Chemical Engineering - Separations 5

1. Chemical Engineering - Separations 5 Lecture 4 – non-ideal mixtures John Christy

2. Activity coefficients & non-ideality • Non-ideal liquids • presence of species A affects volatility of species B • disruption or formation of H-bonds • steric effects (different volumes and shapes) • can express in terms of activity coefficients • related to Excess Gibbs Free Energy • strong effect of interaction  i>>1 or i<<1

3. What leads to non-ideality? • Essentially due to dissimilarity of molecules in mixture: i.e. the forces between different molecules  forces between same molecule • dispersive forces (non-polar) • induction (molecule with permanent dipole induces dipole in another molecule) • orientation (due to dipoles) • Hydrogen Bonding – main force strong enough to influence volatility

4. Hydrogen bonds • Bond energies typically 0.2 – 2 kJ/mole compared with C-H or N-H bond of 20 kJ/mol • requires active H & donor atoms (eg Cl, O, N, F) • leads to higher energy requirement to vaporise, equilibrium partial pressure reduced • if mixing reduces overall number or strength of H-bonds, get +ve deviations from Raoults Law & vice versa

5. Effect of Hydrogen bonds • Ideal. eg n-hexane/n-heptane. Similar size and shape f molecules: n-C6 doesn’t notice whether neighbours are n-C6 or n-C7 – molecular forces much the same • Positive (weak). eg methanol/waterH2O: strong polar bondsMeOH: weaker polar bonds; CH3 not like H mixing leads to weaker bonding and mixture is more volatile.

6. Effect of Hydrogen bonds • Positive (strong). eg ethanol/waterH2O: strong polar bondsEtOH: weak polar bonds; C2H5 less like H even stronger disruption of H-bonds on mixing and mixture is even more volatile.If vapour pressure of mixture is plotted against composition, the positive deviation leads to a maximum vapour pressure at an intermediate composition.

7. Effect of Hydrogen bonds • Negative (strong). eg acetone/chloroform(CH3)2O: no H-bondsCHCl3: no H-bondson mixing strong H-bonds form and mixture is much less volatile.If vapour pressure of mixture is plotted against composition, the negative deviation leads to a minimum vapour pressure at an intermediate composition.

8. Effect of Hydrogen bonds • Summary • If H-bonds broken or weakened on mixing, the mixture vaporises more easily and the vapour pressure rises. A minimum boiling azeotrope may form. • If H-bonds are formed on mixing, the mixture vaporises less easily and the vapour pressure drops. A maximum boiling azeotrope may form.

9. Classification of liquids (Berg) • Class I • Liquids capable of forming 3-dimensional networks of strong H-bonds • e.g. water, glycol,glycerol, amino alcohols, hydroxylamine, hydroxy acids, polyphenols, amides, etc (but not nitromethane, acetonitrile) • Class II • other H-bond forming liquids • e.g. alcohols, acids, phenols, p & s amines, oximes, ammonia, hydrazine, HF, HCN, etc

10. Classification of liquids (Berg) • Class III • Liquids with donor atoms but no active H • e.g. ethers, ketones, aldehydes, esters, tertiary amines,nitro compounds and nitriles without -hydrogen atoms, etc • Class IV • Liquids with active H but no donor atoms • molecules having 2 or 3 Cl atoms on same C as a H atom or one Cl on the same atom and one or more Cl on adjacent C atoms. e.g. CHCl3, CH2Cl2, CH3CHCl2, CH2Cl-CH2Cl

11. Classification of liquids (Berg) • Class V • all other liquids, i.e. liquids with no H-bonding capabilities • e.g. hydrocarbons, CS2, sulphides, mercaptans, halohydrocarbons not in class IV, non-metallic elements I2, P, S

12. Deviations from Raoult’s Law

13. Formation of azeotropes • Formation of azeotrope requires: • boiling point range between pure components to be small • significant effect of mixing on volatility