2.7 Inorganic chemistry of group 7
(limited to chlorine, bromine and iodine)
Cro2012
Group 7 – the halogens
The elements in group 7 of the periodic table, on the right,
are called the halogens.
F
fluorine
Cl
chlorine
Br
bromine
I
At
iodine
astatine
a.
recall the characteristic physical properties of the elements limited to the
appearance of solutions of the elements in water and hydrocarbon solvents
In water:
In hydrocarbon solvents:
• Chlorine solution is pale green
• Bromine solution is yellow to
orange
• Iodine solution is brown.
• Chlorine solution is pale green
• Bromine solution is yellow to orange
• Iodine solution is pink-purple.
Oxidation Reaction of Halogens
b.
describe and carry out the following chemical reactions of halogens:
i.
oxidation reactions with metal and non-metallic elements and ions such as iron(II) and iron(III) ions in solution
With Metals:
React strongly with electropositive metals, removing the outer electrons to
become reduced themselves.
Oxidation Reaction of Halogens
b.
describe and carry out the following chemical reactions of halogens:
i.
oxidation reactions with metal and non-metallic elements and ions such as iron(II) and iron(III) ions in solution
With Non-Metals:
The halogen usually achieves a noble gas configuration by forming a
covalent bond:
Reactivity decreases
Oxidation Reaction of Halogens
b.
describe and carry out the following chemical reactions of halogens:
i.
oxidation reactions with metal and non-metallic elements and ions such as iron(II) and iron(III) ions in solution
With Iron(II) chloride solution:
Disproportionation
b.
describe and carry out the following chemical reactions of halogens:
ii. disproportionation reactions with cold and hot alkali, eg hot potassium hydroxide
with iodine to produce potassium iodate(V)
Give the oxidation
numbers for the chlorine
containing species.
Disproportionation
b.
describe and carry out the following chemical reactions of halogens:
ii. disproportionation reactions with cold and hot alkali, eg hot potassium hydroxide
with iodine to produce potassium iodate(V)
Iodine/thiosulfate titration
c.
carry out an iodine/thiosulfate titration, including calculation of the results and evaluation of the procedures involved,
eg determination of the purity of potassium iodate(V) by liberation of iodine and titration with standard sodium
thiosulfate solution
Remember that when carrying out this titration the starch is added near the end
point (when the solution is pale yellow).
Addition of starch to a solution that contains iodine or triiodide ion forms a
reversible blue complex. The disappearance of this blue coloured complex is a
much more sensitive method of determining the end point.
However, if the starch is added to a solution which contains a great deal of
iodine, the complex which forms may not be reversible. Therefore, the starch is
not added until shortly before the end point is reached.
Reaction with conc. Sulphuric Acid
d.
describe and carry out the following reactions:
i.
potassium halides with concentrated sulfuric acid, halogens and silver nitrate solution
White steamy fumes
• With sodium bromide/iodide, the hydrogen halides are
oxidised by the acid:
Orange/brown
fumes or colour
Purple fumes
and/or black solid
Displacement (Redox) Reactions
d.
describe and carry out the following reactions:
i. potassium halides with concentrated sulfuric acid, halogens and silver
nitrate solution
• Potassium halides react with
other halogens:
• What gets reduced and what gets
oxidised?
A more reactive halogen will displace a less
reactive halogen from it’s halide in solution.
So:
• Cl2 displaces Br2 from Br- and I2 from I• Br2 displaces I2 from I-
d.
Testing for Halides: Silver
Halides
describe and carry out the following reactions:
i.
potassium halides with concentrated sulfuric acid, halogens and silver nitrate solution
ii.
silver halides with sunlight and their solubilities in aqueous ammonia solution
Addition of dilute nitric acid
Then add aqueous silver
Nitrate solution.
Testing for Halides: Silver
Halides
Equations
Partial decomposition turns AgCl into
grayish precipitate in sunlight.
d.
describe and carry out the following reactions:
ii. silver halides with sunlight and their solubilities in aqueous ammonia
solution
The silver halides are unstable in the presence of sunlight. They
decompose forming silver (seen as dark specs) and the halogen,
for example:
2AgI(s)  2Ag(s) + I2(s)
Reaction of Hydrogen Halides
d.
describe and carry out the following reactions:
iii.
hydrogen halides with ammonia and with water (to produce acids)
• Hydrogen halides react with
ammonia gas to form ammonium
halides:
• Hydrogen chloride dissolves in
water to form hydrochloric acid.
e.
make predictions about fluorine and astatine and their compounds based on
the trends in the physical and chemical properties of halogens.
What are the general properties of
the halogens?
All the halogens are:
 Diatomic non-metals and so do not conduct electricity
 brittle and crumbly when solid
 poisonous and smelly.
 Very reactive and strong oxidising agents.
They become darker in colour down the group:
is pale yellow
is yellow-green
is red-brown
is grey
What is the electron structure of the
halogens?
All halogens have seven
electrons in their outer
shell. This means that:
 They can easily obtain a
full outer shell by gaining
one electron.
 They all gain an electron
in reactions to form
negative ions with a -1
charge.
 They have similar
chemical properties.
fluorine
2,7
chlorine
2,8,7
bromine
2,8,8,7
The reactivity of alkali metals decreases going down the
group. What is the reason for this?
 The atoms of each element get
F
larger going down the group.
 This means that the outer shell gets
further away from the nucleus and
is shielded by more electron shells.
Cl
 The further the outer shell is from
the positive attraction of the
nucleus, the harder it is to attract
another electron to complete the
outer shell.
 This is why the reactivity of the
halogens decreases going down
group 7.
Br
decrease in reactivity
How does electron structure affect reactivity?
What is the physical state of the
halogens?
The melting and boiling points of the halogens increase
down the group, as the molecules become bigger.
Halogen
Relative
size
Melting
point (°C)
Boiling
point (°C)
State
-220
-118
gas
-101
-34
gas
-7
59
liquid
114
184
solid
What is the state of each halogen at room temperature?
Halogen Solubility
• Solubility in water decreases down group.
• Chlorine reacts in water to form chlorine water ( a mixture of
HCl and chloric (I) acid:
• Chloric (I) acid is what gives a solution of chlorine its bleaching
properties.
• Bromine reacts in a similar way, but to a lesser extent.