Group VII Elements - Halogens

Group VII Elements - Halogens

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Group VII Elements - Halogens The Group VII elements are called HALOGENS. The name Halogen means
"salt - former" and is based on the fact that the elements combine
with most metals to form salts. (halides)



ELEMENT
=======



FLUORINE
========

CHLORINE



BROMINE
=======



IODINE
======

State at 20oC


GAS
---


GAS
---


LIQUID
------


SOLID
-----


Colour


PALE YELLOW


PALE GREEN

RED-BROWN


BLACK


Melting Point

(0C)

- 220

- 101

- 7

113


Boiling Point

(0C)

- 188

- 35

59

183

ELECTRON AFFINITY

Electron affinity is the enthalpy change when gaseous atoms of an
element gain electrons to become negative ions.

The FIRST ELECTRON AFFINITY of an element is the ENTHALPY CHANGE WHEN
ONE MOLE OF GASEOUS ATOMS GAINS ELECTRONS TO FORM ONE MOLE OF GASEOUS
IONS.

Electron affinity DECREASES as you DESCEND the group VII:

F à 333kJ

Cl à 346kJ of energy is released for every one mole of F/Cl/Br/I

Br à 324kJ atoms changing to a mole of F/Cl/Br/I ions.

I à 295kJ

This trend can be explained by the fact that as you descend the group,
the X- ion becomes less stable and is therefore less likely to form. H
is less stable because electrons are less strongly attracted due to
the fact that the valence electrons are further from the attractive

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positive nucleus, and because there is an increase in the number of
inner shells which, in turn, increases the shielding effect.

HALOGENS AS OXIDISING AGENTS

Since their atoms accept electrons, the halogens are oxidising agents
(OIL), and in a reaction they are reduced (RIG). Of the halogens,
fluorine is the most powerful oxidising agent, and astatine, the
least. This can be explained by the relative size of their atoms. The
fluorine atom is the smallest (i.e., the outer electrons are closer to
the nucleus), with fewer inner - shell shielding electrons, so it's
nucleus can have a greater attraction for an extra electron.

EXPLAINING THE CHANGE IN STATE FROM F2 TO I2

F2 [IMAGE] Cl2 [IMAGE] Br2 [IMAGE] I2

Pale Yellow Pale Green Red-Brown Black

Gas Gas Liquid Solid

This can be explained by instantaneous dipole-induced dipole forces.

The likelihood is that, at any instant, the charge distribution of an
atom will not be symmetrical. If electrons are thought of as " charge
clouds", wherein the electrons are in constant motion, then it is
entirely plausible that, at any instant, there will be more electrons
in one area than in another. Therefore, at that instant, the atom is
polarized.

This INSTANTANEOUS DIPOLE can affect the electron distribution in
nearby atoms, so that they too are distorted. The result of this is to
INDUCE DIPOLES in nearby atoms ( the area of instantaneous high
electron density in the electron cloud will repel the electrons in
nearby atoms and so polarize these atoms too ). These atoms will then
be attracted to the original dipoles.

The instantaneous dipole - induced dipole forces are the forces that
hold halogen atoms together whether as gases, liquids or solids. The
forces increase as you go down the group.

As the atomic number of an element increases, the element becomes more
POLARIZABLE and the instantaneous dipole - induced dipole forces
become stronger. This is because, as the number of electrons
increases, so too does the likelihood that the charge distribution of
an atom will not be symmetrical. It also follows that, with more
electrons, the forces will be stronger. Therefore, as you descend
Group VII the elements change from being gases to being solid at room
temperature.

THE HALOGEN DISPLACEMENT REACTION

A halide ion can be displaced from it's compound by any halogen higher
in Group VII. Since fluorine is the most reactive halogen, it can
react with the halide ion of any of the other halogens. Fluorine
becomes the fluoride ion, and the free halogen (chlorine, bromine or
iodine ) is formed from the halide ion.

F2 + 2NaCl à 2NaF + Cl2

F2 + 2Cl- à 2F- + Cl2

Cl2 + 2NaBr à Br2 + 2NaCl

Cl2 + 2Br - à Br2 + 2Cl -

IDENTIFYING HALIDE IONS

The presence of Cl -, Br -, and I- in aqueous solution can be
confirmed by the formation of a precipitate with silver nitrate
solution :

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

Silver nitrate Halide ion Silver halide Nitrate ion

SILVER CHLORINE --- WHITE WHY

SILVER BROMIDE --- CREAM CAN'T

SILVER IODIDE --- YELLOW YOU

Silver Bromide (cream) and Silver Iodide (yellow) can be difficult to
distinguish by eye. This situation is remedied by the fact that these
silver halides have different solubilities in ammonia solution.

