Ions in Solution

 

    Water have relative perimittivity and high dipole moment which makes them a good solvent for ions. Water molecules orientate themselves around ions to form hydrated species.

    Acid-Base behaviour:

 

    Lattice enthalpy is always exothermic and depends upon the strangth of the force of attration between ions (ionic bond) which is given by:

 

where
E - Energy of interaction              Q.q - Magnitudes of the charges
r - Distance between ions           εo - permittivity of free space ~ 8.85x10-12 Fm-1

    Electrostatic attrations are strong and long-range; the electrostatic repulsion between two protons is 1036 stronger than the gravitational attration. However, fixed charges cause orientation of permanent/ induced dipoles, shielding the charges and weakening the interaction.

    The greater the relative permittivity of the media, the stronger it shields the interaction.

    The Bjerrum length, lB - the charge separation at which the coulomb energy between the ions is equal to the thermal energy, RT.

 

Solubilities of ionic solids in water

i) All group 1A, NH4+ and NO3- salts are soluble in water
ii) Most chloride, bromide and iodide salts are soluble except Ag+ and Hg22+
iii) Most sulfates are soluble except Ba2+, Pb2+, Hg2+, Bi3+, Sn2+. CaSO4 is fairly soluble
iV) Most carbonates, oxides, hydroxides, phosphates and sulfides are insoluble, subject to (i). Ca(OH)2 and Ba(OH)2 are fairly soluble

 

Solubility constant, Ksp 

 

    Limitations:

1. Underestimate the solubility of weak electrolytes
2. Common ion effect - the dissolution of a slightly soluble salt is inhibited if it dissolves into a solution containing one of its component ions
3. When the anion is from a weak acid, effect of pH decreases

 

Definitions

  • Bronsted-Lowry acid - substance that donates an H+ ion
  • Bronsted-Lowry base - substance that accepts an H+ ion
  • Lewis acid - electron pair acceptors
  • Lewis base - electron pair donors
  • Strong acid is totally ionised in aqueous solution
  • A buffer solution is a solution of known pH which has the ability to resist changing pH when small amounts of acid or base are added

 

Ionisation and acidity

i) pH of stong acid and bases

ii) pH of weak acids

    -Weak acids are partially ionised:

    -Since there are equal amount of H+ and A-:

    -The acid dissociation constant, Ka:

 

Henderson - Hasselbalch equation

 

    -pH and pKa are important in understanding drug absorption, protein structure and properties, biomaterials.

Contact

Larona BME

Website Administrator

 

Got a smartphone and want to visit us on the go?

https://m.larona.co/