1. Rate Orders

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

3. Rate vs Rate Law
A + B C
The rate of the reaction is proportional to changes measured in A, B and C
Just watching how [A] changes is a good indication of how fast the reaction proceeds, but it doesn’t take into account how reactant B might be affecting things as well

4. Rate Law is an expression that shows the effects of all reactants on reaction rate simultaneously.
Rate = k[A]x[B]y
where: k - rate constant
[A]/[B] - reactant concentrations
x/y - reaction orders
A rate law is used to calculate the rate of a reaction not just a single reactant

5. The rate constant (k) is a calculated value that is unique for each reaction at a specific temperature.
A + B C
Since temperature affects rate (remember collision theory),
the constant is a way to account for this influence:
changing the temperature changes the value of k
Like temperature: each reactant affects the rate differently
Rate = k[A]x[B]y
The orders of a reaction (x and y) indicate how much each [reactant] affects the rate of a reaction.
The rate constant (k) and the order (x/y) can only be determined experimentally

6. Trial
[A] (mol/L)
[B] (mol/L)
Initial Rate (mol/Ls) 1
0.10
0.20
2.0 2
0.30
18.0 3
0.40
16.0
Note: Data tables like this will be used to calculate the constant (k) and orders (x/y) of the rate law
“Trial” means this experiment was done three separate times, each time varying concentrations to see what effect each reactant had on the overall rate

7. Calculating Orders

8. What change in the rate is seen when [A] is tripled?
Determining rate orders (using initial rates method):
Measure the effect of changes in concentration of one reactant on rate, while
keeping the other reactant constant.
What change in the rate is seen when [A] is tripled?
Trial
[A] (mol/L)
[B] (mol/L)
Initial Rate (mol/Ls) 1
0.10
0.20
2.0 2
0.30
18.0 3
0.40
16.0
same
Analyzing the changes across two trials
Can be done with math or in your head
4 possible outcomes for the order (x/y)

9. Rate = k[A] [B] 1 x y 1. First order reaction ( x = 1)
The change in reactant concentration produces a directly proportional change in reaction rate.
Trial
[A] (mol/L)
[B] (mol/L)
Initial Rate (mol/Ls) 1
0.10
0.20
2.0 2
4.0
same
When [A] is doubled ~ the rate doubles 21 = 2
When [A] is tripled ~ the rate triples 31 = 3 x y
Rate = k[A] [B] 1

10. Rate = k[A] [B] 2 y x 2. Second order reaction ( x = 2)
The change in reactant concentration produces a proportional squared change in reaction rate.
Trial
[A] (mol/L)
[B] (mol/L)
Initial Rate (mol/Ls) 1
0.10
0.20
2.0 2
0.30
18.0
same
[A] is doubled ~ the rate quadruples = 4
[A] is tripled ~ the rate increases 9x = 9 x y
Rate = k[A] [B] 2

11. Rate = k[A] [B] 3 y x 3. Third order reaction ( x = 3)
The change in reactant concentration produces a proportional cubed change in reaction rate.
Trial
[A] (mol/L)
[B] (mol/L)
Initial Rate (mol/Ls) 1
0.10
0.20
2.0 2
16.0
same
[A] is doubled ~ the rate increases 8x = 8
[A] is tripled ~ the rate increases 27x = 27 x y
Rate = k[A] [B] 3

12. *Zero-order reactants are NOT included in rate law
4. Zero order reaction ( x = 0)
The change in reactant concentration produces
NO change in reaction rate.
Trial
[A] (mol/L)
[B] (mol/L)
Initial Rate (mol/Ls) 1
0.10
0.20
2.0 2
0.40
same
[A] is doubled ~ the rate doesn’t change = 1
[A] is tripled ~ the rate doesn’t change = 1 x y
Rate = k
[A]
[B]
*Zero-order reactants are NOT included in rate law

13. 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