Lecture 19: Water and Solvation

Read section 6.1, 6.2, 6.4, 6.7 from your textbook.

Water is sometimes called the universal solvent. This week we will examine solvation by water and other common solvents. Solvents interact with solutes through relatively weak forces, such as dipole-dipole interactions. Important solvent properties include polarity, basicity, dipole moment, dielectric constant. The properties change with temperature and pressure in the supercritical state.

Cations dissolved in these solvents are more strongly bonded to some of the solvent molecules. Crystal field, ligand field, and molecular orbital theories describe the structure and bonding of coordination compounds.

Liquid Water

Water has an ordered packing in solution that consists of a diamond lattice of oxygen atoms bridged to the other oxygens by hydrogen. Each oxygen is bonded to 4 hydrogen atoms in a tetrahedron. Liquid water is more dense than solid water and includes an additional intersitial water in the unit cell.

Unlike solid water, the bonds in liquid water are constantly breaking and forming. It is a dynamic but ordered material.


Hydrogen bonding, the ability of hydrogen atoms bonded to nitrogen, oxygen, or fluorine to bridge to other nitrogen, oxygen, or fluorine atoms is important in the structure of water and in water's ability to solvate ionic substances.


Solvent Properties

What are the forces that exist between atoms and molecules in condensed phases?
    interaction strength distance function
    covalent very strong complex
    ionic very strong 1/r
    ion-dipole strong 1/r2
    dipole-dipole moderate 1/r3
    ion-induced dipole weak 1/r4
    dipole-induced dipol very weak 1/r6
    London forces very weak 1/r6

Molecules dissolve in liquids (or solids) when the solute/solvent forces are greater than the forces holding the material in its original state.

Typical Solvents

Compound

pKa

Dipole moment

Dielectric constant

H2O

water

15.7

1.84

81.7

CS2

carbon disulfide

  

0

2.64

H2SO4

sulfuric acid

-9

  

101

NH3

ammonia

38 ([NH4]+= 9.25)

1.47

26.7

CH3CO2H

acetic acid

4.75

1.74

6.19

CCl4

carbon tetrachloride

  

0

2.23

CHCl3

chloroform

  

1.87

4.70

CH3CH2OH

ethanol

16

1.69

24.3

CH2Cl2

methylene chloride

  

1.60

8.9

C6H5CH3

toluene

  

0.36

2.38

CH3CN

acetonitrile

25

3.92

36.2

n-C6H14

hexane

44

0.08

1.89

cyclo-CH2[CH2]3O

tetrahydrofuran

  

1.63

7.32

C5H5N

pyridine

([pyH]+ = 5.29)

2.19

12.3


Ions in Solution

Ions form solids due to stabilization within the lattice of the ionic bonds. (LATTICE ENERGY). They will dissolve in a liquid on when the forces between the ions and the solvent are greater than those between ions in the crystal. This depends greatly on the dielectric constant that indicates how well ions are shielded from each other and the ability of the solvent to coordinate and stabilize the ions.



Supercritical Fluids

A supercritical fluid is any substance that is above its critical temperature (Tc) and critical pressure (Pc). In the critical region there is only one phase and it possesses properties of both gas and liquid. Just above the critical temperature of the substance, liquid-like densities are rapidly approached with modest increases in pressure in the range of 0.7 to 2 times the critical pressure. Higher pressures are required to attain liquid-like densities for temperatures further above critical. Solvent power of a supercritical fluid unlike a liquid increases with density at a given temperature and increases with temperature at a given density. The increase in density is not linear with pressure, but rather the rate of increase is much greater in the vicinity of the critical point.



A supercritical fluid exhibits properties intermediate between those of liquids and gases. Mass transfer is rapid with supercritical fluids. Their dynamic viscosities are nearer to those found in normal gaseous states. The diffusion coefficient is more than ten times that of a liquid.



There are two supercritical fluids that have the most current applications:
  1. Supercritical water
    • high temperature and pressure
    • very corrosive
    • hydrolysis and wet air oxidation applications


  2. Supercritical carbon dioxide
    • relatively low pressure and temperature
    • solvent for polar organic compounds
    • extraction, dry cleaning

Coordination Compounds

The covalent interaction between a solvent such as water or another Lewis base in solution and cation gives a coordination compound. Different cations and their ligands have different coordination environments.

Hard/soft refers to polarizability of the electron cloud. Highly polarizable Lewis bases tend to coordinate well to large, highly polarizable Lewis acids.

Hard-Soft Acids and Bases

Hard Acids

H+

Na+

Ca2+

Zr4+

Fe3+

Hard Bases

H2O

NH3

F-

OH-

CH3CO2-

Borderline Acids

Fe2+

Cu2+

Sn2+

Borderline Bases

Br-

N3-

pyridine

Soft Acids

Pd2+

Cu+

BH3

Ag+

Soft Bases

I-

CN-

CO

PR3


Reactions of Metals with Water, Ammonia






Acid-base reactions



Example: Cu2+
  • in water

  • add ammonia

  • add HCl