AN INTRODUCTION TO GROUP VII KNOCKHARDY PUBLISHING 2008

AN INTRODUCTION TO GROUP VII KNOCKHARDY PUBLISHING 2008 SPECIFICATIONS

KNOCKHARDY PUBLISHING GROUP 7 (HALOGENS) INTRODUCTION This Powerpoint show is one of several produced to help students understand selected topics at AS and A 2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards. Individual students may use the material at home for revision purposes or it may be used for classroom teaching with an interactive white board. Accompanying notes on this, and the full range of AS and A 2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at. . . www. knockhardy. org. uk/sci. htm Navigation is achieved by. . . either clicking on the grey arrows at the foot of each page or using the left and right arrow keys on the keyboard

GROUP VII 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 – Ag. NO 3 • Testing for halides - concentrated H 2 SO 4 • Volumetric analysis of chlorate(I) in bleach

GROUP PROPERTIES GENERAL • non-metals • exist as separate diatomic molecules… eg Cl 2 • all have the electronic configuration. . . ns 2 np 5 TRENDS • • appearance boiling point electronic configuration electronegativity atomic size oxidising power

GROUP TRENDS APPEARANCE F 2 Cl 2 Br 2 I 2 Colour Yellow Green Red/brown Grey State (at RTP) GAS LIQUID SOLID BOILING POINT Boiling point / °C F 2 Cl 2 Br 2 I 2 - 188 - 34 58 183 INCREASES down Group • increased size makes the van der Waals forces increase • more energy is required to separate the molecules

GROUP TRENDS ELECTRONIC CONFIGURATION F Cl Br I Atomic Number 9 17 35 53 Old 2, 7 2, 8, 18, 7 New … 2 s 2 2 p 5 … 3 s 2 3 p 5 … 4 s 2 4 p 5 … 5 s 2 5 p 5 • electrons go into shells further from the nucleus

GROUP TRENDS ATOMIC & IONIC RADIUS Atomic radius / nm Ionic radius / nm F Cl Br I 0. 064 0. 099 0. 111 0. 128 F¯ Cl¯ Br¯ I¯ 0. 136 0. 181 0. 195 0. 216 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

GROUP TRENDS ELECTRONEGATIVITY Electronegativity F Cl Br I 4. 0 3. 5 2. 8 2. 5 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

GROUP TRENDS OXIDISING POWER • halogens are oxidising agents • they need one electron to complete their octet • the oxidising power gets weaker down the group

GROUP TRENDS OXIDISING POWER • halogens are oxidising agents • they need one electron to complete their octet • the oxidising power gets weaker down the group • 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

GROUP TRENDS OXIDISING POWER • halogens are oxidising agents • they need one electron to complete their octet • the oxidising power gets weaker down the group • 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 REACTIONS THE DECREASE IN REACTIVITY DOWN THE GROUP IS DEMONSTRATED USING DISPLACEMENT REACTIONS. . .

HALOGENS - 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 HALIDES ARE SALTS FORMED FROM HALOGENS (see next slide for examples) A MORE REACTIVE HALOGEN WILL DISPLACE A LESS REACTIVE ONE

HALOGENS - 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 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

HALOGENS - 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 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 Cl 2(aq) + 2 Na. Br(aq) BROMINE + SODIUM CHLORIDE ——> Br 2(aq) + 2 Na. Cl(aq)

HALOGENS - 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 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 Cl 2(aq) + 2 Na. Br(aq) BROMINE + SODIUM CHLORIDE ——> Br 2(aq) + 2 Na. Cl(aq) Cl 2(aq) + 2 Br¯(aq) ——> Br 2(aq) + 2 Cl¯(aq)

HALOGENS - 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 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 CHLORINE + SODIUM IODIDE BROMINE + SODIUM CHLORIDE IODINE + SODIUM CHLORIDE

HALOGENS - 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 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 CHLORINE + SODIUM IODIDE BROMINE + SODIUM CHLORIDE IODINE + SODIUM BROMIDE

HALOGENS - 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 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 CHLORINE + SODIUM IODIDE BROMINE + SODIUM IODIDE BUT BROMINE + SODIUM CHLORIDE IODINE + SODIUM BROMIDE CHLORINE + SODIUM BROMIDE (Bromine is below chlorine in the Group so is less reactive)

GROUP TRENDS OXIDISING POWER Chlorine oxidises bromide ions to bromine Cl 2 + 2 Br¯ ——> Br 2 + 2 Cl¯ Chlorine oxidises iodide ions to iodine Cl 2 + 2 I¯ ——> I 2 + 2 Cl¯ Bromine oxidises iodide ions to iodine Br 2 + 2 I¯ ——> I 2 + 2 Br¯

