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Heat of solution, also known as the enthalpy change of solution, is the change in enthalpy that occurs when a given solute is dissolved in a solvent to form a solution. Enthalpy is a term used in thermodynamics to describe the energy in a system. One cannot directly measure the total enthalpy of a system, so the change in enthalpy is used for measurements such as heat of solution rather than the total enthalpy of the system. There are several processes that occur when a solute is dissolved in a solution, and each is able to change the enthalpy of the solution. In many cases, a variety of chemical bonds are broken and new bonds are formed, all of which result in a change of enthalpy.
There are three primary aspects of the dissolution of a solute in a solvent that contribute to the heat of solution. First, when the solute is added, the chemical interactions linking solute molecules break, which requires the consumption of some energy. Next, chemical attractions linking solvent molecules also break as the solute molecules enter the system, again requiring energy consumption. Lastly, after these attractions are broken, new interactions between the solvent and solute molecules are formed, resulting in the release of some energy.
The first two aspects of dissolution require energy input and are referred to as endothermic processes. The third, by which attractions form between solvent and solute molecules, is referred to as an exothermic process, as it releases energy into the system. To determine the total heat of solution, one can simply take the sum of each enthalpy change. In some cases, the first two parts of dissolution require more energy input than the formation of new attractions releases, resulting in a process that is endothermic overall. In others, the final release of energy is greater than the energy required to break the solute-solute and solvent-solvent attractions, so the process is exothermic overall.
It also is possible to measure the heat of solution based on temperature changes in a solution. A predominantly exothermic process will release energy into the system and will, therefore, increase the temperature of the solution. A primarily endothermic process, on the other hand, will consume energy and, therefore, reduce the temperature of the reaction. If one knows various properties of the solute and solvent in advance, one can use the change in temperature to determine the heat of solution with reasonable accuracy.