Solutions - Comprehensive Notes

Solutions

The Solution Process

• Solution = homogeneous mixture of solute (present in the smallest amount) and solvent (present in the largest amount).
• Solutes and solvents are components of the solution.
• In making solutions with condensed phases, intermolecular forces are rearranged.
• A major factor in determining whether a solution forms is the relative strength of intermolecular forces between solute and solvent particles.
• Solutions form when the magnitudes of the attractive forces between solute and solvent particles are comparable or greater than those that exist between solute particles themselves or between the solvent particles themselves.
• Essentially, solutions form when the forces pulling together solute and solvent are similar/stronger than the forces holding the solute particles together or the solvent particles together.
• "Like dissolves like" refers to polarity.

Saturated Solutions and Solubility

• Solubility: the amount of solute required to form a saturated solution.
• Dissolve: solute + solvent → solution.
• Crystallization: solution → solute + solvent.
• Saturation: crystallization and dissolution are equal, resulting in equilibrium.
• Supersaturated: a solution formed when more solute is dissolved than in a saturated solution.

Factors Affecting Solubility - Solute-Solvent Interaction

• Polar liquids tend to dissolve polar solutes.
• Miscible liquids: mix in any proportions.
• Immiscible liquids: do not mix.
• Intermolecular forces are important: water and ethanol are miscible because the broken hydrogen bonds in both pure liquids are reestablished in the mixture.
• The number of carbon atoms in a chain affects solubility: the more C atoms, the less soluble in water.
• The number of -OH groups within a molecule increases solubility in water.
• More polar bonds in the molecule, the better it dissolves in a polar solvent.
• Less polar the molecule, the worse it dissolves in a polar solvent, and the better it dissolves in a non-polar solvent.
• Network solids do not dissolve because the strong intermolecular forces in the solid are not reestablished in any solution.

Pressure Effects

• The solubility of a gas in a liquid is a function of the pressure of the gas.
• Henry's Law: $\frac{S<em>1}{P</em>1} = \frac{S<em>2}{P</em>2}$

Temperature Effects

• As temperature increases, the solubility of solids generally increases.
• Sometimes, solubility decreases as temperature increases (e.g., $Ce<em>2(SO</em>4)_3$).

Concentration Units

• The concentration of a solution is the amount of solute present in a given quantity of solvent or solution.
• Percent by mass: \text{% by mass} = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100\%
• Mole fraction (X): $X_A = \frac{\text{moles of A}}{\text{sum of moles of all components}}$
• Molarity (M): $M = \frac{\text{moles of solute}}{\text{liters of solution}}$
• Molality (m): $m = \frac{\text{moles of solute}}{\text{mass of solvent (kg)}}$

Colligative Properties of Solutions

• Colligative properties depend on the amount of solute dissolved rather than the nature of the solute.
• Evaporation: the turning of water into water vapor.
• Equilibrium: when two opposing processes occur at the same rate.
• A new equilibrium leads to a new pressure.
• There is an exponential relationship between temperature and vapor pressure.
• Raoult's Law: $P<em>{\text{solvent}} \cdot X</em>{\text{solvent}} = P_{\text{solution}}$
• $X<em>{\text{solvent}} = \frac{n</em>{\text{solvent}}}{n<em>{\text{solvent}} + n</em>{\text{solute}}}$
• More solute dissolved in a solvent results in lower vapor pressure.
• Boiling: when the particles within the liquid exert a pressure just equal to the external pressure.
• The higher the pressure, the higher the boiling temperature.
• Boiling Point Elevation: $\Delta T<em>B = k</em>B \cdot m \cdot i$
  • $k = 0.52 \text{ C}$
  • "i" depends on the solute.
• Freezing Point Depression: $\Delta T<em>f = k</em>f \cdot m \cdot i$
  • $k = -1.86 \text{ C}$
  • Molality (m): $m = \frac{\text{moles of solute}}{\text{mass of solvent (kg)}}$

What to Study

• Solution process
• Saturated solutions
• Solubility
• Concentrations
• Factors affecting solubility
  • Surface area
  • Agitation (stirring a solution)
  • Temperature (to increase solubility of gases, temperature DECREASES)
  • Henry's Law (solubility is directly proportional to pressure)
• Solute-solvent interaction
• Parts of a solution
• Dilute and concentrated solutions
• Similar intermolecular forces
• Solids dissolve better in warmer temperatures and are not significantly affected by pressure