3.5 Intermolecular forces

Question 1

What are the three types of intermolecular forces and where do they act

Answer 1

• Van der Walls:
  Act between all molecules
• Dipole-Dipole:
  Act only between polar molecules
• Hydrogen bonds:
  Act only between certain types of molecules

Question 2

What is the weakest intermolecular force

What is the strongest

Answer 2

van Der Walls forces are the weakest

Hydrogen bonds are the strongest

Question 3

What is a dipole moment

Answer 3

Polarity is a property of a bond however molecules with a polar bond may not always have a dipole moment.

Dipole moment sums up the effect of the polarity of all the bonds in the molecule.

Question 4

Why may a molecule with
dipole-dipole bonds not have a dipole moment

What does it depend on

Answer 4

In symmetrical molecules with more than one polar bond, the effects of each bond may cancel out, leaving the molecule with no dipole moment.

It depends on the shape of the molecule.

If a molecule has no lone pairs and all bonds are identical, so it is symmetrical.

Question 5

Carbon dioxide is a linear molecule, does it have a dipole moment

Answer 5

No, the dipoles cancel as the two O- are at opposite ends of the molecule.

The oxygen has f- and the carbon has f+ (the f’s show the partial charges, as the O is more electronegative than the C)

Question 6

Tetrachloromethane is a tetrahedral, why is there no dipole moment

Answer 6

Because the tetrahedral structure means each atom is equally far enough from the others.

This symmetrical shape cancels the dipole charges out

Question 7

Why does dichloromethane, a tetrahedral have a dipole moment

Answer 7

The dipoles don’t cancel out because it is not symmetrical because there is hydrogen and chlorine bonded to the central carbon atom.

. Carbon forms a bond with two hydrogen and two chlorine molecules

The chlorine atom is more electronegative than hydrogen, so a dipole moment occurs as there is a separation of charge

Question 8

Where do dipole-dipole forces act and how.
Eg in HCL

Answer 8

Between molecules that have permanent dipoles, so they will attract each other.

Eg in HCl, chlorine is more electronegative than hydrogen, so the electrons are pulled towards the chlorine rather than the hydrogen atom.

The molecule has a dipole and is written Hf+ and Clf-

Whatever their starting positions, the molecules with dipoles will flip to give an arrangement where the two dipoles can attract. Eg so the H and Cl are facing each other, and not two Cl’s as they would repel

Question 9

What are van der Waals forces

Answer 9

All atoms and molecules are made of positive and negative charges even though they’re neutral overall.

These charges produce very weak electrostatic attractions between all atoms and molecules.

Question 10

How can an atom with no overall charge have a dipole within it

Answer 10

. Eg in a helium atom, it has two positive charges on its nucleus and two negatively charged electrons.

Overall, the atom is neutral, however at any time the electrons could be anywhere .
This means the distribution of charge is changing all the time.

. This means the atom could have a dipole at any moment however an instant later, this dipole could move to a different direction. Overall, the atom will always have a dipole somewhere.

Question 11

How do van der Walls form between molecules as a result of dipoles

Answer 11

The dipole formed in an atom from electrons moving within, can affect electron distribution in nearby atoms.

This is because they are attracted to the original atom for that instant.
The original atom has induced dipoles in nearby atoms, some will be attracted to the nearby one.

These forces can be called induced dipole forces.
Or van der Walls

Question 12

What are the characteristics of van der Walls forces

Answer 12

. They act between all atoms or molecules at all times

. They are in addition to any other intermolecular forces

. The dipole is caused by the changing position of the electron cloud, so the more electrons there are, the larger the instantaneous dipole will be.

So as a result the size of van der Walls forces increases with the number of electrons.So atoms with larger atomic masses have stronger van der walls forces.

Question 13

The strength of van der walls forces increases with the atomic number, or number of electrons in an atom.

How does this affect boiling points of noble gases and hydrocarbons

Answer 13

. Noble gases increase as the atomic numbers of the noble gases increase

. The boiling points of hydrocarbons increase with increased chain length.

Question 14

What is hydrogen bonding

Answer 14

It is a special type of intermolecular force, with some characteristics of dipole-dipole attraction and some of a covalent bond.

Consists of a hydrogen atom ‘sandwiched’ between two very electronegative atoms.

Question 15

What conditions must there be for a hydrogen bond to occur

Answer 15

. You need a very electronegative atom with a lone pair of electrons covalently bonded to a hydrogen atom.

Question 16

Why does water fit the conditions needed for a hydrogen bond

Answer 16

. Oxygen is much more electronegative than hydrogen so water is polar.

Question 17

Why is intermolecular bonding much stronger in water than in other compounds

Answer 17

. Oxygen atoms in water have a lone pair of electrons

. The hydrogen atoms are highly electron deficient because the more electronegative oxygen atoms attract the shared pair of electrons in the bond towards it.

The hydrogen atoms in water are very small and positively charged.
These exposed protons have a very strong electric field because of their size.

As a result the lone pair of electrons on the oxygen atom of another water molecule is strongly attracted to the electron deficient hydrogen atoms.

These strong bonds are called hydrogen bonds. These are stronger than dipole dipole and van der Walls however are weaker than covalent bonds.

Question 18

When do hydrogen bonds form,
what two things do they need

Answer 18

. A hydrogen atom that is bonded to a very electronegative atom. This will produce a strong partial positive charge on the hydrogen atom.

. A very electronegative atom with a lone pair of electrons. These will be attracted to the partially charged hydrogen atom in another molecule and form the bond.

Question 19

What are the only atoms that are electronegative enough to form hydrogen bonds

Answer 19

. Oxygen, nitrogen, and fluorine.

For example ammonia molecules (NH3) can form hydrogen bonds with water molecules (H20)

The nitrogen-hydrogen-oxygen system is linear because the pair of electrons in the N-H covalent bond repels those in the hydrogen bond between nitrogen and the hydrogen of the water.
This linearity is always the case with hydrogen bonds.

Question 20

Describe the boiling points of the hydrides

Answer 20

. Compare the boiling point of the hydrides with the boiling point of noble gases.
The noble gases show an increase in boiling point down the group because the only forces acting between the atoms are van der Walls, and they increase with the number of electrons present.

However, the boiling points of H2O, HF, and NH3 are all much higher than those of the hydrides of other elements.
They would be lower if only van der Walls were operating.

This is because the hydrogen bonds are present in each molecule of each compound, so these stronger intermolecular forces of attraction make the molecule more difficult to separate.

Question 21

Why is hydrogen bonding important

Answer 21

. Although hydrogen bonds are only around 10% of the strength of covalent bonds, their effect can be significant.
Especially when there are lots of them.

They can make or break under conditions that don’t affect covalent bonds so are very significant

Question 22

Why is ice less dense than water

Answer 22

As ice forms a crystalline structure, the lattice of it contains more hydrogen bonds than water.

This structure forces the water molecules further apart with hydrogen bonds so there are more empty spaces in ice so it is less dense than water

Question 23

How does ice help ponds

Answer 23

ice molecules are less dense than water molecules to fit into the 3D network of covalent bonds,

So ice forms on top of ponds rather than at the bottom.
This insulates ponds and enables fish to survive in winter

Question 24

Why do giant covalent structures eg silicon dioxide have higher melting points than simple molecular eg H2O

Answer 24

The covalent bonds in silicon dioxide are much stronger than the intermolecular forces in ice eg hydrogen bonding

So much more energy is required to break these bonds