Unit 8: Solutions & Reactions Notes

Solutions

• Solution: A homogeneous mixture of a solvent (the substance doing the dissolving) and solute (the substance being dissolved).
• Examples:
  • Solute: sugar
  • Solvent: water

Types of Solutions

• Gas/Gas: Example - Air
• Gas/Liquid: Example - CO2 in H2O
• Liquid/Liquid: Example - Alcohol in water
• Liquid/Solid: Example - Mercury in silver (dental amalgam)
• Solid/Solid: Example - Brass (zinc and copper)

Aqueous Solutions

• Aqueous Solutions: Solutions where water is the solvent.
• Properties:
  • Smaller ions attract more water molecules, slowing movement compared to larger ions due to higher effective nuclear charge.

Electrolytic Solutions

• Electrolytic Solutions: Consisting of ionic compounds dissolved in a polar solvent, typically water.
• Conduct electricity due to dissociated ions (electrolytes).
• Higher ion concentration leads to higher electrical conductivity.

Classifying Electrolytes

• Strong Electrolytes: Complete dissociation (e.g., strong acids, bases, and soluble salts).
• Weak Electrolytes: Partial dissociation (e.g., weak acids, weak bases).
  • Examples: Ammonia (NH3), weak acids that do not belong to the strong category.

Solubility and Solubility Product

• Solubility: Ability of a substance to dissolve in a solvent.
• Ksp: Solubility Product Constant indicates how soluble a salt is in water.
• Solubility Standards:
  • > 0.1M: Soluble
  • 0.001M - 0.1M: Slightly Soluble
  • < 0.001M: Insoluble

Factors Affecting Rate of Dissolving

• Surface Area: Increased surface area speeds up dissolution.
• Agitation: Stirring or shaking increases contact between solute and solvent.
• Temperature: Higher temperatures increase solubility, especially for solids; may decrease for gases.
• Pressure: Mainly affects gases - increasing pressure increases solubility in liquids.

Saturation Levels of Solutions

• Unsaturated: Can still dissolve more solute.
• Saturated: Maximum amount of solute dissolved at specific temperature.
• Supersaturated: Contains more solute than normally possible; can crystallize upon disturbance.

Concentration of Solutions

• Concentration: Quantity of solute per quantity of solvent.
• Formulas:
  • Molarity (M): moles of solute/volume of solution (L)
  • Molality (m): moles of solute/mass of solvent (kg)
  • Mass % = mass of compound/total mass of solution
  • Mole Fraction = moles of specific substance/total moles in solution

Dilutions

• Dilution Formula: M1V1 = M2V2
  • M1: initial molarity
  • V1: initial volume
  • M2: final molarity
  • V2: final volume
• Process: Always add acid to water, not the other way around.

Colligative Properties

• Properties that depend on the number of solute particles:
  • Vapor Pressure Reduction: Lower vapor pressure of solution compared to pure solvent.
  • Boiling Point Elevation: Higher boiling point; calculated as ΔTb = Kb * m.
  • Freezing Point Depression: Lower freezing point; calculated as ΔTf = Kf * m.
  • Osmotic Pressure: Pressure needed to stop solvent movement through a semipermeable membrane. Higher solute concentration leads to higher osmotic pressure.

Summary of Key Equations

• Boiling Point Elevation: ΔTb = Kb * m (where Kb=0.512°C/m for water)
• Freezing Point Depression: ΔTf = Kf * m (where Kf=1.86°C/m for water)
• For ionic compounds, effective molality is multiplied by the number of ions produced.
• Sample Calculation: To find the boiling point of a solution, calculate the amount of elevation based on solute concentration and Kb value.