GROUP VII: THE HALOGENS

element	atomic #	elec config
fluorine	9	1s22s22p5
chlorine	17	[Ne]3s23p5
bromine	35	[Ar]3d104s24p5
iodine	53	[Kr]4d105s25p5

Volatility and colour

element	colour	state at rtp	mp/°C	bp/°C
Cl2	yellow-green	gas	-101	-34.5
Br2	reddish brown	liquid	-72	58.8
I2	black	solid	114	184

volatility decreases down group due to increase mass (actually e^-s) > increase in VDW's forces > less volatile
bromine: corrosive, toxic, volatile liquid, iodine: heat > sublimes > violet vapour

Reactivity as oxidising agents
halogens: gain e^-s >(reduced) > anions: X₂(g) + 2e^- > 2X^-(aq)
oxidising power decreases down group (chlorine & bromine: strong oxidising agents)
oxidising strength given by:
-enthalpy change of reaction,ΔH: larger ΔH >more exothermic > more easily reduced > stronger oxidising agent
-standard electrode potential, : more +ve > stronger oxidising agent

relative oxidising strength of halogens: illustrated by displacement reactions where more reactive (oxidising) halogen displaces a less reactive halide:
Cl₂(g) + 2Br^-(aq) > 2Cl^-(aq) + Br₂(l), Cl₂(g) + 2I^-(aq) > 2Cl^-(aq) + I₂(aq), Br₂(g) + 2I^-(aq) > 2Br^-(aq) + I₂(aq)
(chlorine displaces bromide and iodide ions, bromine displaces iodide ions)
liberation of iodine and bromine- indicated by colour of org solvent (1,1,1-TCE- trichloroethane) added to mixture
bromine > orange org layer, iodine > violet org layer

Reaction w/ hydrogen
halogen + hydrogen > hydrogen halides, HX (hydrides of halogens)
Chlorine: explosive reaction w/ H₂ in strong sunlight (UV light) > HCl, in dark at temp < 200°C- slow reaction: Cl₂(g) + H₂(g) > 2HCl(g)
Bromine & iodine: react readily w/ H₂ at 300°C, w/ Pt catalyst
Br₂(g) + H₂(g) >(300°C, Pt)> 2HBr(g) I₂(g) + H₂(g) <-(300°C, Pt)-> 2HI(g) [reversible reaction] Thermal stabilities of hydrogen halides
-decreases down group (HCl > HBr > HI)
HCl: doesn't decompose readily below 1500°C
HBr: decomposes to reddish-brown Br₂(g) and H₂(g) at 800°C
HI: decomposes to violet I₂(g) and H₂(g) at 180°C
-due to increasing cov bond length down group > decrease in bond strength > easier to break H-X bond

Reactions of halides
w/ Ag⁺(aq)        X^-(aq) + Ag⁺(aq) > AgX(s)

halogen	ppt colour	photosensitive	reaction w/ aq NH3
Cl	white	yes	aq NH3 > soluble: AgCl + 2NH3(aq) > Ag(NH3)2+(aq) + Cl-
Br	pale yellow/cream	yes	conc aq NH3 > soluble: AgBr + 2NH3(aq) > Ag(NH3)2+(aq) + Br-
I	yellow	no	insoluble

AgCl, AgBr, AgI: insoluble in dil HNO₃
AgCl, AgBr:- photosensitive- decomposed when exposed to sunlight (AgCl: white > purple-grey, AgBr: cream > green-yellow)
[Ag(NH₃)₂⁺(aq): diammine silver(I) ions]
w/ conc H₂SO₄(aq)
step 1: acid + solid halide > formation of gaseous hydrogen halide
X^-(s) + conc H₂SO₄(aq) > HSO₄^-(s) + HX(g) [HSO₄^-(s): hydrogen sulphate ion in solid form]
step 2: H₂SO₄ = oxidising agent > reduces HBr & HI
Cl:
Cl^-(s) + H₂SO₄(aq) > HSO₄^-(s) + HCl(g)
Br:
Br^-(s) + H₂SO₄(aq) > HSO₄^-(s) + HBr(g)
HBr(g) + H₂SO₄(aq) > H₂O(l) + Br₂(g) + SO₂(g) [Br₂(g)- red-brown vapour]
I:
I^-(s) + H₂SO₄(aq) > HSO₄^-(s) + HI(g)
2HI(g) + H₂SO₄(aq) > 2H₂O(l) + I₂(s) + SO₂(g) [I₂(s)- brown solid w/ violet vapour]
8HI(g) + H₂SO₄(aq) > 4H₂O(l) + 4I₂(s) + H₂S(g) [HI: strong reducing agent- reduces H₂SO₄ > H₂S, SO_2, S]

Industrial manufacturing of chlorine from brine (conc NaCl sol) by diaphragm cell

-porous asbestos diaphragm separates cathode & anode
-purified saturated brine added to anode compartment (purified to remove Ca²⁺, Mg²⁺ which would formed insoluble hydroxide ppt > can block pores of diaphragm > useless)
-level of sol in anode part kept higher: facilitates ion movement
formation of ions: NaCl(aq) > Na⁺(aq) + Cl^-(aq), H₂O « H⁺(aq) + OH^-(aq)
at anode: Cl^- & OH^- move here, Cl^- more conc > oxidised to chlorine gas : 2Cl^-(aq) - 2e^- > Cl₂(g)
at cathode: H⁺ & Na⁺ collect here, H⁺ reduced to H₂: 2H⁺(aq) + 2e^- > H₂(g)
products of electrolysis: H₂, Cl₂, NaOH (from resulting mixture left)

Reaction of chlorine w/ NaOH
disproportionation reaction: same element oxidised and reduced- Cl₂: reduced to Cl^-, oxidised to ClO^- / ClO₃^-
15°C: cold, dil NaOH (aq) > sodium chlorate(I), NaClO
Cl₂(g) + OH^-(aq)(dil) > Cl^-(aq) + ClO^-(aq) + H₂O(l) [ClO^-(aq): chlorate(I) ion]
70°C: hot, conc NaOH (aq) > sodium chlorate(V), NaClO₄
3Cl₂(g) + 6OH^-(aq)(conc) > 5Cl^-(aq) + ClO₃^-(aq) + 3H₂O(l) [ClO₃^-(aq): chlorate(V) ion]

Uses of halogens & halogeno cpds
-chlorine = a disinfectant- kills bact; to sterilise water & swimming pools
-Cl cpds: organic degreasing solvents (CCl₄), PVC, CFC's, disinfectants, antiseptics, domestic bleach, weed-killers (DDT)
-1,2-dibromoethane in leaded petrol to remove lead as volatile PbBr₂
-AgBr, AgI in photographic films
-iodine: tincture of iodine [sol of iodine in ethanol and KI(aq)]- disinfectant for wounds
I₂ needed in diet for synthesis of hormone thyroxine (produced by thyroid gland) which regulates body activities
lack of iodine > thyroxine deficiency > mental retardation & goitre


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