1. A Model for Reaction Rates

2. Expressing Reaction Rates
Average Rate = quantity
time
The amount of increase or decrease depends on mole ratios
Units =Molarity/s or mol/Ls

3. Expressing Reaction Rates Cont.
As a reaction proceeds, there is a decrease in concentration of reactants and an increase in the concentration of the products
N H2  2NH3
N2 and H2 decrease in concentration over time, while NH3 increases in concentration over time.

4. Expressing Reaction Rates
Average reaction rate
Concentration M Concentration M1
t2 - t1
[Concentration]
time
Reaction rates must always be positive
If you get a negative reaction rate, change the sign to positive

5. The Collision Theory Summary
Reacting substances (atoms, ions, or molecules) must collide.
Reacting substances must collide with the correct orientation
Reacting substances must collide with sufficient energy to form the
activated complex.

6. Activation Energy
The minimum amount of energy that reacting particles must have to form the activated complex and lead to a reaction
Symbol: Ea
Direct influence on the rate of a reaction

7. Exothermic
inc energy
inc time

8. Endothermic
inc energy F
inc time

9. Calculating Average Reaction Rates
[Concentration]
time
Given H2 + Cl2 2HCl
Calculate the average reaction rate expressed in moles H2 consumed per liter per second
Calculate the average reaction rate expressed in moles Cl2 consumed per liter per second
0.025 – 0.060
5.00 – 0.00
= M/s
0.035 – 0.070
5.00 – 0.00
= M/s

10. Rate Law
An expression that relates rate of a reaction and reactant concentration
Rate of reaction depends on reactant concentrations
never includes products
Given: 2A + 3B 2C
In general, rate = k [A]x [B]y

11. Rate Constant Symbol: k (lowercase)
A constant specific and unique for every reaction
If k is large, products form quickly
If k is small, products form slowly

12. Order of reaction Exponents in rate law (x and y)
Determine how much the rate depends on the respective concentration(s)
Can ONLY be determined by experiment
Overall order of a reaction is the sum of all orders (x + y)

13. Example 1 2A + 3B 2C Skeleton Rate = k [A]x [B]y
Overall order of reaction (x + y)
If x=1, 1st order in A
If y=2, 2nd order in B
Overall order = 3rd order

14. Example 2 Write the skeleton rate law rate = k [H2O]x
2H2O(l) 2H2(g) + O2(g)
rate = k [H2O]x

15. Relationship of order and rate
Example 1: rate = k [A] 1st order
If concentration of A doubles, rate doubles
If concentration of A triples, rate triples
If concentration of A is halved, rate halves
Example 2: rate = k [A]2 2nd order
If concentration of A doubles, rate quadruples
If concentration of A triples, rate is 9 times as fast
**directly proportional
**exponentially proportional

16. Example 3: rate = k [A]0 0th order
Recall: anything raised to the 0th power = 1
If the concentration of A changes in any way NOTHING happens to the rate
Rate is not dependent on concentration
Rate = k