AN INTRODUCTION TO GROUP VII KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS.

AN INTRODUCTION TOAN INTRODUCTION TO

GROUP VIIGROUP VII

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING2008 2008

SPECIFICATIONSSPECIFICATIONS

INTRODUCTION

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KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

GROUP 7 (HALOGENS)GROUP 7 (HALOGENS)

CONTENTS• Trends in appearance

• Trends in electronic configuration

• Trends in in Atomic and Ionic radius

• Trends in Electronegativity

• Trends in oxidation power

• Displacement reactions

• Other reactions

• Testing for halides – AgNO3

• Testing for halides - concentrated H2SO4

• Volumetric analysis of chlorate(I) in bleach

GROUP VIIGROUP VII

GROUP PROPERTIESGROUP PROPERTIES

GENERAL • non-metals

• exist as separate diatomic molecules… eg Cl2

• all have the electronic configuration ... ns2 np5

TRENDS • appearance

• boiling point

• electronic configuration

• electronegativity

• atomic size

• oxidising power

GROUP TRENDSGROUP TRENDS

INCREASES down Group

• increased size makes the van der Waals forces increase• more energy is required to separate the molecules

Yellow

Red/brown

GAS GAS LIQUID SOLID

Colour

State (at RTP)

APPEARANCEAPPEARANCE

BOILING POINTBOILING POINT

183Boiling point / °C

• electrons go into shells further from the nucleus

… 2s2 2p5

…3s2 3p5

… 4s2 4p5

… 5s2 5p5

2,7 2,8,7 2,8,18,7 2,8,18,18,7

ELECTRONIC CONFIGURATIONELECTRONIC CONFIGURATION

9 17 35 53Atomic Number

ATOMIC RADIUS INCREASES down Group

IONIC RADIUS INCREASES down Group

• the greater the atomic number the more electrons there are these go into shells increasingly further from the nucleus

• ions are larger than atoms - the added electron repels the others so radius gets larger

F Cl Br I

ATOMIC & IONIC RADIUSATOMIC & IONIC RADIUS

0.064 0.099 0.111 0.128Atomic radius / nm

F¯ Cl¯ Br¯ I¯

0.136 0.181 0.195 0.216Ionic radius / nm

DECREASES down Group

• the increasing nuclear charge due to the greater number of protons should attract electrons more, but there is an ...

an increasing number of shells;

more shielding and less pull on electrons

an increasing atomic radius

attraction drops off as distance increases

F Cl Br I

ELECTRONEGATIVITYELECTRONEGATIVITY

4.0 3.5 2.8 2.5Electronegativity

• halogens are oxidising agents

• they need one electron to complete their octet

• the oxidising power gets weaker down the group

OXIDISING POWEROXIDISING POWER

• the trend can be explained by considering the nucleus’s attraction for the incoming electron which is affected by the...

• increasing nuclear charge which should attract electrons more

but this is offset by • INCREASED SHIELDING• INCREASING ATOMIC RADIUS

• the trend can be explained by considering the nucleus’s attraction for the incoming electron which is affected by the...

• increasing nuclear charge which should attract electrons more

but this is offset by • INCREASED SHIELDING• INCREASING ATOMIC RADIUS

This is demonstrated by reacting the halogens with other halide ions.

HALOGENS - DISPLACEMENT REACTIONSHALOGENS - DISPLACEMENT REACTIONS

THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED USING DISPLACEMENT REACTIONS...

