Transcript Document

Chapter 17
Reaction Rates and
Equilibrium
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Collision Theory
When one substance
is mixed with
another, the two
substances do not
react on a
macroscopic basis,
but react as their
individual particles
(atoms, molecules, or
ions) come together.
Collision theory
video
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Lab 1
Factors Affecting Rates of Reaction
 Create Data Tables
 Do Part C,B,D,A
 Graph Parts C, D
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The factors that affect how
these particles collide are the
factors that influence the rate
of a reaction.
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Factors influencing the likelyhood
of a reaction occurring:
1) Force of collision is sufficient
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2) Orientation of colliding
particles during collision is exact
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3) Must meet the minimum
activation energy requirement for
the specific reaction
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activated complex
In order for the reaction to occur, the
particles involved must collide with
each other, the more often the
particles collide, the faster the
reaction occurs. Not every collision
results in a reaction.
The activated complex is the
temporary arrangement of atoms as
they change from reactants into
products.(transition state)
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Orientation
Successful
collision
Unsuccessful
collision
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activation energy
The colliding molecules
must have enough energy
to react and form an
activated complex. This
minimum amount of
energy is called the
activation energy.
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Rate of a Reaction = speed of reactants
turning into products
Can be measured in :
grams of reactant consumed/second
or
grams of product produced/second
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Effect of the Nature of the Reactant
The nature of the reactants involved will
determine the kind of reaction that occurs.
Reactions with bond rearrangements or
electron transfer take longer than reactions
without these changes. Ionic reactions (such
as double displacement or neutralization) occur
almost instantly.
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Effect of Temperature on Reaction Rate
Increasing the temperature increases
the rate of a reaction. At higher
temperatures, the particles have more
energy, move faster and collide more
frequently.
Increasing the temperature 10 degrees
celcius generally doubles the reaction rate.
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Effect of Concentration on Reaction Rate
Increasing the concentration of the
reactants increases the rate of a
reaction. When there are more particles
per unit volume, they will collide more
frequently, causing the rate to increase.
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Effect of Particle Size/Surface Area
Decreasing particle size/increasing surface
area increases the rate of a reaction.
When the surface area is increases, there
is more contact between the reactants, the
number of collisions increase, and therefore
the rate of reaction also increases.
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Not Cattle List!!
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Catalyst
- any substance that is added to the reaction to
INCREASE the rate. The catalyst DOES NOT react
with any substance to produce product.
Inhibitor
- Any substance added to slow down the rate of
reaction. Works against the catalyst.
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C2H2
H2
C2H4
catalyst
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A catalyst increases the rate of a reaction
by lowering the activation energy.
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Rate Law Expression
Equation relating the rate of a
reaction to the concentrations of
the reactants and the specific
rate constant.
2NO + 2H2
N2 + 2H20
Rate = k[NO]2[H2]2
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Rate = k[NO]2[H2]2
Rate = speed of reactants turning into products
[ ] = the concentration of
k
= specific rate constant for a reaction
- if the rate is fast k will be high
- if the rate is slow k will be low
2 (exponent) is used to predict the order of the
reaction. The order reflects the affect of doubling
the concentration of a reactant on the overall speed
or rate of reactants becoming products.
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Order of reaction
0
1st
affect on the rate of rxn
when [ ] of reactant is “doubled”
No affect
Rate doubles
(2)1 = 2x faster
2nd
Rate quadruples (2)2 = 4x faster
3rd
Rate octuples (2)3 = 8x faster
4th
Rate goes crazy (2)4 = 16x faster
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Rate =
2
k[NO] [H
2
]
2
What is the affect of doubling the concentration
Of each of the reactants in the above expression?
N2 + 3H2
2NH3
Write the rate law expression for the above
reaction and predict the affect of doubling the
concentration of each of the reactants.
Also predict the overall reaction rate
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In-Depth Rate law
 Rate Laws
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Reaction Mechanism
Most reactions occur in a series of steps.
Each step normally involves the collision
of only two particles.
There is little chance of three or more
particles colliding with the proper
position and sufficient energy to cause a
reaction.
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Reaction Mechanism
If a reaction consists of several steps
such as the following:
A
B
C
B
C
final product
One of the steps will be slower than all the
others. This step is called the rate determining
step. The other faster steps will not affect the
rate of the reaction. The series of steps that
must occur for a reaction to go to completion
is called the reaction mechanism.
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Old Lady Driver
or
The rate of a reaction is
determined by the slowest
step in the reaction mechanism
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Elementary steps -1st step is the
rate determining
step
intermediate
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Assignment 1
 Calculate tangents (rates of reaction) on Part C




graphs of Lab 1
(see figure 17.8 as an example)
Do Q 4,6,21-28 Chp 17
Do 17A
OBWS 1-4
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Entropy
A measure of the disorder of system
Law of disorder
Systems tend to go from a state of
order(low entropy)To a state of
maximum disorder (high entropy)
spontaneously.
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Entropy changes can be predicted
I2 (s)
I2 (g)
Solids – low entropy
Liquids –avg entropy
Gases – high entropy
Or
You can predict the entropy by comparing
The # of moles in the equation.
2H20(g) + 2Cl2(g) 4HCl(g) + O2(g)
Fewer moles means lower entropy
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Entropy( S) Calculations
2H2S + 3O2
2H2O + 2SO2
2mol (205.6J/Kmol) +3 (205.O) = 2 (188.7) + 2 (248.5)
1026.2 J/K
=
874.4J/K
S = Products - Reactants
S =874.4J/K - 1026.2J/K
S = -151.8J/K
A (-) entropy indicates a decrease in entropy
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Spontaneous Reactions
are reactions that are known to
produce the written products.
