|
INORGANIC Part 9
Group 7/17 Halogens sub-index: 9.1 Introduction, trends
& Group 7/17 data * 9.2
Halogen displacement
reaction and reactivity trend
* 9.3 Reactions of
halogens with other elements * 9.4
Reaction between halide salts and conc.
sulfuric acid *
9.5 Tests for halogens and halide ions *
9.6 Extraction of halogens from natural sources
* 9.7 Uses of halogens & compounds * 9.8
Oxidation & Reduction - more on redox reactions
of halogens & halide ions * 9.9 Volumetric
analysis - titrations involving halogens or halide ions * 9.10
Ozone, CFC's and halogen organic chemistry
links * 9.11 Chemical bonding in halogen
compounds * 9.12 Miscellaneous aspects of
halogen chemistry Advanced
Level Inorganic Chemistry Periodic Table Index *
Part 1
Periodic Table history
* Part 2
Electron configurations, spectroscopy,
hydrogen spectrum,
ionisation energies *
Part 3
Period 1 survey H to He *
Part 4
Period 2 survey Li to Ne * Part
5 Period 3 survey Na to Ar *
Part 6
Period 4 survey K to Kr and important trends down a
group *
Part 7
s-block Groups 1/2 Alkali Metals/Alkaline Earth Metals *
Part 8
p-block Groups 3/13 to 0/18 *
Part 9
Group 7/17 The Halogens *
Part 10
3d block elements & Transition Metal Series
*
Part 11
Group & Series data & periodicity plots * All
11 Parts have
their own sub-indexes near the top of the pages
9.6
The extraction of halogens from natural sources
The extraction of bromine from seawater
-
Extracting bromine from the
sea (e.g. from the Irish sea) which is done in 4 stages
-
Stage 1 Oxidation of bromide
ions Br- to bromine Br2:
-
Filtered sea water is
acidified with sulphuric acid to minimise reaction of halogens with
water, which leads to loss of bromine and wasted chlorine
-
X2(aq) + H2O(aq) 
HX(aq) + HXO(aq) (X = Br or Cl)
-
Increasing
the H+ concentration moves equilibrium to the left, 'suppresses' H+XO-
acid formation.
-
Excess chlorine added to
displace the bromine
-
Cl2(aq) + 2Br-(aq)
==> 2Cl-(aq) + Br2(aq)
-
In redox terms and oxidation state changes
-
Stage 2 Removal of bromine
vapour:
-
Stage 3 Reduction of bromine
Br2 to hydrobromic acid HBr:
-
Sulphur dioxide gas is
mixed with the air/bromine mixture and both are in contact with fresh
water and the mixture condensed to form a concentrated solution of
hydrobromic acid
-
This reduces the bromine to
bromide (as hydrobromic acid) and also produces a much more concentrated
solution of the bromide ion.
-
Br2(aq) + SO2(g) + 2H2O(l)
==> 2HBr(aq) + H2SO4(aq)
-
or as an ionic redox
equation
-
Br2(aq) + SO2(g) + 2H2O(l)
==> 4H+(aq) + 2Br-(aq) + SO42-(aq)
-
oxidation
state/number changes: S from +4 to +6, and Br from 0 to -1
-
Note: sulphur dioxide =
sulfur dioxide = sulphur(IV) oxide = sulfur(IV) oxide!
-
Stage 4 oxidation of
hydrobromic acid HBr to bromine Br2:
-
Steam and chlorine are
blown through the acid solution to reform bromine by oxidation of the
bromide ion.
-
Cl2(aq) + 2HBr(aq) ==> 2HCl(aq) + Br2(g)
-
or as an ionic redox
equation
-
Cl2(aq) + 2Br-(aq) ==> 2Cl-(aq) + Br2(g)
-
The hot vapour mixture is
condensed forming an upper aqueous layer and a lower more dense wet
bromine layer
-
The bromine layer is dried with concentrated sulphuric acid
(a powerful dehydrating agent).
