Halogens Revision Chart
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1
2
35. Halogens
Fluorine (F
): very pale yellow gas. It is highly reactive
2
Chlorine : (Cl
) greenish, reactive gas, poisonous in high concentrations
2
Bromine (Br
) : red liquid, that gives off dense brown/orange poisonous fumes
2
Iodine (I
) : shiny grey solid sublimes to purple gas.
2
Trend in electronegativity
Trend in melting point and boiling point
Electronegativity is the relative tendency of an atom in a
Increase down the group
molecule to attract electrons in a covalent bond to itself.
As the molecules become larger they have
As one goes down the group the electronegativity of the
more electrons and so have larger van der
elements decreases.
waals forces between the molecules. As the
intermolecular forces get larger more energy
As one goes down the group the atomic radii increases due
has to be put into break the forces. This
to the increasing number of shells. The nucleus is therefore
increases the melting and boiling points
less able to attract the bonding pair of electrons
1. The displacement reactions of halide ions by halogens.
A halogen that is a strong oxidising agent will
The oxidising strength decreases down the group.
displace a halogen that has a lower oxidising
Oxidising agents are electron acceptors.
power from one of its compounds
know these
Chlorine will displace both bromide and iodide ions; bromine will displace iodide ions
observations !
Chlorine (aq)
Bromine (aq)
Iodine (aq)
The colour of the solution in
the test tube shows which free
potassium
Very pale green
Yellow solution, no
Brown solution,
halogen is present in solution.
chloride (aq)
reaction
no reaction
solution, no
Chlorine =very pale green
reaction
solution (often colourless),
Bromine = yellow solution
potassium
Yellow solution, Cl
Yellow solution, no
Brown solution,
Iodine = brown solution
bromide (aq)
has displaced Br
reaction
no reaction
(sometimes black solid
potassium
Brown solution, Cl
Brown Solution, Br
Brown Solution,
present)
iodide (aq)
has displaced I
has displaced I
no reaction
be able to write these reactions as two half
–
–
Cl
(aq) + 2Br
(aq)
2Cl
(aq) + Br
(aq)
2
2
equations showing oxidation or reduction
–
–
-
-
Cl
(aq) + 2I
(aq)
2Cl
(aq) + I
(aq)
e.g. 2Br
(aq)
Br
(aq)+ 2e
2
2
2
–
–
Br
(aq) + 2I
(aq)
2Br
(aq) + I
(aq)
-
-
Cl
(aq)+ 2e
2Cl
(aq)
2
2
2
2. The reactions of halide ions with silver nitrate.
The role of nitric acid is to react with any carbonates
present to prevent formation of the precipitate
This reaction is used as a test to identify which halide ion
Ag
CO
. This would mask the desired observations
2
3
is present. The test solution is made acidic with nitric
2 HNO
+ Na
CO
2 NaNO
+ H
O + CO
3
2
3
3
2
2
acid, and then Silver nitrate solution is added dropwise.
The silver halide precipitates can be treated with ammonia
Fluorides produce no precipitate
solution to help differentiate between them if the colours look
Chlorides produce a white precipitate
similar:
+
-
Ag
(aq) + Cl
(aq)
AgCl(s)
Bromides produce a cream precipitate
Silver chloride dissolves in dilute ammonia to form a
+
-
Ag
(aq) + Br
(aq)
AgBr(s)
complex ion
+
-
Iodides produce a pale yellow precipitate
AgCl(s) + 2NH
(aq)
[Ag(NH
)
]
(aq) + Cl
(aq)
3
3
2
+
-
Ag
(aq) + I
(aq)
AgI(s)
Colourless solution
Silver bromide dissolves in concentrated ammonia to form a
complex ion
+
-
AgBr(s) + 2NH
(aq)
[Ag(NH
)
]
(aq) + Br
(aq)
3
3
2
Colourless solution
Silver iodide does not react with ammonia – it is too insoluble.
N Goalby
1
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