Potassium bromide (KBr) is virtually insoluble in acetone due to the fundamental mismatch between the strong ionic salt and the organic solvent. Acetone is a common solvent, but it lacks the molecular characteristics needed to effectively pull apart and stabilize the highly charged ions of KBr. Solubility is determined by the energetic requirements for dissolving a crystalline solid in a liquid. The minimal solubility that exists is insufficient for practical chemical purposes.
Understanding Potassium Bromide: The Ionic Solute
Potassium bromide is a classic ionic compound, existing as a solid crystalline structure composed of positively charged potassium ions (K+) and negatively charged bromide ions (Br-). These ions are held together by powerful electrostatic forces, forming a highly ordered crystal lattice. This arrangement is incredibly stable and requires substantial energy to break apart.
The stability of the crystal lattice is quantified by its lattice energy, which for KBr is approximately 670 kilojoules per mole. For KBr to dissolve, the energy released when the ions interact with the solvent (solvation energy) must be high enough to overcome this lattice energy. If the solvent-ion interaction is weak, the solid salt remains intact.
Understanding Acetone: The Polar Aprotic Solvent
Acetone is an organic solvent containing a carbonyl group. This group creates a significant separation of charge, giving the molecule a high dipole moment and classifying it as polar. Acetone is categorized as a polar aprotic solvent because it lacks a hydrogen atom bonded to an electronegative atom like oxygen or nitrogen.
The aprotic nature means acetone cannot form strong hydrogen bonds, which are crucial for dissolving many salts. While polar, its dielectric constant is relatively low, around 21, compared to water’s value of about 78. The dielectric constant measures a solvent’s ability to reduce the force of attraction between charged particles, effectively shielding the ions from each other.
Why Strong Ionic Salts Resist Acetone
The primary reason KBr does not dissolve in acetone is the vast energetic mismatch between the solute’s high lattice energy and the solvent’s low solvating power. For dissolution to occur, the solvation energy—the energy released when acetone molecules stabilize the separated K+ and Br- ions—must exceed the energy required to break the crystal lattice. Acetone cannot provide enough solvation energy.
Acetone’s lower dielectric constant means it is poor at shielding the strong electrostatic attraction between the separated ions. The ions quickly re-associate to form the stable solid crystal.
As an aprotic solvent, acetone can effectively stabilize the positive K+ cation using the negative end of its carbonyl dipole. However, it is significantly less effective at stabilizing the negative Br- anion because it cannot form strong hydrogen bonds with it.
This poor stabilization, combined with insufficient shielding, results in a solvation energy too low to overcome KBr’s high lattice energy. In contrast, water, a polar protic solvent, has a high dielectric constant and forms strong hydrogen bonds with both positive and negative ions. Water’s high solvation energy allows it to dissolve KBr readily, a capability acetone fundamentally lacks. The measured solubility of KBr in acetone is extremely low, with values around 0.0045 grams per 100 grams of solvent, confirming its practical insolubility.