Chapter 6
The Periodic Table: Group7
6.1 The Halogens
6.2 Reactions of The Halogens and Their Ions
6.1 What is the outcome from syllabus?
Candidates should be able to:
♣ describe the trends in volatility and colour of chlorine, bromine
and iodine
♣ interpret the volatility of the elements in terms of van der Waals’
forces
♣ describe the relative reactivity of the elements as oxidising agents
♣ describe and explain the reactions of the elements with
hydrogen
♣ describe and explain the relative thermal stabilities of the
hydrides and their relative stabilities in terms of bond energies
6.1 What is the outcome from syllabus?
All the elements in Group 7 are
nonmetals except for astatine,
which is a radioactive metalloid.
These elements are called
halogens, which means
“salt-former.” All of the
halogens form salts with
sodium and with the other
alkali metals.
6.1 The Halogens
greenish gas
red-brown liquid
black solid
6.1 The Halogens
Covalent
radius/nm
Tm/K
Tb/K
Cl
0.099
172
238
Br
0.114
266
332
I
0.133
387
457
The melting and boiling temperatures increase as going down the
group, because larger atoms makes the van der Waals forces
between the molecules stronger.
6.1 The Halogens
+ e-
X
halogen
→ Xˉ
reduction
halide
F
Cl
Br
Increasing
oxidising
power
This is because the electronegativity
increases as we go up the group, which
means that the elements gain electrons
more easily.
I
For example: Cl2 (aq) + 2Brˉ (aq) → Br2 (aq) + 2Clˉ(aq)
6.1 The Halogens
6.1 The Halogens
Formation of hydrogen hylides with hydrogen
H2(g) + Cl2 (g)
hν or heating
HCl (g)
vigorously reaction
H2(g) + F2 (g)
hν or heating
HF (g)
hard to control
H2(g) + Br2 (g)
H2(g) + I2 (g)
hν or heating
hν or heating
rather slowly,
almost no reactions
6.1 The Halogens
Laboratory preparations of HX
CaF2 + H2SO4(conc.) → CaSO4 + 2HF(g)
NaCl + H2SO4(conc.) → NaHSO4 + HCl(g)
∵ 2HBr(g) + H2SO4(conc.) → SO2(g) + 2H2O(g) + Br2(l)
8HI(g) + H2SO4(conc.) → H2S(g) + 4H2O(g) + 4I2(l)

∴ NaBr(s) + H3PO4(l) → NaH2PO4(aq) + HBr(g)

NaI(s) + H3PO4(l) → NaH2PO4(aq) + HI(g)
6.1 The Halogens
Acidity of hydrogen hylides in aqueous solution
HF
H2O
Hydrofluoric acid
Hydrogen fluoride
HCl
H2O
Hydrogen chloride
HBr
H2O
Hydrogen bromide
HI
Hydrogen iodide
The strong H—F bond must
be broken to release H+
H2O
Weak acid
Hydrochloric acid
Hydrobromic acid
Hydroiodic acid
Strong acid
6.1 The Halogens
Thermal Stability of HX
Bond
Tabulated bond
energy/kJ mol-1
H-F
568
H-Cl
432
H-Br
366
H-I
298
2HX(g) → H2(g) + X2(g,l,s)
By plunging a red-hot wire into a
test tube of the gas:
HI: easily decomposed.
HBr: may or may not decompose
depending on the exact temperature
of the wire.
HCl and HF: not decomposed.
