mechanism of chemical change
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THE MECHANISM OFCHEMICAL REACTIONS
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The steps that atoms go through as their
arrangement changes from reactants to
products is known as the MECHANISM of
the reaction.
On the microscopic level a number of stepsare essential before a reaction will occur.
The model used to explain reactionmechanism is THE COLLISION THEORY.
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The Collision Theory
reacting atoms, molecules or ions mustcollide with each other.
the particles must have sufficient energy
and must be oriented correctly.
bonds in the original molecules must break
and new bonds must form.
electrons must re-arranged in order to form
new bonds.
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Amount of energy and orientation of colliding
particles leads to some collisions being
effective and others not.
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Particles with sufficient energy:
This Boltzmann curve shows that there is only
a small fraction of molecules that have enough
energy for the reaction
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THE ACTIVATED COMPLEX
All reactions need energy to start. The
reacting atoms must pass over an energy
hill to change from reactants to products.
The transition state that atoms must pass
through is called the ACTIVATED
COMPLEX.
The activated complex is an unstable
structure where bonds are forming and
breaking at the same time.
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ENERGY DIAGRAM showing the changes in
the potential energy of the reacting substances
during the reaction.
In an exothermic reaction the energy of products is
less than the energy of the reactants.
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In an endothermic reaction the energy of
products is greater than the energy of the
reactants.
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Know these definitions:
ACTIVATION ENERGY the difference inenergy between the reactants and the
activated complex.
Ea is measured in kJ/mol.
Ea is also referred to as the minimum
amount of energy required for a molecule to
react.
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HEAT OF REACTION difference in
energy between reactants and products.
H = energy products energy reactants
H is measured in kJ/molH > 0 for endothermic reactions
H < 0 for exothermic reactions
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Comparing exothermic and endothermic
reactions:Exothermic
reaction
Endothermic
reaction
H < 0
negative
H > 0
positiveEp of the reactants
decreases as products
form
Ep of the reactants
increases as products
form
Net energy is given
out (liberated)
Net energy is
taken in (gained)
The reaction flask
becomes hot
The reaction flask
becomes cold
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What are you expected to do?
state the general steps in the collisiontheory.
state the factors necessary for an
effective collision.
describe and explain energy changes in
a chemical reaction.
interpret graphs showing energy changes
in chemical reactions.