Enthalpy Change of Solution

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Enthalpy change of solution, one mole of a compound is dissolved in water under standard conditions.

The process of dissolving can be exothermic or endothermic.

When an ionic solid dissolves in water, two processes take place:

breakdown of the ionic lattice into gaseous ions

hydration of the ions

The ionic solid must be broken down. This involves breaking down the crystal lattice and separatinf the ions. Overcoming the attractive forces between the oppositely charged ions requires energy. The process is the exact opposite of that producing the lattice enthalpy:

enthalpy change involved in breaking ionic lattice

so the energy has the same magnitude as lattice enthalpy but the sign is the opposite.

Hydration

The second stage of the dissolving process involves the hydration of the gaseous ion. In this process the gaseous ions bdon with the water molecules. The positively charged ions will be attracted to the slightly negative oxygen atoms in water, and the negatively charged ions will be attracted to the slightly positive hydrogen atoms in water.

Standard enthalpy change of hydration

Energy is released when the ions form bonds with water molecules. This energy is known as the enthalpy change of hydration - different ions have different enthalpy changes of hydration.

For the standard enthalpy change of hydration, one mole of aqueous ions are formed from their gaseous ions under standard conditions.

Hydration is an exothermic process.

The lattice enthalpy of an ionic solid can be calculated using:the enthalpy changes of hydration of the constituent gaseous ions and the enthalpy change of solution of the ionic solid.

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