We have studied in section 28.1 that in saturated solution. of ionic solid, an equilibrium exists between the undissolved solute and its ions in solution.
The forward process is called dissolution and the backward process is called crystallisation. Both these processes take place at the same rate at the equilibrium. We have also studied that the maximum amount of a solute which dissolves in a given volume of solvent changes with temperature. This means that the saturated solutions formed at different temperatures, contain different amounts of solute. When a saturated solution is cooled, crystallisation takes place. Sometimes when the cooling is done carefully (without stirring, or without seeding for crystallization to start). The resulting solution contains more solute than it can normally hold and is known as a supersaturated solution. A supersaturated solution is not a true saturated solution. It quickly forms crystals when stirred or when a small crystal of solute for seeding is added. Crystallisation can also be affected by decreasing the volume of a solution through evaporation of solvent.
TEMPERATURE AT WHICH CRYSTALLISATION STARTS
Consider the solubility curve of solute A, whose solubility increases with increase in temperature (Fig 28.4).
Fig. 28.4. Solubility curve for the substance A
Point X represents an unsaturated solution at about 98°C. On cooling, to about 70°C, at point Y, a saturated solution consisting of about 0.33 mol of A in 1 dm3 of solution is formed. On further cooling, crystals begin to form and the concentration of the solution follows the curve. At point Z, the saturated solution contains about 0.10 mol of A; approximately 0.27 mol (0.37- 0.10 ) of substance A would have crystallized out at about 25°C.
SEPARATION OF TWO SALTS OF DIFFERENT SOLUBILITIES
Consider the solubility curves of two solutes A and B having different solubilities, as shown in Fig. 28.5.
Let 1 dm3 solution containing 350 g of solute B and 1100 g of solute B at 70°C was allowed to cool. At 63°C crystals of A start to crystallize out. At room temperature of about 30°C, the solubility of A is approximately 450 g dm-3. Therefore (1100- 450) = 650 g of A will have crystallized out. At about 28°C solute B is an unsaturated solution. Thus, no crystallization of B occurs. This means a mixture of two salts with different solubilities can be separated by
crystallization. A proportion of the more soluble salt crystallizes out and the less soluble salt stays in the solution. This technique of separation is known as fractional crystallization. In some cases it may be necessary to repeat the process several times to obtain pure products.
Till now, we have been discussing the solubility of ionic substances in water. Solutes readily crystallize from solution when there is a significant decrease in solubility at low temperatures. However, a solute can also be recrystallized from the solvents other then water provided its solubility is high at high temperatures and low at low temperatures. The concentration of any soluble impurity in the solute will decrease in each recrystallization process.
To sum up,
• Recrystallisation is a technique used to purify various substances.
• The substance to be purified must fulfill following requirements.
(a) It should be present in larger amount in the mixture.
(b) It should have low solubility (i.e., almost sparingly soluble) at low temperature and high solubility at high temperature.
(c) It should neither decompose, nor react with solvent at higher temperature.
Recrystallization technique can be used to purify both inorganic as well organic solutes.
ENERGY CHANGES DURING CRYSTALLISATION
Crystallisation is a reverse of dissolution. For crystallisation process the solute-solvent interactions have to be overcome. For this process, the energy equivalent to hydration energy (Δ Hyd) is required to be added to the system. The release of energy occurs during solute-solute interactions. Quantitatively this release of energy is expressed in terms of lattice energy (Δ LH). The net energy change during crystallisation Δ Cry H is the difference of energy released and that which is required
For dissolution process
Δ sol H = Δ hyd H +ΔLH
For crystallisation process
Δ Cry H = – Δ sol H
= – (Δ Hyd H + ΔLH)
Crystallisation, as well as recrystallisation is possible if solubility of solute is high at higher temperature and low at lower temperature. This means Ll501H for such a process is +ve. Consequently, Δ CRYH should be -ve. In other words, crystallization is governed by -ve sign of enthalpy change.