Rate Laws
Rate Laws
Rate = k [A]x [B]y
[A] and [B] represent the concentrations
of reactants
x and y often (but not always) represent
the coefficients in front of [A] and [B]
k is a constant specific to the reaction.
Rate Laws
Rate = k [A]x [B]y
k


A constant specific to the reaction.
Rate constants may have a variety of
strange units using L, mol, and sec. This is
so the rate will always work out in mol/L
sec.
Rate Laws
For example:
2NO + O2  2 NO2
The rate expression is:
Rate = k [NO]2[O2]
Rate Laws
For example:
2 N2O5  4 NO2 + O2
The rate expression is
Rate = k [N2O5]2
Rate Laws
Order in reactions:
refers to the exponents on the
compounds in the rate law
Rate = k [NO]2[O2]
This reaction is second order for NO and
first order for O2
Rate Law
This rate expression
Rate = k [NO]2[O2]
is third order overall.
Rate Laws
If a reactant is first order, doubling the
concentration will double the rate.
If a reactant is second order, doubling the
concentration will quadruple the rate.
If changing the concentration has no effect
the chemical does not appear in the rate
law.
Rate Laws
To determine the rate constant, one
must collect data using different
concentrations of reactants.
Rate Laws
For example:
H2O2 + 2HI  2H2O + I2
Trial
[H2O2]
[HI]
1
0.1 M
0.1 M
2
0.1 M
0.2 M
3
0.2 M
0.1 M
Rate
.0076
.0152
.0152
Rate Laws
If [HI] doubles, the rate doubles, so the
rate expression is first order for HI.
If [H2O2] doubles, the rate doubles, so
the rate expression is first order for
H2O2.
Rate Laws
The rate expression must be
Rate = k [H2O2] [HI]
Rate Laws
Plug in data from any trial to solve for k:
.0076 = k (0.1M) (0.1M)
.0076 = .01k
K = .76
Rate Laws
If a reaction has two or more steps, the
rate-determining step is the slowest
one.
Rate Laws
Rate-determining Step
2 NO(g) + 2H2(g)  N2(g) + 2 H2O(g)
Elementary Steps
2 NO  N2O2 (fast)
N2O2 + H2  N2O + H2O + H2 (slow)
N2O + H2  N2 + H2O (fast)
Rate Laws
Rate-determining Step
Which is the rate-determining step?
2 NO  N2O2 (fast)
N2O2 + H2  N2O + H2O + H2 (slow)
N2O + H2  N2 + H2O (fast)
____________________________________________________________________________________
2 NO(g) + 2H2(g)  N2(g) + 2 H2O(g)
Rate Laws
Rate-determining Step
Which is the rate-determining step?
2 NO  N2O2 (fast)
N2O2 + H2  N2O + H2O + H2 (slow)
N2O + H2  N2 + H2O (fast)
____________________________________________________________________________________
2 NO(g) + 2H2(g)  N2(g) + 2 H2O(g)