AN INTRODUCTION TO
GROUP VII
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS
KNOCKHARDY PUBLISHING
GROUP 7 (HALOGENS)
INTRODUCTION
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GROUP VII
CONTENTS
• Trends in appearance
• Trends in electronic configuration
• Trends in in Atomic and Ionic radius
• Trends in Electronegativity
• Trends in oxidation power
• Displacement reactions
• Other reactions
• Testing for halides – AgNO3
• Testing for halides - concentrated H2SO4
• Volumetric analysis of chlorate(I) in bleach
GROUP PROPERTIES
GENERAL
• non-metals
• exist as separate diatomic molecules… eg Cl2
• all have the electronic configuration ... ns2 np5
TRENDS
•
•
•
•
•
•
•
appearance
boiling point
electronic configuration
electronegativity
atomic size
atomic size
oxidising power
GROUP TRENDS
APPEARANCE
F2
Cl2
Br2
I2
Colour
Yellow
Green
Red/brown
Grey
State (at RTP)
GAS
GAS
LIQUID
SOLID
BOILING POINT
Boiling point / °C
F2
Cl2
Br2
I2
- 188
- 34
58
183
INCREASES down Group
• increased size makes the van der Waals forces increase
• more energy is required to separate the molecules
GROUP TRENDS
ELECTRONIC CONFIGURATION
F
Cl
Br
I
Atomic Number
9
17
35
53
Old
2,7
2,8,7
2,8,18,7
2,8,18,18,7
New
… 2s2 2p5
…3s2 3p5
… 4s2 4p5
… 5s2 5p5
• electrons go into shells further from the nucleus
GROUP TRENDS
ATOMIC & IONIC RADIUS
Atomic radius / nm
Ionic radius / nm
F
Cl
Br
I
0.064
0.099
0.111
0.128
F¯
Cl¯
Br¯
I¯
0.136
0.181
0.195
0.216
ATOMIC RADIUS
INCREASES down Group
IONIC RADIUS
INCREASES down Group
• the greater the atomic number the more electrons there are
these go into shells increasingly further from the nucleus
• ions are larger than atoms - the added electron repels the
others so radius gets larger
GROUP TRENDS
ELECTRONEGATIVITY
Electronegativity
F
Cl
Br
I
4.0
3.5
2.8
2.5
DECREASES down Group
• the increasing nuclear charge due to the greater number of protons
should attract electrons more, but there is an ...


an increasing number of shells;
more shielding and less pull on electrons
an increasing atomic radius
attraction drops off as distance increases
GROUP TRENDS
OXIDISING POWER
• halogens are oxidising agents
• they need one electron to complete their octet
• the oxidising power gets weaker down the group
GROUP TRENDS
OXIDISING POWER
• halogens are oxidising agents
• they need one electron to complete their octet
• the oxidising power gets weaker down the group
• the trend can be explained by considering the nucleus’s attraction
for the incoming electron which is affected by the...
• increasing nuclear charge which should attract electrons more
but this is offset by
• INCREASED SHIELDING
• INCREASING ATOMIC RADIUS
GROUP TRENDS
OXIDISING POWER
• halogens are oxidising agents
• they need one electron to complete their octet
• the oxidising power gets weaker down the group
• the trend can be explained by considering the nucleus’s attraction
for the incoming electron which is affected by the...
• increasing nuclear charge which should attract electrons more
but this is offset by
• INCREASED SHIELDING
• INCREASING ATOMIC RADIUS
This is demonstrated by reacting the halogens with other halide ions.
HALOGENS - DISPLACEMENT REACTIONS
THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED
USING DISPLACEMENT REACTIONS...
HALOGENS - DISPLACEMENT REACTIONS
THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED
USING DISPLACEMENT REACTIONS...
A SOLUTION OF THE HALOGEN IS ADDED TO A SOLUTION OF A HALIDE
HALIDES ARE SALTS FORMED FROM HALOGENS (see next slide for examples)
A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE
HALOGENS - DISPLACEMENT REACTIONS
THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED
USING DISPLACEMENT REACTIONS...
