lecture three [dissolution & colligative properties] | notes

Define solute and solvent. Give examples for air and saltwater.

Solute = smaller amount (O₂ in air, NaCl in saltwater). Solvent = larger amount (N₂ in air, H₂O in saltwater). The solution's phase is usually the same as the solvent's.

What are the three key characteristics of a solution?

1) Homogeneous mixture. 2) Components dispersed on molecular scale. 3) Composition/concentration can vary; solute won't separate out.

What two criteria favor spontaneous solution formation?

1) Decrease in internal energy (ΔU decreases, i.e., exothermic). 2) Increase in dispersal of matter (disorder/entropy increases) – this is always true.

What are the three types of IMFs involved in dissolution?

1) Solute-solvent, 2) Solute-solute, 3) Solvent-solvent.

Why might a solution NOT form?

Not enough energy for steps 1 and 2 (solute-solute separation and solvent-solvent separation are endothermic) compared to step 3 (solvation is exothermic).

KNO₃ dissolved in water makes the solution colder. Is this endothermic or exothermic? What does it indicate about IMFs?

Endothermic (absorbs energy from water, cooling it). Conclusion: solute-solute interactions are STRONGER than solute-solvent interactions.

What is an electrolyte? Give examples.

A substance that dissociates into ions when dissolved, conducting electricity. Examples: strong electrolytes (NaCl, HCl), weak electrolytes (acetic acid).

How do ionic vs. covalent compounds behave as solutes?

Ionic compounds typically dissociate into ions (electrolytes). Covalent compounds usually remain as molecules (non-electrolytes, except acids).

Determine if two liquids are soluble or insoluble. What is the key rule?

"Like dissolves like" – polar solvents dissolve polar/ionic solutes; nonpolar solvents dissolve nonpolar solutes.

Define molality (m). How does it differ from molarity (M)?

Molality (m) = moles solute / kg solvent. Molarity (M) = moles solute / L solution. Molality is temperature-independent (mass doesn't change with T); molarity changes with T because volume changes.

Convert between: mass fraction → molality → molarity. What information do you need?

Mass fraction → molality: need molar mass of solute. Molality → molarity: need density of solution (to convert kg solvent + mass solute → total volume).

How does increasing temperature affect solubility of most solids? Of gases?

Solids: solubility usually INCREASES with temperature. Gases: solubility DECREASES with increasing temperature.

How does pressure affect solubility of gases in liquids?

Increasing pressure INCREASES gas solubility (Henry's Law: C = k·P, where C = concentration, k = Henry's constant, P = partial pressure).

What are colligative properties? Name the four main ones.

Properties that depend ONLY on the number of solute particles, not their identity. 1) Vapor pressure lowering, 2) Boiling point elevation, 3) Freezing point depression, 4) Osmotic pressure.

Calculate vapor pressure lowering for a solution. What is Raoult's Law?

Raoult's Law: P_solution = χ_solvent × P°_solvent. Vapor pressure lowering ΔP = P°_solvent - P_solution = χ_solute × P°_solvent.

What is the equation for boiling point elevation? Direction of change?

ΔT_b = i·K_b·m. Boiling point INCREASES (solution boils at higher T than pure solvent).

What is the equation for freezing point depression? Direction of change?

ΔT_f = i·K_f·m. Freezing point DECREASES (solution freezes at lower T than pure solvent).

K_b and K_f are constants. What do they depend on?

They depend ONLY on the solvent, not the solute.

What is the van't Hoff factor (i)? Give i for NaCl and CaCl₂.

i = moles of particles in solution / moles of formula units dissolved. NaCl → i ≈ 2 (Na⁺ + Cl⁻). CaCl₂ → i ≈ 3 (Ca²⁺ + 2Cl⁻).

How do you use colligative property data to calculate molar mass of an unknown compound?

Use ΔT = i·K·m → solve for m = (ΔT)/(i·K). Then m = moles solute / kg solvent → moles solute = m × kg solvent. Finally, molar mass = (mass solute in g) / (moles solute).

For a non-electrolyte, i = ? For an electrolyte that doesn't fully dissociate, i is what?

Non-electrolyte: i = 1. Electrolyte less than 100% dissociated: i is between 1 and the theoretical maximum (e.g., for NaCl, i between 1 and 2).

At extremely low temperatures, would a salt with a higher or lower van't Hoff factor (i) be more effective at de-icing? Why?

HIGHER van't Hoff factor (i) is more effective. ΔT_f = i·K_f·m → higher i gives greater freezing point depression per mole of salt, so ice melts at lower temperatures.

Why is CaCl₂ often preferred over NaCl for de-icing roads in very cold regions?

CaCl₂ has i ≈ 3 (vs. NaCl i ≈ 2), producing more particles per mole → greater freezing point depression → effective at lower temperatures.

Write all four colligative property equations with the van't Hoff factor included.

• Vapor pressure lowering: ΔP = i·χ_solute·P°_solvent

• Boiling point elevation: ΔT_b = i·K_b·m

• Freezing point depression: ΔT_f = i·K_f·m

• Osmotic pressure: Π = i·M·R·T

(M = molarity, R = gas constant, T = Kelvin)