Reaction Mechanismsand Catalysts

(p.26)

Intro to Rxn Mechs (p.26t)• In order for rxns to occur, molecules must collide. • Three molecules colliding simultaneously is 1000

times less likely than two molecules. Four or more molecules colliding simultaneously is practically impossible.

• A reaction like:5 C2O4

2- + 2 MnO4- + 16 H+ 10 CO2 + 2 Mn2+ + 6 H2O

can’t react in one step because there are 23 reactant molecules which couldn’t possibly collide simultaneously.

• Therefore, anything more complicated than a 2 reactant reaction MUST take place in more than 1 step.

• The series of steps that make up a reaction is called the “reaction mechanism”

• You will never be asked to predict a mechanism. Each rxns mechanism is found by years of research.

Intro to Rxn Mechs (p.26t)

Example of a Mechanism (p.26b)4 HBr + O2 2 H2O + 2 Br2 has the following mechanism:

(dotted lines are where bonds are breaking or forming)

Step 1: found to be slow

Step 2: found to be fast

Step 3: found to be fast

(happens twice)

Notes (p.27m)• Each step is called an elementary process• The slowest step is called the rate-determining

step (Step 1 in previous ex.)• If “frozen”, very little HOOBr or HOBr are present• To speed up a rxn, add a reactant to speed up the

rate-determining step.• The overall rxn is found by adding all steps.• HOOBr and HOBr are called “reaction

intermediates” or just “intermediates”• An activated complex’s formula is found by adding

the formulas of the reactants.

Energy diagrams of Rxn Mechs

• What would an energy diagram of this 3 step rxn look like? How would it show that step 1 is the rate-determining (slow) step?

• Each step is shown as a separate hump on a diagram.

• Each step has its own Ea.

• Step 1 has the highest Ea b/c it is the slowest.

• Each step can be endo or exothermic.

Energy diagrams of Rxn Mechs (p.29t)

Step Activation Energies (p.30t)

• It is not correct to show Step 2’s Ea as going from the original reactants to the top of its peak

• The overall Ea goes from the original reactants to the top of the HIGHEST peak.

Catalysts (p.30b)

• Catalysts speed up rxns but this is insufficient for a definition. We go with:

• A catalyst is a substance which provides an overall reaction with an alternative mechanism having a lower activation energy.

Diagram of a Catalyzed Rxn (p.31t)• A catalyst makes a new pathway of lower

Ea but does not alter the H of the rxn.

• The catalyzed rxn always has at least two humps. (Why?)

• A catalyst will increase both forward and reverse rxn rates. (Why?)

• Uncatalyzed pathway is still available.

Example of a Catalyst (p.32t)• The rxn: H2 + O2 H2O has many steps.

• When a catalyst is added there are 2 steps as shown below.

• The rxn: OCl- + I- OI- + Cl- occurs very slowly in one step. Therefore, its PE diagram looks as follows:

Example of a Catalyst #2 (p.32b)

• When a catalyst (water in this case) is added the new mechanism has 3 steps but occurs faster. (Why faster?)

Example of a Catalyst #2 (p.33t)

Catalyzed PE Diagram (p.33m)• The following represents the PE diagram

of this catalyzed rxn.

• Looking at this diagram, what is the:– fastest step? – rate-determining step?

– overall Ea

Notes About Catalyzed Rxns (p.33b)

• A catalyst is part of a rxn but is used up in one step and regenerated in a later step.

• The H is the same for catalyzed or not.• Both rxn intermediates and catalysts

cancel out when individual steps are added so they never appear in overall rxn.

• Intermediates get produced and then used up in a subsequent step.

• Catalysts get added and then reproduced in a subsequent step.

Example (p.33t)

• Find the overall rxn, rxn intermediates and catalysts for this rxn mechanism.

Uses of Catalysts- Catalytic Converters (p.36t)

• Catalytic Converters are used in vehicles to reduce harmful emissions.

• They are part of the exhaust system.

• Catalytic converters convert CO, unburned hydrocarbons and nitrogen oxides into CO2, H2O and N2.

Uses of Catalysts- Catalytic Converters (p.36t)

• The catalyst is Pt, Pd and Rh based and is attached to a honeycomb material to maximize surface area

Homework

• Hebden #46-49, 52, 55, 56 odds, 57, 62 (read part c) on p.35).

• Unit I Test on Friday.