Download Halogens

The Halogens
Element
Ionization
Enthalpy
(kJ/mole)
Electron
Affinity
(kJ/mol)
P
Ionic
Radius Å
Common
Oxidation
#’s
F
1687
334
3.98
1.17
-1
Cl
1257
355
3.16
1.67
-1,1,3,5,7
Br
1146
325
2.96
1.82
-1,1,3,5,7
I
1015
295
2.66
2.06
-1,1,3,5,7
At
270
Fluorine 20-2
(L. and F. fluere, flow or flux) In 1529, Georigius Agricola
described the use of fluorspar(CaF2) as a flux, and as early as
1670 Schwandhard found that glass was etched when
exposed to fluorspar treated with acid. Scheele and many
later investigators, including Davy, Gay-Lussac, Lavoisier,
and Thenard, experimented with hydrofluoric acid, some
experiments ending in tragedy. The element was finally
isolated in 1886 by Moissan after nearly 74 years of
continuous effort.
Fluorine (continue) 20-2

Fluorine is the most electronegative
and reactive of all elements. It is a pale
yellow, corrosive gas, which reacts with
practically all organic and inorganic
substances. Finely divided metals,
glass, ceramics, carbon, and even
water burn in fluorine with a bright
flame.
Chlorine (20-2)
(Gr. chloros, greenish yellow) Discovered in 1774 by Scheele,
who thought it contained oxygen; named in 1810 by Davy,
who insisted it was an element. In nature it is found in the
combined state only, chiefly with sodium as common salt
(NaCl), carnallite, and sylvite. It is a member of the halogen
(salt-forming) group of elements and is obtained from
chlorides by the action of oxidizing agents and more often by
electrolysis; it is a greenish-yellow gas, combining directly
with nearly all elements.
Bromine (20-2)
Gr. bromos, stench) Discovered by Balard in 1826, but not
prepared in quantity until 1860. A member of the halogen
group of elements, it is obtained from natural brines from
wells in Michigan and Arkansas. Bromine is the only liquid
nonmetallic element. It is a heavy, mobile, reddish-brown
liquid, volatilizing readily at room temperature to a red vapor
with a strong disagreeable odor, resembling chlorine, and
having a very irritating effect on the eyes and throat; it less
active than chlorine but more so than iodine; it unites readily
with many elements and has a bleaching action; when spilled
on the skin it produces painful sores. It presents a serious
health hazard, and maximum safety precautions should be
taken when handling it
Iodine
(Gr. iodes, violet) Discovered by Courtois in 1811, Iodine
occurs sparingly in the form of iodides in sea water from
which it is assimilated by seaweeds, in Chilean saltpeter and
nitrate-bearing earth, known as caliche, in brines from old
sea deposits, and in brackish waters from oil and salt wells.
Ultra pure iodine can be obtained from the reaction of
potassium iodide with copper sulfate. Iodine is a bluish-black,
lustrous solid, volatilizing at ordinary temperatures into a
blue-violet gas with an irritating odor; it forms compounds
with many elements, but is less active than the other
halogens, which displace it from iodides. Iodine exhibits
some metallic-like properties.
Astatine
(Gr. astatos, unstable) Synthesized in 1940 by D.R. Corson, K.R.
MacKenzie, and E. Segre at the University of California by
bombarding bismuth (Bi) with alpha particles. The longest-lived
isotopes are found with naturally occurring uranium and thorium
isotopes, and traces of At-217 are found in equilibrium with U-233
and Np-239. The total amount of astatine present in the earth's
crust, however, is less than 1 oz.
The Halogens
Occurrence and Isolation
•The halogens are so reactive that they are never
found pure in nature, only as compounds. The elements
are found mainly as halides in nature, but the most easily
oxidized element, I, also is found as the iodate, MIO3 (M =
Na, K)
•Their abundance in the earth’s crust decreases
steadily with atomic number from fluorine to iodine
(0.065, 0.031, 1.6E-4, 5E-8 %)
•All of the dihalogens are produced commercially on a
large scale with chlorine production the greatest
The Halogens
Occurrence and Isolation
•Chlorides, bromides and iodides are generally
