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Starter
Write down
everything you knowabout the elements
in group 7.
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Group 7 Elements: RedoxReactions
Lesson Objectives:
Explain the trend in boiling points of Cl2, Br2 and I2.
Explain the trend in reactivity for the Group 7 elements.
Describe the redox reactions of the Group 7 elements withother halide ions.
Describe and interpret, using oxidation numbers, thereaction of chlorine with water and aqueous sodiumhydroxide.
Key Words: reactivity, displacement reaction,disproportionation, precipitation reaction
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What are the halogens?
The halogens are the elements in Group 7 of the
periodic table.
The name halogen comes from the Greek
words for salt-making.
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Group 7 Elements:The Halogens
Physical Properties:
Low melting and boiling points Exist as diatomic molecules
On descending the group, no. of electrons increasesincreasing van der Waals forces between molecules.
Boiling points increase on descending the group. The physical states of the halogens at room temperature;
Gas Liquid SolidOn descending the group
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Physical properties of halogens
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Trends in boiling point
Halogen molecules increase in size down the group. This
leads to greater van der Waals forces between molecules,
increasing the energy needed to separate the molecules
and therefore higher melting and boiling points.
fluorine
atomic radius = 42 10-12m
boiling point = -118C
iodine
atomic radius = 115 10-12m
boiling point = 184C
van der
Waals forces
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Trends in electronegativity
Electronegativity of the halogens decreases down the group
due to an increase in atomic radius.
fluorine
atomic radius = 42 10-12m
electronegativity = 4.0
iodine
atomic radius = 115 10-12m
electronegativity = 2.5
Increased nuclear charge has no significant effect because
there are more electron shells and more shielding. Iodine
atoms therefore attract electron density in a covalent bond
less strongly than fluorine.
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Astatine
The name astatine comes from the Greek word for unstable.
It was first made artificially in 1940, by bombarding 209Bi with
a-radiation. What do you predict for these properties of
astatine?
Astatine exists in nature in only very tinyamounts. It is estimated that only 30 grams of
astatine exist on Earth at any one time. This is
because it is radioactive, and its most stable
isotope (210
At) has a half-life of only 8 hours.
electronegativity.
state at room temperature
colour
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Halogens: true or false?
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Group 7 Elements: ElectronConfigurationElement Electron
Configuration
F [He]2s22p5
Cl [Ne]3s23p5
Br [Ar]4s2
4p5
I [Kr]5s25p5
At [Xe]6s26p5
7 electrons in the outer shell, highest energy electrons in a p sub-shell Each element has two fewer electrons than the nextnoble gas
Outer p sub-shell containing 5 electrons (needs 1 more to form a -1 ion)
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The Halogens: OxidisingAgents
The halogens are the most reactive non-metals inthe periodic table
Strong oxidising agents (they gain electrons)
Oxidising power decreases down the group.Therefore less reactive down the group
Cl2(g) + e- Cl-(g) (-1 oxidation state)
Reactivitydecreases downthe group because; Atomic radius decreases Electron shielding increases Ability to gain an electron decreases
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Electron structure and reactivity
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Halogen displacement reactions
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Halogen displacement reactions
Halogen displacement reactions are redox reactions.
Cl2 + 2KBr 2KCl+ Br2
To look at the transfer of electrons in this reaction, the
following two half equations can be written:
Chlorine has gained electrons, so it is reduced to Cl- ions.
What has been oxidized and what has been reduced?
2Br- Br2 + 2e-Cl2 + 2e- 2Cl-
Bromide ions have lost electrons, so they have been
oxidized to bromine.
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Oxidizing ability of halogens
fluorine
increasi
ngo
xidizinga
bility
iodine
bromine
chlorine
In displacement reactions between
halogens and halides, the halogen
acts as an oxidizing agent.
