Transcript Chapter 07

Fundamentals of General, Organic
and Biological Chemistry
5th Edition
Chapter Seven
Chemical Reactions: Energy,
Rates, and Equilibrium
James E. Mayhugh
Oklahoma City University
2007 Prentice Hall, Inc.
There are many questions about
chemical reactions
2 Au (s) + 3 H2O (l)  Au2O3 (s) + 3 H2 (g)
This reaction is balanced but does not occur. Your gold
jewelry is safe in the shower.
1. Why do reactions occur?
2. If it is balanced does that mean it will occur?
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Chapter 7 Goals
►1. What Energy changes take place during a
reaction? Explain the 2 factors that influence energy
in a chemical reaction
►2. What is “Free Energy,” and what is the criterion
for spontaneity? Define enthalpy, entropy, and free
energy change and explain how these values affect a
chemical reaction
►3. What determines the rate of a chemical reaction?
Explain activation energy, and the other factor that
determines the rate.
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Chapter 7 Goals
►4. What is a chemical equilibrium? What occurs in a
reaction at equilibrium and write the equilibrium
constant expression.
►5. What is Le Châtelier’s Principle? State Le
Châtelier’s Principle, use it to predict the effect of
temperature, pressure, and concentration on a
reaction.
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Outline
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7.1 Energy and Chemical Bonds
7.2 Heat Changes during Chemical Reactions
7.3 Exothermic and Endothermic Reactions
7.4 Why Do Chemical Reactions Occur? Free Energy
7.5 How Do Chemical Reactions Occur? Reaction Rates
7.6 Effects of Temperature, Concentration, and Catalysts on
Reaction Rates
► 7.7 Reversible Reactions and Chemical Equilibrium
► 7.8 Equilibrium Equations and Equilibrium Constants
► 7.9 Le Châtelier’s Principle: The Effect of Changing
Conditions on Equilibria
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Chapter 7
►2 factors determine if a reaction goes to products
1. Total Energy
2. Is the reaction fast or slow
►Can we influence the 2 factors above?
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7.1 Energy and Chemical Bonds
► There are two fundamental kinds of energy.
► Potential energy is stored energy. The water in a
reservoir behind a dam, an automobile poised to
coast downhill, and a coiled spring have potential
energy waiting to be released.
► Kinetic energy is the energy of motion. When the
water falls over the dam and turns a turbine, when
the car rolls downhill, or when the spring uncoils
and makes the hands on a clock move, the potential
energy in each is converted to kinetic energy.
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►1. What Energy changes take place during a
reaction? Explain the 2 factors that influence energy
in a chemical reaction
The first factor is Heat. The term in chemistry is called
enthalpy, and it’s symbol is “H”. Mathematically, it is
always treated as ΔH (Delta H, mean change in heat).
We always compare the amount of heat at the
beginning of a reaction, we call it bond
disassociation, and compare it to the amount of heat
at the end, we call it bond forming, of a chemical
reaction; (the second factor is “S”, slide 19).
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7.2 Heat Changes During Chemical
Reactions
► The first step in any reaction is Bond dissociation
energy: The amount of energy that must be
supplied to break a bond and separate the atoms in
an isolated gaseous molecule. The second step will
be Bond Formation energy.
► The triple bond in N2 has a bond dissociation
energy 226 kcal/mole, while the single bond in Cl2
has a bond dissociation energy 58 kcal/mole.
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Measure of Heat in a reaction
►We always want to know if the total heat is:
►Endothermic: A process or reaction that absorbs
heat and has a positive DH.
►Exothermic: A process or reaction that releases heat
and has a negative DH.
Examples of each follow
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7.3 Exothermic and Endothermic
Reactions
The seconds step is bond formation. When the
total strength of the bonds formed in the products
is greater than the total strength of the bonds
broken in the reactants, energy is released and a
reaction is exothermic.
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When the total energy of the bonds formed in the
products is less than the total energy of the bonds
broken in the reactants, energy is absorbed and the
reaction is endothermic.
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A thermite reaction is:
Fe2O3 + Al  Fe + Al2O3
ΔH = -851.3 KJ/mol
Is this endothermic or
exothermic?
Reactions that give off heat
exothermic
are _________________?
When potassium is added to water contained in a
beaker, the reaction shown below occurs, and the
beaker feels hot to the touch.