water-soluble and these anions are found in
abundance in the oceans and in brines
•The primary source of fluorine is calcium fluoride,
CaF2 which is highly insoluble and often found in
sedimentary deposits
The Halogens
Because of the high standard oxidation potentials of the
halides:
F: +2.87 V
Cl: +1.36 V
only electrolytic oxidation is feasible.
Therefore, the principal method of isolation of fluorine,
chlorine is via electrolysis of solutions of halide salts.
The Halogens
Electrolysis of water-based solutions of fluorides is a
problem, since water is more easily oxidized than F(+1.23 V vs +2.87 V)
Fluorine is therefore prepared by the electrolysis of a
solution of potassium fluoride in liquid hydrogen fluoride
Chlorine is produced by electrolysis of aqueous NaCl
solutions
Bromine and iodine are obtained by the chemical oxidation
of their anions in sea water or brines using the more
strongly oxidizing halogen, chlorine
The Halogens
Electrolysis cell for fluorine
Chlor-alkali cell
The Halogens
Trends in Properties
Structures:
•Remarkably similar
•All occur as diatomic molecules
•Increasing tendency to intermolecular
interactions as the group is descended
•Iodine has mildly metallic characteristics
The Halogens
Molecular Properties
•Gap between g* and u*
decreases descending the
group – gives rise to changing
colours
•For F2, the order of g and g
is reversed
MO energy diagram for X2
Molecular Properties
Relative to
the other
halogens,
the F-F
bond and
even some
X-F bonds
is
surprisingly
weak
The Halogens
Hydrogen-halogen
Carbon-halogen
Halogen-halogen
F bonds are weakened by strong repulsions between nonbonding electrons
The Halogens
Molecular Properties
Cl2
Br2
I2
Temp/°C
-160
-106
-163
Bond Length
Å
1.98
2.27
2.72
Nonbonding
distance Å
3.32
3.32
3.50
Ratio
1.68
1.46
1.29
Molecular structure of Cl2, Br2, I2
Reactivity
The Halogens
Fluorine is the most reactive element known
•Stongest oxidizing agent among halogens
•May be due to low kinetic barrier
•Weak F-F bond
•Fluorine can be handled in metal containers
•Forms passivating layers
•Fluorocarbon polymers also useful in handling
fluorine – Teflon, polytetrafluoroethylene
The Halogens
Reactivity
C-Pressure gauge
A-Monel valves
B-Nickel
condensation
tube
D-Nickel
Gas
Storage
Cont’r
GNeutralizer
tube
F-Ni Rx Vessel
E-PTFE Rx tube
Vacuum line for Fluorine Reactions
Special Properties of Fluorine
The Halogens
1. High volatility
F2
-188.2
H2
-252.8
Cl2
-34.0
CF4
-127.9
CH4
-161.5
CCl4
76.7
PF3
-101.5
PH3
-87.7
PCl3
75.5
Boiling points of some fluorine compounds and their
analogs
Special Properties of Fluorine
The Halogens
Enhanced Bronsted Acidity
HSO3CF3 – pKa = 3.0 in nitromethane
HSO3CH3 – pKa = 6.0 in nitromethane
High Lewis Acidity
SbF5 one of the strongest Lewis acids while SbCl5 is
much weaker
Fluorine second only to oxygen in ability to stabilize high
oxidation states
IF7, PtF6, BiF5, KAgF4 – highest known oxidation state
The Hydrogen Halides (HX)
The hydrogen halides are compounds
that contain hydrogen attached to one
of the halogens (HF, HCl, HBr, and HI).
These compounds are all colorless
gases, which are soluble in water. Up to
512 mL of HCl gas can dissolve in a
single mL of water at 0oC and 1 atm, for
example. Each of the hydrogen halides
ionizes to at least some extent when it
dissolves in water.
 HCl(g)+H2O(l) H+(aq)+ Cl-(aq)

Several of the hydrogen halides can be prepared directly from the
elements. Mixtures of H2 and Cl2, for example, react with
explosive violence in the presence of light to form HCl.
H2(g)+Cl2(g)  2 HCl(g)
Because chemists are usually more interested in aqueous
solutions of these compounds than the pure gases, these
compounds are usually synthesized in water. Aqueous solutions
of the hydrogen halides are often called mineral acids because
they are literally acids prepared from minerals.
Sulfuric for F and Cl
Phosphoric for Br and I
•All are good solvents for ionic
compounds
Relative acid strength of HX
HF << HCl < HBr < HI.
 HI strongest as H-I bond is weakest.