-
The used sea water is
treated with sulphur dioxide to remove traces of toxic chlorine or
bromine (X = Cl or Br).
-
X2(aq) + SO2(g) + 2H2O(l)
==> 2HX(aq) + H2SO4(aq)
-
or as an ionic redox
equation (X = Cl or Br)
-
X2(aq) + SO2(g) + 2H2O(l)
==> 4H+(aq) + 2X-(aq) + SO42-(aq)
-
and the waste fluid
containing residual chloride and bromide ions is discharged - although quite acidic, it is rapidly diluted in
the sea.
-
Consider +/- factors in make bromine from
any seawater or Dead sea water in Israel
-
No evaporation needed
for sea water BUT more (e.g. chlorination) stages needed in the process.
-
Dead Sea water is much more
concentrated in bromide and one chlorine treatment sufficient but there
is still the energy costs for evaporation and separation of the bromine.
-
See below for
9.7 Uses of halogens and their compounds.
The industrial production of chlorine
9.7 USES of
HALOGENS and their COMPOUNDS
- Sodium chloride is an
important
raw material found as 'rock salt' or in seawater.
- Salt is used directly for food flavouring
and food preservation.
- It is also used for
de-icing roads because it lowers the freezing point of water
(related to general effect of an impurity lowering the melting
point of a substance).
- Most sodium chloride is
mainly converted into other products by electrolysis (see above for the production of
chlorine,
hydrogen and sodium hydroxide by electrolysis of brine (brine
= aqueous
sodium chloride solution).
-
The hydrogen formed
can be used to make hydrochloric acid via hydrogen chloride gas
-
H2(g)
+ Cl2(g) ==> HCl(g), then, HCl(g) + aq ==>
HCl(aq) hydrochloric acid
- Hydrochloric acid is a very important acid used in the
chemical industry to make chloride salts.
-
or the hydrogen can
be used to hydrogenate unsaturated fats (e.g. vegetable oils) to
more saturated fats like margarine
-
Haber Synthesis
manufacture of ammonia for fertilisers
-
The sodium hydroxide is
used in the manufacture of paper, soluble sodium salts of acidic
materials (eg soluble aspirin), ceramics, soaps and detergents
- All
the Halogens are potentially harmful substances and chlorine in
particular is highly toxic. It is dangerous to ingest
halogens or breathe in any halogen gas or vapour.
-
Chlorine has many
'positive' uses
eg manufacturing hydrochloric acid,
the useful hard wearing plastic PVC, chlorinated organic
solvents like trichloroethane for cleaning, bleaches,
disinfectants (TCP, Dettol etc.), water treatment to kill
bacteria -
-
BUT the use of chlorinated
organic pesticides is not so welcome due ecosystem damage increasing up
the food chains.
- Chlorine
is used to kill bacteria and so sterilise
water for domestic supply or in in swimming pools,
chlorine's bacteria 'killing' action is due to its powerful
oxidising power which will disrupt any cell's chemistry
including ours! However, traces in our drinking water should be
insufficient to do us any harm and the benefits of water
chlorination far outweigh the risk as long as the water
treatment systems are properly managed.
-
The sodium hydroxide
and chlorine can be chemically combined at room temperature to make the
bleach,
sodium chlorate(I) NaClO. This is used in some domestic
cleaning agents e.g. bleaching agents, it chemically 'scours' and chemically 'kills'
germs!
- Organic phenolic chlorine compounds are used
antiseptics and disinfectants
like 'Dettol' or 'TCP'
- Organic chlorine compounds are used
as pesticides, including the now mainly banned DDT.
- Chlorine is used in making CFC
refrigerant gases/liquids but their production and use are being
reduced. They break down in the upper atmosphere and the chlorine
atoms catalyse the destruction of ozone O3 which absorbs
harmful uv radiation.
- Liquid organic chlorine
compounds are used as dry cleaning or de-greasing solvents.
