REACTION MECHANISMS
• Chemists believe that most chemical reactions occur as a sequence of elementary steps.
An elementary step involves a one, two, or three particle collision.
• A reaction mechanism is the series of elementary steps that make up the overall
reaction. It is a one-step or multi-step pathway that a reaction follows, indicating the order
in which collisions occur.
• If one reactant molecule is involved in the reaction mechanism, the elementary step is
called unimolecular. If two reactant molecules are involved in the reaction mechanism,
the elementary step is called bimolecular. If three reactant molecules are involved in the
reaction mechanism, the elementary step is called termolecular.
• Reaction mechanisms are best-guesses at how the reaction occurs and can only be
determined from the experimentally determined rate law.
Step 1: A + B - C (slow)
Step 2: A + C - D (fast)
Overall reaction: 2A
+
B - D
• Each step is called an elementary step – most often an elementary step
involves 1 or 2 particles colliding. Three particle collisions are possible, but collisions with
more than three reactant molecules are very unlikely.
• A reaction intermediate is always formed. An intermediate is a substance
which is formed and then in a later step is used up again. Which substance is
the reaction intermediate in the above example?
• One step is always slower than the rest and an important part of the mechanism is to
indicate which steps are fast and which is slow. The slowest step of the reaction mechanism
is called the rate determining step. The reaction rate can only go as fast as the slowest
step.
• A rate law can be written for each elementary step using the idea that:
rate α # of collisions
Elementary Step (rxn)
A - products
Rate Law
Rate = k[A]
A + B - products
Rate = k[A][B]
2A - products
Rate = k[A]2
2A + B - products
Rate = k[A]2[B]
RATE DETERMINING STEP
• Is the slowest elementary step in the reaction mechanism
• The rate law from the slowest step IS the rate law for the overall reaction
Consider the following reaction mechanism:
Step 1: A + B - C
(slow)
ΔH = -40 kJ
Step 2: A + C - D (fast)
ΔH = -60 kJ
___________________________________________
Overall reaction: 2A + B - D
ΔH = ?
Try drawing the overall PE diagram and then the 2-stepped PE
Evaluating a Reaction Mechanism
If a reaction mechanism meets the following criteria, it is said to be reasonable
● The equations for the elementary steps add up to the overall reaction equation.
● The proposed elementary steps must be reasonable (no more than three reactant
molecules)
● The reaction mechanism must support the experimentally determined rate law. Rate law
for the slowest step must match the experimentally determined rate law.
Ex.1 Is this a reasonable mechanism?
2 NO2(g) +
Cl2(g) -
2NO2Cl(g)
Rate = k[NO2][Cl2]
Overall reaction
Experimentally Determined Rate Law
Proposed Mechanism
Step 1
NO2(g) +
Cl2(g) -
NO2Cl(g)
Step 2
NO2(g) +
Cl(g) -
NO2Cl(g)
+
Cl(g)
(slow)
(fast)
Ex.2 Is this a reasonable mechanism?
2 NO(g)
+
2 H2(g)
Rate = k[NO]2[H2]
-
N2(g)
+
2 H2O(g)
Overall reaction
Experimentally Determined Rate Law
Proposed Mechanism
Step 1
2 NO(g) +
Step 2
N2O(g) +
H2(g) -
H2(g) -
N2O(g)
N2(g)
+
+
H2O(g)
H2O(g)
(slow)
(fast)