Enthalpy change of solution
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The enthalpy change of solution is the quantity of heat produced or absorbed when one mole of a substance is dissolved in a large volume of a solvent at constant pressure.
The heat of solution is one of the three dimensions of solubility analysis. It is most often expressed in kJ/mol at constant temperature. Just as the energy of forming a molecular bond is the difference between electron affinity and ionization energy, the heat of solution of a substance is defined as the difference between the energy absorbed, or endothermic energy, and energy released, or exothermic energy (expressed in "negative" kJ/mol).
Because heating decreases the solubility of a gas, dissolution of gases is exothermic. Consequently, as a gas continues to dissolve in a liquid solvent, temperature will decrease, while the solution continues to release energy. This is an effect of the increase in heat or of the energy required to attract solute and solvent molecules--in other words, this energy outweighs the energy required to separate solvent molecules. When the gas is "completely" dissolved (this is purely theoretical as no substance can infinitely dissolve)--the heat of solution will be at its acme.
Dissolution can be viewed as occurring in three steps:
- Breaking solute-solute attractions (endothermic)
- Breaking solvent-solvent attractions (endothermic)
- Forming solvent-solute attractions (exothermic)
The value of the overall enthalpy change is sum of the individual enthalpy changes of each step.