6.1 The Halogens
√
√
6.2 What is the outcome from syllabus?
Candidates should be able to:
♣ describe and explain the reactions of halide ions with aqueous silver
ions followed by:
(i) aqueous ammonia
(ii) concentrated sulphuric acid
♣ outline a method for the manufacture of chlorine from brine by a
diaphragm cell
♣ describe and interpret in terms of changes of oxidation number the
reaction of chlorine with cold, and with hot, aqueous sodium hydroxide
♣ explain the use of chlorine in water purification
♣ recognise the industrial importance and environmental significance
of the halogens and their compounds, (e.g. for bleaches; PVC;
halogenated hydrocarbons as solvents, refrigerants and in aerosols)
6.2 Reactions of the Halogens and Their Ions
6.2 Reactions of the Halogens and Their Ions
Test for halide ions
The presence of Cl-(aq), Br-(aq) and I-(aq) can be confirmed by adding a few drops of
silver nitrate solution (fluorides are soluble):
AgNO3(aq) + X-(aq)
AgX(s) + NO3-(aq)
Silver halide
Colour
Chloride
White
Bromide
Cream
Iodide
Yellow
Silver chloride and bromide dissolve in concentrated ammonia, but
the iodide does not.
AgBr(s) + 2NH3(aq)
[Ag(NH3)2]+(aq) + Br-(aq)
6.1 Reactions of Halogens and their ions
Reactions of halide ions with conc. H2SO4
CaF2 + H2SO4(conc.) → CaSO4 + 2HF(g)
NaCl + H2SO4(conc.) → NaHSO4 + HCl(g)
With bromides and iodides, a redox reaction occurs:
NaBr(s) + H2SO4(conc.) → NaHSO4 + HBr(g)
2HBr(g) + H2SO4(conc.) → SO2(g) + 2H2O(g) + Br2(l)
NaI(s) + H2SO4(conc.) → NaHSO4 + HI(g)
8HI(g) + H2SO4(conc.) → H2S(g) + 4H2O(g) + 4I2(l)
6.2 Reactions of the Halogens and Their Ions
Colours of Silver halides
Ag+(aq) + Cl-(aq)
Ag+(aq) + Br-(aq)
Ag+(aq) + I-(aq)
AgCl(s)
AgBr(s)
AgI(s)
6.2 Manufacture of Chlorine from brine
An Anode (+): 2Cl-(aq)
At Cathode (-): 2H2O(l) + 2e-
Cl2(g) + 2e-
2OH-(aq) + H2(g)
Overall:
2NaCl(aq) + 2H2O(l)
Cl2(g) + H2(g) + 2NaOH(aq)
6.2 Reactions of Halogens and their ions
Reactions of chlorine with Alkalis
Cold dilute alkali:
Cl2(g) + 2NaOH(aq)
Cl-(aq) + ClO-(aq) + H2O(l)
Hot concentrated alkali:
3Cl2(g) + 6NaOH(aq)
5Cl-(aq) + ClO3-(aq) + H2O(l)
Both examples of disproportionation
Cl-: Cl (-I)
Cl2: Cl (0)
ClO-: Cl (+I)
ClO3-: Cl (+V)
(Roman numerals: I = 1; V = 5)
6.2 Reactions of Halogens and their ions
Reactions of chlorine with Alkalis
Cold dilute alkali:
Cl2(g) + 2NaOH(aq)
Cl-(aq) + ClO-(aq) + H2O(l)
Hot concentrated alkali:
3Cl2(g) + 6NaOH(aq)
5Cl-(aq) + ClO3-(aq) + H2O(l)
Both examples of disproportionation
Cl-: Cl (-I)
Cl2: Cl (0)
ClO-: Cl (+I) chlorate(I)
ClO3-: Cl (+V) chlorate(V)
(Roman numerals: I = 1; V = 5)
6.2 Reactions of Halogens and their ions
Commercial uses of halogens and their compounds
Chlorine is used in water purification as it destroys harmful
bacteria that could accumulate in old and unrecycled
drinking water.
Sodium chlorate(I), NaClO(aq) is used in bleaches.
Halogenated hydrocarbons are used as solvents
(dichloromethane CH2Cl2 dissolves many organic compounds).
CFCs were used as refrigerants and aerosols; section 12.3
They do not contain hydrogen atoms!
Some are used as anaesthetics: Fluothane - CF3CHBr.