Group VII Chemistry: By Riona Mc Cann.

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)

                   PHYSICAL  PROPERTIES

                        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 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                    Cl₂                   Br₂                   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 I_2. (Refer to ILPAC Exp. 55)

Cl                   The oxidising ability of the halogens decreases

                       as you descend the group. If chlorine water is

                Br                   added to a salt of bromine, eg.NaB. (aq) or

                                      Iodine, eg.KI. (aq), then the chlorine will displace

                 I                     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.

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)

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