the halogens
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The Halogens. C omplete this table. Can you explain the trend in boiling point?. The Halogens. C omplete this table. Can you explain the trend in boiling point?. Temporary dipole occurs in one molecule owing to an unequal distribution of the electron cloud around it. - PowerPoint PPT PresentationTRANSCRIPT
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