Chemistry 212

1. Background: Kinetics vs. Thermodynamics: See also (CGWW CH 13: pp. 305-323) and the Lab Discussion Handout for Exploration 8:

1. Thermodynamics deals with the  for a reaction.

The extent of the reaction => How Far the Reaction will Go.
 

Keq

=

 is a state function, that is, it depends only upon the initial and final conditions and not on the path that the reaction follows. Thus,  will predict the amounts of reactants and products present at equilibrium, but not how fast they are consumed or produced.

So far, all reactions with which we have dealt have been thermodynamically controlled.

2. Kinetics: Deals with the rates of reactions.

How Fast the Reaction Proceeds

The rate will depend upon the variation of the energy of the reactants as they proceed toward products.

The Rate Depends Upon the Reaction Path (Mechanism).

a. ONE STEP RXN
 

=

The transition state, the highest energy point along the most favorable reaction path.

In General, the rxn rate will decrease as the difference in free energy between the reactants and the (DG) increases.

= Free Energy of Activation = G- G^Reactants

b. MULTISTEP REACTIONS:

• Each step has a
• Each intermediate product (e.g. C or D) occurs at a minimum in the energy curve.
• The "slow" step in the reaction is the one with the highest energy
• (In the example it is step 2). This step is often termed the rate determining step (RDS.).
• The for the reaction is the energy difference between theof the RDS and the reactants of the initial step (In the example, = G ² - (G^A + G^B).

c. Causes of

• As the is approached, bonds are broken and made and the molecular structures are deformed.
• Before the , the energy increase, due to bond breaking and molecular distortion, is more important than the energy decrease, due to bond making.

The total energy increases.
• After the , the energy decrease, due to the making of new bonds, is more important than the energy increase, due to bond breaking.

The total energy decreases.

d. Predicting Relative Rxn. Rates:

• Since the rate will depend upon the =G- G^Reactants, knowledge or estimates of the relative free energies of the reactants and the 's can provide predictions of the relative rates of reactions.

• Relative energies of Reactants:

These values are predicted, as in equilibrium controlled reactions, from relative abilities of molecules to decrease the energy of their highest energy electrons.
• Relative Energies of 's:

What do the's look like and how can their relative energies be estimated?

 
 

The is part way between reactants and products of the rxn step.

Bonds to be broken are partially broken and bonds to be made are partially made (----- indicates a partial bond). Also, electrons to be transferred are partially transferred (= partial charge). Energies of these partial lone pairs of electrons or partial bonds are proportional to those of full lone pairs of electrons or bonds. Thus, a partial lone pair of electrons has lower energy on an oxygen atom than it has on a nitrogen atom, but the difference in energy between the two partial lone pairs is smaller than with two full lone pairs because there is less electron density to be affected.
 
 

For each of the following pairs of reaction steps:

1. Write a partially bonded structure for the reaction's 
2. By comparing the relative energies of the reactants and 's of the pair of reaction steps, predict which reaction step should occur faster and explain your reasoning.

 

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Class Group Activity # 21 - Out of Class Applications:

Class Group Activity # 21 - Summary of Class Discussion: