Heat of Solution



 

Learning Objective

  • Define heat of solution

Key Points

    • Enthalpy of solution, or heat of solution, is expressed in kJ/mol, and it is the amount of heat energy that is released or absorbed when a solution is formed.
    • There are three steps in solvation: the breaking of bonds between solute molecules, the breaking of intermolecular attractions between solvent molecules, and the formation of new solute-solvent attractive bonds. Energy is absorbed during the first two steps, and it is released during the last step.
    • Depending on the relative amounts of energy required to break bonds initially, as well as how much is released upon solute-solvent bond formation, the overall heat of solution can either be endothermic or exothermic.

Terms

  • heat of solutionThe enthalpy change associated with the dissolution of a substance in a solvent at constant pressure, resulting in infinite dilution.
  • solvationThe process of attraction and association of molecules of a solvent with molecules or ions of a solute; also called dissolution.

The heat of solution, also referred to the enthalpy of solution or enthalpy of dissolution, is the enthalpy change associated with the dissolution of a solute in a solvent at constant pressure, resulting in infinite dilution. The heat of solution, like all enthalpy changes, is expressed in kJ/mol for a reaction taking place at standard conditions (298.15 K and 1 bar).

Three-Step Process of Dissolution

The heat of solution can be regarded as the sum of the enthalpy changes of three intermediate steps:

  1. The breaking of bonds within the solute, such as the electrostatic attraction between two ions (endothermic)
  2. The breaking of intermolecular attractive forces within the solvent, such as hydrogen bonds (endothermic)
  3. The formation of new attractive solute-solvent bonds in solution (exothermic)

The value of the overall heat of solution, [latex]\Delta H^\circ_{sol}[/latex], is the sum of these individual steps. Depending on the relative signs and magnitudes of each step, the overall heat of solution can be either positive or negative, and therefore either endothermic or exothermic. This depends entirely on if more energy was used to break the solute-solute and solvent-solvent bonds, or if more energy was released when solute-solvent bonds were formed.

If more energy is released in making bonds than is used in breaking bonds, the overall process is exothermic, and ∆Hsol is negative. If more energy is used in breaking bonds than is released upon solute-solvent bond formation, then the overall process is endothermic, and ∆Hsol is positive.

Examples

  • Dissolution of sodium chloride (table salt) in water is endothermic. This is because the amount of energy used to break apart the hydrogen bonding interactions between water molecules, as well as the energy used to break apart the electrostatic attractions between sodium and chloride ions, is greater than the amount of energy released when new solute-solvent attractions are formed between water molecules and aqueous ions in solution.
  • Dissolving potassium hydroxide is exothermic. This is because more energy is released upon formation of solute-solvent bonds than was required to break apart the hydrogen bonds in water, as well as the ionic bonds in KOH.
Dissolution of NaCl in waterDissolution of sodium chloride in water is endothermic. Solute-solvent attractive bond formation (the exothermic step in the process of solvation) is indicated by dashed lines.