1. Reaction Mechanics

2. OUTCOME QUESTION(S): C12-3-10 RATE MECHANICS Vocabulary & Concepts
Explain the concept of a reaction mechanism.
Include: rate determining step, intermediates, and
catalysts
Determine the reaction orders and rate law of a chemical reaction from experimental data.
Explain the scientific process connecting a chemical reaction to its experimental rate law, and to the prediction of an appropriate reaction mechanism.
Include: connecting the rate law to the RDS
Vocabulary & Concepts
elementary reaction complex reaction

3. Are chemical reactions really as simple as the equation suggests?
2 NO (g) + O2 (g) 2 NO2 (g) +
Potential
Energy
Kinetic Energy
Kinetic Energy
Kinetic Energy

4. Three particle collisions are quite rare.
2 NO (g) + O2 (g) 2 NO2 (g)
For this reaction to occur in one step:
3 particles must collide - at exactly the same time with the correct orientation and enough energy.
Three particle collisions are quite rare.
Consider how hard it would be to time the collision of three pool balls, rolled with different speeds, to strike each other at the same exact moment their numbers faced inward…
Reactions typically proceed as a series of simpler steps each consisting of bimolecular collisions.

5. 2 NO (g) + O2 (g) 2 NO2 (g) Elementary reaction:
A simple reaction that takes place in one step.
Complex reaction:
A multi-step reaction made of elementary steps.
The reaction mechanism is an outline of the series of elementary steps that make the overall reaction.
In baking, the recipe (mechanism) explains how (steps) to make the cake (overall reactions)
2 NO (g) + O2 (g) NO2 (g)

6. Sum of the steps of the mechanism equals the overall or net equation.
2 NO (g) + O2 (g) NO2 (g)
N2O2 - is called a reaction intermediate:
A compound produced in one step that is required as a reactant of a subsequent step.
Do not confuse intermediate with the activated complex that occurs during every collision as reactants form products

7. Neither intermediates nor catalysts appear in the Net Reaction
The overall reaction is created by cancelling identical reactants and products found across the reaction arrow
Decomposition of ozone, with chlorine catalyst.
1: Cl2 + O ClO + O2
2: O O2 + O
3: ClO + O Cl2 + O2
2 O3 (g) O2 (g)
ClO and O – intermediates
Cl2 – catalyst
Neither intermediates nor catalysts appear in the Net Reaction

8. NO2 + CO NO + CO2 Rate Determining Step
Each elementary step has its own EA needs and proceeds at its own rate.
The step with the slowest rate (greatest EA) is called the rate determining step (RDS):
1: NO2 + NO2 NO3 + NO (slow)
2: NO3 + CO NO2 + CO2 (fast)
RDS
NO2 + CO NO + CO2

9. NO2 + CO NO + CO2 Rate Determining Step 1: NO2 + NO2 NO3 + NO (slow)
RDS has the greatest affect on the overall rate and so determines the overall “speed” of a complex reaction.
Changes to the reactants in the other steps will have very little effect on the overall rate.
1: NO2 + NO2 NO3 + NO (slow)
2: NO3 + CO NO2 + CO2 (fast)
RDS
NO2 + CO NO + CO2
Consider  [CO]: increasing the rate of the second step has little effect overall, as the reaction still waits for the slow step

10. 1 2 1: NO2 + NO2 NO3 + NO (slow) 2: NO3 + CO NO2 + CO2 (fast)
endothermic
exothermic
-ΔH overall
Creating a PE diagram for a complex reaction consists of combining the diagrams of each elementary step 1 2

11. A + 2B C 1. B + B E fast endothermic 2. E + A C slow endothermic ΔH3 4
A B C
+ ΔH 5
The diagram must contain all the information that is provided in the question – and be labeled 3
EA2 4 PE
(kJ)
products 1 2 ΔH 5
EA1
reactants
Reaction Coordinates

12. a) What is the net reaction?
P Q X T (slow, endo)
X P Y R (fast, exo)
Y S T (moderate, exo)
overall
- ΔH
a) What is the net reaction?
a) 2P + Q + S  2T + R
b) What are the reaction intermediates?
b) X + Y
c) First Step
c) Which is the rate determining step?
d) What would be the effect of increasing [P]?
d) Increases the overall rate (it’s in the RDS)
e) Decreases the overall rate (it’s in the RDS)
e) What would be the effect of decreasing [Q]?
f) What would be the effect of increasing [S]?
f) No significant overall change in rate
g) What would a PE graph look like for this reaction?

13. P + Q X + T (slow, endo) X + P Y + R (fast, exo)
Y S T (moderate, exo)
overall
- ΔH
g) What would a PE graph look like for this reaction?
EA2
EA1 PE
(kJ)
EA3
reactants ΔH
products
Reaction Coordinates

14. RECAP:
Most reactions occur in steps - mechanism
Each step is usually bimolecular
Sum of these steps must equal the net equation
Rate determining step (RDS) is the slowest step
and has the greatest effect on the overall rate

15. CAN YOU / HAVE YOU? C12-3-10 RATE MECHANICS Vocabulary & Concepts
Explain the concept of a reaction mechanism.
Include: rate determining step, intermediates, and
catalysts
Determine the reaction orders and rate law of a chemical reaction from experimental data.
Explain the scientific process connecting a chemical reaction to its experimental rate law, and to the prediction of an appropriate reaction mechanism.
Include: connecting the rate law to the RDS
Vocabulary & Concepts
elementary reaction complex reaction