Chapter 5: Solutions and Their Properties – General Chemistry Study Notes

Solutions

Definition and Components

A solution is a homogeneous mixture composed of a solute (minor component) dissolved in a solvent (major component). The solute is uniformly distributed within the solvent. An aqueous solution is one where water is the solvent.

• Solute: The substance being dissolved (e.g., salt in salt water).

• Solvent: The substance doing the dissolving (e.g., water in salt water).

• Concentration: Indicates how much solute is dissolved in the solvent.

• Unsaturated solution: Less than the maximum solute dissolved.

• Saturated solution: Maximum solute dissolved; excess remains undissolved.

• Supersaturated solution: More solute dissolved than normally possible, often by heating.

• Dynamic equilibrium: Rate of dissolution equals rate of precipitation in a saturated solution.

Solubility

Solubility is the ability of a substance to dissolve in a solvent, especially water. Different salts have varying solubility in water, governed by solubility rules.

• Soluble salts: Group 1 metal cations, NO3-, ClO4-, C2H3O2-, NH4+

• Insoluble salts: Ag+, Pb2+, Hg22+, OH-, S2-, CO32-, PO43-

• Rule: Soluble component generally trumps insoluble component.

Electrolytes

Types of Electrolytes

Electrolytes are substances that dissociate into ions in water, conducting electricity. They are classified as:

• Strong electrolytes: Fully dissociate (soluble ionic compounds, strong acids, strong bases).

• Weak electrolytes: Partially dissociate (weak acids and bases).

• Non-electrolytes: Do not dissociate (e.g., glucose, methanol).

Solubility and Phase Rules

Temperature and Pressure Effects

• Solids: More soluble at higher temperatures.

• Gases: More soluble at lower temperatures and higher pressures.

Polarity and Intermolecular Forces

Polarity

Polarity arises from differences in electronegativity and molecular geometry. Polar molecules have a positive and negative end, while non-polar molecules have an equal distribution of charge.

• Polar molecule example: Water (H2O) – oxygen is partially negative, hydrogens are partially positive.

• Non-polar molecule example: Hydrogen (H2).

Intermolecular Forces

• London Dispersion Forces: Weakest, arise from temporary dipoles.

• Dipole-Dipole Interactions: Medium strength, occur between polar molecules.

• Hydrogen Bonding: Strongest, occurs when H is bonded to N, O, or F.

"Like Dissolves Like" Principle

• Polar solvents dissolve polar solutes.

• Non-polar solvents dissolve non-polar solutes.

• Polar and non-polar substances do not mix.

Classification of Matter

Types of Matter

Matter is classified as pure substances (elements and compounds) or mixtures (homogeneous and heterogeneous).

• Element: One type of atom (e.g., Na).

• Compound: Two or more elements chemically combined (e.g., NaCl).

• Homogeneous mixture: Uniform composition (e.g., air, solutions).

• Heterogeneous mixture: Non-uniform composition (e.g., salad).

Concentration Calculations

Molarity (M)

Molarity is moles of solute per liter of solution.

• Formula: $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

Molality (m)

Molality is moles of solute per kilogram of solvent. Used when temperature changes, as mass is unaffected by temperature.

• Formula: $m = \frac{\text{moles of solute}}{\text{kilograms of solvent}}$

Dilution

Dilution reduces the concentration of a solution by adding more solvent.

• Formula: $M_1V_1 = M_2V_2$

Types of Solutions

• Diluted: Low solute concentration.

• Concentrated: High solute concentration.

• Saturated: Maximum solute dissolved; excess visible.

Normality (N)

Normality is the concentration in terms of equivalents per liter of solution, used for acids and bases.

• Formula: $N = n \times M$

Mole Fraction (X)

Mole fraction is the ratio of moles of one component to total moles in solution.

• Formula: $X_A = \frac{\text{moles A}}{\text{total moles}}$

Colligative and Non-Colligative Properties

Non-Colligative Properties

• Surface tension: Resistance to external force due to cohesion.

• Viscosity: Resistance to flow.

• Solubility: Ability to dissolve.

• Volatility: Ease of evaporation.

• Color & Density: Depend on identity of molecules.

Colligative Properties

• Vapor-pressure depression: Lower vapor pressure in solution than pure solvent.

• Boiling point elevation: Higher boiling point in solution.

• Freezing point depression: Lower freezing point in solution.

• Osmotic pressure: Pressure needed to stop osmosis.

Vapor-Pressure Depression

• Raoult’s Law: $P_A = X_A P_A^\circ$

• Vapor pressure and boiling point are inversely related.

Boiling Point Elevation

• Formula: $\Delta T_b = k_b i m$

• Van’t Hoff factor (i): Number of particles solute dissociates into.

Freezing Point Depression

• Formula: $\Delta T_f = -k_f i m$

• Negative sign indicates decrease in freezing point.

Osmotic Pressure

• Formula: $\Pi = i M R T$

• Osmosis: Water moves from low to high solute concentration across a semipermeable membrane.

Net Ionic Equations

Molecular, Ionic, and Net Ionic Equations

• Molecular equation: All compounds written as molecules.

• Ionic equation: Ionic compounds written as separate ions.

• Spectator ions: Ions not involved in the reaction.

• Net ionic equation: Only shows ions and compounds directly involved in the reaction.

Example:

• Molecular: NaCl(aq) + AgNO3(aq) ⇌ NaNO3(aq) + AgCl(s)

• Ionic: Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq) ⇌ Na+(aq) + NO3-(aq) + AgCl(s)

• Net ionic: Cl-(aq) + Ag+(aq) ⇌ AgCl(s)