Rate Orders and Rate
Laws
Reaction Rates
Are measured as the change in concentration
over time. ∆[reactants]
∆time
We can measure rate as the rate of
disappearance of reactants or the
appearance of products
The reactants concentration decreases per unit
time and the product concentration increases
over time.
Reaction Rate Graph
[Products]
Increase
[Reactants]
Decrease
H2 + I2 2HI
Rate Order – a specific exponential
relationship between the concentration
of a chemical and the rate of a reaction
We look to see how the rate of a reaction
changes in relationship to a change in the
concentration of a reactant.
Rate Orders – Tell us how the
rate changes with changes in
reactant concentration
Zero Order (0o)– as concentration changes,
the reaction rate does not change
First Order (1o) – as the concentration
changes, the rate changes by the same factor.
(double concentration = double rate)
Second Order (2o) – as the concentration
changes, the rate changes by the same factor,
squared.
(double concentration = rate increases by a factor
of 22 (or 4x)
Or triple concentration = rate increase by a factor
of 32
Third Order (3o)– as the concentration changes,
the rate changes by the same factor, cubed.
(double concentration = rate increases by a factor
of 23 )
Rate Laws
A rate law is an equation that tells us the rate
order of each reactant in a reaction.
Its general form is
Rate = k [X]a[Y]b
Where ‘a’ and ‘b’ represent the order of the
reactant X and Y respectively.
[X] means the concentration of reactant X in
Molarity. (same thing for [Y])
‘k’ is the rate constant which is different for
every reaction and depends on temperature.
The rate constant and rate orders for a reaction
can only be determined by experimentation
Example
For the reaction X + Y Z
The following rate data was obtained.
Determine the rate order of each reactant, and
then write the rate law.
[X] [Y] Rate (M/sec)
1 1 4
2 1 16
1 3 12
3 3 ___?___