Dependence of Rate on Concentration
- To mathematically calculate or show the dependence of rate on
concentration, a rate law is used. This is an equation that relates
the rate of a reaction to the concentration of reactants and
catalyst raised to various powers.
2NO2 + F2  2NO2F
rate law: rate = k [NO2] [F2]
where k = rate constant
- rate constant is a constant value at a given temperature. Its
value varies with temperature. Its units depend upon the rate
law.
k =
rate
=
mol/(L•s)
= L / (mol•s)
[NO2] [F2]
(mol/L) (mol/L)
- Generally…
E
aA + bB
→
cC + dD
rate = k [A]m [B]n [E]p
where m, n, p are usually integers and must be determined
experimentally!
- Reaction order is the exponent of the concentration of that
species in the rate law.
H+
CH3COCH3 + I2 → CH3COCH2I + HI
acetone
Rate = k [CH3COCH3][H+]
Reaction is FIRST ORDER in acetone.
Reaction is ZERO ORDER in iodine (reaction rate not dependent
on [I2])
Reaction is FIRST ORDER in [H+].
Reaction is SECOND ORDER overall.
Determining the Rate Law
2 N2O5 (g) → 4 NO2
Trial 1
Trial 2
change
exponent
Initial concentration
of N2O5
1.0 × 10-2 mol/L
2.0 × 10-2 mol/L
2?
(g)
+ O2
(g)
Initial Rate of N2O5
=
? = 1
4.8 × 10-6 mol/(L•s)
9.6 × 10-6 mol/(L•s)
2
Exponent of the concentration in the Rate Law is 1, therefore,
reaction is FIRST ORDER.
Rate = k [N2O5]1
What is the reaction order?
If the initial concentration is doubled and
the rate is multiplied by a factor of …
The resulting reaction
order is….
½
-1
1
0
2
1
4
2
Order
of
Reaction
Rate Constant (k )
Rate = k [A]
First
k = Rate /[A]
Second
Rate = k [A]2 or
Rate = k [A] [B]
k = Rate /[A]2
Rate = k [A]2 [B] or
Rate = k [A] [B] 2
Third
k = Rate /[A]2 [B]
Rate
Constant
Units
Rate Constant Units
mol dm-3s-1
mol dm-3
s-1
mol dm-3s-1
(mol dm-3)2
dm3mol-1s-1
mol dm-3s-1
(mol dm-3)3
dm6mol-2s-1