K(s) + 2 H2O(l)  2 KOH(aq) + H2(g)
This reaction is
1.
2.
3.
4.
endothermic and DH = –
endothermic and DH = +
exothermic and DH = –
exothermic and DH = +
When potassium is added to water contained in a
beaker, the reaction shown below occurs, and the
beaker feels hot to the touch.
K(s) + 2 H2O(l)  2 KOH(aq) + H2(g)
This reaction is
1.
2.
3.
4.
endothermic and DH = –
endothermic and DH = +
exothermic and DH = –
exothermic and DH = +
This desert has 550 kcal of potential energy, heat
releasing calories.
Food is “potential energy.” Does our body use food in an
endothermic or exothermic manner?
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Chapter Seven
We’ve seen how heat is a factor in a chemical reaction
(bond breaking, bond forming). Heat releasing
(exothermic) reactions are often “favorable” to us.
However, there is one more factor in a chemical
reaction.
►1. What Energy changes take place during a
reaction? Explain the 2 factors that influence energy
in a chemical reaction
The second influence on energy is called Spontaneity.
The symbol is S.
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►Once at the top will water flow down
7.4 Why Do Chemical Reactions
Occur? Free Energy
► Spontaneous process: A process that, once started,
proceeds without any external influence.
► Spontaneity does not care about how long it takes,
only once it starts, it will go. For instance, it takes a
long time for a car to rust, though it is spontaneous.
Once gasoline is ignited, it will continue to burn on it’s
own.
► We measure spontaneity by
measuring the “disorder”
of a system.
Once at the top will water flow down?
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7.4 Why Do Chemical Reactions
Occur? Free Energy
► Entropy: The symbol S is used for entropy and it
has the unit of cal/mole·K. The physical state of a
substance and the number of particles have a large
impact on the value of S. An example: solid CO2 has
a lower entropy value then gaseous CO2…why?
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►If I shake the beaker on the right, will they ever line
up like the beaker on the left?
►The beaker on the right has a more “+” entropy
value
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7.4 Why Do Chemical Reactions
Occur? Free Energy
► Entropy: The symbol S is used for entropy and it
has the unit of cal/mole·K. The physical state of a
substance and the number of particles have a large
impact on the value of S. An example: solid CO2 has
a lower entropy value then gaseous CO2…why? Gas
molecules are more disordered then solids.
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Entropy
►In 7.4 your book talks about melting ice. Taking ice
from the freezer and letting it melt on the table is an
endothermic process—so in your head, you might
think that this is “Not Favorable,” in the sense that
burning a match is heat “favorable.” However, the
ice does melt. It melts because liquid molecules are
more disordered then solid molecules. At room
temperature, the ice cube will not only melt but
Evaporate into a gas…why? Going from order to
disorder is a powerful driving factor in a chemical
reaction.
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Chapter Seven
Entropy
►Is this an entropy favorable, S = +, reaction?
Dynamite, better known as TNT
2 C7H5N3O6 (s)  3 N2 (g) + 5 H2O (g) + 7 CO (g) + 7 C (g)
It goes from 2 moles of a soild to 22 moles of a gas.
Prespective: it starts out as a cup of solid, and ends up
as 2083 cups of gas
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When potassium is added to water contained in a
beaker, the reaction shown below occurs, and the
beaker feels hot to the touch.
K(s) + 2 H2O(l)  2 KOH(aq) + H2(g)
During this reaction
1.
2.
3.
4.
entropy decreases and DS = –
entropy decreases and DS = +
entropy increases and DS = –
entropy increases and DS = +
When potassium is added to water contained in a
beaker, the reaction shown below occurs, and the
beaker feels hot to the touch.
K(s) + 2 H2O(l)  2 KOH(aq) + H2(g)
During this reaction
1.
2.
3.
4.
entropy decreases and DS = –
entropy decreases and DS = +
entropy increases and DS = –
entropy increases and DS = +
►2. What is “Free Energy,” and what is the criterion
for spontaneity? Define enthalpy, entropy, and free
energy change and explain how these values affect a
chemical reaction
►Both Heat and Entropy affect the Total Energy of a
chemical reaction.
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► Free energy change (DG): Free energy change is used to
describe spontaneity of a process. It takes both DH and DS
into account.
