonding. Such an alignment puts the water molecules in a more open crystal lattice (as compared to the liquid phase). For a given mass, this crystal lattice occupies a greater volume and therefore the solid phase is less dense.

Question 18

Explain an important consequence of the fact that the solid phase for water is less dense than the liquid phase.

Answer 18

There are two major consequences. One is that solid ice floats on top of water, which is in contrast to most substances. This allows aquatic life to survive during winter because warmer water sinks to the bottom of a frozen lake. Secondly is the fact that when water freezes, the ice occupies a greater volume (due to its lower density). This is important in that we usually need to drain liquid water from pipes when there is a risk that the liquid water will freeze. Such freezing can cause the pipes to break, which will result in leaks and costly repairs.