A SOLUTION OF THE HALOGEN IS ADDED TO A SOLUTION OF A HALIDE HALIDES ARE SALTS FORMED FROM HALOGENS (see next slide for examples)

A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE

e.g. CHLORINE + SODIUM BROMIDE BROMINE + SODIUM CHLORIDE

Cl2(aq) + 2NaBr(aq) ——> Br2(aq) + 2NaCl(aq)

Cl2(aq) + 2Br¯(aq) ——> Br2(aq) + 2Cl¯(aq)

Cl2(aq) + 2NaBr(aq) ——> Br2(aq) + 2NaCl(aq)

CHLORINE + SODIUM IODIDE IODINE + SODIUM CHLORIDE

BROMINE + SODIUM IODIDE IODINE + SODIUM BROMIDE

BUT BROMINE + SODIUM CHLORIDE CHLORINE + SODIUM BROMIDE

(Bromine is below chlorine in the Group so is less reactive)

Chlorine oxidises bromide ions to bromine

Cl2 + 2Br¯ ——> Br2 + 2Cl¯

Chlorine oxidises iodide ions to iodine

Cl2 + 2I¯ ——> I2 + 2Cl¯

Bromine oxidises iodide ions to iodine

Br2 + 2I¯ ——> I2 + 2Br¯

SODIUM CHLORIDE

DISPLACEMENT REACTIONS - EXPERIMENT

CHLORINE

SODIUM BROMIDE SODIUM IODIDE

BROMINE

SODIUM CHLORIDE

DISPLACEMENT REACTIONS - EXPERIMENT

CHLORINE

SODIUM BROMIDE SODIUM IODIDE

Solution stays colourless

NO REACTION

Solution goes from colourless to orange-

yellow

NO REACTION

yellow

BROMINE FORMED

yellow

NO REACTION

Solution goes from colourless to red

IODINE FORMED

BROMINE

IODINE FORMED

OTHER REACTIONS OF CHLORINEOTHER REACTIONS OF CHLORINE

Water Halogens react with decreasing vigour down the group as their oxidising power decreases

Litmus will be turned red then decolourised in chlorine water

Cl2(g) + H2O(l) HCl(aq) + HOCl(aq)

strong acid bleaches by oxidation

0 -1 +1

This is an example of DISPROPORTIONATION …‘simultaneous oxidation and reduction of a species’

0 -1 +1

This is an example of DISPROPORTIONATION …‘simultaneous oxidation and reduction of a species’

Alkalis Chlorine reacts with cold, aqueous sodium hydroxide.

2NaOH(aq) + Cl2(g) —> NaCl(aq) + NaOCl(aq) + H2O(l)

0 -1 +1

TESTING FOR HALIDES – AgNOTESTING FOR HALIDES – AgNO33

• make a solution of the halide

• acidify with dilute nitric acid – this prevents the precipitation of other salts

• add a few drops of silver nitrate solution

• treat any precipitate with dilute ammonia solution

• if a precipitate still exists, add concentrated ammonia solution

CHLORIDE white ppt of AgCl soluble in dilute ammonia

BROMIDE cream ppt of AgBr insoluble in dilute ammonia but soluble in conc.

IODIDE yellow ppt of AgI insoluble in dilute andconc. ammonia solution

CHLORIDE white ppt of AgCl soluble in dilute ammonia

BROMIDE cream ppt of AgBr insoluble in dilute ammonia but soluble in conc.

IODIDE yellow ppt of AgI insoluble in dilute andconc. ammonia solution

halides precipitate as follows Ag+(aq) + X¯(aq) ——> Ag+X¯(s)

when they dissolve in ammonia a colourless diammine complex is formed [Ag(NH3)2]+(aq)

PLACE A SOLUTION OF THE HALIDE IN A TEST TUBE

CHLORIDE BROMIDE IODIDE

ADD SOME DILUTE NITRIC ACID

ADD SILVER NITRATE SOLUTION

WHITE PRECIPITATE OF SILVER CHLORIDE AgCl

CREAM PRECIPITATE OF SILVER BROMIDE AgBr

YELLOW PRECIPITATE OF SILVER IODIDE AgI

ADD DILUTE AMMONIA SOLUTION

WHITE PRECIPITATE OF SILVER CHLORIDE - SOLUBLE

CREAM PRECIPITATE OF SILVER BROMIDE - INSOLUBLE

YELLOW PRECIPITATE OF SILVER IODIDE - INSOLUBLE

ADD CONCENTRATED AMMONIA SOLUTION

WHITE PRECIPITATE OF SILVER CHLORIDE - SOLUBLE

CREAM PRECIPITATE OF SILVER BROMIDE - SOLUBLE

YELLOW PRECIPITATE OF SILVER IODIDE - INSOLUBLE

• an alternative test for halides

• add concentrated sulphuric acid carefully to a solid halide

• H2SO4 displaces the weaker acids HCl, HBr, and HI from their salts

• as they become more powerful reducing agents down the group they can react further by reducing the sulphuric acid to lower oxidation states of sulphur