-
PVC: Chlorine from electrolysis
of NaCl, and ethene from
cracking oil fractions, are used to make a chemical called chloro(ethene),
which used to be called vinyl chloride, this is then converted
into the plastic-polymer poly(chloroethene) or PVC,
because it is shorthand for the old name polyvinylchloride!
(equation below)
-
- Poly(chloroethene), old names PVC, from
chloroethene (vinyl chloride) is much tougher than poly(ethene)
and very hard wearing with good heat stability. so it is used
for covering electrical wiring and plugs. It is also replacing
metals for use as gas and water drain pipes and has found a use
as artificial leather and readily dyed to bright colours! (old
names : polyvinyl chloride, shortened to PVC)
- PVC
is very tough
hard wearing useful plastic and a good electrical insulator and is used for water piping, window frames, part of
electrical fittings e.g. plug covers etc.
- Chlorine is used in the
manufacture of potassium chlorate(V) weed killer, KClO3.
-
The precipitation of silver
salts is important and know the observations eg the colours of the precipitate
for halide ion identification (AgX salts are used
in photography, see CS M1)
-
AgNO3(aq) +
NaX(aq) ==> AgX(s) + NaNO3(aq)
-
where
X is the halogen Cl, Br and I, the latter is better expressed as the
ionic (NOT redox) equation
-
Ag+(aq) + X-(aq)
==> AgX(s)
-
When silver salts are
exposed to light, silver is formed - the basis of using silver salts in
photography.
-
Silver
chloride (AgCl), silver bromide (AgBr) and silver iodide (AgI) are
all sensitive to light ('photosensitive'), and all three are used
in the production of various types of photographic film used to
detect visible light and beta and gamma radiation from radioactive
materials.
-
Each silver halide salt has a different sensitivity to
light.
-
When radiation hits
the film the silver ions in the salt are reduced by electron
gain to silver
-
Ag+ + e-
==> Ag
-
(the halide ion is oxidised to the halogen molecule
2X- ==> X2 + 2e-)
-
AgI is the
least sensitive and used in X-ray radiography.
-
AgCl is the most
sensitive and used in 'fast' film for cameras
-
AgBr is used in most
standard photographic films.
-
BUT, much of
their use is being superceded by digital cameras!
-
Fluorine is used as
fluoride salts in toothpaste or added to domestic water supplies to
strengthen teeth enamel helping to minimise tooth decay. (e.g.
potassium fluoride).
-
Fluorine is
used in the manufacture of the tough non-stick plastic PTFE
coating of cooking pans.
-
Fluorine is
used in manufacture of aerosol propellants and refrigerant
gases.
-
Bromine is a dense, dark and
dangerous liquid BUT useful:
-
Apart from its silver salt use in photography,
bromine is used to manufacture organic
-
Organic bromine
compounds are used as
...
-
flame inhibitor
chemicals (flame retardants like TBBA) for plastic products to
reduce their flammability
-
petrol additives
like 1,2-dibromoethane, to reduce the
build-up of lead oxide in car engines (a use decreasing as 'green'
unleaded fuels are becoming more popular),
-
agri-chemicals like the
pest
fumigant bromomethane, organobromo compounds are used as pesticides
and fungicides because of their poisonous nature, so they
are not good for us either!
-
but bromine is an ozone
'destroyer' so its use is hoped to be phased out.
-
silver halide salts are
used in photographic film
- when visible light, uv, X-rays or gamma
rays hit silver halide crystals metallic silver is formed. An electron
is knocked off the halide ion and reduces the silver ion to silver: Ag+X-
+ hv ==> Ag + X (Assignment 5 tests you on the ideas)
-
Bromine and
iodine
are both used in 'halogen' car
headlamps.
-
Iodine is
used in hospitals in the mild antiseptic solution 'tincture
of iodine'.
- Sodium
hydroxide is used in the manufacture of soaps, detergents,
paper, ceramics and to make soluble salts of organic acids with low
solubility in water (e.g. soluble Aspirin).
