Solutions and Colloids

Solutions and Colloids

Review of Solutions

• Solutions are homogeneous mixtures composed of two or more substances.
• Solute: The substance being dissolved.
  • Typically present in a smaller amount.
• Solvent: The substance doing the dissolving.
  • Typically present in the largest amount.
• Solutions can exist in all three physical states:
  • Solid solution: Examples include metal alloys (mixtures of two or more metals).
  • Gas solution: Example is air, which is a mixture of nitrogen, oxygen, and other gases.
  • Liquid solution: The primary focus of this chapter, especially aqueous solutions where water is the solvent.

Traits of Solutions

• Homogeneous throughout.
• Mixed at the molecular level, involving intermolecular attractions between neighboring particles.
• Particle sizes are on a molecular scale, typically $0.2 - 2 \text{ nm}$ in diameter (individual molecules).
• Transparent to light (though they may be colored, they will be clear).
• The physical state of the solution is usually the same as that of the solvent, but not always.
• The dissolved solute will not settle out or separate from the solution.
  • This is because the random motions of the particles are greater than the force of gravity.
• The components of a solution are dispersed and consist of a mixture of separate molecules, atoms, and/or ions.

The Dissolution Process

• Solutions where water acts as the solvent are termed aqueous solutions.
• Examples of dissolution:
  • Sucrose dissolving in water: $\text{C}<em>{12}\text{H}</em>{22}\text{O}<em>{11}(\text{s}) \rightarrow \text{C}</em>{12}\text{H}<em>{22}\text{O}</em>{11}(\text{aq})$
  • Aluminum sulfate dissolving and dissociating in water: $\text{Al}<em>2(\text{SO}</em>4)<em>3(\text{s}) \rightarrow 2\text{Al}^{3+}(\text{aq}) + 3\text{SO}</em>4^{2-}(\text{aq})$

Intermolecular Attractive Forces Influencing Solution Formation

• Three types of intermolecular forces (IMFs) are involved:
  1. Solute-solute IMFs: Must be overcome to separate solute particles.
     • Energy is consumed (endothermic process). The sign of $\Delta H$ for this step is positive ($+$).
  2. Solvent-solvent IMFs: Must be overcome to create space for solute particles.
     • Energy is consumed (endothermic process). The sign of $\Delta H$ for this step is positive ($+$).
  3. Solute-solvent IMFs (Solvation): Attractive forces are established between solute and solvent particles.
     • Energy is released (exothermic process). The sign of $\Delta H$ for this step is negative ($-$).
• The enthalpy of solution ($\Delta H_{\text{solution}}$) is the sum of these energy changes:
  • $\Delta H<em>{\text{solution}} = \Delta H</em>{\text{solute}} + \Delta H<em>{\text{solvent}} + \Delta H</em>{\text{solvation}}$

The Formation of Solutions: Energy Balance

• The relative magnitudes of the energy changes for the three steps determine the overall nature of solution formation:
  • Exothermic dissolution (Likely to dissolve): If the energy released during solvation (Step 3) is significantly greater than the energy consumed in overcoming solute-solute and solvent-solvent IMFs (Steps 1 & 2), the overall $\Delta H_{\text{solution}} < 0$. This is often described as