1. Rate Law

2. rate a [Br2]
rate = k [Br2]

3. The rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers.
aA + bB cC + dD
Rate = k [A]x[B]y
Reaction is xth order in A
Reaction is yth order in B
Reaction is (x +y)th order overall

4. Examples
Write the general rate law for the following chemical reactions
F2(g) + 2 ClO2(g)  2 FClO2(g)
Decomposition of liquid water into hydrogen and oxygen gas

5. Relationship between order and rate
A  B
rate = k[A]x
if x=1
Rate is directly proportional to [A]
If x=2
Rate varies as the square of [A]
If x=0
Rate is independent of [A]

6. Review of Concepts
A reaction is second order in A and first order in B. If the concentration of A doubles and the concentration of B is halved, overall what has happened to the rate?

7. Method of Initial Rates
F2 (g) + 2ClO2 (g) FClO2 (g)
rate = k [F2]x[ClO2]y
Double [F2] with [ClO2] constant
Quadruple [ClO2] with [F2] constant
Rate doubles
Rate quadruples
x = 1
y = 1
rate = k [F2][ClO2]
2nd order overall

8. Example 1
3A B
Trial
Initial [A] (M)
Initial Rate (M/s) 1
0.050
3.0 x 10 -4 2
0.10
12 x 10 -4 3
0.20
48 x 10 -4
Fill in from CP textbook pg 577
Use the data provided to identify the order with respect to A, overall order, and rate law.

9. Initial Rate (mol/Ls)
Example 2
2NO(g) + 2H2(g) N2(g) + 2H2O(g)
Trial
Initial [NO] (M)
Initial [ H2] (M)
Initial Rate (mol/Ls) 1
0.100
2.00 x 10 -3 2
0.200
4.00 x 10 -3 3
16.0 x 10 -3
Fill in from H textbook pg 544 table 17-3
Use the data provided to identify the order with respect to each reactant, overall order, and rate law.

10. General rate law: rate = k [NO]x[H2]y
Pick appropriate experiments to compare
*For NO, pick trials 1&2 because the H2 concentration doesn’t
change
Exp2: 4.00×10−3 = k [0.200]X [0.100]Y
Exp1: 2.00×10−3 = k [0.100]X [0.100]Y
2 = 2x
X=1; 1st order in NO
*Same terms on the top and bottom cancel out

11. For H2, pick trials 2&3 because the concentration of NO doesn’t change
Exp3: 16.0×10−3 = k [0.200]X [0.200]Y
Exp2: 4.0×10−3 = k [0.200]X [0.100]Y
4 = 2y
y=2; 2nd order in H2
(better) rate=k[NO][H2] rd order overall
Pick any trial, plug in all the numbers, solve for k
k=2.00 1/M2s
rate=2.00[NO][H2]2
*Same terms on the top and bottom cancel out
*Calculating the units for k is a bonus
FINIAL ANSWER