1. Rate Law and Mechanics
A B C
2 B E (slow)
E A C (fast)

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

3. aA + bB cC + dD rate = k[A]a[B]b
Remember from before - Elementary reaction:
A simple reaction that takes place in one step.
For reactions that occur in a single step (elementary):
The order of each reactant in the rate law is equal to the coefficient in the balanced equation.
aA + bB cC + dD
rate = k[A]a[B]b

4. 2 H2O(l) → H3O+(aq) + OH-(aq) O3(g) + NO(g) → NO2(g) + O2(g)
Determine a rate law for the following elementary reactions:
The self-ionization of water:
2 H2O(l) → H3O+(aq) + OH-(aq)
Smog – a reaction of ozone and nitrogen monoxide:
O3(g) + NO(g) → NO2(g) + O2(g)
rate = k[H2O]2
rate = k[O3][NO]

5. With its own energy requirements and elementary rate law
Each step in a mechanism is elementary.
With its own energy requirements and elementary rate law
A B C
1. 2 B E (slow)
2. E A C (fast)
Rate1 = k[B]2
Rate2 = k[E][A]
RateOverall = ?
But the overall rate law for a complex reaction must be determined experimentally…

6. A + 2B C 1. 2 B E (slow) 2. E + A C (fast) RateOverall = k[B]2
Rate1 = k[B]2
Rate2 = k[E][A]
Trial
[A] mol/L
[B] mol/L
Rate (mol/Lmin) 1
1.0
0.50 2
3.0 3
2.0 2
RateOverall = k[B]2
The overall rate law matches the information of the rate determining step.

7. A + 2B C 1. E (slow) 2 B 2. E + A C (fast) Rate = k[B]2 WHY?
We already determined:
RDS has the greatest effect on the overall rate.
Changes to the reactants in the other steps will have no effect.
E (slow)
2 B
2. E A C (fast)
A B C
Rate = k[B]2
Reactants not found in the RDS (E and A)
- Have no measurable effect on rate
- Would be zero order (and not appear in the rate law)
The RDS is the only step that affects rate; the rate law is an equation of the only reactants that affect rate…
– they have to match –

8. Putting it all together
Chemists first determine the rate law experimentally though trials ... THEN deduce a RDS and a hypothetical mechanism of how the reaction could occur.
A chemist creates a reaction by putting reactants in a beaker…after some time all that is left is nitryl fluoride –microscopic molecular collisions can’t be viewed, so how might the reaction proceed?
2 NO F NO2F

9. 2 NO2 + F2 2 NO2F Rate = k[NO2][F2] RDS: NO2 + F2 
Analysis of trial data has shown the rate law to be:
Rate = k[NO2][F2]
The RDS must have NO2 and F2 – using coefficients (1) that match the first orders.
RDS: NO2 + F2 
The RDS and the rate law must display the same message of what is important

10. 2 NO2 + F2 2 NO2F The proposed mechanism is then:
Rate = k[NO2][F2]
Of course, the proposed steps must add up to the net equation for the mechanism to be acceptable
Step 1: NO2 + F NO2F + F- (RDS)
Step 2: NO2 + F NO2F
2 NO F NO2F
Chemists would now stop the reaction early to look for the intermediate (F-) to prove that this mechanism is correct – if not found, a new mechanism is proposed.

11. NO2 + CO  NO + CO2 Reaction rate data has shown the rate law to be:
Rate = k[NO2]2
The proposed mechanism is might be:
Step 1: NO2  NO3 + NO (RDS)
Step 2: NO3 + CO  NO2 + CO2
You will be asked to make the connection between rate law and the RDS – but not asked to create a mechanism
NO2 + CO  NO + CO2

12. 3 M + N P + 2 Q
If the experimental rate law was determined to be:
Rate = k[M]2
a) What would be the effect of doubling the [N]?
b) What would be the effect of tripling the [M]?
c) What is the stoichiometry of the RDS?
a) No significant effect (zero order – not in RDS)
b) 9x increase (2nd order – proportional squared)
c) RDS: 2M 

13. A + B C Rate = k[A][B] If the rate law for this reaction is:
What does this evidence suggest?
If the rate law already matches the coefficients of the overall reaction – then this is the ONLY step and it is an elementary reaction.

14. 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