THE VARIATION IN SOLUBILITY OF THE SILVER HALIDES IN WATER AND IN
DILUTE AND CONCENTRATED AMMONIA

AgCl

White precipitate in water

Soluble in dilute NH3

Soluble in concentrated NH3

AgBr

Cream precipitate in water

Slightly soluble in dilute NH3

Soluble in concentrated NH3

AgI

Yellow precipitate in water

Insoluble in dilute NH3

Insoluble in concentrated NH3

OXIDISING NATURE OF THE HALOGENS

The following is a brief description of an experiment that can be
carried out to demonstrate the different oxidizing abilities of the
halogens Cl2 to I2.

The oxidising ability of the halogens decreases s you descend the
group. If chlorine water is added to a salt of bromine, eg.NaB. (aq)
or Iodine, eg.KI. (aq), then the chlorine will displace the halogen
from it's compound. The colourless Solution will turn brown as bromine
is formed, or red as iodine is formed.

Cl2 (aq) + 2NaBr (aq) à 2NaCl (aq) + Br2 (aq)

Bromine water

(BROWN)

Cl2 (aq) + 2KI (aq) à 2KCl (aq) + I2 (actually KI3 (aq) as I2 does not
dissolve In water.)

(RED)

Bromine water could be used to displace iodine from an iodide salt,
such as KI, but it could not displace chlorine from a chloride salt
such as KCl.

Br2 (aq) + 2KI (aq) à 2KBr (aq) + I2 (aq)

Brown Colourless Colourless Red

Halogens form diatomic molecules.

[IMAGE]They form HOMO - NUCLEAR DIATOMIC MOLECULES :

Br - Br

I -- I

i.e., they have no permanent dipole and are therefore non - polar.
They therefore dissolve better in non - polar solvents. Having said
this, the halogens DO dissolve in water, a polar solvent, because of
instantaneous dipoles. They dissolve much more readily and easily,
however, in non - polar solvents.

Hexane is a non - polar solvent. In this experiment, hexane is put in
a test tube with the aqueous solution of a halide ion:

[IMAGE]

Hexane (non - polar)

Aqueous (polar)

The test tube is shaken and the hexane is physically mixed through the
aqueous solution. As it is mixed through the solution, the halide ions
move from the aqueous solution to the hexane, because they dissolve
much more readily in the non - polar solvent.Hexane is less dense than
water and, when the shaking stops, the hexane floats to the surface
with the dissolved halide ions in it. The initially colourless hexane
now takes on the colour of the ions dissolved in it, and so
facilitates observations to be made.

PREPARATION OF THE HYDROGEN HALIDES

Preparations of ALLthe hydrogen halides needto be carried Out in a
FUME CUPBOARD. (Mosthydrogen halides can be produced by reacting the
metal halide with concentratedH2SO4)

HYDROGEN FLUORIDE

HF (g) is acolourless acidic gas with a pungent odour.

NaF(s) + H2SO4(l) = NaHSO4(aq) + HF (g)

HYDROGENCHLORIDE

Thisis prepared by a similar process - reacting NaCl with H2SO4(l)

NaCl(s) + H2SO4(l) = NaHSO4(aq) + HCl (g)

HCl(g) is a colourless acidic gas that has an acid odour. It fumes in
moist airgiving droplets of hydrochloric acid. (Same apparatus as for
HF)

HYDROGENBROMIDE

Ametal bromide is reacted with concentrated H2SO4using the apparatus
shownfor the preparation of HF.

KBr(s) + H2SO4(l) = KHSO4(aq) + HBr (g)

HydrogenBromide has a similar appearance to Hydrogen Chlorine.

N.B. The concentrated sulphuric acid will oxidise some of the HBr as
follows:

2HBr + H2SO4 à Br2 + SO2 + 2H2O

Therefore,one will observe aReddish Vapourdue to some brominebeing
present.

