Chapter 6 The Periodic Table: Group7

Chapter 6 The Periodic Table: Group7

6.1 The Halogens

6.2 Reactions of The Halogens and Their Ions

6.1 What is the outcome from syllabus?

Candidates should be able to:

♣ describe the trends in volatility and colour of chlorine, bromine and iodine

♣ interpret the volatility of the elements in terms of van der Waals’ forces

♣ describe the relative reactivity of the elements as oxidising agents

♣ describe and explain the reactions of the elements with hydrogen

♣ describe and explain the relative thermal stabilities of the hydrides and their relative stabilities in terms of bond energies

All the elements in Group 7 are nonmetals except for astatine, which is a radioactive metalloid.

These elements are called halogens, which means “salt-former.” All of the halogens form salts with sodium and with the other alkali metals.

6.1 What is the outcome from syllabus?

6.1 The Halogens

greenish gas red-brown liquid black solid

6.1 The Halogens

Covalent radius/nm

Tm/K Tb/K

Cl 0.099 172 238

Br 0.114 266 332

I 0.133 387 457

The melting and boiling temperatures increase as going down the group, because larger atoms makes the van der Waals forces between the molecules stronger.

6.1 The Halogens

X + e- → Xˉ

halogen halide

F

Cl

Br

I

Increasing oxidising power

This is because the electronegativity increases as we go up the group, which means that the elements gain electrons more easily.

For example: Cl2 (aq) + 2Brˉ (aq) → Br2 (aq) + 2Clˉ(aq)

reduction

6.1 The Halogens

6.1 The Halogens

Formation of hydrogen hylides with hydrogen

H2(g) + Cl2 (g)hν or heating

HCl (g)

vigorously reaction

H2(g) + F2 (g)hν or heating

HF (g)

hard to control

rather slowly, almost no reactions

H2(g) + Br2 (g)hν or heating

H2(g) + I2 (g)hν or heating

6.1 The Halogens

Laboratory preparations of HX

CaF2 + H2SO4(conc.) → CaSO4 + 2HF(g)

NaCl + H2SO4(conc.) → NaHSO4 + HCl(g)

∵ 2HBr(g) + H2SO4(conc.) → SO2(g) + 2H2O(g) + Br2(l)

8HI(g) + H2SO4(conc.) → H2S(g) + 4H2O(g) + 4I2(l)

∴ NaBr(s) + H3PO4(l) → NaH2PO4(aq) + HBr(g)

NaI(s) + H3PO4(l) → NaH2PO4(aq) + HI(g)

6.1 The Halogens

HFHydrogen fluoride

Hydrogen chloride

Hydrogen bromide

Hydrogen iodide

HCl

HBr

HI

H2O

H2O

H2O

H2O

Hydrofluoric acid

Hydrochloric acid

Hydrobromic acid

Hydroiodic acid

Strong acid

Weak acid

Acidity of hydrogen hylides in aqueous solution

The strong H—F bond must be broken to release H+

6.1 The Halogens

BondTabulated bond energy/kJ mol-1

H-F 568

H-Cl 432

H-Br 366

H-I 298

Thermal Stability of HX

2HX(g) → H2(g) + X2(g,l,s)

HI: easily decomposed.HBr: may or may not decompose depending on the exact temperature of the wire.HCl and HF: not decomposed.

By plunging a red-hot wire into a test tube of the gas:

6.1 The Halogens

√√

6.2 What is the outcome from syllabus?

♣ describe and explain the reactions of halide ions with aqueous silver ions followed by: (i) aqueous ammonia (ii) concentrated sulphuric acid

♣ recognise the industrial importance and environmental significance of the halogens and their compounds, (e.g. for bleaches; PVC; halogenated hydrocarbons as solvents, refrigerants and in aerosols)

♣ explain the use of chlorine in water purification

♣ describe and interpret in terms of changes of oxidation number the reaction of chlorine with cold, and with hot, aqueous sodium hydroxide

Candidates should be able to:

♣ outline a method for the manufacture of chlorine from brine by a diaphragm cell

6.2 Reactions of the Halogens and Their Ions

6.2 Reactions of the Halogens and Their Ions

Test for halide ionsThe presence of Cl-(aq), Br-(aq) and I-(aq) can be confirmed by adding a few drops of silver nitrate solution (fluorides are soluble):

AgNO3(aq) + X-(aq) AgX(s) + NO3-(aq)

Silver halide Colour

Chloride White

Bromide Cream

Iodide Yellow

Silver chloride and bromide dissolve in concentrated ammonia, but the iodide does not.

AgBr(s) + 2NH3(aq) [Ag(NH3)2]+(aq) + Br-(aq)

6.1 Reactions of Halogens and their ions

Reactions of halide ions with conc. H2SO4

CaF2 + H2SO4(conc.) → CaSO4 + 2HF(g)

NaCl + H2SO4(conc.) → NaHSO4 + HCl(g)

NaBr(s) + H2SO4(conc.) → NaHSO4 + HBr(g)2HBr(g) + H2SO4(conc.) → SO2(g) + 2H2O(g) + Br2(l)

NaI(s) + H2SO4(conc.) → NaHSO4 + HI(g)8HI(g) + H2SO4(conc.) → H2S(g) + 4H2O(g) + 4I2(l)

With bromides and iodides, a redox reaction occurs:

6.2 Reactions of the Halogens and Their Ions

Colours of Silver halides

Ag+(aq) + Cl-(aq) AgCl(s)Ag+(aq) + Br-(aq) AgBr(s)Ag+(aq) + I-(aq) AgI(s)

6.2 Manufacture of Chlorine from brine

An Anode (+): 2Cl-(aq) Cl2(g) + 2e-

At Cathode (-): 2H2O(l) + 2e- 2OH-(aq) + H2(g)

Overall:

2NaCl(aq) + 2H2O(l) Cl2(g) + H2(g) + 2NaOH(aq)

6.2 Reactions of Halogens and their ions

Reactions of chlorine with Alkalis

Cold dilute alkali:Cl2(g) + 2NaOH(aq) Cl-(aq) + ClO-(aq) + H2O(l)

Hot concentrated alkali:3Cl2(g) + 6NaOH(aq) 5Cl-(aq) + ClO3

-(aq) + H2O(l)

Both examples of disproportionation

Cl-: Cl (-I)Cl2: Cl (0)ClO-: Cl (+I)ClO3

-: Cl (+V)

(Roman numerals: I = 1; V = 5)

6.2 Reactions of Halogens and their ions

Reactions of chlorine with Alkalis

Cold dilute alkali:Cl2(g) + 2NaOH(aq) Cl-(aq) + ClO-(aq) + H2O(l)

Hot concentrated alkali:3Cl2(g) + 6NaOH(aq) 5Cl-(aq) + ClO3

-(aq) + H2O(l)

Both examples of disproportionation

Cl-: Cl (-I)Cl2: Cl (0)ClO-: Cl (+I) chlorate(I)ClO3

-: Cl (+V) chlorate(V)

(Roman numerals: I = 1; V = 5)

6.2 Reactions of Halogens and their ions

Commercial uses of halogens and their compounds

Chlorine is used in water purification as it destroys harmful bacteria that could accumulate in old and unrecycled drinking water.

Sodium chlorate(I), NaClO(aq) is used in bleaches.

Halogenated hydrocarbons are used as solvents (dichloromethane CH2Cl2 dissolves many organic compounds).

CFCs were used as refrigerants and aerosols; section 12.3They do not contain hydrogen atoms!

Some are used as anaesthetics: Fluothane - CF3CHBr.