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Enthalpy Changes During Dissolution

(a) Enthalpy of Solution (A Sol H)

It is the enthalpy change accompanying the dissolution of one mole of a substance in large excess of a solvent so that further addition of solvent does not involve any more enthalpy change. It may be noted that the enthalpy change accompanying the dissolution of 1 mole of substance in specific number of moles of solute is called integral enthalpy of solution whereas the enthalpy change observed by dissolving one mole of substance in infinite amount of solvent so that interactions between solute molecules are negligible, is called enthalpy of solution or more specifically, the enthalpy at infinite dilution.

 

If water is the solvent, then the symbol aq (aqueous) is used to represent large or infinite dilutions. For example, the thermochemical equations for dissolution of KCl and CuSO4.5H2O are:

Some notable features about A501H are:

• Δ Sol of hydrated salts like CuSO 4.5H2p, CaCl2.6H2O, etc., or the salts which do not form hydrates like NaCl, KCl, NH4Cl, etc., is generally positive.

• Δ Sol of anhydrous salts which form hydrates like CuSO 4′ etc., is negative.

While doing thermochemical calculations, if any equation is multiplied by any integer, the word aq which represents large dilutions is not multiplied by it and is retained as such. In other words aq does not have quantitative significance.

 

(b) Dissolution of Ionic Salts

When ionic compounds dissolves in a solvent, the ions leave their ordered position in the crystal lattice and start moving freely in the solution. In the solution, the ions remain surrounded by the envelop of solvent molecules (see properties of ionic compounds in unit-7). This interaction between the ions and the solvent molecules is known as solvation. In case the solvent is water this process is called hydration. The dismentalling of ionic crystal, i.e., breaking away the ions requires energy which is generally expressed in terms of lattice energy (ΔLH) and solvation/hydration of ions releases energy, which is generally expressed in terms of solvation energy or hydration energy, Δ Hyd H in case solvent is water. For aqueous solutions, these energy changes can be related to Δ sol H according to energy level diagram as shown in Fig. 16.6

It is quite clear that

Δ sol H =ΔL H + Δ Hyd H

The value of 6.501 H can be +ve or -ve depending upon the magnitudes of Δ sol H and ΔL H. Dissolution of salt may not occur if the value of Δ Hyd H is very high.

 

(c) Enthalpy of Hydration of Anhydrous Salt (A Hyd H) It is the enthalpy change accompanying the hydration of one mole of an anhydrous salt by combining with specific number of moles of water. For example, enthalpy of hydration of anhydrous copper(Il) tetraoxosulphate(VI) can be

represented as:

CuSO4(s) + 5Hp(l)à CuSO4.5H2O(s);

ΔH =- 78.2 kJ.