41 The Halogens 41.1 Characteristic Properties of the Halogens

41 The Halogens 41.1 Characteristic Properties of the Halogens
41.2 Variation in Properties of the Halogens 41.3 Comparative Study of the Reactions of Halide Ions 41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride

Characteristic Properties of the Halogens
41.1 Characteristic Properties of the Halogens

Introduction Group VIIA elements include  fluorine  chlorine
41.1 Characteristic Properties of the Halogens (SB p.78) Introduction Group VIIA elements include  fluorine  chlorine  bromine  iodine  astatine

Introduction Astatine  not much is known  radioactive
41.1 Characteristic Properties of the Halogens (SB p.78) Introduction Astatine  not much is known  radioactive

Introduction Group VIIA elements  also called halogens
41.1 Characteristic Properties of the Halogens (SB p.78) Introduction Group VIIA elements  also called halogens

41.1 Characteristic Properties of the Halogens (SB p.78)

Introduction All halogens
41.1 Characteristic Properties of the Halogens (SB p.79) Introduction All halogens  outermost shell electronic configuration of ns2np5  one electron short of the octet electronic configuration

Introduction In the free elemental state  form diatomic molecules
41.1 Characteristic Properties of the Halogens (SB p.79) Introduction In the free elemental state  form diatomic molecules  complete their octets by sharing their single unpaired p electrons

Introduction When halogens react with other elements
41.1 Characteristic Properties of the Halogens (SB p.79) Introduction When halogens react with other elements  complete their octets  depending on the electronegativity of the element

Introduction Either  gaining an additional electron to form halide ions  or sharing their single unpaired p electrons to form single covalent bonds

Appearances of halogens at room temperature and pressure: chlorine
41.1 Characteristic Properties of the Halogens (SB p.79) chlorine Appearances of halogens at room temperature and pressure: chlorine

Appearances of halogens at room temperature and pressure: bromine
41.1 Characteristic Properties of the Halogens (SB p.79) bromine Appearances of halogens at room temperature and pressure: bromine

Appearances of halogens at room temperature and pressure: iodine
41.1 Characteristic Properties of the Halogens (SB p.79) iodine Appearances of halogens at room temperature and pressure: iodine

High Electronegativity
41.1 Characteristic Properties of the Halogens (SB p.79) High Electronegativity Electronegativity is the relative tendency of an atom to attract bonding electrons towards itself in a covalent bond.

High Electronegativity
41.1 Characteristic Properties of the Halogens (SB p.79) High Electronegativity All halogens  high electronegativity values  high tendency to attract an additional electron to achieve the stable octet electronic configuration  highest among the elements in the same period

Electronegativity values of halogens
41.1 Characteristic Properties of the Halogens (SB p.79) Electronegativity values of halogens Halogen Electronegativity value F Cl Br I At 4.0 3.0 2.8 2.5 2.2

High Electron Affinity
41.1 Characteristic Properties of the Halogens (SB p.79) High Electron Affinity Electron affinity is the enthalpy change when one mole of electrons is added to one mole of atoms or ions in the gaseous state.

41.1 Characteristic Properties of the Halogens (SB p.79) High Electron Affinity Its value  indicates the ease of formation of anions

41.1 Characteristic Properties of the Halogens (SB p.79) High Electron Affinity All halogens  negative values of electron affinity  high tendency to attract an additional electron to form the respective halide ions

Electron affinities of halogens
41.1 Characteristic Properties of the Halogens (SB p.79) Electron affinities of halogens Halogen Electron affinity (kJ mol–1) F Cl Br I At –348 –364 –342 –314 –285

Bonding and Oxidation State
41.1 Characteristic Properties of the Halogens (SB p.80) Bonding and Oxidation State Halogens  gain an additional electron to form the halide ions  combine with metals to form metal halides  held together by ionic bonding

41.1 Characteristic Properties of the Halogens (SB p.80) Bonding and Oxidation State The oxidation states of the halogens = –1

41.1 Characteristic Properties of the Halogens (SB p.80) Bonding and Oxidation State The halogen atoms  share their unpaired p electrons with a non-metallic atom  form a covalent bond

41.1 Characteristic Properties of the Halogens (SB p.80) Bonding and Oxidation State Halogens (except fluorine)  exhibit an oxidation state of –1 or +1 in the covalent molecules formed  depend on the electronegativity of the elements that are covalently bonded with the halogens

41.1 Characteristic Properties of the Halogens (SB p.80) Bonding and Oxidation State All halogens (except fluorine)  can expand their octets of electrons by utilizing the vacant, low-lying d orbitals

41.1 Characteristic Properties of the Halogens (SB p.80) Bonding and Oxidation State Each of these halogen atoms  have variable numbers of unpaired electrons to pair up with electrons from other atoms  able to form compounds of different oxidation states

“Electrons-in-boxes” diagrams of the electronic configuration of a halogen atom of the ground state and various excited states

Various oxidation states of halogens in their ions or compounds
41.1 Characteristic Properties of the Halogens (SB p.81) Various oxidation states of halogens in their ions or compounds Oxidation state of halogen Ion / Compound –1 F– Cl– Br– I– HF HCl HBr HI OF2 F2 Cl2 Br2 I2 +1 Cl2O Br2O HOCl HOBr OCl– OBr– +3 HClO2 ClO2–

Various oxidation states of halogens in their ions or compounds
41.1 Characteristic Properties of the Halogens (SB p.81) Various oxidation states of halogens in their ions or compounds Oxidation state of halogen Ion / Compound +4 ClO2 BrO2 +5 HClO3 HBrO3 I2O5 ClO3– BrO3– HIO3 IO3– +6 Cl2O6 BrO3 +7 Cl2O7 H5IO6 HClO4 HIO4 ClO4– IO4–

41.1 Characteristic Properties of the Halogens (SB p.81) Bonding and Oxidation State Fluorine  cannot expand its octet  no low-lying empty d orbitals available  the energy required to promote electrons into the third quantum shell is very high

41.1 Characteristic Properties of the Halogens (SB p.81) Bonding and Oxidation State Fluorine  the most electronegative element  only one unpaired p electron available for bonding  oxidation state is limited to –1

Colour All halogens  coloured
41.1 Characteristic Properties of the Halogens (SB p.82) Colour All halogens  coloured  the absorption of radiation in the visible light region of the electromagnetic spectrum

Colour The absorbed radiation
41.1 Characteristic Properties of the Halogens (SB p.82) Colour The absorbed radiation  the excitation of electrons to higher energy levels

Colour Fluorine atom  smaller size
41.1 Characteristic Properties of the Halogens (SB p.82) Colour Fluorine atom  smaller size  absorb the radiation of relatively high frequency (i.e. blue light)  appears yellow

Colour Atoms of other halogens  larger sizes
41.1 Characteristic Properties of the Halogens (SB p.82) Colour Atoms of other halogens  larger sizes  absorb radiation of lower frequency

Colour Iodine  absorbs the radiation of relatively low frequency (i.e. yellow light)  appears violet Let's Think 1

