Starter• Define the groups of compounds shown by the

three lines• What could you label the x-axis as?• Describe the pattern shown in the three lines

and what types of bonding are present?

Starter answers•Red = Group 5 hydrides•Green = Group 6 hydrides•Blue = Group 7 hydrides or Hydrogen halides•X-axis could be labelled as “Period” (2,3,4

and 5)

•NH3, H2O, and HF all polar.•The rest are non-polar but b.p. increases due

to increased spontaneous dipole-induced dipole (Van der Waals) bonding

Dipole-Dipole Interactions & Hydrogen Bonding

ObjectivesBe able to...•Apply knowledge of electronegativity to

determine polarity of covalent bond•Describe permanent dipole-induced

dipoles•Define hydrogen bonding as a special type

of permanent dipole-induced dipole •Be able to recognise biochemical and

material examples of hydrogen bonding

Specification...Where this comes from...

Intermolecular Forces•Van der Waals forces (instantaneous dipole-

induced dipole interactions) exist between all molecules whether polar or non polar

•They are:

▫Weak intermolecular forces

▫Caused by very small or instantaneous dipoles in molecules , caused by the random movement of electrons within electron clouds

Induced Dipoles

•Movement of electrons produces an

oscillating dipole (temporary) – this

oscillating dipole induces a dipole in a

neighbouring molecule which is induced

onto further molecules – induced dipoles

attract one another.

•As electrons are constantly moving, dipoles are always being created and destroyed, however the overall effect is constant and the atoms are attracted to each other.

Factors which affect the strength:•The greater the number of electrons in

each molecule

•The larger the oscillating and induced dipoles

•The greater the attractive forces between molecules

•How well the molecules fit together in 3D space... Tetris theory!

Questions

•Grades C-D

•Grades A-B

Permanent Dipole-Dipoles

•What is a polar molecule?

▫(a molecule which atoms have significant differences in their electronegativity and a dipole is created)

• + and - charges on polar molecules cause weak electrostatic forces of attraction between molecules E.g. in HCl(g)

H-Cl….. H-Cl….. H-Cl

•Although they are weak forces they are still much greater then VdWaals

Hydrogen Bonding• Special type of permanent dipole – dipole

interaction

• Hydrogen bonding can only occur when Hydrogen is bonded covalently to Fluorine, Nitrogen, or Oxygen.

• Hydrogen has a high charge density as it is so small and F, N & O are highly electronegative.

• The bond is highly polarised

• Molecules which have hydrogen bonding are usually organic

Examples of H-Bonding

Effects of H-Bonding

•Hydrogen bonding has a huge effect on the properties of substances. They are soluble in water and have higher boiling and freezing points than non-polar of similar size.

•Why does ice float? ▫ (less dense than liquid water due to rigid

Hydrogen bonds which are relatively long, molecules held in an open lattice structure)

H-bonding and Ice

•Ice has relatively high melting point, and water a relatively high boiling point

•Relatively strong H-bonds between H2O molecules

•Extra forces on top of VdWaals•The extra intermolecular forces from H-

bonds also explains high surface tension and viscosity in water

Exam Question

•State and explain the overall trend in

boiling points for Group 6 Hydrides. Why

is water’s boiling point higher than

expected in comparison to the other

group 6 hydrides?

[Total 5 marks]

Exam Answers

1. Except for water there is an increase in boiling point going down the group

2. The increase in the size/Mr and number of electrons…

3. Leads to an increase in Van der Waals

4. More energy…

5. Is needed to break the hydrogen bonds between water molecules

Three types of intermolecular bonding...•Instantaneous dipole-induced dipole

(otherwise known as Van der Waals)•Permanent dipole-induced dipole•Permanent dipole-Permanent dipole (of

which Hydrogen bonding is an extreme example)

Three types of intermolecular bonding...

Home Learning

•Complete worksheet on Intermolecular Bonding for next lesson.

ObjectivesBe able to...•Apply knowledge of electronegativity to

determine polarity of covalent bond•Describe permanent dipole-induced

dipoles•Define hydrogen bonding as a special type

of permanent dipole-induced dipole •Be able to recognise biochemical and

material examples of hydrogen bonding

Millard’s

Theory