TESTING FOR HALIDES – Conc. HTESTING FOR HALIDES – Conc. H22SOSO44

SummarySummary

Halide Observation(s) Product O.S. Reaction type

NaCl misty fumes HCl -1 Displacement of Cl¯

NaBr misty fumes HBr -1 Displacement of Br¯

brown vapour Br2 0 Oxidation of Br¯

colourless gas SO2 +4 Reduction of H2SO4

NaI misty fumes HI -1 Displacement of I¯

purple vapour I2 0 Oxidation of I¯

colourless gas SO2 +4 Reduction of H2SO4

yellow solid S 0 Reduction of H2SO4

bad egg smell H2S -2 Reduction of H2SO4

TESTING FOR HALIDES – Conc. HTESTING FOR HALIDES – Conc. H22SOSO44

HYDROGEN HALIDES - PROPERTIESHYDROGEN HALIDES - PROPERTIES

Boiling points At room temp. and pressure HCl, HBr, HI arecolourless gases, HF a colourless liquid.

b. pts ... HF 20°C HCl -85°C HBr -69°C HI -35°C

HF value is higher than expected due to hydrogen bonding

HYDROGEN HALIDES - PROPERTIESHYDROGEN HALIDES - PROPERTIES

Boiling points At room temp. and pressure HCl, HBr, HI arecolourless gases, HF a colourless liquid.

b. pts ... HF 20°C HCl -85°C HBr -69°C HI -35°C

HF value is higher than expected due to hydrogen bonding

Reducingability Increases down the group as bond strength decreases

bond energy H-F 568 H-Cl 432 H-Br 366 H-I 298 / kJ mol-1

HYDROGEN HALIDES - PREPARATIONHYDROGEN HALIDES - PREPARATION

Directcombination Hydrogen halides can be made by direct combination

H2(g) + X2(g) ——> 2HX(g)

• fluorine combines explosively, even in the dark • chlorine combines explosively when heated or in sunlight • bromine is fast at 200°C with a catalyst • iodine reaction is reversible.

H2(g) + X2(g) ——> 2HX(g)

Displacement Chlorides are made by displacing the acid from its salt

NaCl(s) + conc. H2SO4(l) ——> NaHSO4(s) + HCl(g)

H2(g) + X2(g) ——> 2HX(g)

Displacement Chlorides are made by displacing the acid from its salt

NaCl(s) + conc. H2SO4(l) ——> NaHSO4(s) + HCl(g)

HBr and HI are not made this way as they are more powerfulreducing agents and are oxidised by sulphuric acid to the halogen

2HBr(g) + conc. H2SO4(l) ——> 2H2O(l) + SO2(g) + Br2(g)

HALOGENS & HALIDES - USESHALOGENS & HALIDES - USES

Chlorine, Cl2 • water purification• bleach• solvents• polymers - poly(chloroethene) or PVC• CFC’s

Fluorine, F2 • CFC’s• polymers - PTFE poly(tetrafluoroethene) as used in...

non-stick frying pans, electrical insulation, waterproof clothing

Fluoride, F¯ • helps prevent tooth decay - tin fluoride is added to toothpaste - sodium fluoride is added to water supplies

Hydrogenfluoride, HF • used to etch glass

Silverbromide, AgBr • used in photographic film

VOLUMETRIC ANALYSIS OF CHLORATE(I)VOLUMETRIC ANALYSIS OF CHLORATE(I)

Introduction Chlorate(I) ions are oxidising agentsIn acid solution they end up as chloride ions