Colour Halogens  different colours when dissolved in different solvents

Colour Halogens  non-polar molecules
41.1 Characteristic Properties of the Halogens (SB p.82) Colour Halogens  non-polar molecules  not very soluble in polar solvents (such as water)  but very soluble in organic solvents (such as 1,1,1-trichloroethane)

Colours of halogens in pure form and in solutions
41.1 Characteristic Properties of the Halogens (SB p.82) Colours of halogens in pure form and in solutions Halogen Colour in pure form in water in 1,1,1-trichloroethane F2 Pale yellow Cl2 Greenish yellow Yellow Br2 Reddish brown Orange I2 Violet black Yellow (only slightly soluble) Violet

Colours of halogens in water: (a) chlorine; (b) bromine; (c) iodine
41.1 Characteristic Properties of the Halogens (SB p.82) (a) (b) (c) Colours of halogens in water: (a) chlorine; (b) bromine; (c) iodine

Colours of halogens in 1,1,1-trichloroethane: (a) chlorine; (b) bromine; (c) iodine

Check Point 41-1

Introduction All halogens  exist as diatomic molecules
41.2 Variation in Properties of the Halogens (SB p.83) Introduction All halogens  exist as diatomic molecules

Introduction In the diatomic molecules
41.2 Variation in Properties of the Halogens (SB p.83) Introduction In the diatomic molecules  the halogen atoms are held together by strong covalent bonds

Introduction The molecules
41.2 Variation in Properties of the Halogens (SB p.83) Introduction The molecules  only held together by weak van der Waals’ forces (i.e. instantaneous dipole-induced dipole interaction)

Introduction The physical properties of halogens
41.2 Variation in Properties of the Halogens (SB p.83) Introduction The physical properties of halogens  strongly affected by the way that the atoms are joined together  the interactions that hold the molecules together

Some physical properties of the halogens
41.2 Variation in Properties of the Halogens (SB p.83) Some physical properties of the halogens Halogen Colour and state at room temperature and pressure Atomic radius (nm) Ionic radius (nm) Fluorine Chlorine Bromine Iodine Astatine Pale yellow gas Greenish yellow gas Reddish brown liquid Violet black solid – 0.072 0.099 0.114 0.133 0.181 0.195 0.216

Some physical properties of the halogens
41.2 Variation in Properties of the Halogens (SB p.83) Some physical properties of the halogens Halogen Melting point (C) Boiling point (C) Density at 20 C (g cm–3) Fluorine Chlorine Bromine Iodine Astatine –220 –101 –7.2 114 302 –188 –34.7 58.8 184 380 1.11 1.56 3.12 4.93 –

Variation in Physical Properties
41.2 Variation in Properties of the Halogens (SB p.84) Variation in Physical Properties 1. Melting Point and Boiling Point Halogens  exist as non-polar diatomic molecules

1. Melting Point and Boiling Point
41.2 Variation in Properties of the Halogens (SB p.84) 1. Melting Point and Boiling Point Going down the group  the melting points and boiling points of halogens increase

41.2 Variation in Properties of the Halogens (SB p.84) 1. Melting Point and Boiling Point These physical properties depend on  the strength of van der Waals’ forces holding the halogen molecules together

41.2 Variation in Properties of the Halogens (SB p.84) 1. Melting Point and Boiling Point Going down the group  the molecular size increases  the electron clouds of the molecules become larger  more polarizable

41.2 Variation in Properties of the Halogens (SB p.84) 1. Melting Point and Boiling Point Instantaneous dipoles  more readily formed  the instantaneous dipole-induced dipole interaction between the molecules is stronger

41.2 Variation in Properties of the Halogens (SB p.84) 1. Melting Point and Boiling Point A greater amount of energy is required  separate the molecules in the processes of melting and boiling  the melting points and boiling points increase progressively from fluorine to astatine

Variations in melting point and boiling point of the halogens
41.2 Variation in Properties of the Halogens (SB p.84) Variations in melting point and boiling point of the halogens

41.2 Variation in Properties of the Halogens (SB p.84)
2. Electronegativity Electronegativity is the relative tendency of the nucleus of an atom to attract bonding electrons towards itself in a covalent bond.

2. Electronegativity Going down the group
41.2 Variation in Properties of the Halogens (SB p.84) 2. Electronegativity Going down the group  the electronegativity values of halogens decrease

2. Electronegativity Going down the group  the atomic size increases
41.2 Variation in Properties of the Halogens (SB p.84) 2. Electronegativity Going down the group  the atomic size increases  the number of electron shells increases  creates a greater screening effect

2. Electronegativity The atomic size increases
41.2 Variation in Properties of the Halogens (SB p.84) 2. Electronegativity The atomic size increases  The tendency of the nucleus of the halogen atom attract bonding electrons towards itself in a covalent bond decreases

Variations in electronegativity value of the halogens
41.2 Variation in Properties of the Halogens (SB p.85) Variations in electronegativity value of the halogens

3. Electron Affinity Electron affinity of halogens is the enthalpy change when one mole of electrons is added to one mole of halogen atoms or ions in the gaseous state.

3. Electron Affinity The electron affinity
41.2 Variation in Properties of the Halogens (SB p.85) 3. Electron Affinity The electron affinity  increases from fluorine to chlorine  decreases from chlorine to astatine

3. Electron Affinity The general decrease in electron affinity
41.2 Variation in Properties of the Halogens (SB p.85) 3. Electron Affinity The general decrease in electron affinity  the atomic size increases  the number of electrons shells down the group increases  the effective nuclear charge decreases  tendency of the nuclei of halogen atoms to attract additional electrons decreases

3. Electron Affinity Fluorine  abnormally low electron affinity
41.2 Variation in Properties of the Halogens (SB p.85) 3. Electron Affinity Fluorine  abnormally low electron affinity

3. Electron Affinity Fluorine atom  very small atomic size
41.2 Variation in Properties of the Halogens (SB p.85) 3. Electron Affinity Fluorine atom  very small atomic size  energy is required to overcome the repulsion between the additional electron and the electrons present in the electron shell

Variations in electron affinity of the halogens
41.2 Variation in Properties of the Halogens (SB p.85) Variations in electron affinity of the halogens

Check Point 41-2A

Variation in Chemical Properties
41.2 Variation in Properties of the Halogens (SB p.86) Variation in Chemical Properties Halogens  the most reactive group of non- metallic elements  all halogens have one electron short of the octet electronic configuration  tend to attract an additional electron to attain the octet electronic configuration

41.2 Variation in Properties of the Halogens (SB p.86) Variation in Chemical Properties Halogens  highly electronegative  highly negative electron affinity values  strong oxidizing agents

41.2 Variation in Properties of the Halogens (SB p.86) Variation in Chemical Properties Fluorine  very strong oxidizing agent

41.2 Variation in Properties of the Halogens (SB p.86) Variation in Chemical Properties Other elements that combine with fluorine  have their highest possible oxidation numbers

