learning target: refine the design of a chemical system by specifying a change in conditions that...

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)

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Polar water molecules.

Copyright© by Houghton Mifflin Company. All rights reserved.

The heating/cooling curve for water heated or cooled at a constant rate.

Heats of Phase ChangesMolar heat of vaporization, DHvap: the heat needed

to vapourize one mole of a substance at its normal boiling point.

Molar heat of fusion, DHfus: the heat needed to melt one mole of a substance at its normal melting point.

Molar heat of sublimation, DHsub: the heat needed to sublime one mole of a substance from the solid phase to the gas phase (skips the liquid phase).

DHsub DHvap + DHfus


As a phase change occurs, temperature remains constant


Temperature and Phase Changes


Both liquid water and gaseous water contain H2O molecules.


Microscopic view of a liquid near its surface.


Figure 14.10: Behavior of a liquid in a closed container.


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.

Vapour Pressure

Vapour pressure (pvap): 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


Temperature and Phase Changes


Bubble expands as H2O molecules enter.


The packing of Cl¯ and Na+ ions in solid sodium chloride.


Dissolving of solid sodium chloride.


Polar water molecules interacting with positive and negative ions of a salt.

9.4 Colligative PropertiesThe 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.


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:

vap, solution A vap, Ap = X p


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.


Freezing Point Depression

where b is the molality (moles of solute/kg of solvent)

Kf is the cryoscopic constant of the solvent

i is the van’t Hoff factor:

e.g. i = 3 for Na2SO4 because Na2SO4 → 2 Na+ + SO42-


ffT = iK b

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

solutem

solvent

nc =

m


Boiling Point Elevation

Kf is the ebullioscopic constant of the solvent.


b bT = iK b

Freezing Point Depression

Calculate ΔT (freezing point depression) if 100g NaCl is dissolved in 1kg of water.

ΔT = i Kf m

Kf water 1.86

Boiling Point Elevation

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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 Kb m

The temperature change from the pure to the solution is equal to two constants times the molality of the solution. The constant Kbis 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.

These are some sample ebullioscopic constants:

Substance Kb 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.

http://www.chemteam.info/Solutions/Solutions.html

http://en.wikipedia.org/wiki/Ebullioscopic_constant

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