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1.5 Reaction Mechanisms, Catalysis, and Rate Laws Warm Up Study the following series of molecular collisions representing the catalyzed decomposition of formic acid.
+
+
+
+
H+
HCOOH2+
HCOOH
H2O
HCO+
H2O
CO
H+
1. How many reactions are shown in the series above? ______________ 2. In the format provided below question 3, write the balanced chemical equation for each reaction in the series. As it is a product of the overall series of collisions, the H2O produced in the second step should not be shown as a reactant or product in the third step. 3. Algebraically sum the three equations to give the overall reaction. (Species that appear as reactants in one step and products in another should be cancelled.) What is the overall reaction once the three steps have been added together? Step 1:
→
Step 2:
→
Step 3:
→_____________________________________
Overall Reaction:
→
A balanced equation for a chemical reaction shows us what substances are present before and after
Reaction Mechanisms the reaction. Unfortunately, it tells us nothing about the detailed steps occurring at the molecular
level in between. Studies have shown that most reactions occur by a series of steps called the reaction mechanism. The individual steps in such a series are called elementary processes. These steps are themselves individual balanced chemical reactions that may be added together algebraically to give the overall balanced chemical equation. A reaction mechanism is a series of steps that may be added together to give an overall chemical reaction.
Successful collisions require sufficient energy and appropriate orientation. As a consequence, the simultaneous collision of more than two reactant molecules with good geometry and Ea is highly unlikely. It’s not surprising then that reactions having more than two reactant particles almost always involve more than one step in their reaction mechanism.
50 Chapter 1 Reaction Kinetics
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