This means that the halogen:
What is the order of oxidizing
ability of the halogens?
is reduced to form the halide ion.
gains electrons
oxidizes the halide ion to thehalogen
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The Halogens: Recognisingthem
The halogens form solutions with different colours. A colour
change will show if a reaction has taken place.Halogen Water cyclohexane
Cl2 Pale-green Pale-green
Br2 Orange Orange
I2 Brown violet
If you shake thereaction mixture withan organic solvent itcan help distinguishbetween halogens thathave reacted
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Oxidizing ability of halogens
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Chlorine and disproportionation
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Reaction of chlorine with water
Chlorination of drinking water raises questions about individual
freedom because it makes it difficult for individuals to opt out.
Chlorine is used to purify water supplies
because it is toxic to bacteria, some of
which can cause disease. Adding it towater supplies is therefore beneficial for
the population.
However, chlorine is also toxic to humans,so there are risks associated with gas leaks
during the chlorination process. There is
also a risk of the formation of chlorinated
hydrocarbons, which are also toxic.
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Bleach and the chlorate(I) ion
Household bleach commonly contains the chlorate(I) ion,
ClO-, in the form of sodium chlorate(I), NaOCl.
ClO-
+ H2O +
Cl-
+ 2OH-
The chlorine has been reduced because it has gained
electrons. Its oxidation state has decreased from +1 in
ClO- to 1 in Cl-.
How many electrons are needed to balance this equation?
The chlorate(I) ion behaves as an oxidizing agent. It oxidizes
the organic compounds in food stains, bacteria and dyes.
Has the chlorine been oxidized or reduced in the reaction?
2e
-
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U d H lid T
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Uses and Halide Tests
Lesson Objectives:
Interpret and make predictions from the chemical andphysical properties of the Group 7 elements/compounds.
Contrast the benefits and risks of chlorines use as a water
treatment. Describe the precipitation reactions of aqueous anions Cl,Br and I with aqueous silver ions, followed by aqueousammonia.
Recognise the use of these precipitation reactions as a test
for different halide ions.
Key Words: reactivity, displacement reaction,disproportionation, precipitation reaction
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Halides
When halogens react with metals, they form compounds
called halides. Many naturally-occurring halides have
industrial, household and medical applications.
caesium chloride
sodium
hexafluoroaluminate
titanium(IV) chloride
lithium iodide
potassium bromide
Halide Formula Uses
CsCl
NaAlF6
TiCl4
LiI
KBr
Extraction and
separation of DNAElectrolysis of
aluminium oxide
Extraction of titanium
Electrolyte in batteries
Epilepsy treatment in
animals
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Identifying halide ions
Halides can be identified by their reaction with acidified
silver nitrate solution to form silver halide precipitates.
Silver chloride has a low solubilityin water, so it forms a white
precipitate: the positive result in
the test for chloride ions.
KCl(aq) + AgNO3(aq) KNO3(aq)+ AgCl(s)
potassium
chloride
silver
chloride+
potassium
nitrate
silver
nitrate+
Id if i h lid i
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Identifying halide ions
Id tif i h lid i
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Identifying halide ions
H d h lid
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Hydrogen halides
The hydrogen
halides are
colourless gasesat room
temperature.
Hydrogen fluoride has an
unexpectedly high boiling point
compared to the other
hydrogen halides. This is due to
hydrogen bonding between the
HF molecules.
Hydrogen halide Boiling point (C)
HF
HCl
HBr
HI
20
-85
-67
-35
H lid d i t
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Halides as reducing agents
A substance that donates electrons
in a reaction (i.e. is oxidized) is a
reducingagent because itreduces the other reactant.
fluoride
incre
asingr
educing
abil
ity
iodide
bromide
chloride
The larger the halide ion, the easier
it is for it to donate electrons andtherefore the more reactive it is.
This is because its outermost
electrons are further from the
attraction of the nucleus and moreshielded from it by other electrons.
The attraction for the outermost
electrons is therefore weaker.
H lid t f l ?
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Halides: true or false?
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Gl
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Glossary
Wh t th k d?
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Whats the keyword?
Multiple choice quiz
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Multiple-choice quiz
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