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 → 2NaF + Cl2
F2 + 2Cl - → 2F- + Cl2
Cl2 + 2NaBr → Br2 + 2NaCl
Cl2 + 2Br - → 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) → 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
White precipitate in water
Soluble in dilute NH3
Soluble in concentrated NH3
Cream precipitate in water
Slightly soluble in dilute NH3
Soluble in concentrated NH3
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) → 2NaCl (aq) + Br2 (aq)
Cl2 (aq) + 2KI (aq) → 2KCl (aq) + I2 (actually KI3 (aq) as I2 does not dissolve In water.)
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) → 2KBr (aq) + I2 (aq)
Brown Colourless Colourless Red
Halogens form diatomic molecules.
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:
Hexane (non – 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)
HF (g) is acolourless acidic gas with a pungent odour.
NaF(s) + H2SO4(l) = NaHSO4(aq) + HF (g)
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)
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 → Br2 + SO2 + 2H2O
Therefore,one will observe aReddish Vapourdue to some brominebeing present.
Metal iodides give a complex series of reactions with concentrated sulphuric acid. In addition to HI, one also obtains some :-
The reactions may be summarised thus:
∙ KI(s) + H2SO4(l) → KHSO4 + HI (g)
∙ H2SO4(l) + 2HI (g) → SO2 + I2 + 2H20
∙ H2SO4(l) + 6HI (g) → S + 3I2 + 4 H20
∙ H2SO4(l) + 8HI (g) → 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.
H2(g) + I2(g) H is positive
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
❑ CONCENTRATED SULPHURIC ACID AS THE OXIDISINGAGENT
∙ HF } + H2SO4(l) → NO REACTION
∙ HCl } NOT OXIDISED
Oxidation No. increases
Bromine is oxidised
-1 +6 0 +4
∙ 2HBr + H2SO4(l) → Br2 + SO2 + 2H2O
Oxidisation No. decreases Sulphur is reduced
8HI + H2SO4(l) → H2S + 4I2 + 4H20
∙ 2HI + H2SO4(l) → SO2 + I2 + 2H20
∙ 6HI + H2SO4(l) → 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 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
All the hydrogenhalides dissolve in water to form acid solutions:
HX (g) + H2O → 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.