Group 7A (17)

HALOGENS (nS2nP5)

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In nature, always found combined with other elements:

oChlorine, bromine, and iodine occur as halides in seawater

oFluorine occurs in the minerals fluorite (CaF2) and cryolite (Na3AlF6).

oAll isotopes of astatine (As) are radioactive

atomic properties

physical properties

Oxidation numbers range from −1 to +7 except F which can only be 0 (in F2) and −1, in all compounds.

BOND ENERGIES AND BOND LENGTHS OF THE HALOGENS

F2 shows an anomalous bond energy. The F-F bond is weaker than expected since the lone pairs on the small F atom repel each other more than the lone pairs of other halogens.

F2 is the most and I2 the least reactive halogen!

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Preparation – determined by their strong oxidizing capability

Fluorine

• From liquid HF

• At 70oC

17

Chlorine

• Electrolysis of molten NaCl

• Overall reaction

HALIDS

Either ionic or covalent.

The fluorides and chlorides especially those belonging to the alkali metal and alkaline earth metal are ionic compounds (except halides of Be).

Most of the halides of nonmetals are covalent compounds.

Preparation from elements – can occur violently

The Hydrogen Halides (HX) are formed from the reaction of metal halides and a concentrated acid

CaF2(s) + H2SO4(l) CaSO4(s) + 2HF(g)

2NaBr(s) + H3PO4(l) Na3PO4(s) + 3HBr(g)

• When dissolved in water, the hydrogen halides behave as acids, and all except hydrogen fluoride are completely dissociated.

THE RELATIVE OXIDIZING ABILITY OF THE HALOGENS

Halogens are strong oxidizing agents. The oxidizing ability of X2 decreases down the group while the reducing ability of X- increases.

Halogen Oxides Group 7A Halogens form many Oxides that are powerful

oxidizing agents (they are reduced by gaining the electrons lost by the oxidized species)

The Oxides form Acids with water

Dichlorine Monoxide (Cl2O) & Chlorine Dioxide (ClO2) are used to bleach paper

2NaClO3 + SO2 + H2SO4 2ClO2 + 2NaHSO4

Halogen Oxyacids and Oxyanions

All halogens except fluorine combine with

various numbers of oxygen atoms to form

oxyacids.

Strengths of oxyacids vary directly to the

number of oxygen atoms bonded to the

halogen - acid strength increases as more

oxygens are added.

The relative strength of halogen oxoacids depends on both the electronegativity and the oxidation state of the halogen.

For oxoacids with the halogen in the same oxidation state, acid strength decreases as the halogen electronegativity decreases. HClO3 > HBrO3 > HIO3

For oxoacids of a given halogen, acid strength decreases as the oxidation state of the halogen decreases. HClO4 > HClO3 > HClO2

The Known Oxyacids of the Halogens

Oxoacids and Oxoanions are formed by reacting the Halogens and their Oxides with Water

Most Oxoacids are stable only in solution

There are four Oxoacids & Oxoanions

Acid Hypochlorous, Chlorous Chloric Perchloric

Salt Sodium Hypochlorite, Sodium Chlorite, Sodium Chlorate, Sodium Perchlorate

The known Halogen Oxoacids

• Hypochlorous acid (HOCl) + 1

• Chlorous Acid (HClO2) + 3

• Chloric acid (HClO3 ) + 5

• Perchloric acid (HClO4 ) + 7

The chemistry of fluorine differs from that of the rest of the halogens in the following ways:

Fluorine is the most reactive due to the relative weakness of the F−F bond.

The difference in reactivity between fluorine and chlorine is greater than that between chlorine and bromine.

Highlights of Fluorine Chemistry

Fluorine is the most electronegative element

Because of the weak bond, F2 reacts with every element, except the noble gases, in many cases explosively

Fluorine uniquely reacts with cold sodium hydroxide solution to produce oxygen difluoride as follows:

Hydrogen fluoride (HF) has a relatively high boiling point (19.5°C)

Hydrofluoric acid is a weak acid, all other hydrohalic acids are strong acids.

Silver fluoride (AgF) is soluble. All other silver halides (AgCl, AgBr, and AgI) are insoluble.

Most important halogen

Laboratory preparation from MnO2, NaCl and H2SO4:

2NaCl + MnO2 + 2H2SO4 Cl2 + MnSO4 + Na2SO4 + 2H2O

Industrial production:

Chlorine is a by-product in the electrolysis of NaCl, MgCl2, CaCl2, ScCl3, etc.

Highlights of Chlorine Chemistry

Production of Bleach Solution

Cl2(g) + 2NaOH(aq)

NaOCl(aq) + NaCl(aq) + H2O(l)

2Cl2(g) + 2Ca(OH)(aq)

Ca(OCl)2(s) + CaCl2(aq) + 2H2O(l)

The halogens undergo disproportionation in water:

Cl2 + H2O →HCl + HClO

Oxides and Oxyacids of Chlorine

• Oxides of chlorine and its oxidation number (in parenthesis):

• Cl2O (+1), Cl2O3 (+3), ClO2 (+4; unstable), Cl2O5 (+5), Cl2O7 (+7; highest possible)

• Chlorine oxyacids in increasing acid strength

• HOCl < HClO2 < HClO3 < HClO4;

• HClO4 is a strong oxidization agent

Important Compounds of Chlorine

• NaCl – for electrolyte balance

• NaOCl – household bleach solution

• Ca(OCl)2 – bleach for water & sewage treatment

• ClO2 – bleach for paper production

• NaClO3 – production of industrial bleach (ClO2)

• KClO3 – oxidizer in fireworks and matches

• NaClO4 – production of HClO4 and NH4ClO4

• NH4ClO4 – oxidizer in booster rocket fuel

Uses of the halogens Fluorine

• UF6 separating isotopes of U • Production of polytetrafluorethyline (Teflon ©)

Chlorine

• Biological role as Cl−(aq) • Industrial bleaching – Cl2 • Household bleach is a dilute 5.25% solution of

Sodium Hypochlorite (NaClO) • Water purification – Cl2, ClO

− (chlorination) • Organic solvents – CHCl3 • Polymer production – PVC • Hydrochloric Acid occurs in animal stomach fluids and

has many industrial uses

Bromine

• Insecticides (BrCH2CH2Br)

• Scavenger for Pb in gasoline

• Photographic films (AgBr)

Iodine

• Antiseptic (tincture of iodine)

• Thyroxine (thyroid hormone derivative)

• Cloud seeding (AgI)

Industrial uses of hydrogen fluoride (HF)

• Reactive enough to etch glass

• Used in the manufacture of Freons