Chemical properties of Aluminium

Aluminium forms one series of salts, in which it is tervalent. The salts are derived from the basic oxide Al₂O₃. Those derived from colourless acids are themselves colourless. The soluble salts from hydrates containing considerable water of crystallisation, and the anhydrous salts, dissolve in water with the evolution of a large amount of heat. The salts are not poisonous.

On the ionic hypothesis the salts of aluminium are dissociated in aqueous solution, giving rise to a colourless cation Al^•••: -

AlX₃ ⇔ Al^••• + 3X'.

The ionic mobility of the cation, ⅓ Al^•••, is 40.4 at 18° C. Despite the high position occupied by aluminium in the electromotive series, aluminium oxide is only a weak basic oxide and aluminium hydroxide but a feeble base. Accordingly, in aqueous solution, aluminium salts are appreciably hydrolysed, and those derived from moderately strong acids give a decidedly acid reaction. A solution of 1 gram-molecule of aluminium chloride or bromide in 1000 litres of water is hydrolysed to the extent of 4 per cent, at 25°; while the percentage hydrolysis (x) of the chloride in aqueous solution at 99.7 varies with the dilution (v litres per gram-molecule), as follows: -

v	32	64	128	256	512
x	8.0	13.2	19.7	28.2	41.4

Aluminium hydroxide is a stronger base than ferric hydroxide, but is weaker than beryllium hydroxide. It is possible that, owing to the transformation of aluminium hydroxide into the form of a colloidal hydrosol, the degree of hydrolysis of aluminium salts gives a somewhat exaggerated idea of the weakness of the hydroxide as a base.

Owing to the appreciable hydrolysis of aluminium salts, the soluble salts derived from volatile acids cannot be prepared in the anhydrous state by the evaporation of their aqueous solutions on the steam-bath.

Aqueous solutions of aluminium salts dissolve an appreciable amount of aluminium hydroxide.

The heats of formation, etc., of the more important compounds of aluminium are given in the following table, in kilogram-calories: -

Compound.	Heat of Formation. Cals.	Heat of Solution. Cals.
AlF3	249.0	+ 31.3
AlF3.3.5H2O	...	- 1.7
AlCl3	161.4	+ 77.6
AlBr3	122.0	+ 85.3
AlI3	70.3	+ 89.0
Al2O3	380.2	...
Al(OH)3	297.0	...
Al2S3	126.4	...
Al2(SO4)3	879.7	...
Al2(SO4)3.6H2O	...	+ 56.0
Al2(SO4)3.18H2O	...	+ 9.2
Al4C3	244.8	...
KAl(SO4)2.12H2O	...	-10.0
(NH4)Al(SO4)2.12H2O	...	- 9.6

The following data refer to heats of reaction: -

[Al2O3] cryst. + [Na2O]	+48.0 Cals.
[Al2O3] amorp. + [Na2O]	+55.1 Cals.
[Al] + 3HCl.2OH2O	+126.0 Cals.
[Al] + 3HCl.200H2O	+127.0 Cals.
⅓[Al(OH)3] + HCl aq.	+9.2 Cals.
⅓[Al(OH)3] + HF aq.	+11.7 Cals.
⅓[Al(OH)3] + ½H2SO4 aq.	+10.5 Cals.
2[AlCl3] + 2⅓(NH3)	+167.5 Cals.
2[AlCl3] + 6(NH3)	+245.2 Cals.
2[AlCl3] + 10(NH3)	+268.2 Cals.
2[AlCl3] + 12(NH3)	+317.9 Cals.
2[AlCl3] + 18(NH3)	+9.5 Cals.
2[AlCl3] + (H2S)	+9.5 Cals.
2[AlCl3] + (SO2)	+18.3 Cals.
2[AlCl3] + 2(SO2)	+28.9 Cals.
2[AlCl3] + 2[KCl]	+26.4 Cals.
2[AlCl3] + 2[NaCl]	+11.9 Cals.
2[AlCl3] + 2[NH4Cl]	+26.5 Cals.
2[AlCl3] + 3[NaCl]	+15.3 Cals.
2[AlCl3] + 3[KCl]	+30.5 Cals.
2[AlCl3] + 6[NaCl]	+19.5 Cals.
2[AlCl3] + 6[KCl]	+366 Cals.
2[AlCl3] + 2[AgCl]	+5.0 Cals.
2[AlCl3] + 1.5[CaCl2]	+9.3 Cals.
2[AlCl3] + 1.5[SrCl2]	+8.7 Cals.
2[AlCl3] + [BaCl2]	+5.3 Cals.
2[AlF3] + 6[NaF]	+40.7 Cals.
2[AlF3] + 6[KF]	+88.8 Cals.
2[AlF3] + 7H2O	+65.0 Cals.