Chapter 13: Osmotic PressureChapter 13: Osmotic PressureChapter 14: KineticsChapter 14: Kinetics

But first- NOYCE

Osmosis is the movement of water Osmosis is the movement of water through a semi-permeable membranethrough a semi-permeable membrane

Osmotic pressure is used to Osmotic pressure is used to determine the molecular weight of determine the molecular weight of polymerspolymers

=cRT= osmotic pressure in atmc= concentration in mol/LR= 0.0821Latm/KmolT= temperature in Kelvin

12.5g of a cellulose derivative was dissolved in 1L of water. The osmotic pressure of the solution was 0.00210 atm at 30°C. What is the molar mass of this molecule?

Low levels of protein in blood cause Low levels of protein in blood cause edema due to osmotic pressure edema due to osmotic pressure differencesdifferences

CONTROL OF CONTROL OF REACTIONSREACTIONS

Thermodynamics tells us if the products are more/less stable than the reactants

Kinetics tells us if the reaction goes fast enough for us to notice

Back to our “organizational chart” Back to our “organizational chart” of what controls reactivity…of what controls reactivity…

Preview: Preview: The rate of a reaction The rate of a reaction is described by a rate equation is described by a rate equation (rate law)(rate law)

orderRT

E

reactantsAeRatea

Will an increase in Will an increase in temperature increase a temperature increase a reaction rate?reaction rate?

1 2

18%

82%1. Yes.2. No.

Will an increase in Will an increase in concentration increase a concentration increase a reaction rate?reaction rate?

1 2

53%

47%

1. Yes.2. No.

Will increasing the pressure Will increasing the pressure increase a reaction rate?increase a reaction rate?

1 2

25%

75%1. Yes.2. No.

Collision theory is a qualitative Collision theory is a qualitative explanation of how reactions occur explanation of how reactions occur and why rates differand why rates differReactions occur when

◦Molecules collide…

◦In the correct orientation…

◦With enough energy

Consider: NO + O3 NO2 + O2

Molecules collide Bonds are formed and break

product molecules separate

What can control how fast a reaction What can control how fast a reaction occurs?occurs?

So, what controls the rate of a So, what controls the rate of a reaction?reaction?

Number of collisionsHow often they collide in a shape that

allows new bonds to formThe energy of the colliding reactant

molecules

We’ll consider dependence on:1. Concentration

a. Rate laws b. Concentration vs. time relationships

2. Temperature and activation energy3. Mechanism

Concentration DependenceConcentration Dependence

It makes sense that as concentration increases, the number of collisions per second will increase

Therefore, in general, as concentration increases, rate increases

But, it depends on which collisions control the rate

So, you can’t predict concentration dependence: it must be measured experimentally

TypesTypes of measured rates: of measured rates:

Rate over time:

Instantaneous rate:

Initial rate:

concentrationrate =

time

Example of rate measurement:Example of rate measurement:

Rate Laws Rate Laws (also called Rate Equations)(also called Rate Equations)

For the reaction: 2 N2O5 4 NO + O2

Rate = k[N2O5]

For the reaction: NO2 NO + ½ O2

Rate = k[NO2]2

For the reaction: CO + NO2 CO2 + NO

Rate = k[CO][NO2]

first order reaction

second order reaction

first order in CO and in NO2; second order overall

Determining a Rate LawDetermining a Rate Law

Determining the rate law must be done by experiment; the reaction equation does not tell you the rate law

Two methods: Initial Rates and the Graphical Method

Method of Initial Rates• Measure the rate of the reaction right at

the start.• Vary the starting concentrations• Compare initial rates to initial

concentrations

Determining a Rate Law: Initial Rate MethodDetermining a Rate Law: Initial Rate Method

Isolation of variables: Vary only one concentration at a time and keep temperature constant

If concentration doubles and:◦ Rate does not change, then zero order◦ Rate doubles, then first order◦ Rate quadruples, then second order

General Rule:

Initial Rate Method: Example 1Initial Rate Method: Example 1

What is the rate law?

Initial Rate Method: Example 2Initial Rate Method: Example 2

Concentration-Time Concentration-Time RelationshipsRelationships

-ktt o[R] = [R] e

Example 1Example 1

-ktt o[R] = [R] e

The decomposition of nitrous oxide at 565 oC,

2 N2O 2 N2 + O2

is second order in N2O. If the reaction is initiated with [N2O] equal to 0.108 M, and drops to 0.940 M after 1250 s have elapsed, what is the rate constant?

Example 2