Chemistry of non aqueous solvents part-1

for university students

1. CHEMISTRY OF NON-AQUEOUS SOLVENTS Part-1 Dr.S.Z.Bootwala Wilson College

2. SOLVENT IS DEFINED AS A SUBSTANCE WHICH HAS THE POWER OF DISSOLVING OTHER SUBSTANCES. Solvent is a liquid, solid or a gas that dissolves another solid, liquid or gaseous solute, resulting into a solution.

3. Water - Universal Solvent Water is easy available , neutral, odourless , non – toxic Can be handled safely has wide range of liquid state (0 – 100℃ )  is liquid at ordinary temperature , high dielectric constant (78.5 )  is capable of reducing forces of electrostatic attraction binding the charged ions in electrolytes in the solid state. thus , salts and other electrolytes get dissociated into ions when they dissolve in water and the solution conducts electricity. high dipole moment, heat of vapourisation ability to dissolve a large number of substances. water is not always a suitable solvent Some reagents may react with H 2 O (e.g. the alkali metals ) non-polar molecules are insoluble in water . Cannot precipitate KCl , BaCl 2 etc.

4. NON-AQUEOUS SOLVENTS The solvents other than water which are used for carrying out chemical reactions are called  non-aqueous solvents .

5. Importance of non-aqueous solvents. water as solvent has few limitations. It cannot be used as solvent during reactions involving acids stronger than H 3 O + and bases stronger than OH - . During the use of strong reducing agents, water cannot be used, as strong reducing agent would react with water.

6. Importance of non-aqueous solvents . In many cases, non-aqueous solvents can serve as better solvent than water. By changing the solvent medium, the product from given set of reaction can be completely changed. This indicates the influence of solvent on chemical reaction. Reaction in water : 2AgNO 3 + BaCl 2 → 2AgCl  + Ba (NO 3 ) 2 Reaction in Ammonia: 2AgCl + Ba (NO 3 ) 2 → BaCl 2  + 2AgNO 3

7. Importance of non-aqueous solvents . It is possible to precipitate KCl in a non-aqueous solvent (liq.NH 3 ), but it is impossible to precipitate KCl in an aqueous medium. Non-aqueous solvent is used to improve the strength of weak acid. For example CH 3 COOH behaves like a weak acid in water medium but the same acid behaves like a strong acid in liquid NH 3 medium. Relative strength of various acids like HClO 4 , HI, HCl , HBr and HNO 3 can be determined in non-aqueous solvent, NH 3 , but not in aqueous medium.

8. Physical Properties of Solvent A )Melting & Boiling Point (Liq. Range) Water has excellent liquid range( 0 to 100℃) at ordinary temp and pressure. Acids can also act as solvent at ordinary temp while liq.NH 3 (-77.7 to -33.4℃) and liq. SO 2 (-75.5 to -10.2℃) can act as solvent only at low temp .

9. Physical Properties of Solvent B) Heat of Fusion & Heat of vaporization: A high heat of vaporization of liquid indicates that the intermolecular forces in it are strong. ∆ H vap / T boiling pt . = Trouton Constant The ratio of the heat of vaporization expressed in joules to the boiling point(K) is a constant known as Trouton Constant. High value of Trouton constant indicates associated molecules. Molar heat of fusion for water (6.0kJ/mol) and ammonia(5.65kJ/mol) are almost same, this means the forces which hold molecules together in H 2 O and NH 3 are of same magnitude.

10. Physical Properties of Solvent C)Dielectric Constant : This property measures the polar nature of solvent and helps to estimate solubility of polar and non polar substances in the solvent. In solvent of high dielectric constants, the ionic substances will dissolve readily. With decrease in dielectric constant, the solubility of ionic compounds decreases. NH 3 and SO 2 with small value of dielectric constant values show decreased tendency to dissolve ionic compounds.

11. Physical Properties of Solvent D)Dipole Moment: Greater is the Polarity between bonds in the molecules ,larger is the dipole moment. And with increase in dipole moment ,tendency to dissolve ionic substance increases. E)Viscosity: It is the measure of fluidity of the solvent. In solvents of low viscosity the operations such as precipitation, crystallization,filtration etc can be easily carried out without any difficulty.