Section 6.7—Properties of Solutions

1. Section 6.7—Properties of Solutions How do all those dissolved things affect the properties of the solvent (water)? • Objectives • Define vapor pressure • Explain how solutes affect vapor pressure, boiling point, and freezing point • Differentiate between the effect electrolytes vs. non-electrolytes have on these properties

2. What’s Vapor Pressure? Vapor pressure is the pressure exerted in a closed container by liquid particles that have escaped the liquid’s surface and entered the gaseous state.

3. Vapor Pressure & Temperature Temperature is a measure of the average kinetic energy of molecules. As temperature increases, the vapor pressure increases. To evaporate, molecules must break intermolecular forces—this requires a minimum amount of energy More molecules will have the minimum needed to evaporate from the liquid As temperature increases, the average energy of the molecules increase

4. Vapor Pressure of Solutions Only particles at the surface can evaporate. Looking down on the top of beaker: If a particle on the top layer has enough energy, it can break the IMF’s and evaporate. Once evaporated, it causes vapor pressure. Beaker with solvent only

5. What happens when a solute is added?

6. Vapor Pressure of Solutions • When a solvent is pure, its particles occupy the entire surface area. • When the solvent contains a solute, a mix of particles occupies the surface area. Fewer solvent particles are there, so fewer are able to enter the gaseous state. • The fewer the number of particles that can evaporate, the lower the vapor pressure.

7. How Solutes Affect Solvents • The vapor pressure of a solution is always less than the pure solvent.

8. When does something boil? Atmospheric pressure pushes down on the top of the liquid Molecules are gaining the energy to break intermolecular forces and become a gas Heat source (usually underneath) heats the molecules closest to it the fastest

9. When does something boil? When enough water molecules turn to gas and create as much pressure as the atmosphere is pushing down with, bubbles form throughout the liquid.

10. Boiling Points of Solutions Boiling occurs when vapor pressure of liquid = atmospheric pressure The temperature needs to be raised until the vapor pressure of the solution = atmospheric pressure Since solutions have lower vapor pressure than the pure solvent, the temperature must be raised in order to raise the vapor pressure.

11. How Solutes Affect Solvents • The vapor pressure of a solution is always less than the pure solvent. 2. The boiling point of a solution is always higher than the pure solvent. 3.

12. Freezing Point • Normally at water’s freezing point, water molecules do not have enough energy to overcome interparticle forces. Therefore, an organized crystal forms. (The water freezes.)

13. If solute particles are added, they interfere with the formation of these intermolecular forces. The liquid will not freeze at the normal freezing point. • The solution must be cooled further (additional energy is removed) before the solution will freeze.

14. How Solutes Affect Solvents • The vapor pressure of a solution is always less than the pure solvent. 2. The boiling point of a solution is always higher than the pure solvent. 3. The freezing point of a solution is always lower than the pure solvent.

15. Colligative Properties • The properties listed in the previous slide are what scientists call colligative properties. • Colligative properties are physical properties of solutions that are affected by the number of particles but not the identity of dissolved solute particles.

16. Colligative Properties • Recall that some compounds, called electrolytes, break up into their ions when dissolved in water. • A solute that is an electrolyte, then, has more particles (the ions) in a solution than does a solute that is a non-electrolyte.

17. Example • For every 10 sugar molecules added to water, for example, there will be 10 sugar molecules in the solution. • However, with electrolytes like NaCl, for every 10 units of NaCl put in water, there will be 20 ions in the solution. 10 NaCl  10Na+ + 10Cl-

18. Colligative Properties • Since colligative properties are affected by the number of particles in solution, electrolytes have a greater impact on them than non-electrolytes. • The more particles in a solution, the lower the vapor pressure, the higher the boiling point, and the lower the freezing point of the solution.

19. Practice: For each of the following, which solute would have the greatest effect on boiling point? Na2SO4 NaOH CH3OH KBr For each of the following, which solute would have the greatest effect on freezing point? Cu(NO3)2 MgCl2 Al(OH)3 C6H6