24: Bond Polarity and Intermolecular Forces

Question 1

What is the definition of electronegativity?

Answer 1

Electronegativity of an element is a measure of the ability of an atom of the element to attract the bonding electrons in a covalent bond towards itself. / affinity of an atom towards the shared pair(s) of electrons in a covalent bond.

Question 2

What is the most electronegative element?

Answer 2

Fluorine, having electronegativity defined as 4.0.

Question 3

What is the least electronegative element?

Answer 3

Francium, having electronegativity 0.7.

Question 4

How does the electronegativity of elements change when the group number is increased?

Answer 4

The larger the group number, the higher the electronegativity.

Question 5

How does the electronegativity of elements change when the period number is increased?

Answer 5

The larger the period number, the smaller the electronegativity.

Question 6

What is the definition of polar bond?

Answer 6

A polar bond is a covalent bond sharing electrons between atoms of two different elements.

Question 7

Where are the partial positive and negative charges on a molecule of hydrogen fluoride?

Answer 7

Partial positive: hydrogen atom, since it has a lower electronegativity
Partial negative: fluorine atom, since it has a higher electronegativity.

Question 8

Describe the direction of the arrow used to notate the bond polarity in a polar bond.

Answer 8

It point from the partial positive side to the partial negative side.

Question 9

If a molecule only consists of non-polar bonds, will it be a polar molecule?

Answer 9

No

Question 10

If a molecule consists of polar bonds, what are the conditions for it to be non-polar?

Answer 10

The bond polarity is perfectly cancelled among the bonds in one molecule.

Question 11

If a molecule consists of polar bonds, what are the conditions for it to be polar?

Answer 11

The bond polarity cannot be perfectly cancelled among the bonds in one molecule.

Question 12

How is a liquid tested to find whether it is polar or not?

Answer 12

Run a stream of liquid from a burette and bring a rod with electrostatic charge close to the liquid stream. A polar liquid will be deflected from its vertical path towards the charged rod whereas a non-polar liquid will not be deflected.

Question 13

What is the difference between the expected results when a positively charged rod and a negatively charged rod is brought close to a stream of water from a burette respectively?

Answer 13

There is no difference in the expected observation, the water paths are both deflected towards the rod.
For the rod with positive charge, the partial negative ends of water molecules are attracted to the rod.
For the rod with negative charge, the partial positive ends of water molecules are attracted to the rod.

Question 14

What are two types of intermolecular forces?

Answer 14

Hydrogen bonding and van der Waals’ forces

Question 15

What are the major categories of van der Waals’ forces in decreasing strength?

Answer 15

Permanent dipole - permanent dipole attraction,
Permanent dipole - induced dipole attraction,
Instantaneous dipole - induced dipole attraction.

Question 16

What are the molecules needed to create permanent dipole - permanent dipole attraction?

Answer 16

Polar molecules

Question 17

What are the molecules needed to create permanent dipole - induced dipole attraction?

Answer 17

A mixture of polar molecules and non-polar molecules

Question 18

What are the molecules needed to create instantaneous dipole - induced dipole attraction?

Answer 18

Any kind of molecules

Question 19

What kind of van der Waals’ forces do polar molecules experience?

Answer 19

Permanent dipole - permanent dipole attraction and instantaneous dipole - instantaneous dipole attraction

Question 20

What kind of van der Waals’ forces do non-polar molecules experience?

Answer 20

Instantaneous dipole - instantaneous dipole attraction only

Question 21

How is instantaneous dipole - induced dipole attraction created?

Answer 21

Each molecule is neutral as a whole but their electrons are constantly in random motion. If at any instant there are more electrons on one side of the molecule, that end has more negative charge than the other. A temporary dipole / instantaneous dipole is set up. This temporary dipole affects the electron distribution in the neighbouring molecules, resulting in the formation of another dipole. As a result, there are forces of attraction between the δ+ end of the instantaneous dipole and the δ- end of the induced dipole.

Question 22

What is a dipole?

Answer 22

A dipole is the separation of opposite electrical charges.

Question 23

What is induction in the formation of an induced dipole?

Answer 23

Induction happens when an existing dipole affects the electron distribution of the neighbouring molecule to cause it to become another dipole.

Question 24

What are the physical states of molecular liquids which van der Waals’ forces are often present?

Answer 24

Solid and liquids; once the van der Waals’ forces are overcome, the molecules move apart from each other and the substance becomes a gas.

Question 25

What are the factors that determine the strength of van der Waals’ forces?

Answer 25

Number of electrons per molecule, and molecular shape.

Question 26

How does the number of electrons per molecule determine the strength of van der Waals’ forces and why?

Answer 26

The more electrons per molecule, the stronger the van der Waals’ forces.
Down the group, the outermost electrons are further away from the nucleus and hence less tightly bound. The electron distribution in a molecule can be distorted increasingly easily. This favours the occurrence of the temporary dipole and the creation of induced dipoles.

Question 27

How does the boiling point of a molecule show the strength of the intermolecular forces among the molecules?

Answer 27

The stronger the intermolecular forces, the higher the boiling point.

Question 28

How does the shape of the molecule determine the strength of van der Waals’ forces and why?

Answer 28

The larger the surface area, the stronger the van der Waals’ forces, since a greater area of contact is allowed with neighbouring molecules.

Question 29

Describe and explain the trend of boiling points of the hydrides of group VI elements.

Answer 29

From hydrogen sulphide down the group, the boiling point increases due to the increasing molecular size (number of electrons per molecule).
Water has an exceptionally high boiling point due to the formation of hydrogen bonding among water molecules.