- It isn't a halogen
compound, but it is made from the electrolysis of salt solution.
-
The sodium hydroxide
and chlorine can be chemically combined at room temperature to make the
bleach,
sodium chlorate(I) NaClO. This is used in some domestic
cleaning agents, it chemically 'scours' and chemically 'kills'
germs!
-
sodium
hydroxide + chlorine ==> sodium chloride + sodium chlorate(I)
+ water
-
Halide salts are formed by
neutralising the corresponding acid and alkali ...
-
eg NaOH(aq) +
HCl(aq) ==> NaCl(aq) + H2O(l)
-
but this is a ridiculously
expensive method of production, evaporation of seawater is much simpler
and cost effective!
PLEASE NOTE
KS4 Science
GCSE/IGCSE/O Level GROUP 7 HALOGENS NOTES are on a separate webpage
WHAT NEXT?
INORGANIC Part 9
Group 7/17 Halogens sub-index: 9.1 Introduction, trends
& Group 7/17 data * 9.2
Halogen displacement
reaction and reactivity trend
* 9.3 Reactions of
halogens with other elements * 9.4
Reaction between halide salts and conc.
sulfuric acid *
9.5 Tests for halogens and halide ions *
9.6 Extraction of halogens from natural sources
* 9.7 Uses of halogens & compounds * 9.8
Oxidation & Reduction - more on redox reactions
of halogens & halide ions * 9.9 Volumetric
analysis - titrations involving halogens or halide ions * 9.10
Ozone, CFC's and halogen organic chemistry
links * 9.11 Chemical bonding in halogen
compounds * 9.12 Miscellaneous aspects of
halogen chemistry
keywords phrases formula oxidation states balanced
symbol equations: X2(aq) + H2O(aq) HX(aq) + HXO(aq) (X = Br or Cl) Br2(aq) +
SO2(g) + 2H2O(l) ==> 2HBr(aq) + H2SO4(aq) Br2(aq) + SO2(g) + 2H2O(l) ==> 4H+(aq)
+ 2Br-(aq) + SO42-(aq) Cl2(aq) + 2HBr(aq) ==> 2HCl(aq) + Br2(g) X2(aq) + SO2(g)
+ 2H2O(l) ==> 2HX(aq) + H2SO4(aq) X2(aq) + SO2(g) + 2H2O(l) ==> 4H+(aq) +
2X-(aq) + SO42-(aq) 2NaOH(aq) + Cl2(aq) ==> NaCl(aq) + NaClO(aq) + H2O(l)
AgNO3(aq) + NaX(aq) ==> AgX(s) + NaNO3(aq) Ag+(aq) + X-(aq) ==> AgX(s) Cl2(aq) +
SO2(g) + 2H2O(l) ==> 2HCl(aq) + H2SO4(aq) Br2(aq) + SO2(g) + 2H2O(l) ==>
2HBr(aq) + H2SO4(aq) Cl2(aq) + SO2(g) + 2H2O(l) ==> 4H+(aq) + 2Cl-(aq) +
SO42-(aq) Br2(aq) + SO2(g) + 2H2O(l) ==> 4H+(aq) + 2Br-(aq) + SO42-(aq)
A level Inorganic Chemistry Group 7 Halogens
Periodic Table Revision notes for GCE Advanced
Subsidiary Level AS Advanced Level A2 IB
Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters
Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre-university students
(equal to US grade 11 and grade 12 and AP Honours/honors level courses)
 Website
content copyright © Dr
Phil Brown 2000-2013 All rights reserved
on ALL
revision notes, puzzles, quizzes, worksheets, x-words etc. Copying of website
material is not permitted
chemhelp@tiscali.co.uk
Alphabetical Index for Science
Pages Content
A
B C D
E F
G H I J K L M
N O P
Q R
S T
U V W
X Y Z |
Doc
Brown's Chemistry