Learning Target: Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Must Do: Agenda: Must Do WWD Demo Properties of solutions Colligative properties lab (freezing point depression)

Polar water molecules. Copyright© by Houghton Mifflin Company. All rights reserved. 3

The heating/cooling curve for water heated or cooled at a constant rate. Copyright© by Houghton Mifflin Company. All rights reserved.

Heats of Phase Changes Molar heat of vaporization,  H vap : the heat needed to vapourize one mole of a substance at its normal boiling point. Molar heat of fusion,  H fus : the heat needed to melt one mole of a substance at its normal melting point. Molar heat of sublimation,  H sub : the heat needed to sublime one mole of a substance from the solid phase to the gas phase (skips the liquid phase).  H sub   H vap +  H fus Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

As a phase change occurs, temperature remains constant Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Temperature and Phase Changes

Both liquid water and gaseous water contain H 2 O molecules. Copyright© by Houghton Mifflin Company. All rights reserved. 7

Microscopic view of a liquid near its surface. Copyright© by Houghton Mifflin Company. All rights reserved. 8

Figure 14.10: Behavior of a liquid in a closed container. Copyright© by Houghton Mifflin Company. All rights reserved. 9

Figure 14.11: (a) Measuring vapor of a liquid by using a simple barometer. (b) The water vapor pushed the mercury level down. (c) Diethyl ether shows a higher vapor pressure than water. Copyright© by Houghton Mifflin Company. All rights reserved. 10

Vapour Pressure Vapour pressure (p vap ): the pressure at which dynamic equilibrium is achieved in a closed container. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

As a phase change occurs, temperature remains constant Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd. Temperature and Phase Changes

Bubble expands as H 2 O molecules enter. Copyright© by Houghton Mifflin Company. All rights reserved. 13

The packing of Cl¯ and Na + ions in solid sodium chloride. Copyright© by Houghton Mifflin Company. All rights reserved. 14

Copyright© by Houghton Mifflin Company. All rights reserved. 15 Dissolving of solid sodium chloride.

Copyright© by Houghton Mifflin Company. All rights reserved. 16 Polar water molecules interacting with positive and negative ions of a salt.

9.4 Colligative Properties The presence of a solute affects some physical properties of the solvent. The physical properties are referred to as colligative properties.  Vapour Pressure (reduced)  Freezing Point (reduced)  Boiling Point (increased)  Osmotic Pressure (increased) The magnitude of the colligative property depends only on the concentration of the solute, not its identity. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Vapor Pressure Reduction A pure solvent in a closed system will reach a dynamic equilibrium between the liquid and vapour phases. The addition of a solute will decrease the vapour pressure because the solute molecules reduce the rate of escape of solvent molecules. Raoult’s Law quantifies this: Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Boiling and Freezing Points Boiling-point elevation – the increase in boiling point of a solvent by adding a solute. Freezing-point depression – the lowering of the freezing-point of a solvent by adding solute. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Freezing Point Depression where b is the molality (moles of solute/kg of solvent) K f is the cryoscopic constant of the solvent i is the van’t Hoff factor: e.g. i = 3 for Na 2 SO 4 because Na 2 SO 4 → 2 Na+ + SO 4 2- Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Molality Useful for applications where the temperature (thus volume) of a solution changes. Defined as the number of moles of solute divided by the mass of the solvent in kg Units are mol/kg Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Boiling Point Elevation K f is the ebullioscopic constant of the solvent. Chemistry, 2nd Canadian Edition ©2013 John Wiley & Sons Canada, Ltd.

Freezing Point Depression Calculate ΔT (freezing point depression) if 100g NaCl is dissolved in 1kg of water. ΔT = i K f m K f water 1.86

Boiling Point Elevation Return to Solutions Menu A solution will boil at a higher temperature than the pure solvent. This is the colligative property called boiling point elevation. The more solute dissolved, the greater the effect. An equation has been developed for this behavior. It is: ΔT = i K b m The temperature change from the pure to the solution is equal to two constants times the molality of the solution. The constant K b is actually derived from several other constants and its derivation is covered in textbooks of introductory thermodynamics. Its technical name is the ebullioscopic constant. The Latin prefix ebulli- means "to bubble" or "to boil." In a more generic way, it is called the "molal boiling point elevation constant." The constant i will be discussed below.ebullioscopic constant. These are some sample ebullioscopic constants: Substance K b benzene 2.53 camphor 5.95 carbon tetrachloride 5.03 ethyl ether 2.02 water 0.52 The units on the constant are degrees Celsius per molal (°C m¯ 1 ). There are some variations on the theme you should also know: 1) K m¯ 1 - the distance between a single Celsius degree and a Kelvin are the same. 2) °C kg mol¯ 1 - this one takes molal (mol/kg) and brings the kg (which is in the denominator of the denominator) and brings it to the numerator. This last one is very useful because it splits out the mol unit. We will be using this equation (or the freezing point) to calculate molecular weights. Keep in mind that the molecular weight unit is grams / mol. Another reminder: molal is moles solute over kg solvent.