HYDROGENIODIDE

Metal iodides give a complex series of reactions with concentrated
sulphuric acid. In addition to HI, one also obtains some :-

IODINE(I2)

HYDROGENSULPHIDE (H2S)

SULPHURDIOXIDE (SO2)

SULPHUR(S)

The reactions may be summarised thus:

· KI(s) + H2SO4(l) à KHSO4 + HI (g)

· H2SO4(l) + 2HI (g) à SO2 + I2 + 2H20

· H2SO4(l) + 6HI (g) à S + 3I2 + 4 H20

· H2SO4(l) + 8HI (g) à H2S + 4I2 + 4 H20

Duringthe reaction one will observe :

¨ VioletIodine Vapourbeing evolved,

¨ Theviolet vapour cooling and subliming to formdarksolidiodine,

¨ Asmell ofrotten eggs(H2S)

¨ Somefree sulphur

¨ SomeHI (g). (it is similar in appearance toHCl (g))


THERMALSTABILITY OF THE HYDROGEN HALIDES

Thethermal stability of the hydrogen halides DECREASES as you DESCEND
THE GROUP.

The decomposition of hydrogen halides is an endothermic process.

LeChatelier's Principle states: - If a system inequilibrium is
subjected to a change,processes will occur which tend to counteract
the change imposed.

2HI(g)

H2(g) + I2(g) H is positive

[IMAGE][IMAGE]Text Box: 2HI (g)Take, for example, the reaction

When the temperatureis increased the equilibrium shifts to the right
because the forward reaction isendothermic. An increase in
temperature, by Le Chatelier's Principle, promotesthe reaction which
will use up this extra energy, i.e., the forward endothermicprocess.
Now we haveestablished how an increase in temperature causes the
decomposition of HI, wemust explain the trend of decreasing thermal
stability as relative molecularmass increases, (i.e., as you go down
the group). This is accountedfor by the decrease in the bond energy
from H - F toH - I.

EXAMININGTHE EASE OF OXIDISATION OF THE HYDROGEN HALIDES

q CONCENTRATED SULPHURIC ACID AS THE OXIDISINGAGENT

· HF } + H2SO4(l) à NO REACTION

· HCl } NOT OXIDISED

Oxidation No. increases

Bromine is oxidised

-1 +6 0 +4

· 2HBr + H2SO4(l) à Br2 + SO2 + 2H2O

Oxidisation No. decreases Sulphur is reduced

Text Box: Oxidation No. increases Bromine is oxidised -1 +6 0 +4 · 2HBr + H2SO4 (l) à Br2 + SO2 + 2H2O Oxidisation No. decreases Sulphur is reduced

8HI + H2SO4(l) à H2S + 4I2 + 4H20

· 2HI + H2SO4(l) à SO2 + I2 + 2H20

· 6HI + H2SO4(l) à S + 3I2 + 4H20

CONCENTRATED SULPHURIC ACID ALONG WITHMANGANESE (IV) OXIDE AS THE
OXIDISING AGENT.

This is an even stronger oxidising solution and it is capable of
oxidising HCl as well as HBr and HI. Concentrated sulphuric acid alone
is not a sufficiently strong oxidising agent to oxidise HCl(g) to Cl2(g).
This isdue to the relatively strong H - Cl bond. In combination with
MnO2it can oxidise HClto Cl2.

2HCl + H2SO4(l) MnO2 CL2 + SO2 + 2H2O
Text Box: 2HCl + H2SO4 (l) MnO2 CL2 + SO2 + 2H2O A COMBINATION OF
HYDROGEN PEROXIDE ANDDILUTE SULPHURIC ACID AS AN OXIDISING SOLUTION.

This oxidisingsolution is only sufficiently strong to oxidise iodide
to iodine. Itcannotoxidise solutions of theother halide ions.

REACTIONWITH WATER OF THE HYDROGEN

HALIDES

All the hydrogenhalides dissolve in water to form acid solutions:

Hydrochloric Acid

Hydrobromic Acid

Hydroiodic Acid

HX (g) + H2O à H3O+(aq) + X-(aq)

(Where X = Cl, Br or I)

The hydrogen halides are covalently bonded, whereas, their
corresponding acids are ionically bonded. Hydroiodic acid is a
stronger acid than Hydrobromic acid, which, in turn, is stronger than
hydrochloric acid. In other words, acid strength increases from HCl
(aq) to HI(aq). This can be explained by virtue of the fact that the H
- I bond is weaker than the H - Cl bond. Therefore, HI dissociates
more fully in water. All three acids are STRONG ACIDS and display the
typical reactions of strong acids. Dilute Hydrofluoric Acid is a WEAK
ACID. Only about 10% of the HF molecules are dissociated in a 0.1 mol
dm-3solution. This is because the H - F bond is very strong and the
presence of strong intermolecular hydrogen bonds hinders dissociation.
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