A SOLUTION OF THE HALOGEN IS ADDED TO A SOLUTION OF A HALIDE
HALIDES ARE SALTS FORMED FROM HALOGENS (see next slide for examples)
A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE
e.g.
CHLORINE + SODIUM BROMIDE
BROMINE + SODIUM CHLORIDE
HALOGENS - DISPLACEMENT REACTIONS
THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED
USING DISPLACEMENT REACTIONS...
A SOLUTION OF THE HALOGEN IS ADDED TO A SOLUTION OF A HALIDE
HALIDES ARE SALTS FORMED FROM HALOGENS (see next slide for examples)
A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE
e.g.
CHLORINE + SODIUM BROMIDE
Cl2(aq) + 2NaBr(aq)
BROMINE + SODIUM CHLORIDE
——> Br2(aq) + 2NaCl(aq)
HALOGENS - DISPLACEMENT REACTIONS
THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED
USING DISPLACEMENT REACTIONS...
A SOLUTION OF THE HALOGEN IS ADDED TO A SOLUTION OF A HALIDE
HALIDES ARE SALTS FORMED FROM HALOGENS (see next slide for examples)
A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE
e.g.
CHLORINE + SODIUM BROMIDE
Cl2(aq) + 2NaBr(aq)
BROMINE + SODIUM CHLORIDE
——> Br2(aq) + 2NaCl(aq)
Cl2(aq) + 2Br¯(aq) ——>
Br2(aq) + 2Cl¯(aq)
HALOGENS - DISPLACEMENT REACTIONS
THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED
USING DISPLACEMENT REACTIONS...
A SOLUTION OF THE HALOGEN IS ADDED TO A SOLUTION OF A HALIDE
HALIDES ARE SALTS FORMED FROM HALOGENS (see next slide for examples)
A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE
e.g.
CHLORINE + SODIUM BROMIDE
CHLORINE + SODIUM IODIDE
BROMINE + SODIUM CHLORIDE
IODINE + SODIUM CHLORIDE
HALOGENS - DISPLACEMENT REACTIONS
THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED
USING DISPLACEMENT REACTIONS...
A SOLUTION OF THE HALOGEN IS ADDED TO A SOLUTION OF A HALIDE
HALIDES ARE SALTS FORMED FROM HALOGENS (see next slide for examples)
A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE
e.g.
CHLORINE + SODIUM BROMIDE
CHLORINE + SODIUM IODIDE
BROMINE + SODIUM IODIDE
BROMINE + SODIUM CHLORIDE
IODINE + SODIUM CHLORIDE
IODINE + SODIUM BROMIDE
HALOGENS - DISPLACEMENT REACTIONS
THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED
USING DISPLACEMENT REACTIONS...
A SOLUTION OF THE HALOGEN IS ADDED TO A SOLUTION OF A HALIDE
HALIDES ARE SALTS FORMED FROM HALOGENS (see next slide for examples)
A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE
e.g.
