The enthalpy of solution, enthalpy of dissolution, or heat of solution is the enthalpy change associated with the dissolution of a substance in a solvent at constant pressure resulting in infinite dilution.
The enthalpy 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 chemical bond is the difference between electron affinity and ionization energy, the heat of solution of a substance is defined as the sum of the energy absorbed, or endothermic energy, expressed in positive values and unit kJ/mol, and energy released, or exothermic energy (negative value).
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Dissolution of most gases is energetically favorable. That is, when a gas dissolves in a liquid solvent, energy is released into the surroundings as heat in an exothermic reaction. As energy leaves the system, the temperature of the solution decreases. Thus, decreasing the temperature of solution increases the solubility of a gas. Alternatively, when a solution is heated, the unfavorable reverse reaction occurs more readily and gas evolves. When the gas is completely dissolved (at a very low temperature, although this is purely theoretical as no substance can infinitely dissolve), the heat of solution is said to be at its maximum (a large negative enthalpy).
Dissolution can be viewed as occurring in three steps:
The value of the overall enthalpy change is the sum of the individual enthalpy changes of each of these steps. For example, dissolving ammonium nitrate in water decreases the temperature of the solution. Solvation does not compensate energy spent in breaking down the crystal lattice, while adding potassium hydroxide will increase it.
Solutions with negative enthalpy changes of solution form stronger bonds and have lower vapor pressure.
The enthalpy of solution of an ideal solution is zero since the attractive and repulsive properties of ideal fluids are equal, irrespective of the compounds, thus mixing them does not change the interactions.
| Enthalpy change of solution for some selected compounds | ||
| hydrochloric acid | -74.84 | |
| ammonium nitrate | +25.69 | |
| ammonia | -30.50 | |
| potassium hydroxide | -57.61 | |
| caesium hydroxide | -71.55 | |
| sodium chloride | +3.88 | |
| potassium chlorate | +41.38 | |
| acetic acid | -1.51 | |
| sodium hydroxide | -44.51 | |
| Change in enthalpy ΔH |
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