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Honors Chemistry
Chapter 18 Notes – Reaction Rates and Equilibrium
(Student edition)
Chapter 18 problem set: 49-52, 55, 56, 70, 75, 76, 78, 79, 81, 83, 86
18.1
Rates of Reaction
Kinetics: the study of how
and how
Reaction rate: the speed with which a reaction takes place
rate =
[R] represents the
Reaction mechanism: the
in a reaction
Thermodynamics: the study of
in chemical reactions
Collision Theory: In order for molecules to react, they must
doesn’t guarantee reaction.
, but
For a reaction to take place,
1. the collisions need enough
2. the particles need proper
Reaction Mechanism: The series of
We have learned the following:
H2
+
I2

H = + 26.5 kJ
in a reaction.
2 HI
However, we now know that it is really:
Step 1
I2
Step 2
I
+
H2
Step 3
H2I
+
I
H2
+
I2




2I
H2I
2 HI
2 HI
: a species that appears in some steps but not in the overall
reaction. It is relatively short lived. So, in the above
example, H2I is the reaction intermediate.
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What affects reaction rate?
The Nature of Reactants
Reaction rates are determined by the type of
.
Reactants that have ionic bonds react
than reactants that have covalent bonds.
Temperature and Reaction Rate
For every 10 C0 increase, the rate of reaction approximately
This is easily explained by
there are more
(souring milk).
. Since particles move faster,
and the molecules have more
.
Concentration of Reactants
An Increase in [R]
increases reaction rate if it is a homogeneous
reaction (all reactants are in the same phase).
Ex.
2 H2(g)
+
O2(g)

2 H2O(g)
NaCl(aq)
+
AgNO3(aq)

NaNO3(aq)
+
AgCl(ppt)
Heterogeneous reaction: when the reactants are in more than one phase.
Ex.
Ex.
4Fe(s) +
3O2(g) 
2Fe2O3(s)
Again….
 [R] leads to  rate. This is explained by
A
B
+

If we double [A], the rate
.
C
.
If we double [B], the rate
.
If we double the [ ] of both reactants, the rate
.
Pressure and Reaction Rate
Pressure only affects
.
For gases, if pressure increases, the amount of reactants in an area
So, the reaction rate
.
.
2
Catalysts and Reaction Rate
Catalyst: A substance that
the reaction.
Inhibitor: A substance that
Catalysts work by
the activation energy (Ea).
a reaction, but is not
in
the rate of reaction.
the mechanism of a reaction and
Activation Energy and the Activated Complex:
Activated Complex
New Products
Collision
Not enough energy to make new products
: a short lived structure existing when old bonds are broken and new
bonds are being formed.
: energy needed to transform reactants into an activated complex.
Reactants must have sufficient
.
3
Energy Diagrams:
50
h 40
e
a 30
t
reactants
20
products
10
time
Energy of the reactants =
Energy of the activated complex =
Change in the heat =
Energy of the products =
Activation energy =
The reaction is
.
50
h 40
e
a 30
t
products
20
reactants
10
time
Energy of the reactants =
Energy of the activated complex =
Change in the heat =
Energy of the products =
Activation energy =
The reaction is
.
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Activation Energy – Temperature and Concentration:
An increase in temperature equals an increase in
.
So, the number of particles that can reach the appropriate Ea
.
#
Of
P
a
r
t
i
c
l
e
s
T1
kinetic energy
Ea
#
Of
P
a
r
t
i
c
l
e
s
T2
kinetic energy
Ea
5
Activation Energy and Catalysts:
Catalysts lower Ea by changing the
.
Lower activation energy means more
more easily, thereby forming
18.2
can reach the
more quickly.
Reversible Reactions
_Fe3O4(s)
+
4 H2(g)

3 Fe(s)
This reaction is also possible in
+
4 H2O(g)
.
If water is removed from the vessel, the reaction shifts to the
closed container…
Fe3O4(s)
+
H2(g)

The double arrows represent
Fe(s)
+
, but if in a
H2O(g)
.
: when the rate of the forward reaction equals rate of the reverse reaction.
r
a
t
e
r
a
t
e
time
Forward reaction:
Reactants combining
to form
r
a
t
e
time
Reverse reaction:
Products combining
to form
time
Equilibrium:
Reactants forming
products forming
and
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: It is used to describe a system undergoing a chemical change.
xA
+

yB
zC
+
aD
the mass action expression 
so for…
2A
+

B
3C
+
2D
the mass action expression 
: It is essentially the same thing as the mass action expression.
For…
H2
Keq
+
I2

2 HI
=
Ex1:
At the beginning of a reaction, the Hydrogen concentration is 1.00 M
(1 mole/Liter) and so is the Iodine concentration. At equilibrium, [H2] = 0.228 M,
[I2] = 0.228 M, and [HI] = 1.544 M. Calculate Keq.
Ex2:
At the beginning of a reaction, the HI concentration is 1.00 M. At equilibrium,
[H2] = 0.114 M, [I2] = 0.114 M, and [HI] = 0.772 M. Calculate Keq.
Keq =
Note:
(
If K1,
are favored.
If K1,
are favored.
,
)
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Applications of Keq
Ex1:
If the concentration of HI at equilibrium is 0.158 M, what are the concentrations
of Hydrogen and Iodine? (Keq = 45.9)
Ex2:
If the concentration of HI at equilibrium is 1.423 M, what are the concentrations
of Hydrogen and Iodine if twice as much Hydrogen was added as Iodine?
Ex3:
For the reaction….
N2O4 
2 NO2
If 1.00 mol of Dinitrogen tetroxide is in a 5.00 dm3 container at 100 C0, it
decomposes into Nitrogen dioxide. At equilibrium, 1.00 mole of the product is
present in the container. Calculate Keq.
[
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Le Chatelier’s Principle:
We’ve learned before, Keq is a
can change.
– however, conditions in a reaction
Le Chatelier’s Principle: When a system at equilibrium is
shifts to
that stress.
, the system
Types of change:
1.  [ ]
for the reaction:
A
+
B

