Nyb F09 Unit 2 Slides 1 25

• The number of solute particles, not the type, changes the properties of a solution, called colligative properties.

3. Freezing point depression

1. Vapour pressure lowering

2. Boiling point elevation

4. Changes in osmotic pressure

Colligative Properties (Ch. 12.7)

Inverse Relationship

• Vapour Pressure: The pressure exerted by a vapour at equilibrium with its liquid in a closed system (Ch. 11)

Pure liquidwater

Water vapourEvaporation

Vapour Pressure Lowering

Water withdissolved, non-volatile solute

• Solutes interfere with and reduce the ability of the water molecules at the surface of the solution to escape into the gas phase.

1. There are simply less water molecules at the surface of the solution that can escape into the gas phase compared to pure water.

2. The solute molecules interact more strongly with the water molecules, “holding” them back from evaporating into the gas state.

Vapour Pressure as a Function of Temperature

Temperature

Vap

our

Pre

ssur

e

1 atm

Pure Solvent

Solution

• Boiling point is defined at the point where the vapour pressure of a substance equals the external pressure, usually that of the atmosphere.

- Since solutes lower vapour pressure, a higher temperature is required for a solution to equal the external pressure

Boiling Point Elevation

ΔTb = Kb . m . i

• To calculate boiling temperature elevation ΔTb :

- where Kb is the boiling point elevation constant (for water it is 0.512oC/m) and m is the molality of the solution.

- i is the vant Hoff factor. It represents the stoichiometry of particles produced when a compound dissolves in solution under ideal conditions

Eg. Sucrose has an i factor of 1 NaCl has an i factor of 2 MgCl2 has an i factor of 3

Tbsolution = Tbsolvent + ΔTb

Boiling pointchange (elevation)Boiling point of

solutionBoiling pointof solvent(usually water100oC)

• The boiling point of a solution will be higher than that of the pure solvent:

Freezing Point Depression

25oCcool

5oC 0oC -10oC

Molecules close and tumbling about

Molecules closer with less movement

Freezing Point:Some water as solid, some as liquid

All solid

• For pure water:

25oCcool

5oC 0oC

Molecules close and tumbling about

Molecules closer with less movement

Solute gets inthe way of water

-10oC

Still slushy

• With solute in water:

ΔTf = Kf . m . i

• To calculate freezing temperature depression:

- where Kf is the freezing poing depression constant (for water it is 1.86oC/m) and m is the molality of the solution.

Tfsolution = Tfsolvent – ΔTf

Freezing pointchange (depression)

Freezing pointof solvent(usually water 0oC)

Freezing pointof solution

• The freezing point of a solution will be lower than that of the pure solvent:

Osmotic Pressure

• Osmosis: the spontaneous movement of water molecules through a semipermeable membrane from an area of low concentration to high concentration of solute

Net movement of water

- Isotonic: same concentration as body fluid

- Hypertonic: more concentrated than body fluid

- Hypotonic: less concentrated than body fluid

π = MRTi

• To calculate osmotic pressure:

- Where π is osmotic pressure, M is concentration (molarity), R is the universal gas constant (8.314 kPa.L.mol-1.K-1), T is the temperature in Kelvin, and i is the vant Hoff factor.

Technically, colligative property calculations apply only in solutions that are “ideal”. This means that the solution cannot be overly concentrated in solute.

Phase Diagram of Solvents (Water)

Molar Mass Determination From Colligative Properties

1. 2.00 g of a salt is dissolved in 15.00 ml of water (assume density is 1.00 g/ml). This salt solution freezes at -3.6oC. What is the identity of the salt? (Kf = 1.86oC/m)

a) BaCl2 or b) MgCl2

208 g/mol 95.3 g/mol

2. 1.158 grams of a non-dissociating protein is dissolved in 50.00 ml of water. This protein solution generates an osmotic pressure of 2.09 kPa at 25oC. What is the molar mass of the protein?

R = 8.314 kPa.L/mol.K oK = oC + 273.15

Kinetics (Ch. 13)

• Kinetics is the study of the change in concentrations of the reactants or products over time, or in other words, the rate of the reaction.

Time (s)

Speed(km/h)

200

150

100

50

250

Car Acceleration Profiles

Car A

Car B

Collision Theory/Model

• The particles involved in a reaction must physically collide with each other with enough energy AND in the proper orientation for a reaction to occur

• Reactions require that the particles/molecules collide in the correct orientation.

• Factors That Affect the Rate of a Reaction:

1. The concentration of reactants: If we have more particles of A and B, it will increase the chance of them colliding together, so the greater the rate of the reaction

2. The physical state (surface area): Certain physical states allow for greater mixing of the reactants, thus increasing rate of reaction

3. Temperature: Not all collisions result in a reaction. The energy of the collision must surpass the activation energy.

4. Catalysts: A substance that increases the rate of the chemical reaction but is not consumed by the reaction. Catalysts can be homogeneous or heterogeneous.

A

B

Reaction Progress Diagram

• Since catalysts are not consumed during the reaction, they are typically written above the reaction arrow.

A catalyst increasesthe rate of a reactionby lowering the Ea

ΔHrxn = enthalpy of reactionEa = activation energy

C2H4(g) + H2(g) C2H6(g)Ni

Eg.

• Expressing Reaction Rate (of Reactants and Products)

- Rate is defined as a change in some variable per unit of time.

- If we measure the change in concentration of A over time, we can determine its reaction rate.

Take the reaction A B

=

[ ], square brackets are short-hand notation for concentration

= Δ[A]

Δt

[A]2 - [A]1

t2 – t1

There is a minus signsince A is disappearingin the reaction

change in [A]

change in time

- Alternatively, we could have also measured the rate of change in B. Since B is being produced, its change is positive

Reaction A B

Finally, in this particular example. Δ[B]

Δt

Δ[A]

Δt=

Since reactants disappear, the change is negativeSince products are produced, the change is positive.

Δ[A]

ΔtTherefore rate = And is measured in

units of M/time (mol.L-1.time-1)

• For any general reaction of say

aA + bB cC + dD

- Where the italicized letter is the stoichiometric coefficient

- The rate expression of all reactants and products would be:

Rate = Δ[B]

Δt

Δ[A]

Δt=

Δ[D]

Δt

Δ[C]

Δt=1

a1b

= 1c

1d

C2H4(g) + O3(g) C2H4O(g) + O2(g)- For the reaction:

- The concentration of ozone (O3) is monitored over time,

at 10.0 second intervals.

• Some Real Kinetics Data and Graphing

Measuring thedisappearanceof O3(g)

Slope of a lineon this graph is also equal to the rate

-Average Rate: rate over a period of time

-Instantaneous Rate: rate at one time