folder/flashfiles/propOfSoln/colligative.html

Colligative Properties of Solutions -properties that depend only on the concentration of solute particles, but not on the type.  The solutes in these solutions are nonvolatile, which means it has little tendency to become a gas. They stay in solution as a liquid. These tend to be ionic substances or polar covalent, like salts and sugars In comparison:Volatiles have low boiling points and evaporate easily because they have weak, nonpolar bonding Ex: CO 2, methane CH 4, diatomic gases

Molecular vs. Ionic solutes:  Sugar C 12 H 22 O 11 (s) is molecular. When 1 mole of it dissolves in water, one mole of particles is produced in solution: C 12 H 22 O 11 (s)  C 12 H 22 O 11 (aq) 1 mol 1 mol  When 1 mole of an ionic substance dissolves in water, 2 or more moles of particles are produced: total moles NaCl (s)  Na + (aq) + Cl - (aq) 1 mol NaCl1 mol Na + 1 mol Cl - 2 CaCl 2 (s)  Ca 2+ (aq) + 2Cl - (aq) 1 mol CaCl 2 1 mol Ca 2+ 2 mol Cl - 3

A.Boiling point elevation - The presence of a nonvolatile solute (like sugar or salt) raises the boiling point of a solvent. Here’s why: The solute particles occupy sites on the surface. This prevents solvent from evaporating. The temperature must then be raised to bring the vapor pressure up to atmospheric pressure, which is what boiling is. Key: = solvent = solute +

 The greater the concentration of solute, the more the boiling point is raised.  1 mole of solute particles elevates the boiling point of 1000 g of water by about ½ 0 C (exact = C). Ex. By about how much will each of the following solutes raise the boiling point of 1000 g of water? a/ 342 g (1 mol) of C 12 H 22 O 11 (s)? b/ 32 g (1 mol) of CH 3 OH (methanol = molecular)? c/ 58.5 g (1 mol) of NaCl? d/ 111 g (1 mol) of CaCl 2 ? e/ 95.2 g (1 mol) of MgCl 2 ?

B. Freezing point depression - The presence of a nonvolatile solute (like sugar or salt) lowers the freezing point of a solvent. Here’s why: Normally, at freezing, the number of H 2 O’s joining an ice crystal to become solid equals the number of H 2 O’s leaving the crystal to join the liquid phase. Solute particles block liquid H 2 0’s from joining the ice, but allow ice H 2 0’s to join the liquid. More liquid forms, which means: it melts. To slow the rate of forming liquid, the temperature must be lowered. liquid H 2 O solid H 2 0 = solute

 The greater the concentration of solute, the more the freezing point is lowered.  1 mole of solute particles depresses the freezing point of 1000 g of water by almost 2 0 C (exact = C). Ex. About much will each of the following solutes lower the freezing point of 1000 g of water? a/ 342 g (1 mol) of C 12 H 22 O 11 (s)? b/ 58.5 g (1 mol) of NaCl? c/ 111 g (1 mol) of CaCl 2 ? d/ 95.2 g (1 mol) of MgCl 2 ? Which are the best to spread on icy roads in order to prevent ice from forming?

Ex. A mixture of ethylene glycol, C 2 H 6 O 2, is used as an antifreeze and a coolant in cars. A typical mixture contains about 18 moles of it in each 1000 g of solution. a/ What is the approximate freezing point of the solution? b/ What is the approximate boiling point of the solution? Ex. If NaCl costs 1.5x a sugar-based ice melt product, which is a better deal? Which is better for the environment?

In sum: boiling point freezing point Pure solvent: elevated boiling point depressed freezing point Solution:  More moles of solute, wider range of liquid state  Ionic substances produce more moles than molecular and so they produce a wider temp. range for the liquid. liquid

C. Osmosis – the process by which solvent molecules pass through a semipermeable membrane from a dilute solution into a more concentrated one Semipermeable membrane – solvent can pass through it, but solute particles cannot Solvent particles flow through the membrane from low to high concentration.

Osmotic pressure is the pressure that must be exerted in order to stop the flow of solvent. It depends only on the concentration of solute. Ex. Fluids move into and out of cells by means of osmosis Ex. Reverse osmosis is used to desalinate seawater. level rises until excess pressure stops the flow

Sports drinks are designed to: 1. make money; and 2. replenish solutes (electrolytes) to prevent cell damage due to osmotic pressure caused by drinking large amts of water.

Why is salt added to boiling water?