1. Determining the Form of the Rate Law

2. How Data is created

3. Method of Initial Rates
Used to find the form of the rate law
Choose one reactant to start with
Find two experiments where the concentration of that reactant changes but all other reactants stay the same
Write the rate laws for both experiments
Divide the two rate laws
Solve for the order
Follow the same technique for other reactants

4. Example Choose one reactant to start with NH4+
Find two experiments where the concentration of that reactant changes but all other reactants stay the same
Exp 2 & 3

5. Example Write the rate laws for both experiments
Exp 2: Rate = 2.70x10-7 = k(0.100)x(0.010)y
Exp 3: Rate = 5.40x10-7 = k(0.200)x(0.010)y
Divide the two rate laws
0.50 = 0.50x
Use log rules to solve for the order
x = 1 so the order for NH4+ is one

6. Example Follow the same technique for other reactants NO2-: Exp 1 & 2
Exp 1: Rate = 1.35x10-7 = k(0.100)1(0.0050)y
Exp 2: Rate = 2.70x10-7 = k(0.100)1(0.010)y
0.5 = 0.5y
y = 1
So Rate = k[NH4+]1[NO2-]1
Overall Reaction Order – sum of orders of reactants

7. Finding k We can find k using values from any of the experiments given
Units will be different for k depending on order of reactants

8. Example
BrO3- : Exp 1 & 2
Exp 1: Rate = 8.0x10-4 = k(0.10)x(0.10)y(0.10)z
Exp 2: Rate = 1.6x10-3 = k(0.20)x(0.10)y(0.10)z
0.50 = 0.50x
x = 1

9. Example
Br- : Exp 2 & 3
Exp 2: Rate = 1.6x10-3 = k(0.20)1(0.10)y(0.10)z
Exp 3: Rate = 3.2x10-3 = k(0.20)1(0.20)y(0.10)z
0.50 = 0.50y
y = 1

10. Example H+ : Exp 1 & 4 Exp 1: Rate = 8.0x10-4 = k(0.10)1(0.10)1(0.10)z
0.25 = 0.50z OR ¼ = (½)z
z = 2

11. Example So Rate = k[BrO3-]1[Br-]1[H+]2 Solve for rate constant, k
Overall order of reaction = 4
Solve for rate constant, k