The excess volume V E is the difference between the real volume of a mixture of chemical substances and the ideal volume, which corresponds to the sum of the volumes of the components before mixing ( pure substance volumes):
If the volume of the real mixture is greater than that of the ideal, the excess volume is positive (volume dilation), in the opposite case it is negative ( volume contraction ). As the difference between the real and ideal behavior of a mixture, the excess volume is an excess quantity .
Excess molar volume
In relation to the amount of substance n in the mixture, one speaks of the molar excess volume :
The molar volume of the mixture is equal to the sum of the partial molar volumes of the components:
The partial molar volume of a substance A is the volume that this substance contributes as a component to the total volume of a mixture of several substances A and B. It depends both on the other substance B and on the mixing ratio and is not always identical to the molar volume that substance A occupies as a pure substance:
- Molar mass M and
- Density .
Scale and examples
The volume effect of mixing pure substances is relatively small. Usually the difference is only around one to two percent.
- Mixtures of non-polar and polar substances usually show a clearly positive excess volume, i.e. H. the volume of the mixture is greater than that of the ideal mixture (volume dilatation). Examples are:
- Methylcyclohexane and 2-pentanol (maximum = +0.50 cm 3 / mol at = 117.98 cm 3 / mol, 298 K = 25 ° C)
- Dichloromethane and 2-butanone (maximum = +0.06 cm 3 / mol at = 72.99 cm 3 / mol, 298 K = 25 ° C)
- Mixtures of small polar components and larger molecules with a polar group often have a negative excess volume, i.e. H. the volume is smaller than that of the ideal mixture ( volume contraction ). Examples are:
- N-methyl-2-oxazolidinone and water (minimum = −0.54 cm 3 / mol, 298 K = 25 ° C)
- Pyridine and methanol (minimum = −0.48 cm 3 / mol at = 57.53 cm 3 / mol, 298 K = 25 ° C)
- Carbon monoxide and methane (minimum = −0.35 cm 3 / mol at = 36.29 cm 3 / mol, 90 K = −183 ° C)