Free Energy of Dissolution
Chemistry: Cambridge International A Level, topic 1.7.9
Free Energy of Dissolution
Dissolution is an example of a spontaneous process that occurs without the requirement of energy from some external source. The free energy change (ΔG°) of dissolution depends on many factors.
Spontaneity
- The formation of a solution is an example of a spontaneous process.
- This is a process that occurs under specified conditions without the requirement of energy from some external source.
- The topic of spontaneity covered more thoroughly in the application of thermodynamics section.
Factors favoring dissolution
- Factors that favor, but do not guarantee, the spontaneous formation of a solution include:
- A decrease in the internal energy of the system, so an exothermic change.
- An increased dispersal of matter in the system.
- This indicates an increase in the entropy of the system, which will be discussed in a later section.
- Dissolution requires a favorable free energy change (ΔG°).
- A reaction that is exothermic and increases dispersal of matter is more likely to be favorable.
Steps involved in dissolution
- The dissolution of a solution involves several steps including:
- Overcoming solute-solute interactions in the substance to be dissolved.
- Rearrangement of solute and solvent molecules.
- Formation of solute-solvent attractions once the substance is dissolved.
- The relative magnitudes of the energy changes associated with these stepwise processes determine whether the dissolution process overall will release or absorb energy.
Dissolving ionic compounds
- Dissolution of an ionic compound involves:
- Overcoming the electrostatic forces between the cations and anions of the compound (solute–solute).
- Establishment of attractive forces between these ions and water molecules (solute–solvent), which is called solvation.
- Hydrogen bonding between water molecules must also be overcome.
Endothermic vs. exothermic
- If the solute’s electrostatic forces are greater than the solvation forces, the dissolution process is endothermic.
- This means the compound may not dissolve to an appreciable extent.
- Calcium carbonate, the major component of coral reefs, is an example of an “insoluble” ionic compound.
- If the solvation forces are much stronger than the compound’s electrostatic forces, the dissolution is exothermic and the compound may be soluble.
- A common example of this type of ionic compound is sodium chloride.
Endothermic vs. exothermic 2
- Spontaneous solution formation is favored, but not guaranteed, by exothermic dissolution processes.
- Some compounds dissolve endothermically. An example is ammonium nitrate (NH4NO3).
- Endothermic dissolutions such as this one require a greater energy input to separate the solute species than is recovered when the solutes are solvated.
- However, they are spontaneous nonetheless due to the increase in disorder that accompanies formation of the solution.
Test yourself on Free Energy of Dissolution
Why can an endothermic dissolution process still be spontaneous?