Intermolecular Forces

Question 1

How do London forces (van der Waal forces) form?

Answer 1

The random distribution of electrons produces an oscillating dipole; a temporary dipole is created which induces a dipole in adjacent molecules.

Question 2

What is a permanent dipole - permanent dipole force?

Answer 2

When there is a non-directional separation of charge (uneven distribution) where partial charges in a molecule attract opposite charges in another molecule.

Question 3

How does a hydrogen bond arise?

Answer 3

Exists between a delta + hydrogen and a highly electronegative element with a lone pair of electrons (N, O or F). Arrises due to hydrogen lacking inner shells (its nucleus is exposed).

Question 4

What factors affect the strength of London forces?

Answer 4

The greater the number of electrons and the greater the electron cloud, the stronger the London forces.

Question 5

Explain the trend in boiling points of hydrogen halides, using their intermolecular forces.

Answer 5

HF has highest bp - H bonds, F is highly electronegative.
HCl has higher than expected bp because H-Cl bond is more polar than HBr & HI.
Increase from HCl, HBr to HI - increasing no. of electrons, size and mass

Question 6

Explain whether halogenoalkanes are:

a) polar
b) soluble in water
c) soluble in organic solvent

Answer 6

a) are polar: because they have a polar C-X bond (except iodine)
b) insoluble in water: the London and permanent dipole forces that need to be broken in haloalkanes are weaker than the hydrogen bonds that need to be broken in water molecules. The new bonds formed are weaker.
c) soluble in organic solvents: new intermolecular attractions have same strength as ones being broken.

Question 7

Why is NaCl more soluble in water than in hexane?

Answer 7

Energy is released when the ions in NaCl are hydrated (exothermic)

Question 8

What makes a good solvent? Would ethanol or water be better at dissolving a halogenoalkane?

Answer 8

A good solvent is one that can form several intermolecular forces (I.e. Propanone forms both London forces and hydrogen bonds with molecules)
Ethanol is better at dissolving because ethanol forms one hydrogen bond per molecule while water forms two. Less energy is required to break the bonds between ethanol molecules.

Question 9

Why does graphite have electrical conductivity in its plane?

Answer 9

Each carbon atom is only bonded to 3 other carbon atoms so the remaining outer electron is free to move within the plane.
Between the layers the energy gap is too large so the electrons are too far apart.