Download equilibrium and activation energy

Chemistry
Chapter 17
Equilibrium and collision
theory
Chemistry
Zuhumdal
Last revision Fall 2014
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Collision Theory


Rates of chemical reactions are related to the
properties of atoms, ions, and molecules
through a model called collision theory.
According to collision theory, atoms, ions, and
molecules can react to form products when
they collide, provided they have enough
kinetic energy.
Think clay


If you throw 2 clay balls together
gently, they do not stick together,
kinda like the lack of reaction between
colliding particles with low energy.
If you throw the same balls of clay
together with great force, they stick
together.
Activation Energy

Activation energy is a barrier that reactants
must cross to be converted to products.
Activated Complex

An activated complex is the arrangement of
atoms at the peak of the activation-energy barrier
Also
called
transition
state.
Factors Affecting Reaction
Rates

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
Temperature
Concentration
Particle Size
Catalysts
Pressure
Temperature

Usually, raising the temperature speeds up the
reactions, while lowering the temperature slows
down the reactions.

This will increase the number of particles that
have enough kinetic energy when they collide.
Concentration

Cramming more particles into a fixed volume
increases the concentration of reactants.
Particle Size


The smaller the particle size, the larger the
surface area for a given mass of particles.
Which burns faster, a piece of wood or a
kindle? Why?
Catalyst

A catalyst is a substance that increases the
rate of a reaction without being used itself.

Catalyst permit reactions to proceed at a lower
energy than required.
Reversible Reaction

Do reactions only go one way or do reactions
go 2 ways?

Let’s look at this equation:


2SO2(g) + O2 -------------- 2SO3
What happens when sulfur dioxide and
oxygen gas are mixed in a sealed container?
Reversible Reactions

As the SO3 concentration increases, a small
amount collides and reverts to SO2 and O2
by the reverse direction!

As the concentration of SO3 becomes higher
and higher, the reverse reaction speeds up!

Eventually SO3 decomposes to SO2 and O2
as fast as it forms SO3.
Dynamic Chemical Equilibrium

Equilibrium occurs when opposing reactions
occur at equal rates and the concentrations of
the products and reactants stop changing.

Reactants and products are not necessarily
equal in amount.
And the forward and reverse reactions have
not stopped.

Le Chatelier’s Principle

Le Chatelier’s Principle


If a stress is applied to a system in
dynamic equilibrium, the system changes
to relieve stress.
Stresses include changes in the
concentration of reactant or products,
changes in temperature, and changes
in pressure
Le Chatelier's Principle
>> Concentration Changes




The concentration can be changed by
adding or removing a substance.
If added, the equilibrium shifts to remove
the substance.
If removed, the equilibrium shifts to
replace it.
Adding or removing (provided some remains)
solid, pure liquid, or solvent does not affect
the equilibrium, since the concentration of these
substances is constant.
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Le Chatelier's Principle
>> Concentration Changes
Example 15
For the reaction,
CH3OH(g) + O2(g)  HCOOH(g) + H2O(g)
a)
b)
c)
d)
e)
What direction does the equilibrium shift if more oxygen
is added?
What direction does the equilibrium shift if water is
removed?
How does the concentration of methanol (CH3OH)
change if more oxygen is added?
How does the concentration of methanol change if more
water is added?
How does the concentration of methanol change when
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more methanol is added?
Le Chatelier's Principle
>> Pressure Changes




Changing pressure only affects gases.
Higher pressures favor fewer moles of
gas
Lower pressures favor more moles of
gas.
Only consider moles of gas (not other
physical states) in determining
equilibrium shifts.
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Le Chatelier's Principle
>> Pressure Changes
Example 16
How does an increase in pressure
affect the concentration of the
reactants in the following reactions?
a) C2H4(g) + H2(g)  C2H6(g)
b) Xe(g) + 3F2(g)  XeF6(g)
c) C(s) + 2F2(g)  CF4(g)
d) H2S(g) + Hg(l)  HgS(s) + H2(g)
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Le Chatelier's Principle
>> Temperature Changes



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The easiest way to predict equilibrium
shifts is to consider energy (or heat) as a
product or a reactant.
Energy is a product in exothermic
reaction (– H)
Energy is a reactant in endothermic
reactions (+ H).
Increasing temperature increases the
"concentration of energy."
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Le Chatelier's Principle
>> Temperature Changes
Example 17
How does the concentration of the
product change if the temperature
increases?
a) Fe2S3  2Fe 3+ + 3S 2–
+H
b) Si + 2F2  SiF4
–H
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