Section 2c: The Group 7 elements — chlorine, bromine and iodine
2.9 recall the colours and physical states of the elements at room temperature
2.10 make predictions about the properties of other halogens in this group
2.13 describe the relative reactivities of the elements in Group 7
Appearance and physical states of the halogens
Study the trends in properties and complete the table.
element
colour
state
fluorine
density (g/cm3)
melting point (in C)
0.001696
-219
chlorine
yellow green
gas
0.003214
- 101
bromine
red brown
liquid
3.122
-7.2
iodine
dark grey
solid
4.809
113.5
7
302
astatine
Another family of elements
Group 7 elements share the following properties:
 they are all diatomic molecules i.e. F2, Cl2, Br2 and I2 , At2.
 react with:
 metals to form ionic compounds called salts
 water to form acidic solutions
 hydrogen to form hydrogen halides such as HF, HCl, HBr and HI.
Trends within the group
… as you go down group 7 …..
reactivity decreases; any halogen displaces a halogen ion from its salt which is lower in the
group as shown by displacement reactions
Displacement reactions
2.14 describe experiments to show that a more reactive halogen will displace a less reactive halogen
from a solution of one of its salts
2.15 understand these displacement reactions as redox reactions
These are reactions between solutions of the halogens (Cl2, Br2 and I2 are only slightly soluble in water
but soluble enough to make a solution) with solutions of their ionic compounds or salts. To be able to
analyze observations the following information is needed:
appearance of halogens
dissolved in water
appearance of halogens
dissolved in methyl benzene
appearance of sodium halides
Cl2 (aq)
colourless
Cl2 (aq)
colourless
NaCl (aq)
Br2 (aq)
red brown
Br2 (aq)
red brown
NaBr (aq)
I2 (aq)
dark red brown
I2 (aq)
purple
NaI (aq)
Section 2c The Group 7 elements — chlorine, bromine and iodine
colourless
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Halogens prefer to be dissolved in methyl benzene instead of water.
The table below summarises the results of a set of displacement experiments
solutions of halogen salts to which halogen is added
NaCl (aq)
NaBr (aq)
NaI (aq)

Cl2 (aq)
no colour change
Br2 (aq)
no colour change
colourless solution goes
yellow/brown
 when shaken with methyl
benzene a red-brown layer
is formed on top of water
no colour change
I2 (aq)
no colour change
no colour change

colourless solution goes dark redbrown
 when shaken with methyl benzene,
a purple layer is formed on top of
water
 colourless solution goes dark redbrown
 when shaken with methyl benzene,
a purple layer is formed on top of
water
no colour change
Redox reactions. The above observations indicate that the following reactions have taken place:
Cl2 (aq) + 2NaBr (aq)  Br2 (aq) + 2NaCl (aq)
During this reaction, the chlorine displaces the bromide ion. The chloride ion is reduced (it gains an
electron) and bromide ion is oxidized (it loses an electron). This displacement reaction is also a redox
reaction:
Half equations:
Ionic equation:
reduction: Cl2 + 2e-  2Cl-
oxidation:
2Br-  Br2
+2e-
Cl2 (aq) + 2Br- (aq)  Br2 (aq) + 2Cl- (aq)
Cl2 (aq) + 2NaI (aq)  I2 (aq) + 2NaCl (aq)
During this reaction, the chlorine displaces the iodide ion. The chloride ion is reduced (it gains an
electron) and iodide ion is oxidized (it loses an electron).
Half equations:
Ionic equation:
reduction: Cl2 + 2e-  2Cl-
oxidation:
2I-  I2
+2e-
Cl2 (aq) + 2I- (aq)  I2 (aq) + 2Cl- (aq)
Br2 (aq) + 2NaI (aq)  I2 (aq) + 2NaBr (aq)
During this reaction, the chlorine displaces the iodide ion. The chloride ion is reduced (it gains an
electron) and iodide ion is oxidized (it loses an electron).
Half equations:
reduction: Br2 + 2e-  2Br-
Ionic equation:
Br2 (aq) + 2I- (aq)  I2 (aq) + 2Br- (aq)
oxidation:
Section 2c The Group 7 elements — chlorine, bromine and iodine
2I-  I2
+2e-
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Hydrogen chloride – a halogen compound
2.11 understand the difference between hydrogen chloride gas and hydrochloric acid
2.12 explain, in terms of dissociation, why hydrogen chloride is acidic in water but not in methylbenzene.
Hydrogen chloride gas, HCl (g), is a covalent compound which has a low melting and boiling point, and
does not conduct electricity.
However when hydrogen chloride gas is dissolved in water and forms hydrogen and chloride ions
(hydrogen chloride dissociates) it becomes hydrochloric acid; it becomes a very corrosive chemical with
a very low pH which reacts easily with metals, bases and metal carbonates. It is the hydrogen ions
which give the hydrogen chloride solution its acidic character.
The equation for the dissociation of hydrogen chloride:
HCl (g)
+
H2O (l)

H3O+ (aq)
+
Cl- (aq)
We can write a simplified version of the above equation:
HCl (aq)

H+ (aq)
+
Cl- (aq)
When hydrogen chloride is added to methylbenzene the dissociation process does not occur as a result
hydrogen chloride does not behave like an acid i.e. does not change the colour of an universal indicator
solution or blue litmus and does not conduct electricity.
Section 2c The Group 7 elements — chlorine, bromine and iodine
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