Acidity of hydrogen fluoride

Aqueous HF is only partly ionized in
water.
HF(aq) <-----> H+(aq) + F-(aq)

F- can now form a hydrogen bond
between itself and a HF molecule
cutting HF available for H+.

F- + HF <-----> FHF-
Binary Metal Halides (20-3)




Predominantly ionic, but some are covalent, and
some have character of both
- As charge/radius ratio of metal ion increases,
covalency of bond increases
F- ion has radius comparable to O2-, thus many
fluorides and oxides have similar structures
Cl-, Br-, and I- resemble S2- more closely, and give
structures similar to its compounds
Cl- can form covalent structures, while F- cannot
Binary Metal Halides

Formation of MX, MX2, and MX3
- Use element and metal hydride
 Formation of MX3, MX4, and MX5
- Five general reactions
 Useful in the Van Arkel process for metal
purification
- Form volatile metal halide
- Decompose metal halide to metal and
halide
Hydrolysis of Molecular Halides (P247)
General properties
BCl3 + 3H2O
B(OH)3 + 3H+ + 3 Cl–
PBr3 + 3H2O
HP(OH)2 + 3H+ + 3 Br–
SiCl4 + 4H2O
Si(OH)4 + 4H+ + 4 Cl–
 Kinetic inert
CCl4 SF6 (SeF6 TeF6)
1E23

Neutral Oxides of the Halogens
(20-4)
• Under certain conditions, it is possible to isolate
neutral oxides of the halogens, such as Cl2O,
Cl2O3, ClO2, Cl2O4, Cl2O6, and Cl2O7. Cl2O7, for
example, can be obtained by dehydrating
perchloric acid, HClO4. O2F2
• These oxides are notoriously unstable
compounds that explode when subjected to
either thermal or physical shock. Some are so
unstable they detonate when warmed to
temperatures above -40oC
Oxyacids of the Halogens and Their Salts
(20-5)
Oxyanions and Oxyacids of Chlorine
Oxyanions
Oxyacids
Oxidation State
of the Chlorine
Compound
Name
Compound
Name
+1
ClO-
hypochlorite
HClO
hypochlorous
acid
+3
ClO2-
chlorite
HOClO
chlorous acid
+5
ClO3-
chlorate
HOClO2
chloric acid
+7
ClO4-
perchlorate
HOClO3
perchloric acid
Disproportionation
• Disproportionation is the oxidation and reduction of
atoms of the same element in the same reaction.
• Chlorine disproportionates in water or alkali to form
chloride (ox no. =-1) and chlorate(I) ions (ox. no. = +1).
Cl2(g) + 2H2O(l) ----> Cl-(aq) + H3O+(aq) + HClO(aq)
Cl2(g) + 2OH-(aq) -----> Cl-(aq) + ClO-(aq) + H2O(l).
• Further disproportionation of ClO- can now occur to form
chlorate(V) ions.
3ClO-(aq) -----> 2Cl-(aq) + ClO3-(aq)
Disproportionation
• The equilibrium constants for X2
Cl2 7.5E15 Br2 2E8 I2 30
• The equilibrium constants for XO–
ClO– 1E27 BrO– 1E15 IO– 1E20
• The reaction rates for XO–
ClO– < BrO– < < IO–
The Perbromate Ion (P251)
• ClO4- and IO4- (H5IO6) are both well-known.
• BrO4- was not synthesised until 1968.
NaBrO3 + F2 + 2NaOH
NaBrO3 + XeF2 + H2O
NaBrO4 + 2NaF + H2O
NaBrO4 + 2HF + Xe
• Very strong oxidising agent (slow kinetics
though):
BrO4- + 2H+ + 2e
BrO3- + H2O
Eo = +1.8 V
• Corresponding values for
• ClO4- Eo = +1.19 V; IO4- Eo = +1.65 V
Perbromate
• Why is BrO4- is different?
– High promotion energy to form Br(+7)
– This is not compensated by the Br-O bond
energies.
– Cl-O better because they have better overlap.
– I-O bonds: poor overlap, but strength
enhanced by the electronegativity difference
between I and O (see Pauling’s equation).
Periodates (p251)
H5IO6 = H+ + H4IO6–
K = 1E-5
 H4IO6– = 2H2O + IO4–
K = 23
 H4IO6– = H+ + H3IO62–
K = 2E-7

