Chemistry on Freezing and Boiling Points

Freezing Point Depression and Boiling Point Elevation

• Freezing point depression is calculated using the equation ∆Tf = -iKfm.
• Boiling point elevation is calculated using the equation ∆Tb = iKbm.
• The molality is calculated by dividing the moles of solute by the kilograms of solvent.
• The change in temperature is calculated by multiplying the van't Hoff factor (i), the molal freezing point depression constant (Kf) or boiling point elevation constant (Kb), and the molality.
• For example, a 5.37 m solution of a non-electrolyte in water will experience a freezing point depression of -10.0 oC and a boiling point elevation of 2.7 oC.
• The final freezing point will be -10.0 oC and the final boiling point will be 102.7 oC.

Solubility

• The larger the gas, the more soluble it will be in water.
• Larger gas particles have a greater surface area, which allows them to interact more strongly with the water molecules.
• Polar organic molecules dissolve in water better than nonpolar organic molecules.
• Hydrogen bonding increases solubility.
• C–C and C–H bonds are not very polar, which makes them less soluble in water.
• Liquids that mix in all proportions are miscible.
• Liquids that do not mix are immiscible.
• Hexane is nonpolar and water is polar, so they are immiscible.

Biological Importance of Solubility

• Fat-soluble vitamins, like vitamin A, are nonpolar and readily stored in fatty tissue in the body.
• Water-soluble vitamins, like vitamin C, need to be included in the daily diet since they are not stored in the body.
• The solubility and polarity of molecules are important for biological processes.

Pressure Effects on Solubility

• The solubility of solids and liquids are not appreciably affected by pressure.
• The solubility of gases is affected by pressure.

Henry's Law

• The solubility of a gas is proportional to the partial pressure of the gas above the solution. - If you double the partial pressure of a gas over a liquid at constant temperature, there will be twice as many gas molecules in the liquid.
• The Henry's law constant is not affected by changes in pressure.
• The Henry's law constant is a proportionality constant that is unique to each gas and solvent system.

Van't Hoff Factor

• The van't Hoff factor (i) takes into account dissociation in solution.
• For example, a 1.00 m solution of NaCl in water will have a van't Hoff factor of 2, as NaCl dissociates into two ions (Na+ and Cl-) in solution.
• The exact amount that particles remain together is dependent on the concentration.

Azeotropes

• Azeotropes are solutions where the components exist in the same mole fraction in the liquid and vapor phase at a particular concentration.
• Once an azeotropic composition is reached, the samples cannot be separated further by distillation.
• Other physical methods are needed to separate azeotropic mixtures.

Osmosis

• Osmosis is the net movement of solvent molecules from a solution of low to high concentration across a semipermeable membrane.
• The applied pressure to stop osmosis is called osmotic pressure.
• Osmotic pressure is calculated using the equation π = iMRT, where:
  • π = osmotic pressure
  • i = van't Hoff factor
  • M = molarity
  • R = ideal gas constant
  • T = temperature in Kelvin.

Types of Solutions and Osmosis

• Isotonic solutions have the same osmotic pressure and solvent passes the membrane at the same rate both ways.
• Hypotonic solutions have lower osmotic pressure and solvent leaves this solution at a higher rate than it enters with.
• Hypertonic solutions have higher osmotic pressure and solvent enters this solution at a higher rate than it leaves with.

Osmosis and Blood Cells

• Red blood cells have semipermeable membranes.
• If red blood cells are stored in a hypertonic solution, they will shrivel as water leaves the cell; this is called crenation.
• If red blood cells are stored in a hypotonic solution, they will grow until they burst; this is called hemolysis.

This quiz covers concepts of freezing point depression and boiling point elevation in chemistry. You'll learn to apply formulas related to molality and understand the principles of solubility. Explore solutions and their effects on temperature changes.