C
+
D
If we increase the amount of A or B, the system shifts to the
If we increase the amount of C or D, the system shifts to the
If we decrease the amount of C or D, the system shifts to the
(a good way to increase
!)
Shift away from an
and towards a
.
Please note: Keq
2. T
The shift depends on whether the reaction is
A
+
B
+
heat
or

C
For an endothermic reaction, if we increase the amount of heat, the system shifts to the
Treat heat like a reactant. It is the
for an exothermic reaction.
For exothermic reactions, removing heat helps increase
Shift away from an
and towards a
.
.
.
.
Please note: Keq
3. P
Pressure only affects
.
An increase in pressure shifts the reaction towards the
.
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For the reaction:
A(g)
+
3B(g) 
2C(g)
An increase in pressure will shift the reaction to the
A decrease in pressure will shift the reaction to the
Please note: Keq
4. Catalysts
It has
effect on equilibrium.
It’s just that the reaction proceeds to equilibrium
.
18.3 Solubility Equilibrium
Equilibrium is also reached when solids dissolve in water.
AgCl(s) +

H2O(l)
Ag+1(aq)
+
Cl-1(aq)
We don’t count solids or liquid water in K expressions, so…
Ksp =
Ksp = solubility product constant (only for products)
K = more soluble
For… Ag2SO4(s)
+
H2O(l)

2 Ag+1(aq)
+
SO4-2(aq)
Ksp =
10
Ex1: If Ksp for CdS = 1.0 x 10 –28, what is the concentration of each ion?
CdS(s) +

H2O(l)
Cd+2(aq)
+
S-2(aq)
Ex2: If the Ksp for Ag2SO4 = 1.10 x 10-12, calculate the [ ] of each ion.
Ex3: If the solubility of BaSO4 is 9.09 g in 100.0 cm3 of water, find Ksp.
18.10 The Common Ion Effect
When a slightly soluble solid is dissolved in solution, and another salt with a
is added, equilibrium shifts
, causing
Ex:
AgCl (s)

Ag+1(aq)
+
.
Cl-1(aq)
Ksp = 1.7 x 10 –11 (very
)
If we add NaCl….
NaCl(s)

Na+1(aq) + Cl-1(aq)
Since we add Cl-1, we are increasing the
shifts
and AgCl
.
(adding a
)
, and the reaction
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18.4
Entropy and Free Energy
In regard to enthalpy,
For example;
is more favorable (
C8H18 +

O2
CO2
).
+
H2O
+
heat
However, this is not the only driving force.
Entropy (S): the measure of
in a system. The higher disorder (more
S), the
likely the reaction is to occur (messy room, leaves on trees).
Systems tend to go towards
energy (
) and
randomness (
).
General trends in entropy:
s

l

g
g

g + g + g
 temperature
=
=
=
Spontaneous reactions take place without outside influence (they can be fast or slow).
Ex1:
2 C8H18(l)
+ 25 O2(g)
 16 CO2(g)
pieces and
Entropy (S) is
and enthalpy (H) is
CO2(g) + N2(g)
+ H2O(g)
Ex3: H2O (s) 
+
.
heat

C3H5(ONO2)3(l)
heat
and enthalpy (H) is
Thus, the reaction is
heat
.
pieces and
Entropy (S) is
+
heat
Thus, the reaction is
Ex2:
+ 18H2O(g)
.
.
H2O (l)
does it happen?
Entropy (S) is
and enthalpy (H) is
.
Thus, we
determine the spontaneity based on the information given.
Gibbs Free energy formula is used to determine the spontaneity.
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Free energy of a system: G = Gibbs Free Energy (combined enthalpy/entropy function)
G =
(use
temperature)
if G is negative, the reaction is
Possible combinations of entropy and enthalpy:
Spontaneous
NonSpontaneous
Maybe
Maybe
Enthalpy
Entropy
What determines a maybe? Temperature
Is this reaction spontaneous?
H2O
+
C

CO
+
H2
H = + 131.3 kJ/mole
S = + 0.134 kJ/mole.K at 25 Co
How about at 900 Co?
18.5
The Progress of Chemical Reactions – Rate Laws
For:
A + B 
rate =
C
(determined experimentally)
If a reaction is one step, then the coefficients equal the exponents.
If a reaction is multi step, then the coefficients do not equal exponents.
Ex.
2 H2
+
2 NO 
N2
+ 2 H2O
If the reaction is one step, the reaction rate formula is …
R=
13
However, experiments show:
2 times [ H2 ] = rate increases 2 times
3 times [ H2 ] = rate increases 3 times
So, rate is proportional to
2 times [ NO ]
= rate increases 4 times
3 times [ NO ]
= rate increases 9 times
So, rate is proportional to
So, the rate is proportional to
Thus, the reaction rate formula is …
Ex.
X
+

2Y
R=
XY2
(a single step reaction)
Write the rate law:
R=
If you double X, the rate
.
If you double Y, the rate
.
If Y is reduced to 1/3, the rate is
.
If X is cut in half and Y is doubled, the rate
.
Reaction Mechanisms and the Rate of Reaction:
Different steps take place at
.
The slowest step is the
step.
Increasing the [ R ] in the
Ex:
So, increasing
step increases the rate of reaction.
A
+
B

Int1
fast step
A
+
Int1

Int2
slow step
C
+
Int2

D
fast step
or
doesn’t speed up the reaction, but increasing
does.
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