Download Unit 9: Thermodynamics Content Outline: The Mechanics of

Unit 9: Thermodynamics
Content Outline: The Mechanics of Chemical Reactions (9.4)
I.
Kinetic Molecular Theory (KMT)
A. A theory that explains that the behavior of physical systems depends on the combined actions of the
molecules constituting the system.
(Velocity/kinetic energy of molecules helps make chemical reactions possible.)
B. The molecules kinetic reaction possibility is dependent upon:
1. Energy pathway. (Is the total reaction a one step, two step, or multiple step process.)
2. Changes at the molecular level when substances interact with one another.
3. Ability to overcome molecular repulsions, due to charge or electron clouds interacting.
II. Collision Theory
A. Theory on the ability to make a chemical reaction take place within a system.
1. The ability to interact is dependent upon:
a. Orientation of molecules.
Remember, from Biology, to build macromolecules the monomers had to be “oriented” so as
to make water form from a dehydration synthesis reaction.
b. The molecules must possess enough energy to initiate a chemical reaction to take place.
i.
This energy is referred to as Energy of Activation (EA).
You should remember this from Biology too, as you discussed EA when you
discussed enzymes. Enzymes (Biological catalysts) lower the EA so as to allow the
chemical reactions of life take place.
ii.
The energy required can be lowered using enzymes or catalysts.
 These can be affected by pH, concentration, and temperature.
iii.
EA is greater for endothermic (non-spontaneous) reactions versus exothermic
(spontaneous) reactions.
Endothermic needs energy input; where as, exothermic has it already… as it will
release the energy as the reaction occurs.
2. Transition State
a. The state of two molecules interacting during a chemical reaction.
3. Activated Complex
a. A transitional structure that results from an effective collision and that persists while “old
bonds” are breaking and “new bonds” are being formed.
III. Reaction Mechanisms (“Stages/Steps”)
A. The step-by-step sequence of reactions for the complete process to occur.
For example: H2(g) + I2(g)  2 HI(g) (The final written chemical reaction.)
Step 1: I2(g)  2I(g)
Step 2: I(g) + H2(g)  H2I(g)
Step 3: H2I(g) + I(g)  2 HI(g)
B. Intermediates
1. These are the chemical molecules found in the formation steps, but not the final net chemical
reaction.
For example: from above- I(g) or H2I(g)
C. Homogenous Reaction
1. All the reactants and products are in the same state of matter. Such as above are all gases.
D. Heterogeneous Reaction
1. Reactants and/or products are in different states of matter.