CHLORINE + SODIUM BROMIDE
CHLORINE + SODIUM IODIDE
BROMINE + SODIUM IODIDE
BUT
BROMINE + SODIUM CHLORIDE
BROMINE + SODIUM CHLORIDE
IODINE + SODIUM CHLORIDE
IODINE + SODIUM BROMIDE
CHLORINE + SODIUM BROMIDE
(Bromine is below chlorine in the Group so is less reactive)
GROUP TRENDS
OXIDISING POWER
Chlorine oxidises bromide ions to bromine
Cl2 + 2Br¯ ——> Br2 + 2Cl¯
Chlorine oxidises iodide ions to iodine
Cl2 + 2I¯
——> I2
+ 2Cl¯
Bromine oxidises iodide ions to iodine
Br2 + 2I¯
——> I2 + 2Br¯
HALOGENS - DISPLACEMENT REACTIONS
DISPLACEMENT REACTIONS - EXPERIMENT
SODIUM CHLORIDE
CHLORINE
BROMINE
SODIUM BROMIDE
SODIUM IODIDE
HALOGENS - DISPLACEMENT REACTIONS
DISPLACEMENT REACTIONS - EXPERIMENT
CHLORINE
BROMINE
SODIUM CHLORIDE
SODIUM BROMIDE
SODIUM IODIDE
Solution stays
colourless
NO REACTION
Solution goes from
colourless to orangeyellow
BROMINE FORMED
Solution goes from
colourless to orangered
IODINE FORMED
Solution goes from
colourless to orangeyellow
NO REACTION
Solution goes from
colourless to orangeyellow
NO REACTION
Solution goes from
colourless to red
IODINE FORMED
OTHER REACTIONS OF CHLORINE
Water
Halogens react with decreasing vigour down the group as
their oxidising power decreases
Litmus will be turned red then decolourised in chlorine water
Cl2(g) + H2O(l)
HCl(aq)
strong acid
+
HOCl(aq)
bleaches by oxidation
OTHER REACTIONS OF CHLORINE
Water
Halogens react with decreasing vigour down the group as
their oxidising power decreases
Litmus will be turned red then decolourised in chlorine water
0
Cl2(g) + H2O(l)
-1
HCl(aq)
strong acid
+1
+
HOCl(aq)
bleaches by oxidation
OTHER REACTIONS OF CHLORINE
Water
Halogens react with decreasing vigour down the group as
their oxidising power decreases
Litmus will be turned red then decolourised in chlorine water
0
Cl2(g) + H2O(l)
-1
HCl(aq)
strong acid
+1
+
HOCl(aq)
bleaches by oxidation
This is an example of DISPROPORTIONATION …
‘simultaneous oxidation and reduction of a species’
OTHER REACTIONS OF CHLORINE
Water
Halogens react with decreasing vigour down the group as
their oxidising power decreases
Litmus will be turned red then decolourised in chlorine water
0
Cl2(g) + H2O(l)
-1
HCl(aq)
strong acid
+1
+
HOCl(aq)
bleaches by oxidation
This is an example of DISPROPORTIONATION …
‘simultaneous oxidation and reduction of a species’
Alkalis
Chlorine reacts with cold, aqueous sodium hydroxide.
2NaOH(aq) + Cl2(g) —> NaCl(aq) + NaOCl(aq) + H2O(l)
TESTING FOR HALIDES – AgNO3
• make a solution of the halide
• acidify with dilute nitric acid – this prevents the precipitation of other salts
• add a few drops of silver nitrate solution
• treat any precipitate with dilute ammonia solution
• if a precipitate still exists, add concentrated ammonia solution
TESTING FOR HALIDES – AgNO3
CHLORIDE
white ppt of AgCl
soluble in dilute ammonia
BROMIDE
cream ppt of AgBr
insoluble in dilute ammonia
but soluble in conc.
IODIDE
yellow ppt of AgIinsoluble in dilute and
conc. ammonia solution
TESTING FOR HALIDES – AgNO3
CHLORIDE
white ppt of AgCl
soluble in dilute ammonia
BROMIDE
cream ppt of AgBr
insoluble in dilute ammonia
but soluble in conc.
