Group VII

The focus for Inorganic Chem (Gp II, VII, Transition Elements) is the trends for the physical and

chemical properties of the elements.

Group VII

Physical Properties

Colour/ State

It is important to memorise the colour/ state of halogens as it is often needed for identification of unknown halogens.

Physical Properties

X2 State at rtp Colour in water Colour in organic solvents (e.g hexane)

F2 Pale yellow gas - -

Cl2 Yellow green gas - Yellow green

Br2 Dark red liquid Brown Brown

I2 Black solid Brown Violet

Physical Properties

bp/ mp

- X2 are have simple molecular structures with dispersion forces between molecules

- Boiling/ melting involves overcoming these intermolecular dispersion forces

- Strength of dispersion forces depends on Mr (size of e– cloud)

Physical Properties

Solubility

- Being non-polar molecules, X2 are insoluble in water (polar solvent)

- However, they can ‘dissolve’ by undergoing reaction with water or ions which may be present in water:

Cl2 + H2O HOCl + HCl⇌

I2 + I– I⇌ 3–

Chemical Properties- Halogens are oxidising agents

Down the group- Oxidising ability ↓- Reactivity ↓

F2 is the strongest oxidising agent amongst the halogens; F− is the weakest reducing agent amongst the halides

I2 is the weakest oxidising agent amongst the halogens; I− is the strongest reducing agent amongst the halides

4 Cl2 + S2O32− + 5H2O → 8 Cl− + 10 H+ + 2 SO4

2−

4 Br2 + S2O32− + 5H2O → 8 Br− + 10 H+ + 2 SO4

2−

I2 + 2 S2O32− → 2I− + S4O6

2−

Chemical Properties

Cl2 and Br2 oxidises S2O32− (-2) to SO4

2− (+6) while I2 can only oxidise S2O3

2− (-2) to S4O62− (-1)

this is the rxn in iodometric titrations.

Chemical PropertiesDisplacement

- A more reactive halogen can displace the less reactive halogen from its aqueous halide ions.- Reaction is redox in nature.

Chemical Properties

Reaction with H2

- H2 + X2 2HX

- X2 that is more strongly oxidising will give rise to a more vigorous reaction

Chemical Properties

Stability of HX

- HX can decompose on heating: 2HX H2 + X2

- Note that decomposition breaks the H–X covalent bond- The stronger the bond, the greater the thermal stability of

the HX

Chemical Properties

Stability of HX

Recall: - Strength of covalent bonds depends on atomic radius of overlapping atoms (chemical bonding)- Boiling overcomes intermolecular bonds; decomposition overcomes intramolecular bonds (Gp II)

- Be clear what the qns is asking for. Student often give the wrong factors/ explanations

AgX (s) ⇌ Ag+ + X–

Ag+ + 2NH3 [Ag(NH⇌ 3)2]+

Chemical Properties

Halide + AgNO3 (aq) + NH3

Cl– (aq) white ppt soluble in aq NH3

Br– (aq) cream ppt insoluble in aq NH3; soluble in conc NH3

I– (aq) yellow ppt insoluble in aq NH3 and conc NH3

Ag+ + X– AgX (s)

Chemical Properties

Halide + conc H2SO4

Cl– NaCl (s) + H2SO4 (aq) HCl (g) + NaHSO4 (aq)

white fumes (HCl) observedBr– NaBr (s) + H2SO4 (aq) HBr (g) + NaHSO4 (aq)

2HBr (g) + H2SO4 (aq) Br2 (g) + SO2 (g) + 2H2O (l)

orange fumes (Br2) observed; pungent gas (SO2) liberated

I– NaI (s) + H2SO4 (aq) HI (g) + NaHSO4 (aq)

8HI (g) + H2SO4 (aq) 4I2 (g) + H2S (g) + 4H2O (l)

violet fumes (I2) observed; pungent gas (H2S) liberated

Chemical Properties

- I– being the most strongest reducing can reduce H2SO4 (+6) to H2S (-2)

- It is important to memorise the observations and equations for the reactions.