The Halogens
20-6
Interhalogens
The halogens react with one another to form interhalogens
0
0
X-X
0
+
0
Y-Y
+1
-1
oxidation no.
2 X-Y
Less electronegative element X is oxidized
Note: actual charge on X is only + and on Y is -
Properties of an interhalogen are between those of the parent
halogens
Can have interhalogens of the form XYn where n can be 3, 5
or even 7 (e.g. IF7) (Name: Iodine heptafluoride)
The more electronegative halogens, Y, are attached to the
central halogen X
The Halogens
Interhalogens
XY
XY3
XY5
ClF, colourless,
b.p. –100 C
ClF3, colourless,
b.p. 12 C
ClF5, colourless,
b.p. –13 C
BrF, light brown,
b.p. –20 C
BrF3, yellow, b.p.
126 C
BrF5, colourless,
b.p. 41 C
IF
(IF3)n, yellow, dec., IF5, colourless, b.p. IF7, colourless, b.p.
-28 C
105 C
5C
BrCl, red brown,
b.p. 5 C
Icl, red solid
IBr, black solid
XY7
I2Cl6, bright yellow,
m.p. 101 C
The Halogens
Interhalogens
All interhalogens are strong oxidizing agents and this is their
main use. e.g. to purify and separate the isotopes of U, UF6 is
often used. This is made by
3ClF3(l) + U(s)
UF6(l)
+ 3ClF(g)
ClF3 and BrF3 are more aggressive fluorinating agents than
BrF5, IF5 and IF7 (ClF3 is more aggressive than F2 in many
cases
ClF3 and BrF3 often react explosively with organics
Structures all conform nicely to VSEPR rules
ClF3: C2v
BrF5: C4v
IF7: D5h
The Halogens
Cationic Polyhalogens
In fuming sulphuric acid, I2 is oxidized to I2+
•Br2+ also known
Higher polyhalogen cations also known:
•Br5+, I3+, I5+ (Name: pentaiodium)
I+
I
I
I
I
I
I +
I
The Halogens
Cationic Polyhalogens
XFn+ compounds also known
ClF3 + SbF5
ClF2SbF6
XF2+
XF4+
XF6+
ClF2+ (name:
Chloridium
difluoride)
BrF2+
ClF4+
ClF6+
BrF4+
BrF6+
IF4+
IF6+
The Halogens
Polyhalides and Halogen Complexes
Add I2 to a sol’n of I- ions  deep brown colour
•this is due to the formation of polyiodide
ions
I3- (Name: triiodide) I5-
I 7-
I 9-
Polyhalides of the other elements are also known
Cl3-
Br3-
BrI2- (Bromine diiodide)
The Halogens
The Halogens
Other Polyhalides:
IF8- ClF6-
BrF6-
IF6-
Halogen Oxides
OF2, O2F2
OF2 Strong fluorinating agent, not so
strong as fluorine
O2F2 better fluorinating agent than ClF3
The identification of halide ions
• Adding aqueous silver nitrate to an aqueous halides mixed with
nitric acid forms a silver halides.
Ag+(aq) + X-(aq) -----> AqX(s)
• Silver chloride is white, silver bromide is buff color and silver iodide
is yellow.
• 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)
• Precipitates of silver chloride and silver bromide react with dilute
and conc. ammonia solution respectively to form the diammine
silver(I)ion [Ag(NH3)2]+(aq) so the precipitates dissolve. Silver iodide
does not dissolve in even conc. ammonia solution.
Halogens in pesticides, polymers and refrigerants
• PVC used as electrical insulator -[-CH2-CHCl-]n- The C-Cl bond is
strong so PVC insulation lasts a long time but when discarded it
does not rot.
• Freon 12 CF2Cl2 is a refrigerant and an example of a
chlorofluorcarbon (CFC). It does not decompose easily so lasts for
the lifetime of a refrigerator. It does not decompose quickly when
discarded but does so in the upper atmosphere. The radicals it
forms react with ozone leading to an increase in UV radiation
reaching the Earth's surface.
• DDT is an pesticide used to kill mosquitos.
CCl3
|
Cl-C6H5-C-C6H5-Cl
|
H
The strong C-Cl bonds give DDT a long life in the field killing
pests. It is however so long lived that it persists in the environment
and builds up in the food chain threatening creatures at the top of
the chain.