IODIDE
yellow ppt of AgIinsoluble in dilute and
conc. ammonia solution
halides precipitate as follows
Ag+(aq) + X¯(aq) ——> Ag+X¯(s)
when they dissolve in ammonia a colourless diammine complex is
formed [Ag(NH3)2]+(aq)
TESTING FOR HALIDES – AgNO3
CHLORIDE
BROMIDE
IODIDE
PLACE A SOLUTION OF THE HALIDE IN A TEST TUBE
TESTING FOR HALIDES – AgNO3
CHLORIDE
BROMIDE
ADD SOME DILUTE NITRIC ACID
IODIDE
TESTING FOR HALIDES – AgNO3
CHLORIDE
BROMIDE
IODIDE
ADD SILVER NITRATE SOLUTION
WHITE PRECIPITATE OF SILVER CHLORIDE
CREAM PRECIPITATE OF SILVER BROMIDE
YELLOW PRECIPITATE OF SILVER IODIDE
AgCl
AgBr
AgI
TESTING FOR HALIDES – AgNO3
CHLORIDE
BROMIDE
IODIDE
ADD DILUTE AMMONIA SOLUTION
WHITE PRECIPITATE OF SILVER CHLORIDE
CREAM PRECIPITATE OF SILVER BROMIDE
YELLOW PRECIPITATE OF SILVER IODIDE
- SOLUBLE
- INSOLUBLE
- INSOLUBLE
TESTING FOR HALIDES – AgNO3
CHLORIDE
BROMIDE
IODIDE
ADD CONCENTRATED AMMONIA SOLUTION
WHITE PRECIPITATE OF SILVER CHLORIDE
CREAM PRECIPITATE OF SILVER BROMIDE
YELLOW PRECIPITATE OF SILVER IODIDE
- SOLUBLE
- SOLUBLE
- INSOLUBLE
TESTING FOR HALIDES – Conc. H2SO4
• an alternative test for halides
• add concentrated sulphuric acid carefully to a solid halide
• H2SO4 displaces the weaker acids HCl, HBr, and HI from their salts
• as they become more powerful reducing agents down the group
they can react further by reducing the sulphuric acid to lower
oxidation states of sulphur
TESTING FOR HALIDES – Conc. H2SO4
Summary
Halide
Observation(s)
Product
O.S.
Reaction type
NaCl
misty fumes
HCl
-1
Displacement of Cl¯
NaBr
misty fumes
brown vapour
colourless gas
HBr
Br2
SO2
-1
0
+4
Displacement of Br¯
Oxidation of Br¯
Reduction of H2SO4
NaI
misty fumes
purple vapour
colourless gas
yellow solid
bad egg smell
HI
I2
SO2
S
H2S
-1
0
+4
0
-2
Displacement of I¯
Oxidation of I¯
Reduction of H2SO4
Reduction of H2SO4
Reduction of H2SO4
HYDROGEN HALIDES - PROPERTIES
Boiling points
b. pts ...
At room temp. and pressure HCl, HBr, HI are
colourless gases, HF a colourless liquid.
HF 20°C
HCl -85°C
HBr -69°C
HI -35°C
HF value is higher than expected due to hydrogen bonding
HYDROGEN HALIDES - PROPERTIES
Boiling points
b. pts ...
At room temp. and pressure HCl, HBr, HI are
colourless gases, HF a colourless liquid.
HF 20°C
HCl -85°C
HBr -69°C
HI -35°C
HF value is higher than expected due to hydrogen bonding
Reducing
ability
bond energy
/ kJ mol-1
Increases down the group as bond strength decreases
H-F 568
H-Cl 432
H-Br 366
H-I 298
HYDROGEN HALIDES - PREPARATION
Direct
combination
Hydrogen halides can be made by direct combination
H2(g) + X2(g) ——> 2HX(g)
•
•
•
•
fluorine combines explosively, even in the dark
chlorine combines explosively when heated or in sunlight
bromine is fast at 200°C with a catalyst
iodine reaction is reversible.
HYDROGEN HALIDES - PREPARATION
Direct
combination
Hydrogen halides can be made by direct combination
H2(g) + X2(g) ——> 2HX(g)
•
•
•
•
Displacement
fluorine combines explosively, even in the dark
chlorine combines explosively when heated or in sunlight
bromine is fast at 200°C with a catalyst
iodine reaction is reversible.
Chlorides are made by displacing the acid from its salt
NaCl(s) + conc. H2SO4(l) ——>
NaHSO4(s) + HCl(g)
HYDROGEN HALIDES - PREPARATION
Direct
combination
Hydrogen halides can be made by direct combination
H2(g) + X2(g) ——> 2HX(g)
•
•
•
•
Displacement
fluorine combines explosively, even in the dark
chlorine combines explosively when heated or in sunlight
bromine is fast at 200°C with a catalyst
iodine reaction is reversible.