► Exergonic: A spontaneous reaction or process that releases
free energy and has a negative DG.
► Endergonic: A nonspontaneous reaction or process that
absorbs free energy and has a positive DG.
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DG = DH - TDS
DH
DS
DG
(-) favorable
(+) favorable
(-) spontaneous always
(+) unfavorable (-) unfavorable
(+) nonspontaneous always
(-) favorable
(-) spontaneous @ Low T
(+) nonspontaneous @ High T
(-) unfavorable
(+) unfavorable (+) favorable
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(+) nonspontaneous @ Low T
(-) spontaneous @ High T
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When potassium is added to water contained in a
beaker, the reaction shown below occurs, and the
beaker feels hot to the touch.
K(s) + 2 H2O(l)  2 KOH(aq) + H2(g)
This reaction is
1.
2.
3.
4.
nonspontaneous and DG = –
nonspontaneous and DG = +
spontaneous and DG = –
spontaneous and DG = +
When potassium is added to water contained in a
beaker, the reaction shown below occurs, and the
beaker feels hot to the touch.
K(s) + 2 H2O(l)  2 KOH(aq) + H2(g)
This reaction is
1.
2.
3.
4.
nonspontaneous and DG = –
nonspontaneous and DG = +
spontaneous and DG = –
form heat a gas and ions
spontaneous and DG = +
Chapter Seven
When is it Spontanious?
►7.40. For the reaction: H2 + Br2  2HBr
∆H = -17.4 kcal/Kmol, ∆S = 27.2 cal/Kmol
1. Is this reaction spontaneous at all temperatures?
No
2. At what temperature is the reaction spontaneous?
When ∆G = 0, anything above 0 is spontaneous,
so solve for T; ∆G = ∆H – T ∆S,
0 = -17.4 kcal/Kmol– T(.0272kcal/Kmol)
= 640. K or 367ºC
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►3. What determines the rate of a chemical reaction?
Explain activation energy, and the other factor that
determines the rate.
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7.5 How Do Chemical Reactions
Occur? Reaction Rates
► The value of DG indicates whether a reaction will
occur but it does not say anything about how fast
the reaction will occur or about the details of the
molecular changes that takes place.
► For a chemical reaction to occur, reactant particles
must collide, some chemical bonds have to break,
and new bonds have to form. Not all collisions lead
to products, however.
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One requirement for a productive collision is that the
colliding molecules must approach with the correct
orientation so that the atoms about to form new
bonds can connect.
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Another requirement for a reaction to occur is that
the collision must take place with enough energy to
break the appropriate bonds in the reactant. If the
reactant particles are moving slowly the particles will
simply bounce apart.
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► Activation energy (Ea): The amount of energy
the colliding particles must have for productive
collisions to occur. The size of the activation
energy determines the reaction rate, or how fast
the reaction occurs.
► The lower the activation energy, the greater the
number of productive collisions in a given
amount of time, and faster the reaction.
► The higher the activation energy, the lower the
number of productive collisions, and slower the
reaction.
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Shown is an energy diagram for a reaction with a
small activation energy and products having less
energy than reactants. This reaction is
1. nonspontaneous
and fast.
2. nonspontaneous
and slow.
3. spontaneous and
fast.
4. spontaneous and
slow.
Shown is an energy diagram for a reaction with a
small activation energy and products having less
energy than reactants. This reaction is
1. nonspontaneous
and fast.
2. nonspontaneous
and slow.
3. spontaneous and
fast.
4. spontaneous and
slow.
Draw a reaction diagram for:
►A reaction with a ∆G of -50 kcal/Kmol and Ea of
25kcal/mol. Label axis, and if the reaction is
endergonic or exergonic.
-50 kcal/mol
25 kcal/mol
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7.6 Effects of Temperature, Concentration,
and Catalysts on Reaction Rates
Reaction rates increase with temperature. With more
energy the reactants move faster. The frequency of
collisions and the force with which collisions occur both
increase. As a rule of thumb, a 10°C rise in temperature
causes a reaction rate to double.
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► A second way to speed up a reaction is to increase
the concentrations of the reactants.
► With reactants crowded together, collisions become
more frequent and reactions more likely.
Flammable materials burn more rapidly in pure
oxygen than in air because the concentration of
molecules is higher (air is approximately 21%
oxygen).
