U1 S1 L4Catalysts and reaction mechanisms

Textbook Readingspage 477: Reaction Mechanisms and Catalysts pages 477- 478: Elementary Reactions pages 479-480: Proposing and Evaluating Reaction Mechanisms pages 481-482: Catalysts

Upon completion of this lesson, you will be able to:

• define a reaction mechanism as a series of elementary processes which determine the reaction rate and when added together result in an overall balanced equation.

• define an elementary process as a single step in a multi-step reaction mechanism

• define reaction intermediate as a substance which is produced by an elementary process and consumed by a subsequent elementary process in a multi-step reaction mechanism.

• define rate determining step as the slowest elementary process in a reaction mechanism or the step that determines the overall reaction rate.

• sketch a potential energy diagram for a given a reaction mechanism which shows an energy barrier for each step and the energy barrier of the rate determining step relative to that of all the other elementary processes.

• given a reaction mechanism with the rate determining step identified, determine all catalysts, intermediates, the overall balanced equation, and state the effect of changing a reactant concentration on the overall rate.

• state that a catalyst speeds up a chemical reaction by means of providing an alternate reaction mechanism ("alternate pathway").

Reaction Mechanisms• A reaction mechanism is a series of steps that make up a

chemical reaction.– The idea here is that a reaction proceeds in a series of two molecule

collisions. – In reactions that involve more than two reactant molecules the reaction

must occur in many steps.

– Each step is called an elementary process

– in this example O is referred to as a reaction intermediate• the O is produced in one step and consumed in another.

Rate determining step

• The overall speed of the reaction is controlled by one step in the reaction this step is known as the rate determining step.

• Simply put the rate determining step is the slowest step in the mechanism.

• Each step in the mechanism has its own specific activation energy with the highest belonging to the slowest step

What is the overall equation?

(fast)

2 2 3

3 2 (fast)(fast)

o

sl wC A G

G R S MS C XS X W A

Catalysts• A catalyst was defined as something that speeds up a chemical reaction by

providing a reaction pathway with lower activation energy than that of the uncatalyzed reaction.

– Use of a catalyst often results in the replacement of a single high activation energy step with two or more lower activation energy steps.

– In a reaction mechanism a catalyst can be identified as a species that is consumed in the first step and then produced in another step, unchanged.

Ozone depletion mechanism

2 O3 (g) + uv light → 3 O2 (g)

Cl• + O3 (g) → ClO + O2 (g) (slow)

ClO + O3 (g) → Cl• + 2 O2 (g) (fast)

1) Which species is the intermediate?a) Cl•b) O3

c) ClOd) O2

2) Which species is the catalyst?

a) Cl•b) O3

c) ClOd) O2

3) Which step is the rate determining step?a) 1b) 2

Homework:

p 478 # 5 – 8 (answers p 487)

p 484 # 9-12

Consider these elementary processes for a reaction mechanism: a. Write the equation for the overall reaction.b. Identify the catalyst.c. Identify a reaction intermediate.d. The uncatalyzed overall reaction is a slow exothermic reaction.

Sketch a potential energy diagram for the uncatalyzed reaction and on the same diagram show the curve for the catalyzed reaction.

Consider this potential energy diagram for a catalyzed three-step reaction:a. Which arrow indicates the activation energy for the first step of the reverse

reaction?  

b. Which arrow indicates the activation energy for the first step of the forward reaction?  

c. Which arrow indicates the activation energy for the second step of the forward reaction? 

d. Which arrow indicates the enthalpy change (ΔH) or "enthalpy change" for the overall forward reaction?  

e. Which arrow indicates the enthalpy change (ΔH) or "enthalpy change" for the overall reverse reaction? 

f. Which arrow indicates the activation energy for the overall forward reaction? 

g. Which step is the rate determining step in the forward reaction?

• This reaction is fast at room temperature:

A student proposes this two-step mechanism for the above reaction:

• Do you agree or disagree with this proposed mechanism? Why?

Homework

Textbook Readingspage 477: Reaction Mechanisms and Catalysts pages 477- 478: Elementary Reactions pages 479-480: Proposing and Evaluating Reaction Mechanisms pages 481-482: Catalysts

  Textbook Practice Items

pages 478: items 5, 6, 7, and 8 pages 484-85: items 5, 6, 7, 8, 9, 10, 11, and 12 pages 486-487: items 8, 9, 10, 11, and 17 pages 538-541: items 2, 3, 23, 29, 35, 36, and 37