HALOGENS - DISPLACEMENT REACTIONS - EXPERIMENT SODIUM CHLORIDE CHLORINE BROMINE SODIUM BROMIDE SODIUM IODIDE

HALOGENS - DISPLACEMENT REACTIONS - EXPERIMENT CHLORINE BROMINE SODIUM CHLORIDE SODIUM BROMIDE SODIUM IODIDE Solution stays colourless Solution goes from colourless to orangeyellow Solution goes from colourless to orangered NO REACTION BROMINE FORMED IODINE FORMED Solution goes from colourless to orangeyellow NO REACTION Solution goes from colourless to red IODINE FORMED

OTHER 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 Cl 2(g) + H 2 O(l) HCl(aq) strong acid + HOCl(aq) bleaches by oxidation

OTHER 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 0 Cl 2(g) + H 2 O(l) -1 HCl(aq) strong acid +1 + HOCl(aq) bleaches by oxidation

OTHER 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 0 Cl 2(g) + H 2 O(l) -1 HCl(aq) strong acid +1 + HOCl(aq) bleaches by oxidation This is an example of DISPROPORTIONATION … ‘simultaneous oxidation and reduction of a species’

OTHER 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 0 Cl 2(g) + H 2 O(l) -1 HCl(aq) strong acid +1 + HOCl(aq) bleaches by oxidation This is an example of DISPROPORTIONATION … ‘simultaneous oxidation and reduction of a species’ Alkalis Chlorine reacts with cold, aqueous sodium hydroxide. 2 Na. OH(aq) + Cl 2(g) —> Na. Cl(aq) + Na. OCl(aq) + H 2 O(l)

TESTING FOR HALIDES – Ag. NO 3 • 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

TESTING FOR HALIDES – Ag. NO 3 CHLORIDE white ppt of Ag. Cl soluble in dilute ammonia BROMIDE cream ppt of Ag. Br insoluble in dilute ammonia but soluble in conc. IODIDE yellow ppt of Ag. Iinsoluble in dilute and conc. ammonia solution

TESTING FOR HALIDES – Ag. NO 3 CHLORIDE white ppt of Ag. Cl soluble in dilute ammonia BROMIDE cream ppt of Ag. Br insoluble in dilute ammonia but soluble in conc. IODIDE yellow ppt of Ag. Iinsoluble in dilute and conc. 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(NH 3)2]+(aq)

TESTING FOR HALIDES – Ag. NO 3 CHLORIDE BROMIDE IODIDE PLACE A SOLUTION OF THE HALIDE IN A TEST TUBE

TESTING FOR HALIDES – Ag. NO 3 CHLORIDE BROMIDE ADD SOME DILUTE NITRIC ACID IODIDE

TESTING FOR HALIDES – Ag. NO 3 CHLORIDE BROMIDE IODIDE ADD SILVER NITRATE SOLUTION WHITE PRECIPITATE OF SILVER CHLORIDE CREAM PRECIPITATE OF SILVER BROMIDE YELLOW PRECIPITATE OF SILVER IODIDE Ag. Cl Ag. Br Ag. I

TESTING FOR HALIDES – Ag. NO 3 CHLORIDE BROMIDE IODIDE ADD DILUTE AMMONIA SOLUTION WHITE PRECIPITATE OF SILVER CHLORIDE CREAM PRECIPITATE OF SILVER BROMIDE YELLOW PRECIPITATE OF SILVER IODIDE - SOLUBLE - INSOLUBLE

TESTING FOR HALIDES – Ag. NO 3 CHLORIDE BROMIDE IODIDE ADD CONCENTRATED AMMONIA SOLUTION WHITE PRECIPITATE OF SILVER CHLORIDE CREAM PRECIPITATE OF SILVER BROMIDE YELLOW PRECIPITATE OF SILVER IODIDE - SOLUBLE - INSOLUBLE

TESTING FOR HALIDES – Conc. H 2 SO 4 • an alternative test for halides • add concentrated sulphuric acid carefully to a solid halide • H 2 SO 4 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. H 2 SO 4 Summary Halide Observation(s) Product O. S. Reaction type Na. Cl misty fumes HCl -1 Displacement of Cl¯ Na. Br misty fumes brown vapour colourless gas HBr Br 2 SO 2 -1 0 +4 Displacement of Br¯ Oxidation of Br¯ Reduction of H 2 SO 4 Na. I misty fumes purple vapour colourless gas yellow solid bad egg smell HI I 2 SO 2 S H 2 S -1 0 +4 0 -2 Displacement of I¯ Oxidation of I¯ Reduction of H 2 SO 4

HYDROGEN HALIDES - PROPERTIES Boiling points b. pts. . . At room temp. and pressure HCl, HBr, HI are colourless gases, HF a colourless liquid. HF 20°C HCl -85°C HBr -69°C HI -35°C HF value is higher than expected due to hydrogen bonding

HYDROGEN HALIDES - PROPERTIES Boiling points b. pts. . . At room temp. and pressure HCl, HBr, HI are colourless gases, HF a colourless liquid. HF 20°C HCl -85°C HBr -69°C HI -35°C HF value is higher than expected due to hydrogen bonding Reducing ability bond energy / k. J mol-1 Increases down the group as bond strength decreases H-F 568 H-Cl 432 H-Br 366 H-I 298

HYDROGEN HALIDES - PREPARATION Direct combination Hydrogen halides can be made by direct combination H 2(g) + X 2(g) ——> 2 HX(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.

