Solutions: Chapter 14

Solutions

Types of Solute-Solvent Interactions

Understanding the types of interactions involved in the solution process is essential for predicting solubility and solution behavior.

• Solute-solute interactions: Forces between solute particles that must be overcome for dissolution.

• Solvent-solvent interactions: Forces between solvent molecules that must be disrupted to accommodate solute particles.

• Solute-solvent interactions: Attractive forces between solute and solvent particles that stabilize the solution.

• Example: Dissolving NaCl in water involves breaking ionic bonds (solute-solute), hydrogen bonds (solvent-solvent), and forming ion-dipole interactions (solute-solvent).

Enthalpy Equations for Solution Formation

The enthalpy change of solution formation can be described by the sum of the enthalpy changes for each interaction.

• Equation: $\Delta H_{solution} = \Delta H_{solute} + \Delta H_{solvent} + \Delta H_{mix}$

• Definitions:

  • $\Delta H_{solute}$: Energy required to separate solute particles.

  • $\Delta H_{solvent}$: Energy required to separate solvent molecules.

  • $\Delta H_{mix}$: Energy released when solute and solvent mix.

• Application: Used to predict whether dissolution is endothermic or exothermic.

Saturated, Unsaturated, and Supersaturated Solutions

Solutions are classified based on the amount of solute dissolved relative to its solubility limit.

• Saturated solution: Contains the maximum amount of solute that can dissolve at a given temperature.

• Unsaturated solution: Contains less solute than the solubility limit.

• Supersaturated solution: Contains more solute than is normally possible; unstable and may precipitate.

• Example: A solution of sugar in water at room temperature can be saturated, unsaturated, or supersaturated depending on the amount of sugar added.

Phases of Solutions

Solutions can exist in different phases depending on the solvent and solute.

• Solid solutions: Alloys such as brass (copper and zinc).

• Liquid solutions: Saltwater, sugar water.

• Gaseous solutions: Air (mixture of gases).

Environmental Pollution and Solutions

Pollutants can dissolve in water, affecting aquatic life and water quality.

• Example: Dissolved oxygen is crucial for fish survival; pollutants can reduce oxygen solubility.

Henry's Law

Henry's Law quantifies the solubility of gases in liquids.

• Equation: $C = k_P P_{gas}$

• Definitions:

  • $C$: Concentration of dissolved gas

  • $k_P$: Henry's Law constant

  • $P_{gas}$: Partial pressure of the gas

• Application: Used to calculate oxygen solubility in water for aquatic environments.

"Like Dissolves Like" Principle

Solubility is influenced by the polarity of solute and solvent.

• Polar solutes dissolve in polar solvents; nonpolar solutes dissolve in nonpolar solvents.

• Example: Salt (ionic, polar) dissolves in water (polar); oil (nonpolar) does not.

Solution Formation and Oil Slicks

Formation of oil slicks is due to the immiscibility of oil (nonpolar) and water (polar).

• Application: Environmental impact and cleanup strategies.

Hydration and Lattice Energy

Hydration energy and lattice energy influence the solubility of ionic compounds.

• Lattice energy ($U$): Energy required to separate ions in a solid.

• Hydration energy: Energy released when ions are surrounded by water molecules.

• Equation: $\Delta H_{solution} = U + \Delta H_{hydration}$

Hydration Strength and Solubility

Hydration strength depends on ion size and charge.

• Smaller, highly charged ions have stronger hydration and are more soluble.

Solubility and Temperature

Solubility of solids and gases varies with temperature.

• Solids: Generally, solubility increases with temperature.

• Gases: Solubility decreases with increasing temperature.

Concentration Units

Several units are used to express solution concentration.

• Molarity (M): $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Molality (m): $m = \frac{\text{moles of solute}}{\text{kg of solvent}}$

• Percent by mass: $\% = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100$

• ppm, ppb: Used for very dilute solutions.

Colligative Properties

Colligative properties depend on the number of solute particles, not their identity.

• Vapor pressure lowering

• Boiling point elevation

• Freezing point depression

• Osmotic pressure

• Equation for boiling point elevation: $\Delta T_b = i K_b m$

• Equation for freezing point depression: $\Delta T_f = i K_f m$

• Equation for osmotic pressure: $\Pi = iMRT$

• Definitions:

  • $i$: van't Hoff factor (number of particles per formula unit)

  • $K_b$, $K_f$: Boiling/freezing point constants

  • $m$: Molality

  • $M$: Molarity

  • $R$: Gas constant

  • $T$: Temperature (K)

Raoult's Law

Raoult's Law describes vapor pressure lowering in solutions.

• Equation: $P_{solution} = X_{solvent} P^0_{solvent}$

• Definitions:

  • $P_{solution}$: Vapor pressure of solution

  • $X_{solvent}$: Mole fraction of solvent

  • $P^0_{solvent}$: Vapor pressure of pure solvent

Electrolytes and van't Hoff Factor

Electrolytes dissociate into ions, increasing the number of particles in solution.

• van't Hoff factor ($i$): Number of particles produced per formula unit.

• Example: NaCl ($i \approx 2$), CaCl$_2$ ($i \approx 3$).

Osmosis and Osmotic Pressure

Osmosis is the movement of solvent through a semipermeable membrane; osmotic pressure is the pressure required to stop this flow.

• Equation: $\Pi = iMRT$

• Example: Red blood cells in hypotonic or hypertonic solutions.

Preparation and Dilution of Solutions

Solutions are often prepared by diluting a concentrated stock solution.

• Equation: $M_1V_1 = M_2V_2$

• Steps:

  1. Calculate the volume of stock solution needed.

  2. Add solvent to reach the desired final volume.

Effect of Temperature on Solubility and Colligative Properties

Temperature affects solubility and the magnitude of colligative properties.

• Solubility: Most solids become more soluble at higher temperatures; gases become less soluble.

• Colligative properties: Depend on temperature and concentration.

Summary Table: Colligative Properties

Property	Equation	Effect
Vapor Pressure Lowering	$P_{solution} = X_{solvent} P^0_{solvent}$	Decreases with added solute
Boiling Point Elevation	$\Delta T_b = i K_b m$	Boiling point increases
Freezing Point Depression	$\Delta T_f = i K_f m$	Freezing point decreases
Osmotic Pressure	$\Pi = iMRT$	Pressure required to stop osmosis

Additional info:

• Some questions reference practical applications, such as environmental pollution and medical scenarios (e.g., IV solutions).

• Students should be able to convert between concentration units and recognize when insufficient information is given to solve a problem.

• Entropy concepts are important for understanding vapor pressure lowering in solutions.