► Hospitals must therefore take extraordinary
precautions to ensure that no flames are used near
patients receiving oxygen.
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► A third way to speed up a reaction is to add a
catalyst—a substance that accelerates a chemical
reaction but is itself unchanged in the process.
► A catalyzed reaction has a lower activation energy.
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The thousands of biochemical reactions continually
taking place in our bodies are catalyzed by large protein
molecules called enzymes, which promote reaction by
controlling the orientation of the reacting molecules.
Since almost every reaction is catalyzed by its own
specific enzyme, the study of enzyme structure, activity,
and control is a central part of biochemistry.
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When a catalyst is added to a reaction to increase its
rate of reaction the
1. activation energy is lowered and DG becomes
more negative.
2. activation energy is lowered and DG remains
unchanged.
3. activation energy is raised and DG becomes more
positive.
4. activation energy is raised and DG remains
unchanged.
When a catalyst is added to a reaction to increase its
rate of reaction the
1. activation energy is lowered and DG becomes
more negative.
2. activation energy is lowered and DG remains
unchanged.
3. activation energy is raised and DG becomes more
positive.
4. activation energy is raised and DG remains
unchanged.
Two curves are shown on the energy diagram. Which
represents the faster reaction and which the slower?
Which represents a spontaneous reaction?
The faster reaction is the one with the smaller activation energy
which is the reaction indicated by the blue line. The spontaneous
reaction should have a negative delta G which would be the
reaction indicated by the red line.
Two curves are shown in the following energy diagram.
Which represents the catalyzed reaction?
Catalysts lower the activation energy, so the curve with
the lower hump is the catalyzed reaction.
►You’ve just digested a lot about chemical reactions.
►Free Energy, ΔG, tells us if we make products or not.
►The Eact gives us the rate, or how fast.
►To complete this chapter, we’re going to look at what
commonly occurs in many chemical reactions, that is,
often there isn’t a large ΔG, and the stability of the
products and reactants is about the same, AND there is
usually a very low Eact. So what happens is, it is about
as easy to make products as reactants
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Be able to answer the following question in the next
section
►4. What is a chemical equilibrium? What occurs in a
reaction at equilibrium and write the equilibrium
constant expression.
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►What occurs in a reaction at
equilibrium.
►When the number of people
moving up is the same as the
number of people moving
down, the number of people
on each floor remains
constant, and the two
populations are in equilibrium.
►Equilibrium occurs when the
forward and reverse reactions
are the same.
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7.7 Reversible Reactions and Chemical
Equilibrium
Imagine the situation if you mix acetic acid and ethyl
alcohol. The two begin to form ethyl acetate and
water. But as soon as ethyl acetate and water form,
they begin to go back to acetic acid and ethyl
alcohol. Such a reaction, which easily goes in either
direction, is said to be reversible and is indicated by
a double arrow in equations; like people on the
escalator.
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Both reactions occur until the concentrations of reactants
and products reach constant values. The reaction vessel
contains both reactants and products and is said to be in a
state of chemical equilibrium. A state in which the rates of
forward and reverse reactions are the same.
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7.8 Equilibrium Equations and
Equilibrium Constants
► Consider the following general equilibrium reaction:
aA + bB + …  mM + nN + …
► Where A, B, … are the reactants; M, N, …. Are the
products; a, b, ….m, n, …. are coefficients in the
balanced equation. At equilibrium, the composition
of the reaction mixture obeys an equilibrium
equation.
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► The equilibrium constant K is the number obtained
by multiplying the equilibrium concentrations of
the products and dividing by the equilibrium
concentrations of the reactants, with the
concentration each substance raised to a power
equal to its coefficient in the balanced equation.
► The value of K varies with temperature.
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The pictures shown represent two similar reactions that
have achieved equilibrium. Which has the larger
equilibrium constant, and which has the smaller
equilibrium constant?
-The
pictures below represent four similar reactions that
have achieved equilibrium. A atoms are unshaded. B, C,
D, and E atoms are shaded. Which reaction has the
largest equilibrium constant?
A 2 + B2 
 2 AB
1.
2.
3.
4.