1. Relative Oxidizing Power of Halogens
41.2 Variation in Properties of the Halogens (SB p.86) 1. Relative Oxidizing Power of Halogens Reactions with Sodium All halogens  combine directly with sodium to form sodium halides  the reactivity decreases down the group from fluorine to iodine

 react explosively to form sodium fluoride
41.2 Variation in Properties of the Halogens (SB p.86) Reactions with Sodium Fluorine  react explosively to form sodium fluoride 2Na(s) + F2(g)  2NaF(s)

 react violently to form sodium chloride
41.2 Variation in Properties of the Halogens (SB p.86) Reactions with Sodium Chlorine  react violently to form sodium chloride 2Na(s) + Cl2(g)  2NaCl(s)

 burns steadily in bromine vapours to form sodium bromide
41.2 Variation in Properties of the Halogens (SB p.86) Reactions with Sodium Bromine  burns steadily in bromine vapours to form sodium bromide 2Na(s) + Br2(g)  2NaBr(s)

 burns steadily in iodine vapours to form sodium iodide
41.2 Variation in Properties of the Halogens (SB p.86) Reactions with Sodium Iodine  burns steadily in iodine vapours to form sodium iodide 2Na(s) + I2(g)  2NaI(s)

Reactions with Iron(II) Ions
41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Iron(II) Ions Aqueous chlorine  oxidizes green iron(II) ions to yellowish brown iron(III) ions 2Fe2+(aq) + Cl2(aq)  2Fe3+(aq) + 2Cl–(aq) = V

41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Iron(II) Ions Aqueous bromine  oxidizes green iron(II) ions to yellowish brown iron(III) 2Fe2+(aq) + Br2(aq)  2Fe3+(aq) + 2Br–(aq) = V

41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Iron(II) Ions Iodine  a mild oxidizing agent  not strong enough to oxidize iron(II) ions.

41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Iron(II) Ions The spontaneity of a reaction can be worked out  adding the standard electrode potentials of the two half reactions concerned

41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Iron(II) Ions If the overall standard electrode potential (i.e. the standard cell electromotive force, ) is a positive value  the reaction is predicted to be spontaneous

41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Iron(II) Ions Aqueous chlorine and bromine  the for both reactions are positive  the oxidation reactions of aqueous iron(II) ions are spontaneous

Standard electrode potentials of some related half reactions
41.2 Variation in Properties of the Halogens (SB p.87) Standard electrode potentials of some related half reactions Half reaction Standard electrode potential (V) Cl2(aq) + 2e–  2Cl–(aq) Br2(aq) + 2e –  2Br–(aq) I2(aq) + 2e–  2I–(aq) Fe3+(aq) + e–  Fe2+(aq) +1.36 +1.07 +0.54 +0.77

41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Iron(II) Ions Aqueous iodine  the = –0.23 V  this reaction is not spontaneous 2Fe2+(aq) + I2(aq)  2Fe3+(aq) + 2I–(aq) = –0.23 V

Reactions with Thiosulphate Ions
41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Thiosulphate Ions Thiosulphate ions  a reducing agent  reacts differently with halogens of different oxidizing power

41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Thiosulphate Ions Iodine  reacts with sodium thiosulphate to form sodium tetrathionate and sodium iodide

41.2 Variation in Properties of the Halogens (SB p.87) Reactions with Thiosulphate Ions This is a typical reaction  determine the concentration of iodine in a solution  by titration with standard thiosulphate solution (iodometric titration) I2(aq) + 2S2O32–(aq)  2I–(aq) + S4O62–(aq)

41.2 Variation in Properties of the Halogens (SB p.88) Reactions with Thiosulphate Ions Chlorine and bromine  more powerful oxidizing agents  oxidize thiosulphate ions to sulphate(VI) ions

41.2 Variation in Properties of the Halogens (SB p.88) Reactions with Thiosulphate Ions Chlorine 4Cl2(aq) + S2O32–(aq) + 5H2O(l)  8Cl–(aq) + 2SO42–(aq) + 10H+(aq)

41.2 Variation in Properties of the Halogens (SB p.88) Reactions with Thiosulphate Ions Bromine 4Br2(aq) + S2O32–(aq) + 5H2O(l)  8Br–(aq) + 2SO42–(aq) + 10H+(aq)

Reactions of halogens with sodium
41.2 Variation in Properties of the Halogens (SB p.88) Reactions of halogens with sodium Halogens Reactant Chlorine Bromine Iodine Sodium They react violently to form sodium chloride Sodium burns in bromine vapour to form sodium bromide Sodium burns in iodine vapour to form sodium iodide

Reactions of halogens with iron(II) ions
41.2 Variation in Properties of the Halogens (SB p.88) Reactions of halogens with iron(II) ions Halogens Reactant Chlorine Bromine Iodine Iron(II) ions The green iron(II) ions are oxidized to yellowish brown iron(III) ions The solution remains green since iron(II) ions are not oxidized by iodine

Reactions of halogens with thiosulphate ions
41.2 Variation in Properties of the Halogens (SB p.88) Reactions of halogens with thiosulphate ions Halogens Reactant Chlorine Bromine Iodine Thiosulphate ions The thiosulphate ions are oxidized to sulphate(VI) ions The thiosulphate ions are oxidized to tetrathionate and iodide ions

41.2 Variation in Properties of the Halogens (SB p.88) Reactions with Thiosulphate Ions The relative oxidizing power of the halogens decreases in the order: F2 > Cl2 > Br2 > I2

2. Disproportionation of the Halogens in Water and Alkalis
41.2 Variation in Properties of the Halogens (SB p.89) 2. Disproportionation of the Halogens in Water and Alkalis Reactions with Water Fluorine  reacts vigorously with water to form hydrogen fluoride and oxygen 2F2(g) + 2H2O(l)  4HF(aq) + O2(g)

 less reactive than fluorine
41.2 Variation in Properties of the Halogens (SB p.89) Reactions with Water Chlorine  less reactive than fluorine  reacts with water to form hydrochloric acid and chloric(I) acid (also known as hypochlorous acid

The oxidation number of chlorine
41.2 Variation in Properties of the Halogens (SB p.89) Reactions with Water The oxidation number of chlorine  decreases from 0 in Cl2(g) to –1 in HCl(aq)  increases from 0 in Cl2(g) to +1 in HOCl(aq)

 simultaneously oxidized and reduced
41.2 Variation in Properties of the Halogens (SB p.89) Reactions with Water Cl2(g)  simultaneously oxidized and reduced  an example of disproportionation

Reactions with Water Disproportionation is a chemical change in which oxidation and reduction of the same species (which may be a molecule, atom or ion) take place at the same time.

 a mixture of hydrochloric acid and chloric(I) acid
41.2 Variation in Properties of the Halogens (SB p.89) Reactions with Water Chlorine water  a mixture of hydrochloric acid and chloric(I) acid

