They have low meting and boiling points.

The atoms within each molecule are held together by strong covalent bonds, but the forces between the molecules ( Van der Waals forces) are very weak.

THE HALOGENSWHY?

Physical Properties of HALOGENS

How does the colour change down the group?

How does the state change down the group?What does that imply about the melting and boiling point down the group?

Make a guess how At would look like (the colour, state and melting point)?

Physical Properties of HALOGENS

Element Melting Point (oC)

Boiling Point (oC)

Colour State at room temp

Flourine -220 -189 Colourless Gas

Chlorine -101 -35 Yellow-greenish

Gas

Bromine -7 59 Reddish-brown

Liquid

Iodine 114 184 Purpleish-black

Solid

Astatine 300 337 Black solid

Halogens in Hexane & Water • When halogens undergo a physical change of moving from a

water solvent to a hexane solvent, they change color.• Halogens are nonpolar, so they are not very soluble in water

(which is polar)• Halogens are very soluble in hexane (which is nonpolar) • When X2 (aq) is mixed (shook!!) with hexane, the X2 moves

out of the water solvent and into the hexane solvent if possible (and also changes color)

Ns2np5Ionic or covalent bond with oxidation state –1 or +1 (F shows –1 only)

Except F, all other halogens can expand their octet byusing the low-lying, vacant d-orbitals to form bonding. Their oxidation states range from –1 to +7.

Bonding and Oxidation State

ns

np

nd

(+3 states)

HClO2, ClO2-

ns

np

nd

(0, +1, -1 states)Cl2, Cl-, OCl2

Cl2(g) + NaOH(aq) NaOCl(aq) + NaCl(aq)Oxidation Cl in NaClOxidation Cl in NaOCl Chlorines Disproportionation in Water The disproportion reaction produces the strong acid HCl(aq) and weak hypochlorous acid HOCl(aq): Cl2 (aq) + H2O(l) HCl(aq) + HOCl(aq)Oxidation Cl in HClOxidation Cl in HOCl

What would you expect to happen if this reaction were monitored using a pH meter and explain why this is?

Disproportionation

+1 -1

-1

+1

• Be able to apply knowledge of hydrogen bonding to HF properties

• Be able to explain the precipitation reactions of silver halides

• Be familiar with the use of silver halides in photography

Objectives: Group 7 Halogens Part II

Bonding in HF

Why is the HF Boiling point so high?

Flourine is the smallest of all group 7 elements in the periodic table.

It is the most charge dense of the halogens because it is the smallest in size with the least electron shells.

Flourine is highly electronegative

Flourine can only take the -1 oxidation state because it has no near-by d-orbitals to promote extra electrons to.

Hydrogen bonding is a stronger intermolecular force than permanent dipole attractions or Van der Waals forces.

Hydrogen bonding only occurs between a hydrogen atom and an electronegative atom e.g. F, N or O

Strong hydrogen bonding causes strong forces between molecules raising their boiling point.

AA BB

CC

DD

EE

FF

GG

Acidity of HF

Explanation of weak acid strength of HF:HF has the greatest bond dissociation energy and exceptionally smallelectron affinity. It has the least exothermic H (dissociation enthalpy)

Due to formation of strong hydrogen bond HF has the smallest decrease of dissociation.

Halogen ReactivityElectronegativity: Electron Affinity:

When halogens react with another substance, they become negative ions, as they are gaining an extra electron. These compounds are HALIDES. Once it has reacted – instead of ending with ‘–ine’, they end with ‘-ide’.

Halogen Halidereaction

fluoride (F-)

chloride (Cl-)

bromide (Br-)

iodide (I-)

(F)

(Cl)

(Br)

(I)

Element EN value

F 4.0

Cl 3.0

Br 2.8

I 2.5

Generally high electron affinity.All are exothermic.

X(g) + e- X-(aq)

Element EA kJ/mol

F -348

Cl -364

Br -342

I -285

A test for Halide Ions….AddAg+(aq)Silver Nitrate

Ag+(aq) + Cl-(aq) AgCl(s) , white precipitate

Ag+(aq) + Br-(aq) AgBr(s) , pale yellow precipitate

Ag+(aq) + I-(aq) AgI(s) , yellow precipitateAs a further test try the precipitates solubility in

ammonia.

• When working with concentrated ammonia take your test tube to the fume hood. • Use the concentrated NH3 in the fume hood and dispose of the test tube contents in the fume hood sink!!!

Expected Results: Halides + Ag+

(aq)

• Electron ejected from chlorine (Oxidation): Ag+ + Cl- + light energy → Ag+ + Cl + 1 electron • Electron captured by silver (Reduction): Ag+ + 1 electron → Ag (metal)• When silver metal is produced as a result of the

electron capture, it forms a dark image on film. Ag+ has been reduced to Ag (metal), and Cl- has been oxidised.

Silver Chloride and Photos

1. -1, +1, +3, +5, +72. Electron affinity decreases as you go down the group3. Br2(l) + H2O(l)HBr(aq) + H0Br(aq)4. Cl2(g)+ 2Br-(aq)2Cl-

(aq) + Br2(aq)

5. Disproportionation6. There is much more hydrogen bonding in HF than in the

other hydrogen halides7. HCl8. The H-X bond energy9. HI – Most readily donates electrons most displaced 10. F2 - oxidisingagent accepts - electrons

Test Answers