A2 + B2  2 AB
A2 + C2  2 AC
A2 + D2  2 AD
A2 + E2  2 AE
A 2 + C2 
 2 AC
A 2 + D2 
 2 AD
A 2 + E2 
 2 AE
-The
pictures below represent four similar reactions that
have achieved equilibrium. A atoms are unshaded. B, C,
D, and E atoms are shaded. Which reaction has the
largest equilibrium constant?
A 2 + B2 
 2 AB
1.
2.
3.
4.
A2 + B2  2 AB
A2 + C2  2 AC
A2 + D2  2 AD
A2 + E2  2 AE
A 2 + C2 
 2 AC
A 2 + D2 
 2 AD
A 2 + E2 
 2 AE
► K larger than 1000: Reaction goes essentially to
completion.
► K between 1 and 1000: More products than
reactants are present at equilibrium.
► K between 1 and 0.001: More reactants than
products are present at equilibrium.
► K smaller than 0.001: Essentially no reaction
occurs.
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Chapter Seven
Chapter Seven
7.9 Le Châtelier's Principle: The Effect of
Changing Conditions on Equilibria
► Le Châtelier's Principle: When a stress is applied to
a system at equilibrium, the equilibrium shifts to
relieve the stress.
► The stress can be any change in concentration,
pressure, volume, or temperature that disturbs
original equilibrium.
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► What happens if the concentration of CO is
increased?
► To relieve the “stress” of added CO, according to Le
Châtelier’s principle, the extra CO must be used up.
In other words, the rate of the forward reaction
must increase to consume CO.
► Think of the CO added on the left as “pushing” the
equilibrium to the right:
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► The forward and reverse reaction rates adjust until
they are again equal and equilibrium is reestablished.
► At this new equilibrium state, the value of [H2] will be
lower, because more has reacted with the added CO,
and the value of [CH3OH] will be higher.
► The changes offset each other, however, so the value
of the equilibrium constant K remains constant.
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► Le Châtelier’s principle predicts that an increase in
temperature will cause an equilibrium to shift in
favor of the endothermic reaction so the additional
heat is absorbed.
► You can think of heat as a reactant or product
whose increase or decrease stresses an equilibrium
just as a change in reactant or product
concentration does.
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► Pressure influences an equilibrium only if one or
more of the substances involved is a gas. As
predicted by Le Châtelier’s principle, increasing the
pressure shifts the equilibrium in the direction that
decreases the number of molecules in the gas phase
and thus decreases the pressure.
► For the ammonia synthesis, increasing the pressure
favors the forward reaction because 4 moles of gas
is converted to 2 moles of gas.
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The effects of changing reaction conditions on
equilibria are summarized below.
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Chapter Seven
Practice
►7.83: For the reaction:
2Al + 3Cl2  2AlCl3 + 336.6 kcal/mol
How much heat is released when 5.00 grams of Al
reacts with excess chlorine?
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Chapter Summary
► The strength of a covalent bond is measured by its
bond dissociation energy.
► If heat is released, DH is negative and the reaction is
said to be exothermic. If heat is absorbed, DH is
positive and the reaction is said to be endothermic.
► Spontaneous reactions are those that, once started,
continue without external influence;
nonspontaneous reactions require a continuous
external influence.
► Spontaneity depends on two factors, the amount of
heat absorbed or released in a reaction and the
entropy change.
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Chapter Summary Contd.
► Spontaneous reactions are favored by a release of
heat, DH <0, and an increase in entropy, DS >0.
► The free-energy change, DG = DH - T DS, takes both
factors into account.
► DG<0 indicates spontaneity, DG>0 indicates
nonspontaneity.
► Chemical reactions occur when reactant particles
collide with proper orientation and energy. The exact
amount of collision energy necessary is the activation
energy.
► Reaction rates can be increased by raising the
temperature, by raising the concentrations of
reactants, or by adding a catalyst.
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Chapter Summary Contd.
► At equilibrium, the forward and reverse reactions
occur at the same rate, and the concentrations of
reactants and products are constant. Every reversible
reaction has an equilibrium constant, K. The forward
reaction is favored if K>1; the reverse reaction is
favored if K<1.
► Le Châtelier’s principle states that when a stress is
applied to a system in equilibrium, the equilibrium
shifts so that the stress is relieved.
► Applying this principle allows prediction of the effects
of changes in temperature, pressure, and
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Chapter Seven
concentration.
End of Chapter 7
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