13.1 - 13.7 Solutions

Solvent in aqueous solution (water or glucose)

Water, which is the majority component.

Solute in aqueous solution (water or glucose)

Glucose, which is the minority component.

Solution

A homogeneous mixture of two or more components.

Solubility

The amount of solute that dissolves in a given solvent; depends on polarity.

What will facilitate the formation of a solution

1. intermolecular forces present in both the solute and solvent

2. the energy differences is close to 0 where solvent-solute interactions are comparable or stronger than solvent-solvent and solute-solute interactions

   1. example: Salt (NaCl) in Water: Water molecules (polar) surround the Na⁺ and Cl⁻ ions, forming strong ion-dipole interactions that compensate for breaking NaCl's ionic bonds and water's hydrogen bonds.

Like dissolves like

Polar solvents dissolve polar/ionic solutes, while nonpolar solvents dissolve nonpolar solutes.

Endothermic Steps in solution formation

Separating solute and solvent particles requires energy. (+)

Exothermic Step in solution formation

Mixing solute and solvent releases energy. (-)

What are the steps to make a solution

1. separate solute particles from each other (+)

2. separate solvent particles from each other (+)

3. let solute and solvent particles interact so new bonds are forming (-)

Saturated Solution

Solute in dynamic equilibrium with undissolved solid; additional solute does not increase concentration.

Unsaturated Solution

Contains less solute than equilibrium; adding solute increases concentration.

Supersaturated Solution

Contains more solute than equilibrium allows; unstable, leading to precipitation of excess solute.

Effect of Temperature on Solubility - Solids

Solubility generally increases with temperature.

Effect of Temperature on Solubility - Gases

Solubility decreases with increasing temperature.

Henry's Law

Gas solubility is directly proportional to gas pressure above the liquid.

Sgas=kHPgas

• Sgas​: Solubility (M).

• kH: Henry’s law constant.

• Pgas​: Partial pressure of gas (atm).

Higher pressure = greater gas solubility

Entropy

Measure of energy dispersal/randomization.

Recrystallization Process

Create a saturated solution at high temperature; excess solid precipitates out upon cooling.

dilute solution

A solution that contains a small amount of solute relative to the solvent, resulting in low concentration.

concentrated solution

A solution that contains a large amount of solute relative to the solvent, resulting in high concentration.

molarity (M) =

(moles of solute) / (volume of solution L)

colligative properties

properties that depend on the number of solute particles in a solution, not their identity. Examples include boiling point elevation and freezing point depression.

What are 4 examples of colligative properties

• Vapor pressure lowering

• Freezing point depression

• Boiling point elevation

• Osmotic pressure

nonelectrolytes

substances that do not dissociate into ions in solution, maintaining their molecular structure.

1 mol dissolves into 1 mol of particles (e.g., sugar in water).

electrolytes

substances that dissociate into ions in solution, conducting electricity.

1 mol dissolves into more particles (e.g., NaCl forms 2 mol of ions: Na⁺ and Cl⁻).

Which has stronger colligative properties: electrolytes or nonelectrolytes

Electrolytes have stronger colligative properties than nonelectrolytes because they dissociate into multiple ions

vapor pressure

Pressure of gas above a liquid when vaporization and condensation are in dynamic equilibrium

Why Does Vapor Pressure Decrease?

• Nature's tendency to mix (greater entropy) causes solvent molecules to move toward the solution.

• Example: In a sealed container with a pure solvent and a concentrated solution:

  • Solvent vaporizes from the pure solvent.

  • Vapor condenses into the solution, diluting it.

• This results in a continuous transfer of solvent molecules, lowering the vapor pressure of the solution compared to the pure solvent.

Raoult’s Law

P_solution​= x_solvent ​* P_solvent

• P_solution​: Vapor pressure of the solution.

• x_solvent​: Mole fraction of the solvent. (only use moles not mass to calculate the mole fraction, as this ensures the accuracy of our vapor pressure predictions)

  • x_solvent is equal to solvent / (solvent + solute)

• P_solvent​: Vapor pressure of the pure solvent.

What is the relation between vapor pressure and moles of solute

As the number of moles of solute increases in a solution, the vapor pressure decreases due to the reduction in the mole fraction of the solvent, which is directly related to Raoult's Law.

What is the significance of Raoult’s Law

It describes how the vapor pressure of a solution is affected by the presence of a solute, allowing for predictions of vapor pressure changes based on solute concentration.

what is the relation of a volatile solvent and solute

They both contribute to the solution's vapor pressure.

Ideal solution (referencing Raoult’s Law)

Following Raoult’s law, solute-solvent interactions are similar to solute-solute and solvent-solvent interactions:

• PA=xAPA

• PB=xBPB

• Total pressure: Ptot=PA+PB

What is a nonideal solution

Stronger solute-solvent interactions: Reduces vaporization, lower vapor pressure than predicted.

Weaker solute-solvent interactions: Increases vaporization, higher vapor pressure than predicted.

What is freezing point depression

Adding a nonvolatile solute lowers the freezing point

What is the equation for the freezing point depression

• Equation: ΔT_f​ = m⋅K_f​⋅i

  • ΔT_f​: Change in freezing point.

  • m: Molality (mol solute/kg solvent).

  • K_f​ Freezing point depression constant (Kf​ for water: 1.86∘C/m).

  • i: Van't Hoff factor (number of particles the solute splits into).

  • THE ANSWER IS ALWAYS NEGATIVE

Boiling Point Elevation

Adding a nonvolatile solute raises the boiling point.

What is the equation for Boiling Point Elevation

• Equation: ΔT_b​=m⋅K_b​⋅ i

  • ΔTb: Change in boiling point.

  • m: Molality- mol/kg

  • Kb​: Boiling point elevation constant (Kb​ for water: 0.512∘C/m).

  • i: Van't Hoff factor, representing the number of particles the solute dissociates into.

Osmosis

The movement of solvent molecules through a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. (low → high concentration)

Osmotic Pressure Equation

π = iMRT, where π is osmotic pressure, i is the van 't Hoff factor, M is the molarity, R is the ideal gas constant, and T is the temperature in Kelvin.

Van’t Hoff Factor (i)

A factor that accounts for the number of particles into which a solute dissociates in solution, influencing colligative properties like boiling point elevation and osmotic pressure.

What is the discrepancy of the Van’t Hoff Factor

Caused by ion pairing: some cations and anions pair together, reducing the total number of free particles in solution.

• Complete dissociation is not achieved in practice.

Hyperosmotic Solutions

Solutions with higher osmotic pressure than a reference solution, typically causing water to move out of cells by osmosis.

hyposmotic solutions

Solutions with lower osmotic pressure than a reference solution, typically causing water to move into cells by osmosis.