TOPIC D:ARRHENIUS EQUATIONAND CATALYSTS

Arrhenius• Said the reaction rate should increase with temperature.• At high temperature more molecules have the energy

required to get over the barrier.• The number of collisions with the necessary energy

increases exponentially.

Arrhenius• Number of collisions with the required

Energy = ze-Ea/RT

• z = collision frequency• e is Euler’s number (opposite of ln)• Ea = activation energy• R = ideal gas constant (8.314 J/K·mol)• T is temperature in Kelvin

•e-Ea/RT represents the fraction of collisions with energy

>Ea at temperature T.

Problems• Observed rate is less than the number of collisions that

have the minimum energy.• Due to Molecular orientation• written into equation as p the steric factor.

ON

Br

ON

Br

ON

Br

ON

Br

O N Br ONBr ONBr

O NBr

O N BrONBr No Reaction

Two Requirements for successful collision

• The collision must have enough energy to produce the reaction [ >Ea ].

• The relative orientation of the reactants must allow formation of any new bonds.

• K = zpe-Ea/RT z = collision freq.

• P = steric factor (fraction of collisions with the correct orientation)

•k = zpe-Ea/RT = Ae-Ea/RT

• A is called the frequency factor = zp

• This equation is in your packet:

• ln k = -(Ea/R)(1/T) + ln A

• Another line !!!!

• ln k vs 1/t is a straight line

• Slope m = -Ea / R

Arrhenius Equation

Ex 12.7 Determining Activation Energy

• The reaction: 2N2O5 (g) → 4NO2 (g) + O2 (g)

was studied as several temperatures, and the following values of k were observed.

K (s-1) T (oC)

2.0 x 10-5 20

7.3 x 10-5 30

2.7 x 10-4 40

9.1 x 10-4 50

2.9 x 10-3 60

Calculate the value of Ea for this reaction.

Catalysts• Speed up a reaction without being used up in the reaction.• Enzymes are biological catalysts.• Homogenous Catalysts are in the same phase as the

reactants.• Heterogeneous Catalysts are in a different phase as the

reactants.

How Catalysts Work

• Catalysts allow reactions to proceed by a different mechanism - a new pathway.

• New pathway has a lower activation energy.• More molecules will have this activation energy.• Do not change E

Lowering of Activation Energy by a Catalyst

Catalysts Increase the Number of Effective Collisions

Pt surface

HH

HH

HH

HH

Heterogenous Catalysts

• Hydrogen bonds to surface of metal.

• Break H-H bonds

Pt surface

HH

HH

Heterogenous Catalysts

C HH C

HH

Pt surface

HH

HH

Heterogenous Catalysts• The double bond breaks and bonds to the catalyst.

C HH C

HH

Pt surface

HH

HH

Heterogenous Catalysts• The hydrogen atoms bond with the carbon

C HH C

HH

Pt surface

H

Heterogenous Catalysts

C HH C

HH

H HH

Homogenous Catalysts• Chlorofluorocarbons catalyze the decomposition of ozone.• Enzymes regulating the body processes. (Protein

catalysts)

Catalysts and rate• Catalysts will speed up a reaction but only to a certain

point.• Past a certain point adding more reactants won’t change

the rate.• Zero Order

Catalysts and rate.

• Rate increases until the active sites of catalyst are filled.

• Then rate is independent of concentration

Concentration of reactants

Rate