1. Rate Law p

2. You have qualitatively observed factors affecting reaction rate but…
There is a mathematical relationship between reaction rate and factors affecting it!
Must be determined empirically (experimentally)

3. The balanced chemical equation is the first step
We know reaction rate depends on [R]
Recall: rate = ∆[R]/∆t
But does [NH4+] have a greater effect than [NO2-]?

4. Note: Exponents do NOT have to equal molar coefficients.
The Rate Law
For a reaction:
aX + bY  products
Rate is proportional to the product of the initial concentrations of the reactants raised to some exponent
r  [X]m[Y]n
r = k [X]m[Y]n
Where k is the rate constant for a specific reaction at a specific temperature.
Note: Exponents do NOT have to equal molar coefficients.

5. Compare Experiments 1 and 2:
To determine the rate law we measure the rate at different starting conditions.
Compare Experiments 1 and 2:
when [NH4+] doubles, the initial rate doubles.

6. Compare Experiments 5 and 6: when [NO2-] doubles, the initial rate doubles.
r = k[NH4+]1[NO2-]1 or
r = k[NH4+][NO2-]
I.e. reaction rate depends equally on the initial concentrations of each reactant

7. Order of Reaction
The exponents in the rate law describe the dependence of rate on initial concentration
Overall order of reaction is the sum of each of the individual orders of reaction.
r = k[NH4+][NO2-]
Order of reaction wrt NH4+ is 1
Order of reaction wrt NO2- is 1
Overall order of reaction is 2 (1+1)

8. For the reaction:
2 X + 2 Y + 3 Z  products
The rate law has been experimentally determined as:
r = k[X]1[Y]2[Z]0
Because r  [X]1
If initial [X] doubles, rate will double too 21 = 2
If initial [X] triples, rate will triple too 31 = 3

9. r = k[X]1[Y]2[Z]0
Because r  [Y]2
If initial [Y] doubles, rate will multiply by 4 (22)
If initial [Y] triples, rate will multiply by 9 (32)
Because r  [Z]0
If initial [Z] doubles, rate will multiply by 1 (20), the rate will remain unchanged
If initial [Z] triples, rate will multiply by 1 (30), the rate will remain unchanged
I.e. [Z] does NOT affect reaction rate
r = k[X]1[Y]2