HYDROGEN HALIDES - PREPARATION Direct combination Hydrogen halides can be made by direct combination H 2(g) + X 2(g) ——> 2 HX(g) • • Displacement 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. Chlorides are made by displacing the acid from its salt Na. Cl(s) + conc. H 2 SO 4(l) ——> Na. HSO 4(s) + HCl(g)

HYDROGEN HALIDES - PREPARATION Direct combination Hydrogen halides can be made by direct combination H 2(g) + X 2(g) ——> 2 HX(g) • • Displacement 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. Chlorides are made by displacing the acid from its salt Na. Cl(s) + conc. H 2 SO 4(l) ——> Na. HSO 4(s) + HCl(g) HBr and HI are not made this way as they are more powerful reducing agents and are oxidised by sulphuric acid to the halogen 2 HBr(g) + conc. H 2 SO 4(l) ——> 2 H 2 O(l) + SO 2(g) + Br 2(g)

HALOGENS & HALIDES - USES Chlorine, Cl 2 • • • water purification bleach solvents polymers - poly(chloroethene) or PVC CFC’s Fluorine, F 2 • 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 Hydrogen fluoride, HF • used to etch glass Silver bromide, Ag. Br • used in photographic film

VOLUMETRIC ANALYSIS OF CHLORATE(I) Introduction Chlorate(I) ions are oxidising agents In acid solution they end up as chloride ions Cl. O¯ + 2 H+ + 2 e¯ ——> Cl¯ + H 2 O

VOLUMETRIC ANALYSIS OF CHLORATE(I) Introduction Chlorate(I) ions are oxidising agents In acid solution they end up as chloride ions Cl. O¯ Oxidation state +1 + 2 H+ + 2 e¯ ——> Cl¯ -1 + H 2 O

VOLUMETRIC ANALYSIS OF CHLORATE(I) Introduction Chlorate(I) ions are oxidising agents In acid solution they end up as chloride ions Cl. O¯ Analysis (1) + 2 H+ + 2 e¯ ——> Cl¯ + H 2 O Add excess potassium iodide; the chlorate oxidises the iodide ions to iodine Cl. O¯ + 2 H+ + 2 e¯ ——> Cl¯ 2 I¯ ——> I 2 + + H 2 O 2 e¯

VOLUMETRIC ANALYSIS OF CHLORATE(I) Introduction Chlorate(I) ions are oxidising agents In acid solution they end up as chloride ions Cl. O¯ Analysis (1) 2 H+ + 2 e¯ ——> Cl¯ + H 2 O Add excess potassium iodide; the chlorate oxidises the iodide ions to iodine Cl. O¯ overall + + 2 H+ + 2 e¯ ——> Cl¯ 2 I¯ ——> I 2 + 2 e¯ I 2 + Cl¯ Cl. O¯ + 2 H+ + 2 I¯ ——> + H 2 O + moles of I 2 produced = original moles of OCl¯ H 2 O -- (i)

VOLUMETRIC ANALYSIS OF CHLORATE(I) Introduction Chlorate(I) ions are oxidising agents In acid solution they end up as chloride ions Cl. O¯ Analysis (2) + 2 H+ + 2 e¯ ——> Cl¯ + H 2 O Titrate the iodine produced with sodium thiosulphate using starch as the indicator near the end point I 2 2 S 2 O 32 - ——> S 4 O 62 - + 2 I¯ 2 e¯ ——> + 2 e¯

VOLUMETRIC ANALYSIS OF CHLORATE(I) Introduction Chlorate(I) ions are oxidising agents In acid solution they end up as chloride ions Cl. O¯ Analysis (2) + 2 H+ 2 e¯ ——> Cl¯ + H 2 O Titrate the iodine produced with sodium thiosulphate using starch as the indicator near the end point I 2 overall + I 2 2 S 2 O 32 - ——> S 4 O 62 - + 2 I¯ 2 e¯ ——> + 2 S 2 O 32 - ——> S 4 O 62 - + 2 e¯ + 2 I¯

VOLUMETRIC ANALYSIS OF CHLORATE(I) Introduction Chlorate(I) ions are oxidising agents In acid solution they end up as chloride ions Cl. O¯ Analysis (2) + 2 H+ I 2 ——> Cl¯ 2 S 2 O 32 - ——> S 4 O 62 - + 2 I¯ 2 e¯ ——> + 2 S 2 O 32 - ——> moles of I 2 from (i) and (ii) 2 e¯ + H 2 O Titrate the iodine produced with sodium thiosulphate using starch as the indicator near the end point I 2 overall + S 4 O 62 - + 2 e¯ + 2 I¯ = ½ x moles of S 2 O 32 - original moles of OCl¯ = -- (ii) ½ x moles of S 2 O 32 -