Download Rate Law

Rate Law
rate a [Br2]
rate = k [Br2]
The rate law expresses the relationship of the rate of a
reaction to the rate constant and the concentrations of the
reactants raised to some powers.
aA + bB
cC + dD
Rate = k [A]x[B]y
Reaction is xth order in A
Reaction is yth order in B
Reaction is (x +y)th order overall
Examples
Write the general rate law for the following
chemical reactions
1.
F2(g) + 2 ClO2(g)  2 FClO2(g)
2.
Decomposition of liquid water into hydrogen
and oxygen gas
Relationship between order and rate
AB
rate = k[A]x
o if
x=1
o Rate
o If
x=2
o Rate
o If
is directly proportional to [A]
varies as the square of [A]
x=0
o Rate
is independent of [A]
Review of Concepts
A reaction is second order in A and first order in B. If the
concentration of A doubles and the concentration of B
is halved, overall what has happened to the rate?
Method of Initial Rates
F2 (g) + 2ClO2 (g)
2FClO2 (g)
rate = k [F2]x[ClO2]y
Double [F2] with [ClO2] constant
Rate doubles
x=1
rate = k [F2][ClO2]
2nd order overall
Quadruple [ClO2] with [F2] constant
Rate quadruples
y=1
7
Example 1
3A
Trial Initial [A] (M)
B
Initial Rate (M/s)
1
0.050
3.0 x 10 -4
2
0.10
12 x 10 -4
3
0.20
48 x 10 -4
Use the data provided to identify the order with
respect to A, overall order, and rate law.
Example 2
2NO(g) + 2H2(g)
N2(g) + 2H2O(g)
Trial
Initial [NO]
(M)
Initial [ H2]
(M)
Initial Rate
(mol/Ls)
1
0.100
0.100
2.00 x 10 -3
2
0.200
0.100
4.00 x 10 -3
3
0.200
0.200
16.0 x 10 -3
Use the data provided to identify the order with
respect to each reactant, overall order, and
rate law.
General rate law: rate = k [NO]x[H2]y
Pick appropriate experiments to compare
*For NO, pick trials 1&2 because the H2 concentration doesn’t
change
*Same terms on
Exp2: 4.00×10−3 = k [0.200]X [0.100]Y
the top and
−3
X
Y
Exp1: 2.00×10 = k [0.100] [0.100]
bottom cancel
out
2 = 2x
X=1; 1st order in NO
For H2, pick trials 2&3 because the concentration of NO doesn’t
change
Exp3: 16.0×10−3 = k [0.200]X [0.200]Y
Exp2: 4.0×10−3 = k [0.200]X [0.100]Y
4 = 2y
y=2; 2nd order in H2
(better) rate=k[NO][H2]2
*Same terms on
the top and
bottom cancel
out
3rd order overall
Pick any trial, plug in all the numbers, solve for k
k=2.00 1/M2s *Calculating the
FINIAL ANSWER
units for k is a bonus
rate=2.00[NO][H2]2