Outline: 2/9/07 Physics Seminar – today @ 3pm Exam 1 – one week from today…

Outline Finish Chapter 15 - Kinetics: - Arrhenius Rate equation & temperature dependence

k = A e Ea/RT

k = Rate law constant A = constant (frequency factor) Ea = Arrhenius Activation Energy

R = 8.315 J/mol K T = Temperature

k also depends on temperature and Arrhenius Activation energy….

15-6

What does Ea mean in terms of rate? k = A e Ea/RT

e.g. big Ea

e (big)

= small k = small rate!

e.g. small Ea

e (small)

= big k = big rate!

The temperature dependence of a rate of reaction is used to determine the Activation Energy:

Example: (eq. 15-8 on page ) Where does this come from?

Algebra …. Rate2 = k2 = A e Ea/RT2

Rate1 = k1 = A e Ea/RT1

divide these equations

21

1

2

11

ln

TT

kk

R

Ea

Algebra …. Rate2 = k2 = A e Ea/RT2

Rate1 = k1 = A e Ea/RT1

divide these equations

21

1

2

11

ln

TT

kk

R

Ea

21

1

2

11

TTR

E

kk

a

e

An example: The colorless gas N2O2 decomposes to the

brown gas NO2 in a first-order reaction. The rate constant k = 4.5 103 s at 274 K and 1.0 104 s at 283 K. What is the activation energy Ea?

Use the correct equation: (eq. 15-8) ln(k2/k1) = Ea/R [1 /T2 1/T1 ]

ln(4.5/10) = Ea/8.315 [1 /283 1/274 ]

Ea= 57205 J = 57.2 kJ Example 15-12 - p. 651

How to study for this exam?

Practice lots of examples….Practice lots of examples….

Problems to try: Chapter 15

15.13, 15.17, 15.19, 15.21, 15.23, 15.13, 15.17, 15.19, 15.21, 15.23, 15.25, 15.27, 15.29, 15.31, 15.33, 15.25, 15.27, 15.29, 15.31, 15.33, 15.39, 15.45, 15.47, 15.51, 15.57, 15.39, 15.45, 15.47, 15.51, 15.57, 15.59, 15.61, 15.63, 15.65, 15.67, 15.59, 15.61, 15.63, 15.65, 15.67, 15.69, 15.73, 15.77, 15.81, 15.85, 15.69, 15.73, 15.77, 15.81, 15.85, 15.87, 15.89, 15.91, 15.95, 15.10315.87, 15.89, 15.91, 15.95, 15.103

Practice Problems: Chapter 14

14.11, 14.15, 14.17, 14.19, 14.23, 14.25, 14.11, 14.15, 14.17, 14.19, 14.23, 14.25, 14.27, 14.31, 14.35, 14.37, 14.38, 14.41, 14.27, 14.31, 14.35, 14.37, 14.38, 14.41, 14.43, 14.49, 14.51, 14.53, 14.55, 14.57, 14.43, 14.49, 14.51, 14.53, 14.55, 14.57, 14.61, 14.65, 14.67, 14.71, 14.75, 14.77, 14.61, 14.65, 14.67, 14.71, 14.75, 14.77, 14.79, 14.81, 14.91, 14.101, 14.10314.79, 14.81, 14.91, 14.101, 14.103

Problems to try: Chapter 13

13.1, 13.3, 13.5, 13.7, 13.9, 13.11, 13.1, 13.3, 13.5, 13.7, 13.9, 13.11, 13.15, 13.19, 13.21, 13.35, 13.43, 13.15, 13.19, 13.21, 13.35, 13.43, 13.45, 13.47, 13.4913.45, 13.47, 13.49

Problems to try: Chapter 12 12.3, 12.5, 12.7, 12.9, 12.11, 12.17, 12.19, 12.3, 12.5, 12.7, 12.9, 12.11, 12.17, 12.19,

12.29, 12.31, 12.33, 12.35, 12.39, 12.41, 12.29, 12.31, 12.33, 12.35, 12.39, 12.41, 12.43, 12.49, 12.51, 12.53, 12.55, 12.6312.43, 12.49, 12.51, 12.53, 12.55, 12.63

Activation Energy

• The rearrangement of methyl isonitrile:

– The energy required for the above twist and break is the activation energy, Ea.

– Once the C-N bond is broken, the NC portion can continue to rotate forming a C-CN bond.

What exactly is EWhat exactly is Eaa??

H3C N CC

NH3C H3C C N

• The E for the reaction is the difference in energy between CH3NC and CH3CN.

• The activation energy is the difference in energy between reactants, CH3NC and transition state.

• The rate of reaction depends on Ea.

k = A e Ea/RT

Activation EnergyActivation Energy

How can a catalyst affect the rate of reaction?

It lowers the Ea ….

And Ea appears in the exponent!!!

k = A e Ea/RT

A catalyst lowers Ea

DEMO: Catalytic Converters

http://www.randomtechnology.com/technical.html

• change the rate of a chemical reaction

• there are two types of catalyst:

– homogeneous (all in same phase)

– heterogeneous (in different phases)

• typically lower the Ea for a reaction

CatalystsCatalysts

• Enzymes are biological catalysts.• Most enzymes are proteins with large

molar masses (104 106 amu).

• Enzymes have very specific shapes and catalyze very specific reactions.

Another Example of Catalysis

• Only substrates that fit into the enzyme lock can be involved in the reaction.

• If a molecule binds tightly to an enzyme so that another substrate cannot displace it, then the active site is blocked and the catalyst is inhibited (enzyme inhibitors).

• The number of events catalyzed (turnover number) is large for enzymes: 103107 sec-1.

EnzymesEnzymes

Quiz #4

Please put away your books and notes….Please put away your books and notes…. When you are finished, you may turn it When you are finished, you may turn it

in to me and leave…in to me and leave…

Have a great Winter Break!