Chlorate(I) ion (also known as hypochlorite ion)  an unstable ion
41.2 Variation in Properties of the Halogens (SB p.89) Reactions with Water Chlorate(I) ion (also known as hypochlorite ion)  an unstable ion  decomposes when exposed to sunlight or high temperatures to give chloride ions and oxygen 2OCl–(aq)  2Cl–(aq) + O2(g)

 able to oxidize dyes to form colourless compounds
41.2 Variation in Properties of the Halogens (SB p.89) Reactions with Water Chlorate(I) ion  able to oxidize dyes to form colourless compounds  bleaching power

Cl2(aq) + H2O(l) 2H+(aq) + Cl–(aq) + OCl–(aq)
41.2 Variation in Properties of the Halogens (SB p.89) Reactions with Water Chlorate(I) ion Cl2(aq) + H2O(l) 2H+(aq) + Cl–(aq) + OCl–(aq) OCl–(aq) + dye  Cl–(aq) + (dye + O) coloured colourless

 only slightly soluble in water
41.2 Variation in Properties of the Halogens (SB p.89) Reactions with Water Bromine  only slightly soluble in water  mainly exists as molecules in saturated bromine water

When the solution is diluted  hydrolysis takes place
41.2 Variation in Properties of the Halogens (SB p.89) Reactions with Water When the solution is diluted  hydrolysis takes place  hydrobromic acid and bromic(I) acid (also called hydrobromous acid) are formed Br2(l) + H2O(l) HBr(aq) + HOBr(aq)

 forms colourless compounds when reacting with dyes
41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Water Bromate(I) ion  also unstable  forms colourless compounds when reacting with dyes OBr–(aq) + dye coloured  Br–(aq) + (dye + O) colourless

 does not react with water  only slightly soluble in water
41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Water Iodine  does not react with water  only slightly soluble in water

 soluble in potassium iodide solution
41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Water Iodine  soluble in potassium iodide solution  exists as triiodide ions in thesolution  often called iodine solution I2(s) + KI(aq)  KI3(aq) iodine solution

Reactions with Alkalis
41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis All halogens  react with aqueous alkalis

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis The reactions between halogens and aqueous alkalis  disproportionation (except fluorine)

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis Halogens  react differently under cold / hot and dilute / concentrated conditions

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis In general  their reactivities decrease down the group

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis Fluorine is passed through a cold and very dilute (2%) sodium hydroxide solution  oxygen difluoride (OF2) is formed 2F2(g) + 2NaOH(aq) cold, very dilute  2NaF(aq) + OF2(g) + H2O(l) – –1

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis When fluorine is passed through a hot and concentrated sodium hydroxide solution  oxygen is formed instead 2F2(g) + 4NaOH(aq) – hot, concentrated  4NaF(aq) + O2(g) + 2H2O(l) –1 0

Let's Think 2

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis Chlorine  reacts with cold and dilute sodium hydroxide solution to form sodium chloride and sodium chlorate(I) (also called sodium hypochlorite) Cl2(aq) + 2NaOH(aq) cold, dilute  NaCl(aq) + NaOCl(aq) + H2O(l) –

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis Chlorine  reacts with hot and concentrated sodium hydroxide solution to form sodium chloride and sodium chlorate(V) 3Cl2(aq) + 6NaOH(aq) hot, concentrated  5NaCl(aq) + NaClO3(aq) + 3H2O(l) –

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis Bromine  undergoes similar reactions with alkalis as chlorine  sodium bromate(I) formed is unstable

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis Sodium bromate(I) formed  disproportionates to form sodium bromide and sodium bromate(V) readily at room temperature and pressure  reversible

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis Br2(aq) + 2NaOH(aq) cold, dilute  NaBr(aq) + NaOBr(aq) + H2O(l) 3NaOBr(aq) NaBr(aq) + NaBrO3(aq)

41.2 Variation in Properties of the Halogens (SB p.90) Reactions with Alkalis The chemical equation for the overall reaction: 3Br2(aq) + 6NaOH(aq) cold, dilute  5NaBr(aq) + NaBrO3(aq) + 3H2O(l) –

41.2 Variation in Properties of the Halogens (SB p.91) Reactions with Alkalis Iodine  behaves similarly as bromine

41.2 Variation in Properties of the Halogens (SB p.91) Reactions with Alkalis Except that the reaction with a cold and dilute alkali  reversible 3I2(aq) + 6NaOH(aq) cold, dilute 5NaI(aq) + NaIO3(aq) + 3H2O(l) –

41.2 Variation in Properties of the Halogens (SB p.91) Reactions with Alkalis The backward reaction  often used to prepare standard iodine solution for iodometric titrations

41.2 Variation in Properties of the Halogens (SB p.91) Reactions with Alkalis Dissolving a known quantity of potassium iodate(V) in excess potassium iodide solution and dilute sulphuric(VI) acid  generated a known amount of iodine solution KIO3(aq) + 5KI(aq) + 6H+(aq)  3I2(aq) + 3H2O(l) + 6K+(aq)

41.2 Variation in Properties of the Halogens (SB p.91) Reactions with Alkalis The iodine generated  used to oxidize reducing agents  such as sulphate(IV) ions and ascorbic acid (vitamin C)

41.2 Variation in Properties of the Halogens (SB p.91) Reactions with Alkalis Excess iodine  can be determined by back titration with sodium thiosulphate solution I2(aq) + 2S2O32–(aq)  2I–(aq) + S4O62–(aq)

Check Point 41-2B

Introduction All metal halides  basically ionic compounds
41.3 Comparative Study of the Reactions of Halide Ions (SB p.91) Introduction All metal halides  basically ionic compounds  the ionic character of metal halides decreases on going down from fluorides to iodides

Introduction Lithium fluoride  ionic
41.3 Comparative Study of the Reactions of Halide Ions (SB p.91) Introduction Lithium fluoride  ionic

Introduction Lithium iodide  considerable covalent character
41.3 Comparative Study of the Reactions of Halide Ions (SB p.91) Introduction Lithium iodide  considerable covalent character

Introduction Chlorides, bromides and iodides
41.3 Comparative Study of the Reactions of Halide Ions (SB p.91) Introduction Chlorides, bromides and iodides  similar solubilities in water

Introduction Fluorides  anomalous properties
41.3 Comparative Study of the Reactions of Halide Ions (SB p.91) Introduction Fluorides  anomalous properties

Introduction Silver chloride, silver bromide and silver iodide
41.3 Comparative Study of the Reactions of Halide Ions (SB p.91) Introduction Silver chloride, silver bromide and silver iodide  insoluble in water

Introduction Silver fluoride  soluble in water
41.3 Comparative Study of the Reactions of Halide Ions (SB p.91) Introduction Silver fluoride  soluble in water

41.3 Comparative Study of the Reactions of Halide Ions (SB p.92)
Reactions of Halogens The reactions of halogens with halide ions follow the order of relative oxidizing power: F2 > Cl2 > Br2 > I2

Reactions of Halogens Fluorine
41.3 Comparative Study of the Reactions of Halide Ions (SB p.92) Reactions of Halogens Fluorine  displace all other halogens from the corresponding halide ions F2(g) + 2Cl–(aq)  2F–(aq) + Cl2(aq) F2(g) + 2Br–(aq)  2F–(aq) + Br2(aq) F2(g) + 2I–(aq)  2F–(aq) + I2(aq)

Reactions of Halogens Chlorine
41.3 Comparative Study of the Reactions of Halide Ions (SB p.92) Reactions of Halogens Chlorine  displace bromine and iodine from bromide and iodide ions respectively Cl2(aq) + 2Br–(aq)  Br2(aq) + 2Cl–(aq) Cl2(aq) + 2I–(aq)  I2(aq) + 2Cl–(aq)

The mixture of chlorine water with (a) potassium bromide solution; (b) potassium iodide solution

Reactions of Halogens Bromine  displace iodine from iodide ions only
41.3 Comparative Study of the Reactions of Halide Ions (SB p.92) Reactions of Halogens Bromine  displace iodine from iodide ions only Br2(aq) + 2I–(aq)  I2(aq) + 2Br–(aq)

Reactions of Halogens Iodine
41.3 Comparative Study of the Reactions of Halide Ions (SB p.92) Reactions of Halogens Iodine  cannot displace the other halogens from the corresponding halide ions

Reactions of Halogens The feasibility of redox reactions at standard states in aqueous solutions  predicted by using the values of standard electrode potentials

Reactions of Halogens Adding up the standard cell electrode potentials of the two corresponding half reactions  obtain the of the overall cell reaction

Reactions of Halogens If the is a positive value  spontaneous
41.3 Comparative Study of the Reactions of Halide Ions (SB p.92) Reactions of Halogens If the is a positive value  spontaneous

Reactions of Halogens Consider whether the following redox reaction will take place at standard states: Br2(aq) + 2I–(aq)  I2(aq) + 2Br–(aq)

Reactions of Halogens Considered as the combination of the following two equilibria competing with one another: Br2(aq) + 2e– Br–(aq) = V I2(aq) + 2e– I–(aq) = V

Reactions of Halogens The half reaction Br2(aq) + 2e– 2Br–(aq)
41.3 Comparative Study of the Reactions of Halide Ions (SB p.92) Reactions of Halogens The half reaction Br2(aq) + 2e– 2Br–(aq)  more positive standard electrode potential

Reactions of Halogens Bromine
41.3 Comparative Study of the Reactions of Halide Ions (SB p.92) Reactions of Halogens Bromine  higher tendency to gain electrons (i.e. stronger oxidizing power)

Reactions of Halogens Therefore Br2(aq) + 2e– 2Br–(aq) = +1.07 V
41.3 Comparative Study of the Reactions of Halide Ions (SB p.92) Reactions of Halogens Therefore Br2(aq) + 2e– Br–(aq) = V I2(aq) + 2e– –) 2I–(aq) = V Br2(aq) + I2(aq) 2Br–(aq) + 2I–(aq) = V

Reactions of Halogens The of the overall reaction  a positive value
41.3 Comparative Study of the Reactions of Halide Ions (SB p.92) Reactions of Halogens The of the overall reaction  a positive value  proceed spontaneously

Reactions of halide ions with halogens
41.3 Comparative Study of the Reactions of Halide Ions (SB p.93) Reactions of halide ions with halogens Aqueous solution Halogen added F2 Cl2 Br2 I2 F– No reaction Cl– A pale yellow solution is formed (Cl2 is formed)

Reactions of halide ions with halogens
41.3 Comparative Study of the Reactions of Halide Ions (SB p.93) Reactions of halide ions with halogens Aqueous solution Halogen added F2 Cl2 Br2 I2 Br– A yellow solution is formed (Br2 is formed) No reaction I– A yellowish brown solution is formed (I2 is formed)

Reactions of Halogens The most convenient way to carry out displacement reactions:  mix aqueous solutions of the halogens with aqueous solutions of potassium iodide, potassium bromide and potassium chloride  shake them well

Reactions of Halogens Iodine  almost insoluble in water
41.3 Comparative Study of the Reactions of Halide Ions (SB p.93) Reactions of Halogens Iodine  almost insoluble in water  dissolves readily in a solution containing iodide ions

Reactions of Halogens The soluble triiodide ion, I3–, is formed in this way: I2(s) + I–(aq) I3–(aq)

Reactions of Halogens Observing the colour changes
41.3 Comparative Study of the Reactions of Halide Ions (SB p.93) Reactions of Halogens Observing the colour changes  difficult to determine whether certain reactions have taken place or not by only

Reactions of Halogens To determine whether the reaction mixture contains bromine or iodine  add a small amount of 1,1,1- trichloroethane to the reaction mixture

Reactions of Halogens Any bromine or iodine present
41.3 Comparative Study of the Reactions of Halide Ions (SB p.93) Reactions of Halogens Any bromine or iodine present  dissolve more readily in the organic solvent than in water

Reactions of Halogens If the reaction mixture contains bromine
41.3 Comparative Study of the Reactions of Halide Ions (SB p.93) Reactions of Halogens If the reaction mixture contains bromine  the bromine will dissolve in the bottom organic layer  give a deep orange colour

Reactions of Halogens If the reaction mixture contains iodine
41.3 Comparative Study of the Reactions of Halide Ions (SB p.93) Reactions of Halogens If the reaction mixture contains iodine  the iodine will dissolve in the bottom organic layer  give a violet colour

(a) If the reaction mixture contains bromine, the bottom organic layer will appear deep orange; (b) If the reaction mixture contains iodine, the bottom organic layer will appear violet.

Check Point 41-3A

Reactions with Silver Ions
41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions Aqueous solutions of chlorides, bromides and iodides  give precipitates when reacting with silver nitrate(V) solution  a characteristic test to show the presence of halide ions (except fluoride ions)

41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions Ag+(aq) + Cl–(aq)  AgCl(s) white precipitate Ag+(aq) + Br–(aq)  AgBr(s) pale yellow precipitate Ag+(aq) + I–(aq)  AgI(s) yellow precipitate

41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions All silver halides  insoluble in acids

41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions Dilute nitric(V) acid should be added before silver nitrate(V) solution  remove interfering ions  like sulphate (IV) ions or carbonate ions

41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions The formation of the white precipitate of silver sulphate(IV) or silver carbonate  may be mistaken as silver halides, can be prevented 2H+(aq) + SO32–(aq)  SO2(g) + H2O(l) 2H+(aq) + CO32–(aq)  CO2(g) + H2O(l)

Formation of silver halides: silver chloride
41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) AgCl(s) Formation of silver halides: silver chloride

Formation of silver halides: silver bromide
41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) AgBr(s) Formation of silver halides: silver bromide

Formation of silver halides: silver iodide
41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) AgI(s) Formation of silver halides: silver iodide

41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions The formation of silver chloride, silver bromide and silver iodide  identified by their colours  or by their reactions with aqueous ammonia

41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions Silver chloride  dissolves readily in aqueous ammonia  the formation of the complex diamminesilver(I) ion ([Ag(NH3)2]+(aq))  soluble in water

41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions AgCl(s) + 2NH3(aq)  [Ag(NH3)2]+(aq) + Cl–(aq)

41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions Silver bromide  slightly soluble in aqueous ammonia

41.3 Comparative Study of the Reactions of Halide Ions (SB p.94) Reactions with Silver Ions Silver iodide  insoluble in aqueous ammonia

41.3 Comparative Study of the Reactions of Halide Ions (SB p.95) Reactions with Silver Ions When exposed to sunlight  silver chloride turns grey  silver bromide turns yellowish grey  silver iodide remains yellow

41.3 Comparative Study of the Reactions of Halide Ions (SB p.95) Reactions with Silver Ions The colour changes of silver chloride and silver bromide  photochemical decomposition of the silver halides into their constituent elements (i.e. silver and halogens) 2AgCl(s)  2Ag(s) + Cl2(g) light 2AgBr(s)  2Ag(s) + Br2(g) light

Action of acidified silver nitrate(V) solution on halides
41.3 Comparative Study of the Reactions of Halide Ions (SB p.95) Action of acidified silver nitrate(V) solution on halides Ion Action of acidified silver nitrate(V) solution on halides Confirmatory test of the product Effect of exposure to sunlight Effect of adding aqueous ammonia Cl– A white precipitate is formed (AgCl is formed) The solution turns grey The white precipitate dissolves Br– A pale yellow precipitate is formed (AgBr is formed) The solution turns yellowish grey The pale yellow precipitate slightly dissolves I– A yellow precipitate is formed (AgI is formed) The solution remains yellow The yellow precipitate does not dissolve

Reactions with Concentrated Sulphuric(VI) Acid
41.3 Comparative Study of the Reactions of Halide Ions (SB p.95) Reactions with Concentrated Sulphuric(VI) Acid Concentrated sulphuric(VI) acid  an oxidizing acid  exhibits both oxidizing and acidic properties

41.3 Comparative Study of the Reactions of Halide Ions (SB p.95) Reactions with Concentrated Sulphuric(VI) Acid On treatment with concentrated sulphuric(VI) acid  fluorides and chlorides give hydrogen fluoride and hydrogen chloride respectively

41.3 Comparative Study of the Reactions of Halide Ions (SB p.95) Reactions with Concentrated Sulphuric(VI) Acid Example: NaF(s) + H2SO4(l)  NaHSO4(s) + HF(g) NaCl(s) + H2SO4(l)  NaHSO4(s) + HCl(g)

41.3 Comparative Study of the Reactions of Halide Ions (SB p.95) Reactions with Concentrated Sulphuric(VI) Acid Bromides and iodides  do not give hydrogen bromide and hydrogen iodide respectively  sulphur dioxide or hydrogen sulphide is formed

Action of concentrated sulphuric(VI) acid on: (a) sodium bromide; (b) sodium iodide

41.3 Comparative Study of the Reactions of Halide Ions (SB p.95) Reactions with Concentrated Sulphuric(VI) Acid Bromides NaBr(s) + H2SO4(l)  NaHSO4(s) + HBr(g) 2HBr(g) + H2SO4(l)  SO2(g) + Br2(g) + 2H2O(l)

41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Reactions with Concentrated Sulphuric(VI) Acid The chemical equation for the overall reaction is 2NaBr(s) + 3H2SO4(l)  2NaHSO4(s) + SO2(g) Br2(g) + 2H2O(l)

41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Reactions with Concentrated Sulphuric(VI) Acid Iodides NaI(s) + H2SO4(l)  NaHSO4(s) + HI(g) 8HI(g) + H2SO4(l)  H2S(g) + 4I2(g) + 4H2O(l)

41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Reactions with Concentrated Sulphuric(VI) Acid The chemical equation for the overall reaction is 8NaI(s) + 9H2SO4(l)  8NaHSO4(s) + H2S(g) I2(g) + 4H2O(l)

41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Reactions with Concentrated Sulphuric(VI) Acid Bromides and iodides  do not react in the same way as fluorides and chlorides  the hydrogen bromide and hydrogen iodide produced are oxidized by concentrated sulphuric(VI) acid to bromine and iodine respectively

41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Reactions with Concentrated Sulphuric(VI) Acid Hydrogen chloride  is not oxidized by concentrated sulphuric(VI) acid

Action of concentrated sulphuric(VI) acid on halides
41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Action of concentrated sulphuric(VI) acid on halides Halide ion Action of concentrated sulphuric(VI) acid Product Confirmatory test of the product Cl– Steamy fumes are formed No green gas is evolved even on heating HCl Dense white fumes are formed with aqueous ammonia It turns blue litmus paper red but not bleached

41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Action of concentrated sulphuric(VI) acid on halides Halide ion Action of concentrated sulphuric(VI) acid Product Confirmatory test of the product Br– Steamy fumes are formed A pungent smell is detected A brown gas is evolved on warming HBr White fumes are formed with aqueous ammonia SO2 It turns orange dichromate(VI) solution green Br2 A red colour is observed when adding hexane

41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Action of concentrated sulphuric(VI) acid on halides Halide ion Action of concentrated sulphuric(VI) acid Product Confirmatory test of the product I– Steamy violet fumes are formed A bad egg smell is detected HI White fumes are formed with aqueous ammonia H2S It turns lead(II) ethanoate paper black I2 A violet colour is observed when adding hexane

41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Reactions with Concentrated Sulphuric(VI) Acid The reaction of chlorides with concentrated sulphuric(VI) acid  used for the preparation of hydrogen chloride in the laboratory

41.3 Comparative Study of the Reactions of Halide Ions (SB p.96) Reactions with Concentrated Sulphuric(VI) Acid Hydrogen bromide and hydrogen iodide  cannot be prepared in this way

Reactions with Phosphoric(V) Acid
41.3 Comparative Study of the Reactions of Halide Ions (SB p.97) Reactions with Phosphoric(V) Acid Phosphoric(V) acid  not an oxidizing agent  reacts with halides to form the corresponding hydrogen halides  a general method to prepare hydrogen halides in the laboratory

Reactions with Phosphoric(V) Acid
41.3 Comparative Study of the Reactions of Halide Ions (SB p.97) Reactions with Phosphoric(V) Acid 3NaCl(s) + H3PO4(l)  Na3PO4(s) + 3HCl(g) 3NaBr(s) + H3PO4(l)  Na3PO4(s) + 3HBr(g) 3NaI(s) + H3PO4(l)  Na3PO4(s) + 3HI(g)

Action of concentrated phosphoric(V) acid on halides
41.3 Comparative Study of the Reactions of Halide Ions (SB p.97) Action of concentrated phosphoric(V) acid on halides Halide ion Action of concentrated phosphoric(V) acid Product Confirmatory test of the product Cl– Steamy fumes are formed on warming HCl White fumes are formed with aqueous ammonia Br– HBr I– HI Check Point 41-3B

Introduction Hydrogen halides
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.98) Introduction Hydrogen halides  prepared by the direct reactions of hydrogen with halogens

Introduction Hydrogen  reacts explosively with fluorine
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.98) Introduction Hydrogen  reacts explosively with fluorine

Introduction Hydrogen chloride is formed
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.98) Introduction Hydrogen chloride is formed  when hydrogen burns in chlorine gas

Introduction Hydrogen
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.98) Introduction Hydrogen  reacts with bromine or iodine in the presence of a catalyst at high temperatures  form hydrogen bromide and hydrogen iodide respectively H2(g) + X2(g)  2HX(g)

Introduction Hydrogen halides
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.98) Introduction Hydrogen halides  prepared by adding concentrated phosphoric(V) acid to the halides 3X–(aq) + H3PO4(l)  PO43–(aq) + 3HX(g)

Some physical properties of hydrogen halides
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.98) Some physical properties of hydrogen halides Hydrogen halide Relative molecular mass Melting point (C) Boiling point (C) Physical state at room temperature and pressure HF 20.0 –83.1 19 Liquid HCl 36.5 –115 –85 Gas HBr 87 80.9 –67 HI 127.9 –50 –36

Acidic Properties of Hydrogen Halides
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.98) Acidic Properties of Hydrogen Halides Hydrogen halides  dissociate in water to form acidic solutions HX(g) + H2O(l) H3O+(aq) + X–(aq)

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.98) Acidic Properties of Hydrogen Halides The larger the acid dissociation constant  the stronger its acid strength

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.98) Acidic Properties of Hydrogen Halides The acid strength of hydrogen halides decreases in the order: HI > HBr > HCl >> HF

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.99) Acid dissociation constants of hydrogen halides and their degrees of dissociation in 0.1 M solutions Hydrogen halide Acid dissociation constant, Ka (mol dm–3) Degree of dissociation in 0.1 M solution (%) Acid strength HF HCl HBr HI 7 × 10–4 1 × 107 1 × 109 1 × 1011 8.5 92 93 95 Low Strong Very strong

Anomalous Behaviour of Hydrogen Fluoride
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.99) Anomalous Behaviour of Hydrogen Fluoride 1. Hydrogen fluoride has abnormally high boiling point and melting point among the hydrogen halides Molecules of all other hydrogen halides  held together by weak van der Waal’s forces only

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.99) Formation of the extensive intermolecular hydrogen bonds among hydrogen fluoride molecules

2. Hydrogen fluoride is soluble in water
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.99) 2. Hydrogen fluoride is soluble in water A dilute solution of hydrogen fluoride  behaves only as a weak acid HF(l) + H2O(l) H3O+(aq) + F–(aq) ..…... (1) Ka = 7 × 10–4 mol dm–3

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.99) 2. Hydrogen fluoride is soluble in water A more concentrated solution of hydrogen fluoride  another equilibrium is established  the fluoride ion form the complex ion [HF2]– F–(aq) + HF(l) [HF2]–(aq) .….... (2) K = 5.1 dm3 mol–1

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.99) 2. Hydrogen fluoride is soluble in water The equilibrium of reaction (2)  shifts to the right  the concentration of hydrogen fluoride increases

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.99) 2. Hydrogen fluoride is soluble in water The consumption of fluoride ions in reaction (2)  the equilibrium of reaction (1) also shifts to the right

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.99) 2. Hydrogen fluoride is soluble in water The acid strength of hydrogen fluoride  enhanced

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.99) 2. Hydrogen fluoride is soluble in water A concentration of approximately 5 to 15 M of hydrogen fluoride  effectively a strong acid

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.100) 3. Other fluorides (e.g. potassium fluoride) also react with hydrogen fluoride to form acid salts containing the stable [HF2]–ion KF(s) + HF(l) KHF2(s)

Heating the solid potassium hydrogen difluoride
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.100) 3. Other fluorides (e.g. potassium fluoride) also react with hydrogen fluoride to form acid salts containing the stable [HF2]–ion Heating the solid potassium hydrogen difluoride  reverses the reaction  a convenient way to obtain anhydrous hydrogen fluoride

41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.100) 4. A special property of hydrofluoric acid is its ability to react with glass Use to etch glass

A glass is etched by hydrofluoric acid
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.100) A glass is etched by hydrofluoric acid

The glass object to be etched
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.100) 4. A special property of hydrofluoric acid is its ability to react with glass The glass object to be etched  coated with wax or a similar acid- proof material  cutting through the wax layer to expose the glass  apply hydrofluoric acid

The principle of etching glass
41.4 Acidic Properties of Hydrogen Halides and the Anomalous Behaviour of Hydrogen Fluoride (SB p.100) 4. A special property of hydrofluoric acid is its ability to react with glass The principle of etching glass CaSiO3(s) + 6HF(aq)  CaF2(aq) + SiF4(aq) + 3H2O(l) Check Point 41-4

The END

What is the predicted colour of astatine?
41.1 Characteristic Properties of the Halogens (SB p.82) Let's Think 1 What is the predicted colour of astatine? Answer The colour of astatine is black. Back

Check Point 41-1 (a) Write the electronic configuration of each of the halogens. What is in common about these electronic configurations? Answer

Electronic configuration
41.1 Characteristic Properties of the Halogens (SB p.83) (a) They have the outermost shell electronic configuration of ns2np5. Halogen Electronic configuration F 1s22s22p5 Cl 1s22s22p63s23p5 Br 1s22s22p63s23p63d104s24p5 I 1s22s22p63s23p63d104s24p64d105s25p5 At 1s22s22p63s23p63d104s24p64d104f145s25p65d106s66p5

Check Point 41-1 (b) Does a halogen atom gain or lose an electron more readily when forming a compound? Answer (b) A halogen atom tends to gain an electron when forming a compound.

Check Point 41-1 (c) The colour of halogens darkens on going down the group. Explain why. Answer (c) Going down the halogen, the sizes of the halogen atoms increase, and the radiation of lower frequency is absorbed. For instance, since fluorine atom has a smaller size, it tends to absorb the radiation of relatively high frequency (i.e. blue light), hence fluorine appears yellow. Atoms of other halogens have larger sizes and they absorb radiation of lower frequency. For example, iodine absorbs the radiation of relatively low frequency (i.e. yellow light), hence iodine appears violet. Back

Check Point 41-2A (a) Explain the term “electron affinity”. Describe the trend of variation in electron affinity of the halogens. Answer

(a) Electron affinity is the enthalpy change when one mole of electrons is added to one mole of atoms or ions in the gaseous state. The electron affinity increases from fluorine to chlorine and then decreases from chlorine to astatine. The general decrease in electron affinity is due to the increases in atomic size and number of electron shells down the group. This leads to a decrease in effective nuclear charge. Therefore, the tendency of the nuclei of halogen atoms to attract additional electrons decreases. On the other hand, fluorine has an abnormally low electron affinity. It is because fluorine atom has a very small atomic size. Energy is required to overcome the repulsion between the additional electron and the electrons present in the electron shell. The electron affinity of fluorine is therefore lower than expected.