Copy table 17.2 page 408
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A reaction tends to be spontaneous if :
-the reaction is exothermic
heat is released
(- Enthalpy)
-the entropy of the products is
greater than the entropy of the
reactants (+ Entropy)
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Free Energy
When a reaction occurs some energy
known as free energy of the system
becomes available to do work.
Free energy may be available but is
not always used efficiently.
- Auto engine uses only 30% of free
energy of burning gasoline. 70% is
lost as friction and heat.
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Spontaneous
reactions
release free
energy and
are said to be
exergonic.
(- Gibbs free energy)
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(+ Gibbs free energy)
Nonspontaneous
reactions absorb
free energy and
are said to be
endergonic.
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Nonspontaneous reactions can be
made spontaneous by:
-increasing the temperature
-by linking it to a
spontaneous reaction that
produces a large amount of
free energy
Which makes the nonspontaneous
reaction go!!
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Gibb’s Free Energy Calculation
C + O2
CO2
(0.0kj/mol) + (0.0kj/mol) =
(-394.4kj/mol)
1mol (0.0kj/mol) + 1 (0.0kj/mol) = 1 (-394.4kj/mol)
G = Products
- Reactants
G = -394.4kj
- 0.0kj
G = -394.4kj
A (-) free energy means the reaction is
spontaneous
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Free Energy Calculations
G= H - T S
G = Gibb’s Free energy (kJ/mol)
Table 17.4 Pg 414
H = Enthalpy (heat energy) (kJ/mol)
Table 8.1 Pg 190
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G= H - T S
S = entropy (disorder) (j/K-mol)
Table 17.1 pg 407
T = temperature (Kelvin)
C + 273= K
*All
are calculated using
Products – Reactants Method
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G= H - T S
CaCO3
CaO + CO2 at 25 C
Is this reaction spontaneous? Try
getting the Gibb’s Free energy.
G = -178.5kJ/mol – [(298K)x(164.7J/Kmol)]
= -178.5 kJ/mol – 49080.6 J/mol
= -178.5 kJ/mol – 49.1 kJ/mol
G = -227.6 kJ/mol
Negative G means the reaction is spontaneous
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Assignment 2
 17B Entropy Problems
 17C Gibb’s Free Energy Problems
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Reversible Reactions
The conversion of reactants into products
and the conversion of products into
reactants occuring at the same time.
A + B
C
Note double arrows
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When the forward and reverse reactions
are occurring at the same rate(speed) the
reaction is said to be in a state of
chemical equilibrium
The equilibrium position can be shown
by the double arrows and a 3rd arrow
A
B
1%
99%
A
B
99%
1%
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Catalysts will have no effect on the
equilibrium because they speed up
both the forward and reverse reactions
equally.
catalyst
A
B
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Assignment 3
 Lab E1 : Analogy for an Equilibrium
Reaction
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Equilibrium constants (Keq)
-Indicate if the reverse reaction will
favor reactants or products at equilibrium
aA + bB
cC + dD
Keq expression
Keq = [C]c [D]d
[A]a [B]b
products
reactants
Keq > 1 more products than reactants at eq
Keq < 1 more reactants than products at eq
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Assignment 4
Iinitial Change Equilibrium
Section 16.3 Equilibrium Calculations for
Homogeneous Systems pg 656-661,665
(see Section 16.3 Summary pg 665)
 17D Equilibrium Calculations
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Henry Louis Le Chatelier
If a chemical system at equilibrium experiences
a change in concentration, temperature, or total
pressure; the equilibrium will shift in order to
minimize that change.
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Le Châtelier’s
Principle
If a stress is
applied to a
system at
equilibrium, the
equilibrium will
shift to relieve
the stress.
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Equilibrium = A state in which no
net change takes place in a
system
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In a reaction equilibrium, both the forward and
reverse reactions are taking place simultaneously,
at the same rate, so that no net change occurs.
N2 (g) + 3H 2(g)
2NH3 (g)
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Also called…
Dynamic
Equilibrium =
An equilibrium
in which two or
more changes
are taking
place
simultaneously,
but at the same
rate.
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Why would anyone want to shift a
system in equilibrium?
To make more product!!
How can you shift a system that
is already in equilibrium?
By adding a stress on the system!!
LIKE…
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Concentration Change
Increase [reactant] = shifts to use up the added reactants and
produce more products Shifts right
Reactants
Products
Increase [product] = shifts to use up added product and produce
more reactants Shifts left
Reactants
Products
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Decrease [reactant] = shifts to produce more
reactants Shifts left
Reactants
Products
Decrease [product] = shifts to produce more product
Shifts right
Reactants
Products
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Adding heat to increase stress
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Effect of Temperature on a System
In Equilibrium
 Video
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Temperature Change
Increase = favors the endothermic direction.
The reaction shifts in the direction that uses up
the added energy.
A + B
C + heat
Decreasing = favours the exothermic direction.
The reaction shifts in the direction that produces
energy.
A + B
C + heat
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NO (g)
colorless
NO2 (g)
brown
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Pressure/Volume Change
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Increasing Pressure or Decreasing Volume favors
fewer gas molecules.
The reaction shifts to relieve the pressure.
2A + B
C
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Decreasing Pressure or Increasing Volume favors
more gas molecules.
The reaction shifts to restore the pressure.
2A + B
C
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Assignment 5
 Online Lab : Le Chatelier’s Principle
 Complete OBWS
 Review Sheet 12
 Test
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