1chemistry 2c lecture 22: may 21 th, 2010 1)arrhenius equation 2)transition state theory...

1Chemistry 2C Lecture 22: May 21th, 2010

1) Arrhenius Equation2) Transition State Theory

3) Molecularity4) Rate limiting steps

5) Reaction mechanisms6) Catalysis

7) Nuclear Introduction

Lecture 22: Kinetics


Temperature Effects on Reaction Rate

“Normal” Catalyzed reaction(inactivation of catalyst)

rare Chain reaction(Explosion)

Not every reaction follows the Arrhenius equation. But most!


Activated Complex-Transition States

The potential energy of the system increases at the transition state because: 1) The approaching reactant molecules must overcome the mutual repulsive

forces between the outer shell electrons of their constituent atoms 2) Atoms must be separated from each other as bonds are broken

In the transition state theory, the mechanism of interaction of reactants is not considered; the important criterion is that colliding molecules must have

sufficient energy to overcome a potential energy barrier (the activation energy) to react.


Activated Complex-Transition StatesThis increase in potential energy corresponds to an energy barrier

over which the reactant molecules must pass if the reaction is to

proceed. The transition state occurs at the maximum of this energy

barrier.

The transition state is an unstable transitory combination

of reactant molecules that occurs at a potential energy maximum

The combination can either go on to form products or fall apart

to return to the unchanged reactants.

The energy difference between the reactants and the potential energy maximum is referred to as the activation energy: Ea or

G‡


Energy barrier from reactants to

products (forward direction)

Reaction Profile

Energy barrier from products to reactants

(reverse direction)

G for reaction

Since G is negative, this is a spontaneous reaction, although its timescale for occurring is dictated by the energy barrier


Arrhenius EquationsHow is Ea measured?

The rate constant is a function of temperature, but Ea is considered to be a constant and depends only on thermodynamics

k=Ae-Ea/RT

This is the form of a line!

RT

EAk

Aek

A

RT

EA

lnln


Arrhenius EquationsHow is Ea measured (measure two rates at two different temperatures?

ln k2 = –Ea/RT2 + ln A

@ T1

@ T2

ln k1 = –Ea/RT1 + ln A

Subtract the two equations:

ln k1 - ln k2 = –Ea/RT1 –Ea/RT2

ln (k1 / k2) = –Ea/R (1/T1 -1/T2)

Can just substitute into this equation! Make sure temperature is in Kelvin!


Chemical Kinetics Molecularity of a Reaction

The reaction order refers to the concentration dependence of the reaction rate and can be an integer or a non-integer and even negative!

The molecularity of a reaction refers to a definite molecular encounter during the course of the reaction. The molecularity has to be an integer

(there are no partial atoms/molecules!)

Unimolecular Reactions: One reactant molecule undergoes transformation into the product(s). Examples are racemizations, thermal

decomposition, or isomerizations.

Bimolecular Reactions: Two reactant molecules collide in one elementary step. Most common type of reaction molecularity.

Termolecular reactions: Three reactants have to collide to lead to an reaction. Extremely rare.

No reactions of molecularities higher than 3 are known.


Reaction MechanismA mechanism describes in detail exactly what takes place at each stage of a chemical transformation. It describes the transition state and which bonds

are broken and in what order, which bonds are formed and in what order, and what the relative rates of the steps are. A complete mechanism must also

account for all reactants used, the function of a catalyst, stereochemistry, all products formed and the amount each.

For example:

CO + NO2 → CO2 + NO

In this reaction, it has been experimentally determined that this reaction takes place according to the rate law R = k[NO2]2. Therefore, a possible

mechanism by which this reaction takes place is:

2 NO2 → NO3 + NO (slow)

NO3 + CO → NO2 + CO2 (fast)

Each step is called an elementary step, and each has its own rate law and molecularity. All of the elementary steps must add up to the original reaction.


Intermediates & Rate Determining StepsReactions may have intermediate species that don’t show up in the final

reaction equation, but play a role in the mechanism. Kindah like electrons in REDOX reactions.

For example: 2 NO2 (g) + F2 (g) → 2NO2F (g)

The first step is slow (k1<<1) and the second step is fast (k2>>1), the rate determining step

NO2 + F2 → 2NO2F + F

Overall Rx.

Step one: k1

F + NO2 → NO2F Step two: k2

rate = k[NO2][F2]

The experimentally determined rate law is:


NO2 + F2 NO2F + F

F + NO2 NO2F

k1

k2

Slow

Fast

2NO2 + F2 2NO2F

rate = k[NO2][F2] according to the elementary reaction, with a rate limiting step

Intermediates & Rate Determining Steps

Each of these steps is an elementary process. That means that those two species must collide for a reaction to occur. In this

example, each step is bimolecular.


Energy Diagram for a two-step Reaction

Reactants -> transition state -> intermediateIntermediate -> transition state -> product


Chemical Kinetics: Enzymes and Catalysis

General principles of catalysis:

• A catalyst works by lowering the Gibbs energy of activation. This enhances the rate of forward and backward reaction.

• The catalyst forms an intermediate with the reactant(s) in the initial step of the reaction and is released in during product formation.

• A catalyst can not affect the enthalpies or the Gibbs energies of the reactants and products.It increases the rate of the approach to equilibrium, but can not change the change the equilibrium constant.


Chemical Kinetics: Enzymes and Catalysis

General principles of catalysis:

Uncatalyzed Reaction Catalyzed Reaction


Types of Catalysis

• Homogeneous Catalysis– The interaction of reactants and catalysts in the

same phase.– e.g., CFC’s (gas/gas)

• Heterogeneous Catalysis– The interaction of reactants and catalysts in

different phases.– e.g., catalytic converters (solid/gas)

Enzymes in the body are biological catalysts!


Reaction Profile for Enzyme

The catalysed reaction pathway goes through the transition states TSc1, TSc2 and TSc3, with an energy barrier Gc*, whereas the uncatalysed reaction goes through the transition state TSu

with a barrier of Gu*. In this example the rate limiting step would be the conversion of ES into EP.


Reaction Profiles

The catalysis of H2O2 decomposition by Br-

2H2O2 -> 2H20 + O2 Overall net Rx.

When Br- is added, the reaction goes

2Br- + H2O2 +2H+ -> Br2 + 2H20

Br2 + H2O2 -> 2Br- + 2H+ + O2

Step 1

Step 2

Br2 is an intermediate because it is produced and then consumed!

Br- is a catalyst because it speeds up the reaction and is neither and is unchanged during the net reaction!

1chemistry 2c lecture 22: may 21 th, 2010 1)arrhenius equation 2)transition state theory...

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