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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


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


electronegativity = 4.0

iodine


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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Id tif i h lid i

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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

17 - halogens

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