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Enthalpy of Solution

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Enthalpy of Solution - Lesson Summary

The amount of heat energy released or absorbed when one mole of a substance dissolves in a specified amount of solvent is known as enthalpy of solution.

Ex: The ions of an ionic compound, AB, are held together by lattice energy. When it is dissolved in a solvent, its ions leave their ordered positions on the crystal lattice.

This conversion of ionic compound to ions is known as change in lattice enthalpy and is denoted by ΔH°.

When water is used as a solvent, the dissolving process is called as hydration. If any other solvent is used, then the process is known as solvation of ions.

The dissolution of AB ions into solvent is known as enthalpy of hydration of ions and is represented as ΔhydH°. This energy change of the solution is known as solution enthalpy (ΔsolH°).

The enthalpy of solution of ionic compound, AB is the sum of change in lattice enthalpy, and enthalpy of hydration of ions.

Enthalpy of hydration ΔhydH of an ion is the amount of energy released when a mole of the ion dissolves in a large amount of water forming an infinite dilute solution in the process.

Lattice enthalpy, ΔH, of an ionic compound is the enthalpy change which occurs when one mole of an ionic compound dissociates into its ions in gaseous state.

The lattice enthalpy of a compound depends on the size of its ions and the charge on them.

Decrease in the size of an ion increases the lattice enthalpy. And increase in charge also increases the lattice enthalpy. It is not possible to determine lattice enthalpy through an experiment but can be calculated by constructing an enthalpy diagram known as Born-Haber cycle. The enthalpy change involved in direct formation of two compounds is called the enthalpy of formation.

"Hess law states that, the enthalpy change in a particular reaction is the same whether the reaction takes place in one step or in number of steps".


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