Solutions

Solutions

1. Solutions are homogeneous mixtures of two or more than two components.

   • Solvent- The component that is present in the largest quantity
   • Solute- One or more components present in the solution other than solvent

2. Concentration of solution

   • Mass percentage(w/w): (Mass of component in solution/Total Mass of the solution)*100
   • Volume percentage(v/v): (Volume of the component/Total volume of solution)*100
   • Mass by volume percentage: It is the mass of solute dissolved in 100 mL of the solution.
   • Parts per million(ppm):(Number of parts of the component/Total number of parts of all components of the solution)*10^6
   • Mole fraction(X):Number of moles of the component/Total number of moles of all the components
   • Molarity(M):Moles of solute/Volume of solution in litre
   • Molality(m):Moles of solute /Mass of solvent in kg
   • Each method of expressing concentration of the solutions has its own merits and demerits. Mass %, ppm, mole fraction and molality are independent of temperature, whereas molarity is a function of temperature. This is because volume depends on temperature and the mass does not.

3. Solubility

   • Solubility of a substance is its maximum amount that can be dissolved in a specified amount of solvent. It depends upon the nature of solute and solvent as well as temperature and pressure.
   • Solubility of a solid in a liquid
     • It is observed that polar solutes dissolve in polar solvents and non polar solutes in nonpolar solvents.Like dissolves like
     • Dissolution:When a solid solute is added to the solvent, some solute dissolves and its concentration increases in solution.
     • Crystallisation: Some solute particles in solution collide with the solid solute particles and get separated out of solution.
     • When both the rates of these processes become equal a state of dynamic equilibrium is reached.
       • At this stage the concentration of solute in solution will remain constant under the given conditions, i.e., temperature and pressure.
       • The solution which is in dynamic equilibrium with undissolved solute is the saturated solution.Thus, the concentration of solute in such a solution is its solubility.
     • Effect of Temperature: In general, if in a nearly saturated solution, the dissolution process is endothermic (Δsol H > 0), the solubility should increase with rise in temperature and if it is exothermic (Δsol H > 0) the solubility should decrease.
     • Effect of pressure Pressure does not have any significant effect on solubility of solids in liquids. It is so because solids and liquids are highly incompressible and practically remain unaffected by changes in pressure.
   • Saturated Solution:Such a solution in which no more solute can be dissolved at the same temperature and pressure
   • Unsaturated Solution:An unsaturated solution is one in which more solute can be dissolved at the same temperature.
   • Solubility of a gas in liquid
     • Effect Of Pressure
       • Henry's Law
         • The solubility of a gas in a liquid is directly proportional to the pressure of the gas.
         • “The partial pressure of the gas in vapour phase (p) is proportional to the mole fraction of the gas (x) in the solution”
           • p = Kh x
             • Kh is the Henry's Law constant which depends on the nature of the gas
         • Industrial application and Biological phenomenon: 1.CO2 dilution in Cold Drinks. 2.Scuba Divers' helium in tanks. 3.High altitude scarcity of O2
       • Dalton's
         • Mole fraction of gas in the solution is proportional to the partial pressure of the gas over the solution.
     • Effect Of Temperature
       • Solubility of gases in liquids decreases with rise in temperature. When dissolved, the gas molecules are present in liquid phase and the process of dissolution can be considered similar to condensation and heat is evolved in this process.As dissolution is an exothermic process, the solubility should decrease with increase of temperature.

4. Vapour Pressure of Liquid Solutions

   • Vapour Pressure of Liquid-Liquid Solutions
     • Raoult's Law
       • For a solution of volatile liquids the partial vapour pressure of each component in the solution is directly proportional to its mole fraction.
         • Thus following conclusions can be drawn:
           • Total vapour pressure over the solution can be related to the mole fraction of any one component.
           • Total vapour pressure over the solution varies linearly with the mole fraction of component 2.
           • Depending on the vapour pressures of the pure components 1 and 2, total vapour pressure over the solution decreases or increases with the increase of the mole fraction of component 1.
   • Raoult’s Law as a special case of Henry’s Law
     • Raoult’s law becomes a special case of Henry’s law in which Kh becomes equal to p1(knot) .
   • Vapour Pressure of Solutions of Solids in Liquids
     • In a pure liquid the entire surface is occupied by the molecules of the liquid. If a non-volatile solute is added to a solvent to give a solution [Fig. 2.4.(b)], the vapour pressure of the solution is solely from the solvent alone. This vapour pressure of the solution at a given temperature is found to be lower than the vapour pressure of the pure solvent at the same temperature. In the solution, the surface has both solute and solvent molecules; thereby the fraction of the surface covered by the solvent molecules gets reduced. Consequently, the number of solvent molecules escaping from the surface is correspondingly reduced, thus, the vapour pressure is also reduced.
       • The decrease in the vapour pressure of solvent depends on the quantity of non-volatile solute present in the solution, irrespective of its nature.

5. Ideal Solutions

   • Obey Raoult’s law over the entire range of concentration
   • ΔmixH = 0, ΔmixV = 0
   • If the intermolecular attractive forces between the A-A and B-B are nearly equal to those between A-B, this leads to the formation of ideal solution.

6. Non-Ideal Solutions

   • Does not obey Raoult’s law over the entire range of concentration, then it is called non-ideal solution
   • Positive Deviation: Higher vapour pressure than predicted by raoult's law
     • The intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules. Resulting in easy escape of solute or solvent molecules.
   • Negative Deviation: Lower vapour pressure than predicted by raoult's law
     • The intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure resulting in negative deviations.

7. Some liquids on mixing, form AZEOTROPES which are binary mixtures having the same composition in liquid and vapour phase and boil at a constant temperature. In such cases, it is not possible to separate the components by fractional distillation.

   • The solutions which show a large positive deviation from Raoult’s law form minimum boiling azeotrope at a specific composition
   • The solutions that show large negative deviation from Raoult’s law form maximum boiling azeotrope at a specific composition.

8. Colligative Properties

   • Relative lowering of vapour pressure.
     • Determination of molar mass of solute.
   • Elevation of boiling point.
     • The boiling point of a solution is always higher than that of the boiling point of the pure solvent in which the solution is prepared
       • The elevation of boiling point also depends on the number of solute molecules rather than their nature
     • The vapour pressure of a liquid increases with increase of temperature.Liquid boils at the temperature at which its vapour pressure is equal to the atmospheric pressure. Vapour pressure of the solvent decreases in the presence of non-volatile solute.
     • ΔTb ∝ m ΔTb = Kb m
       • Kb is molal elevation constant or boiling point elevation constant
         • The unit of Kb is K kg/mol
     • Determination of molar mass of solute
   • Depression of freezing point

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