Chlorides are made by displacing the acid from its salt
NaCl(s) + conc. H2SO4(l) ——>
NaHSO4(s) + HCl(g)
HBr and HI are not made this way as they are more powerful
reducing agents and are oxidised by sulphuric acid to the halogen
2HBr(g) +
conc. H2SO4(l) ——>
2H2O(l) + SO2(g) + Br2(g)
HALOGENS & HALIDES - USES
Chlorine, Cl2
•
•
•
•
•
water purification
bleach
solvents
polymers - poly(chloroethene) or PVC
CFC’s
Fluorine, F2
• CFC’s
• polymers - PTFE poly(tetrafluoroethene) as used in...
non-stick frying pans, electrical insulation,
waterproof clothing
Fluoride, F¯
• helps prevent tooth decay
- tin fluoride is added to toothpaste
- sodium fluoride is added to water supplies
Hydrogen
fluoride, HF
• used to etch glass
Silver
bromide, AgBr
• used in photographic film
VOLUMETRIC ANALYSIS OF CHLORATE(I)
Introduction
Chlorate(I) ions are oxidising agents
In acid solution they end up as chloride ions
ClO¯
+
2H+
+
2e¯
——>
Cl¯
+
H 2O
VOLUMETRIC ANALYSIS OF CHLORATE(I)
Introduction
Chlorate(I) ions are oxidising agents
In acid solution they end up as chloride ions
ClO¯
Oxidation state
+1
+
2H+
+
2e¯
——>
Cl¯
-1
+
H 2O
VOLUMETRIC ANALYSIS OF CHLORATE(I)
Introduction
Chlorate(I) ions are oxidising agents
In acid solution they end up as chloride ions
ClO¯
Analysis (1)
+
2H+
+
2e¯
——>
Cl¯
+
H 2O
Add excess potassium iodide;
the chlorate oxidises the iodide ions to iodine
ClO¯
+
2H+
+
2e¯ ——>
Cl¯
2I¯ ——>
I2
+
+
H2 O
2e¯
VOLUMETRIC ANALYSIS OF CHLORATE(I)
Introduction
Chlorate(I) ions are oxidising agents
In acid solution they end up as chloride ions
ClO¯
Analysis (1)
2H+
+
2e¯
——>
Cl¯
+
H 2O
Add excess potassium iodide;
the chlorate oxidises the iodide ions to iodine
ClO¯
overall
+
+
2H+
+
2e¯ ——>
Cl¯
2I¯ ——>
I2
+
2e¯
——>
I2
+
Cl¯
ClO¯ + 2H+ + 2I¯
+
H2 O
+
moles of I2 produced = original moles of OCl¯
H2 O
-- (i)
VOLUMETRIC ANALYSIS OF CHLORATE(I)
Introduction
Chlorate(I) ions are oxidising agents
In acid solution they end up as chloride ions
ClO¯
Analysis (2)
+
2H+
+
2e¯
——>
Cl¯
+
H 2O
Titrate the iodine produced with sodium thiosulphate
using starch as the indicator near the end point
2S2O32- ——>
I2
+
2e¯ ——>
S4O622I¯
+
2e¯
VOLUMETRIC ANALYSIS OF CHLORATE(I)
Introduction
Chlorate(I) ions are oxidising agents
In acid solution they end up as chloride ions
ClO¯
Analysis (2)
+
2H+
+
2e¯
——>
+
H 2O
Titrate the iodine produced with sodium thiosulphate
using starch as the indicator near the end point
2S2O32- ——>
I2
overall
Cl¯
I2
+
2e¯ ——>
+ 2S2O32- ——>
S4O62-
+
2e¯
+
2I¯
2I¯
S4O62-
VOLUMETRIC ANALYSIS OF CHLORATE(I)
Introduction
Chlorate(I) ions are oxidising agents
In acid solution they end up as chloride ions
ClO¯
Analysis (2)
+
2H+
+
2e¯
——>
I2
I2
+
2e¯ ——>
+ 2S2O32- ——>
moles of I2
from (i) and (ii)
+
H 2O
Titrate the iodine produced with sodium thiosulphate
using starch as the indicator near the end point
2S2O32- ——>
overall
Cl¯
S4O62-
+
2e¯
+
2I¯
2I¯
S4O62-
= ½ x moles of S2O32-
original moles of OCl¯ =
-- (ii)
½ x moles of S2O32-
AN INTRODUCTION TO
GROUP VII
THE END
©2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING