CCEA ADVANCED SUBSIDIARY

CHEMISTRY

MODULE 1

1.8 The Periodic Table

The organisation of elements in the Periodic Table according to their proton numbers and electronic structures. The terms group and period. The trends in the physical properties across the period sodium to argon limited to melting points, electrical conductivity, first ionisation energies and atomic radii.

Group VII (fluorine, chlorine, bromine and iodine)

Practical work restricted to chlorine, bromine and iodine and their compounds.

Trends within the group limited to colour, physical state, melting and boiling points, atomic and ionic radii, first ionisation energies, bond energies of halogen molecules, hydrogen halides and carbon-halogen bonds; electronegativies.

The halogens are a group of reactive non-metals, which are essentially similar to each other with only gradual changes as the atomic number increases.

Physical properties of the halogens

Property	Fluorine F	Chlorine Cl	Bromine Br	Iodine I
Colour	Pale yellow	Pale green	Red/brown	Black
Physical state at R^o	Gas	Gas	Liquid	Solid
Melting point / ^oC	-220	-101	-7	114
Boiling point / ^oC	-188	-34	58	183
Number of protons (atomic number)	9	17	35	53
Outer electron configuration	2s^22p^5	3s^23p^5	3d^104s^24p^5	4d^105s^25p^5
Atomic radius/ nm	0.064	0.099	0.111	0.128
Ionic radius/ nm	0.133	0.181	0.196	0.219
Ionisation energy/ kJ mol^-1	1682	1255	1142	1008
Bond energy X2/ kJ mol^-1	158	242	193	151
Bond energy HX/ kJ mol^-1	562	431	366	299
Bond energy C-X/ kJ mol^-1	484	338	276	238
Electronegativity	4.00	2.85	2.75	2.20

They are all p-block elements with a simple molecular structure consisting of covalently bonded diatomic molecules, X_2.

o o o o

o o o

o X o X o X X

o o o o

There are only weak Van der Waals forces between the molecules. The strength of the forces increases as the number of electrons (and M_r) in the molecule increases.

F₂< Cl₂< Br₂< I₂

In the case of iodine, the forces are sufficiently strong to bind the iodine molecules together in a 3-D crystal lattice. The X-X bond strength decreases down the group

Cl₂> Br₂ > I₂ as the atoms get larger and the attraction of the nucleus for the shared electrons decreases (electronegativity decreases).

There is a slight tendency to metallic character with increasing atomic number. The halogens complete their octet by gaining one electron forming a halide ion, X^- (see electron affinity values) or by sharing one electron.

Solubility in water and non-aqueous solvents eg hexane.

Solubility of the elements

All three elements are only slightly soluble in water because of the relatively strong hydrogen-bonding between the water molecules, which does not exist between the halogen molecules

i.e. solvent-solvent attractions > solute-solvent attractions > solute-solute

attractions.

Cl₂> Br₂>I₂

solubility decreasing

They are soluble in non-polar organic solvents such as toluene and TCE.

(Why?)

Chemical trends: reactivity with hydrogen, sodium and phosphorus.

All the halogens are oxidising agents and combine readily with metals and non-metals.

Reaction of halogens with elements

Hydrogen

The halogens combine enthusiastically with hydrogen, the vigour of the reaction decreasing from fluorine to iodine.

H₂ (g) + X₂ (g) 2HX (g)

Fluorine reacts explosively even in the dark at –200 ^oC.

Chlorine reacts explosively in sunlight, or slowly in the dark below 200 ^oC.

Bromine reacts above 200 ^oC and at lower temperatures with a platinum catalyst.

Iodine reacts to form an equilibrium mixture

H₂ (g) + I₂ (g) ⇌ 2HI (g)

Metals

The halogens combine readily with most metals forming the metal halides.

The vigour of the reaction decreases from fluorine to iodine.

Group I and II halides are ionic.

2Na (s) + Cl₂ (g) 2Na⁺Cl^- (s)

Mg (s) + Cl₂ (g) Mg²⁺2Cl^- (s)

The halides of Group III are predominantly covalent.

2Al (s) + 3Cl₂ (g) 2AlCl₃ (s)

Non-metals

The elements react directly with many non-metals the oxidising power decreasing from fluorine to iodine.

The elements combine directly with phosphorus, the oxidation state of the product depending on the oxidising power of the halogen:

2P (s) + 5Cl₂ (g) 2PCl₅ (s)

2P (s) + 3Br₂ (l) 2PBr₃ (l)

Reactions of the elements illustrated by use of chlorine gas (or chlorine water), bromine water and aqueous iodine (in potassium iodide) with water, aqueous alkalis, other halides in solution and iron (II) and iron (III) ions as appropriate. Disproportionation.

Reaction of halogens with water

Fluorine and chlorine can oxidise water. Fluorine oxidises water to oxygen.

2F₂ (g) + 2H₂O (l) 4HF (aq) + O₂ (g)

Chlorine reacts slowly with water forming hydrochloric acid and chloric(I) acid. This reaction involves disproportionation:- a change in which one particular molecule, atom or ion is simultaneously both oxidised and reduced.

reduction

Cl₂ (g) + H₂O (l) HCl (aq) + HClO (aq) (chlorine water)

o.n. 0 -1 +1

oxidation

Chlorine water contains chloric (I) acid HClO (aq), (hypochlorous acid). This is a weak acid which ionises to give the chlorate (I) ion ClO^-, (hypochlorite ion). The hypochlorite ion is a powerful disinfectant and bleach.

Bromine disproportionates in a similar way but to a lesser extent.

Iodine has a very low solubility in water.

Reaction of halogens with aqueous sodium hydroxide.

Chlorine reacts faster with dilute sodium hydroxide than with water.

When chlorine is added to cold dilute alkali it disproportionates to chloride and chlorate(I).

(i)

reduction

Cl₂ (g) + 2NaOH (aq) NaCl (aq) + NaOCl (aq) + H₂O

o.n. 0 -1 +1

oxidation

( 2OH^- + Cl₂ Cl^- + OCl^- + H₂O )

(ii) In hot concentrated alkali, if the solution is warmed to 70^oC, the chlorate(I) disproportionates further to chlorate(V).

reduction

3NaOCl (aq) 2NaCl (aq) + NaClO₃ (aq)

o.n. +1 -1 +5

oxidation

If chlorine is bubbled directly into hot conc. alkali then

(iii) reduction

3Cl₂ (g) + 6NaOH(aq) 5NaCl (aq) + NaClO₃ (aq)

o.n. 0 -1 +5

oxidation

( 6OH^- + 3Cl₂ 5Cl^- + ClO₃^- + 3H₂O )

For bromine, both reactions (i) and (ii) are fast at 15^oC.

For iodine, decomposition of IO^- occurs rapidly at 0^oC so it is difficult to prepare NaIO free from NaIO₃.

NaClO is a mild antiseptic (Milton).

NaClO₃ is a powerful weed killer.

Displacement reactions of the halogens

Since they are very electronegative, all the halogens are oxidising agents. As the group is descended their oxidising power decreases.

Therefore chlorine oxidises bromide ions to bromine and iodide ions to iodine.

These are displacement reactions.

Cl₂ (g) + 2Br^- (aq) Br₂ (l)+ 2Cl^- (aq)

(colourless) (yellow/orange)

Cl₂ (g) + 2I^- (aq) I₂ (s) + 2Cl^- (aq)

(colourless) (red/brown)

Bromine oxidises iodide to iodine

Br₂ (g) + 2I^- (aq) I₂ (s) + 2Br^- (aq)

Iodine does not oxidise any of the others.

Other oxidising reactions of the halogens

The trend in oxidising power is illustrated by the compounds formed by iron when it combines directly with the halogens.

Fluorine and chlorine form iron(III) fluoride and iron(III) chloride respectively.

2Fe (s) + 3F₂ (g) 2FeF₃ (s)

Bromine forms both iron(II) bromide and iron(III) bromide.

Iodine is too weak an oxidising agent and only forms iron (II) iodide.

Fe (s) + I₂ (g) 2FeI₂ (s)

Aqueous solutions of chlorine, bromine and iodine oxidise iron (II) to iron (III).

Cl₂ (aq) + 2Fe²⁺ (aq) 2Cl^- (aq) + 2Fe³⁺ (aq)

Iodine is so weak an oxidising agent that iron (III) ions oxidise iodide ions to iodine.

2Fe³⁺ (aq) + 2I^- (aq) 2Fe²⁺ (aq) + I₂ (s)

Thermal stability of hydrogen halides related to bond enthalpies. The relative strength of the acids, HF, HCI, HBr and HI.

Thermal stability of hydrogen halides

The thermal stability of the hydrogen halides decreases as the group is descended. This is in keeping with the trend in bond enthalpies

Bond energy HX/ kJ mol^-1

562

431

366

299

The size of the halogen atom increases from fluorine to iodine; therefore the bond length increases and the bond enthalpy decreases.

H