Question 30

Describe and explain the trend of boiling points of the hydrogen halides.

Answer 30

From hydrogen chloride down the group, the boiling point increases due to the increasing molecular size (number of electrons per molecule).
Hydrogen fluoride has an exceptionally high boiling point due to the formation of hydrogen bonding among hydrogen fluoride molecules.

Question 31

What are the criteria for the formation of hydrogen bonds?

Answer 31

1. Hydrogen atom being covalently bonded to one of nitrogen, fluorine, or oxygen (the 3 most electronegative elements)
2. The presence of lone pair(s) of electrons on the atom (N, O, F) which hydrogen is bonded to

Question 32

Do molecules capable of hydrogen bonding experience van der Waals’ forces?

Answer 32

Yes, but they contribute less to the total intermolecular attraction.

Question 33

What is hydrogen bond?

Answer 33

A hydrogen bond is the strong attractive force between a hydrogen atom attached to a highly electronegative atom and the lone pair on another electronegative atom.

Question 34

What is the fluorine compound with intermolecular hydrogen bonds?

Answer 34

Hydrogen fluoride

Question 35

Why does hydrogen fluoride create hydrogen bonds among its molecules instead of covalent bonds?

Answer 35

Although there is a strong attraction between the hydrogen atom and the lone pair of electrons on another fluorine atom, the electron shell of hydrogen is already full and it cannot take more electrons, therefore there is no covalent bond.

Question 36

Why does hydrogen fluoride form 2 hydrogen bonds per molecule instead of 4?

Answer 36

Theoretically, each hydrogen fluoride molecules can form 4 hydrogen bonds; however, if 4 bonds are actually formed, the lone pair on fluorine atoms will come close with another lone pair on another fluorine atom, causing repulsion and making the structure less stable.

Question 37

What is an exception for hydrogen bonding?

Answer 37

The hydrogen bonding between trichloromethane and methoxymethane

Question 38

Why do trichloromethane and methoxymethane form hydrogen bonding when mixed?

Answer 38

In trichloromethane, the central carbon atom is bonded to three chlorine atoms of a higher electronegativity. Thus, the electrons are drawn away from carbon and carbon enters an electron-deficient state, being almost as electronegative as the nitrogen, oxygen, and fluorine. This creates a very large partial positive charge on the hydrogen atom being bonded to the carbon atom.
In methoxymethane, there are two lone pairs of electrons on the oxygen atom, making it available for hydrogen bonding.

Question 39

How can the hydrogen bonding between trichloromethane and methoxymethane be proven?

Answer 39

When mixed together, there is energy released as heat. This shows that the mixture has a lower energy level, which shows that their stability increased, suggesting that there is hydrogen bonding between the molecules.

Question 40

Arrange the compounds in ascending order of boiling points:
Propane, ethanol, methoxymethane

Answer 40

Propane < methoxymethane < ethanol
Ethanol contains a hydrogen atom covalently bonded to oxygen with two lone pairs, which shows that it is capable of forming hydrogen bonds among its molecules.
Methoxymethane is polar and there is permanent dipole - permanent dipole attraction among its molecules.
Propane is non-polar and there is only instantaneous dipole - induced dipole attraction among its molecules, which is the weakest type of intermolecular forces.

Question 41

What is surface tension?

Answer 41

Surface tension is the effect of unbalanced forces acting on molecules at the surface of a liquid, pulling the molecules back into the liquid and causing the surface to tighten like a thin elastic sheet.

Question 42

What is the structure of water molecules on the surface of liquid water?

Answer 42

Through hydrogen bonding, water molecules from very temporary hexagonal arrays across the surface of water.

Question 43

What are the factors that determine the viscosity of a liquid?

Answer 43

The strength of intermolecular forces, and the tendency molecules to become entangled with each other.

Question 44

What is the structure of ice?

Answer 44

In ice, each water molecule is surrounded tetrahedrally by 4 other water molecules joined by intermolecular hydrogen bonding. It has an “open” lattice structure with water molecules held apart and empty space in between.

Question 45

Why does ice have a lower density than water at 0ºC?

Answer 45

When ice melts, the water molecules in liquid water have relative motion and the open structure collapses. The molecules become more closely packed, resulting in a decrease in volume and an increase in density.

Question 46

What is the change in density of ice when it is melted and warmed from 0ºC to 25ºC?

Answer 46

The density increases from 0ºC to 4ºC, since the open structure of ice collapses. Then, from 4ºC to 25ºC, the density decreases, since the speed of the water molecules increase and they become more spread out as the water is heated.

Question 47

Compare the strengths of intermolecular attractions between the compounds:
Propan-1-ol, propane-1,3-diol, propane-1,2,3-triol

Answer 47

The three compounds have 1, 2, and 3, -OH groups per molecule respectively, thus they can form 1, 2, and 3 hydrogen bonds per molecule respectively. Thus the strength of intermolecular attractions is
Propan-1-ol < propane-1,3-diol < propane-1,2,3-triol.

Question 48

Compare the viscosities of intermolecular attractions between the compounds:
Propan-1-ol, propane-1,3-diol, propane-1,2,3-triol

Answer 48

The three compounds have 1, 2, and 3, -OH groups per molecule respectively, thus they can form 1, 2, and 3 hydrogen bonds per molecule respectively. Since the viscosity is dependent on the strength of the intermolecular attraction, the viscosities of the compounds are
Propan-1-ol < propane-1,3-diol < propane-1,2,3-triol.