Enthalpy Change of Solution - Steps in Dissolution

Steps in Dissolution

Dissolution can be viewed as occurring in three steps:

  1. Breaking solute-solute attractions (endothermic), see for instance lattice energy in salts.
  2. Breaking solvent-solvent attractions (endothermic), for instance that of hydrogen bonding
  3. Forming solvent-solute attractions (exothermic), in solvation.

The value of the enthalpy of solution is the sum of these individual steps. Dissolving ammonium nitrate in water is endothermic. The energy released by solvation of the ammonium ions and nitrate ions is less that the energy absorbed in breaking up the ammonium nitrate ionic lattice and the attractions between water molecules. Dissolving potassium hydroxide is exothermic, as more energy is released during solvation than is used in breaking up the solute and solvent.

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 ΔHo in kJ/mol in water at 25°C

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