Check Point 41-2A (b) For each of the following physical properties of the halogens, state the trend down the group. (i) Atomic radius (ii) Ionic radius (iii) Melting point (iv) Electronegativity (v) Colour intensity (b) (i) Increase (ii) Increase (iii) Increase (iv) Decrease (v) Increase Answer Back

Let's Think 2 Why does fluorine always behave differently from chlorine, bromine and iodine? Answer Fluorine cannot expand its octet as there are no low-lying empty d orbitals available, and the energy required to promote electrons into the third quantum shell is very high. Since fluorine is the most electronegative element and there is only one unpaired p electron available for bonding, its oxidation state is limited to –1 when bonded with other elements. Back

Check Point 41-2B (a) Explain why halogens are strong oxidizing agents. Answer (a) It is because all halogens have one electron short of the octet electronic configuration, they tend to attract an additional electron to complete the octet. They have high electronegativity values and highly negative electron affinity values. Halogens are thus strong oxidizing agents.

Check Point 41-2B (b) What chemical species are present in the following solutions? (i) Chlorine water (ii) Bromine water (iii) Iodine in potassium iodide solution Answer (b) (i) Cl2(aq), Cl–(aq), ClO–(aq), H+(aq), H2O(l ) (ii) Br2(aq), Br–(aq), BrO–(aq), H+(aq), H2O(l ) (iii) I–(aq), K+(aq), I3–(aq), H2O(l) Back

Check Point 41-3A (a) Is bromine and iodine more soluble in water or 1,1,1-trichloroethane? Explain your answer. Answer (a) Both bromine and iodine are more soluble in 1,1,1-trichloroethane than in water. It is because both bromine and iodine are non polar molecules, and water is a much polar solvent than 1,1,1-trichloroethane.

Check Point 41-3A (b) Describe a simple way to increase the solubility of iodine in water. Answer (b) The solubility of iodine in water can be increased by adding potassium iodide solution. Iodine exists as triiodide ions in potassium iodide solution as shown in the following equation: I2(s) + KI(aq)  KI3(aq) Back

Check Point 41-3B (a) State any observable changes when the following substances are added into sodium iodide solution. Give appropriate equations, if any. (i) Iron(II) sulphate(VI) solution Answer (a) (i) There is no observable change.

Check Point 41-3B (a) State any observable changes when the following substances are added into sodium iodide solution. Give appropriate equations, if any. (ii) Chlorine water Answer (a) (ii) The solution turns yellowish brown. Cl2(aq) + 2NaI(aq)  I2(aq) + 2NaCl(aq)

Check Point 41-3B (a) State any observable changes when the following substances are added into sodium iodide solution. Give appropriate equations, if any. (iii) Sodium iodate(V) solution and dilute sulphuric(VI) acid Answer (a) (iii) The solution turns yellowish brown. 5I–(aq) + IO3–(aq) + 6H+(aq)  3I2(aq) + 3H2O(l)

Check Point 41-3B (b) If you are given two solutions, sodium fluoride and sodium chloride, how would you distinguish them with a simple chemical test? State all observable changes and write equations where appropriate. Answer (b) Sodium chloride solution and sodium fluoride solution can be distinguished using acidified silver nitrate(V) solution. Sodium chloride solution reacts with acidified silver nitrate(V) solution to form a white precipitate, while sodium fluoride solution does not. Ag+(aq) + Cl–(aq)  AgCl(s) white precipitate Back

Check Point 41-4 (a) For each of the following pairs, which is a stronger acid? Explain your answers. (i) Dilute hydrochloric acid and dilute hydrofluoric acid Answer

(a) (i) Dilute hydrochloric acid is a stronger acid than dilute hydrofluoric acid, as hydrochloric acid has a much larger Ka value than hydrofluoric acid. HCl(aq) + H2O(l) H3O+(aq) + Cl–(aq) Ka = 1 × 107 mol dm–3 HF(l) + H2O(l) H3O+(aq) + F–(aq) Ka = 7 × 10–4 mol dm–3 The small Ka value of hydrofluoric acid indicates that only a small amount of hydrogen fluoride molecules is ionized. Most of hydrogen fluoride molecules still exist in the undissociated form. The large Ka value of hydrochloric acid indicates that almost all hydrogen chloride molecules are ionized in water. This can be explained by the presence of extensive intermolecular hydrogen bonds among hydrogen fluoride molecules.

Check Point 41-4 (a) For each of the following pairs, which is a stronger acid? Explain your answers. (ii) Concentrated hydrochloric acid and concentrated hydrofluoric acid Answer

(a) (ii) Concentrated hydrofluoric acid is a stronger acid than concentrated hydrochloric acid. Hydrogen fluoride is soluble in water. In a dilute solution of hydrogen fluoride, it behaves only as a weak acid. HF(l) + H2O(l) H3O+(aq) + F–(aq) (1) Ka = 7 × 10–4 mol dm–3 However, in a more concentrated solution of hydrogen fluoride, another equilibrium is established with the fluoride ion forming the complex ion [HF2]–. F–(aq) + HF(l) [HF2]–(aq) …… (2) K = 5.1 dm3 mol–1 The equilibrium of reaction (2) shifts to the right as the concentration of hydrogen fluoride increases. With the consumption of fluoride ions in reaction (2), the equilibrium of reaction (1) also shifts to the right. The acid strength of hydrogen fluoride is therefore enhanced. Hence, concentrated hydrofluoric acid is effectively a strong acid.

Check Point 41-4 Answer (b) Explain the following phenomena:
41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) Check Point 41-4 (b) Explain the following phenomena: (i) Hydrogen fluoride is a liquid at room temperature and pressure. Answer (b) (i) Hydrogen fluoride exists as a liquid at room temperature and pressure due to its ability to form extensive intermolecular hydrogen bonds.

41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) Check Point 41-4 (b) Explain the following phenomena: (ii) Hydrogen fluoride forms acid salts – potassium hydrogen difluoride. Answer (b) (ii) Hydrogen fluoride is able to react with other fluorides (e.g. potassium fluoride) to form acid salts containing the stable [HF2]– ion (e.g. potassium hydrogen fluoride). KF(s) + HF(l) KHF2(s)

41.3 Comparative Study of the Reactions of Halide Ions (SB p.100) Check Point 41-4 (b) Explain the following phenomena: (iii) Hydrogen fluoride can be used to etch glass. (b) (iii) The principle of etching glass by hydrofluoric acid can be explained by its reaction with the silicate of glass. CaSiO3(s) + 6HF(aq)  CaF2(aq) + SiF4(aq) + 3H2O(l) Answer

Check Point 41-4 (c) Complete and balance the following equations: (i) F2(g) + H2O(l)  (ii) F2(g) + KOH(aq) (cold, dilute)  Answer (c) (i) 2F2(g) + 2H2O(l )  4HF(aq) + O2(g) (iii) 2F2(g) + 2KOH(aq)  2KF(aq) + OF2(g) + H2O(l) Back

41 The Halogens 41.1 Characteristic Properties of the Halogens

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41 The Halogens 41.1 Characteristic Properties of the Halogens

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