The Halogens

Property Fluorine Chlorine Bromine Iodine

State at room temp Gas

Formula of molecule

Boiling point -188 °C –34 °C 59°C 184°C

Colour of vapour Pale yellow

Electronic configuration [He] [Ne] [Ar] [Kr]

The Halogens

Complete this table. Can you explain the trend in boiling point?

Property Fluorine Chlorine Bromine Iodine

State at room temp Gas Gas Liquid Solid

Formula of molecule

F2 Cl2 Br2 I2

Boiling point -188 °C –34 °C 59°C 184°C

Colour of vapour Pale yellow Yellow/green Brown Black

Electronic configuration [He]2s2 2p5 [Ne] 3s2 3p5 [Ar] 4s2 3d104p5 [Kr] 5s2 4d105p5

The Halogens

Complete this table. Can you explain the trend in boiling point?

Explaining the trend in boiling point

Temporary dipole occurs in one molecule owing to an unequal distribution of the electron cloud around it.

This temporary dipole induces a dipole in another molecule and the two attract each other.

Known as induced dipole-dipole forces or Van der Waals forces.

The strength of van der waals forces increase:

The larger the molecule (due to a larger electron cloud)

DISPLACEMENT REACTIONS

2KCl + I2

2KBr + I2

halogen

chlorine

bromine

iodine

salt (aq)potassium

chloridepotassium

iodidepotassiumbromide

2KCl + Br2

no reactionno reaction

no reaction

The Halogens: Recognising them

• The halogens form solutions with different colours. A colour change will show if a reaction has taken place.

Halogen Water Cyclohexane

Cl2 Pale-green Pale-green

Br2 Orange Orange

I2 Brown violet

If you shake the reaction mixture with an organic solvent it can help distinguish between them

What is the trend in reactivity of the halogens?

How can we explain this trend?

The halogens get less reactive as we descend the group.

This is because…

- atomic size- shielding- nuclear attraction

How does this link to redox?

Displacement: a redox reaction

Write an equation for the reaction between chlorine and bromide ions,

showing the changes in oxidation numbers.

Which species is the oxidising agent?

Cl2 (aq) + 2Br- (aq) 2Cl- (aq) + Br2 (aq)0 -1 -1 0

Chlorine is reducedBromine is oxidised

Chlorine acts as an oxidising agent

TRENDS IN OXIDISING ABILITY

Halogens react by gaining electrons

This means they are oxidising agents

They themselves are reduced

O xidationI sL osingR eductionI sG aining

Cl2 + 2e- 2Cl-

Gain of electrons

Oxidising ability of halogens decreases down the group

Because atoms become larger (and less electronegative)

Oxidising power trend: Cl2 > Br2 > I2

When a halogen acts as an oxidising agent, it gains electrons (taken from the oxidised species).

X2 + 2 e- → 2 X-

Going down the group it becomes harder to gain an electron because:Atoms are larger & there is more shielding (due to extra electron shell)

Cl

Br

I