Chapter 15 Chemical Equilibrium

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Chapter 15ChemicalEquilibrium
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Equilibrium vs. Disequilibrium

• when systems are at equilibrium with their
  surroundings, their conditions are the same as
  the surroundings and they stay that way
• when systems are in disequilibrium with their
  surroundings, their conditions are not the same
  as the surroundings
• systems that are in disequilibrium tend to change
  until they reach equilibrium with their
  surroundings
• living things are in controlled disequilibrium
  with their environment they are not at the same
  conditions as the environment and do not tend to
  change toward those conditions

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Reaction Rates

• some chemical reactions proceed rapidly
• like the precipitation reactions studied in Chp
  7, where the products form practically the
  instant the two solutions are mixed
• other reactions proceed slowly
• like the decomposition of dye molecules of a sofa
  placed in front of a window
• the rate of a reaction is measured in the amount
  of reactant that changes into product in a given
  period of time
• generally moles of reactant used per second
• like miles per hour
• chemists study ways of controlling reaction rates

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Reaction Rates
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2 N2O5 (g) ? 4 NO2(g) O2(g)
Over time, the concentrations of reactants
decrease as products increase.
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2 N2O5 (g) ? 4 NO2(g) O2(g)Rate vs. Time
Because reactant concentrations decrease, the
rates of reactions slow down over time.
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Collision Theory

• in order for a reaction to take place, the
  reacting molecules must collide with each other.
• once molecules collide they may react together or
  they may not, depending on two factors -
• whether the collision has enough energy to start
  to break the bonds holding reactant molecules
  together"
• whether the reacting molecules collide in the
  proper orientation for new bonds to form.

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Effective Collisions

• collisions in which these two conditions are met
  (and therefore the reaction occurs) are called
  effective collisions.
• the higher the frequency of effective collisions
  the faster the reaction rate.
• there is a minimum energy needed for a collision
  to be effective we call this the activation
  energy
• the lower the activation energy, the faster the
  reaction will be

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Activation Energy

• the energy barrier that prevents any collision
  between molecules from being an effective
  collision is called the activation energy
• the larger the activation energy of a reaction,
  the slower it will be
• at a given temperature

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Exothermic Reaction
Activation Energy, large
Activation Energy, small
Reactants
Relative Potential Energy
DHreaction
Products
Progress of Reaction
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Endothermic Reaction
Activation Energy
Products
Relative Potential Energy
DHreaction
Reactants
Progress of Reaction
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Factors Effecting Reaction RateReactant
Concentration

• the higher the concentration of reactant
  molecules, the faster the reaction will generally
  go.
• increases the frequency of reactant molecule
  collisions
• since reactants are consumed as the reaction
  proceeds, the speed of a reaction generally slows
  over time

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Effect of Concentration on Rate
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Factors Effecting Reaction RateTemperature

• increasing temperature increases the reaction
  rate
• for each 10C rise in temperature, the speed of
  the reaction generally doubles
• increasing the temperature increases the number
  of molecules in the sample with enough energy so
  that their collisions can overcome the activation
  energy
• increasing the temperature also increases the
  frequency of collisions so the rate increases
  because the frequency of effective collisions
  increases

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Effect of Temperature on Rate
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Reaction Dynamics

• If the products of a reaction are removed from
  the system as they are made, then a chemical
  reaction will proceed until the limiting
  reactants are used up.
• However, if the products are allowed to
  accumulate they will start reacting together to
  form the original reactants. This is called the
  reverse reaction.

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Reaction Dynamics

• The forward reaction slows down as the amounts of
  reactants decreases.
• At the same time the reverse reaction speeds up
  as the concentration of the products increases.
• Eventually the forward reaction is using
  reactants and making products as fast as the
  reverse reaction is using products and making
  reactants. This is called chemical equilibrium.
• dynamic equilibrium is reached when the rates of
  two opposite processes are the same

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Chemical Equilibrium

• When a reaction reaches equilibrium, the amounts
  of reactants and products in the system stay
  constant
• the forward and reverse reactions still continue
• because they go at the same rate, the amounts of
  materials don't change.

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Equilibrium
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Equilibrium
Initially, only the forward reaction takes place.
As the forward reaction proceeds it makes
products and uses reactants.
Because the reactant concentration decreases,
the forward reaction slows. As the products
accumulate, the reverse reaction speeds up.
Once equilibrium is established, the forward and
reverse reactions proceed at the same rate, so
the concentrations of all materials stay constant.
Eventually, the reaction proceeds in the reverse
direction as fast as it proceeds in the forward
direction. At this time equilibrium is
established.
Rate
Time
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Equilibrium ? Equal

• the rates of the forward and reverse reactions
  are equal at equilibrium
• but that does not mean the concentrations of
  reactants and products are equal
• some reactions reach equilibrium only after
  almost all the reactant molecules are consumed
  we say the position of equilibrium favors the
  products
• other reactions reach equilibrium when only a
  small percentage of the reactant molecules are
  consumed we say the position of equilibrium
  favors the reactants

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An Analogy Population Changes
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Equilibrium Constant

• even though the concentrations of reactants and
  products are not equal at equilibrium, there is a
  relationship between them
• for the general equation aA bB ? cC dD, the
  relationship given below is called the
  equilibrium constant, Keq
• the lower case letters represent the coefficients
  of the balanced chemical equation
• always products over reactants

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Writing Equilibrium Constant Expressions

• for aA bB ? cC dD the equilibrium constant
  expression is

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What Does the Value of Keq Imply?

• when the value of Keq gtgt 1, we know that when the
  reaction reaches equilibrium there will be many
  more product molecules present than reactant
  molecules
• the position of equilibrium favors products
• when the value of Keq ltlt 1, we know that when the
  reaction reaches equilibrium there will be many
  more reactant molecules present than product
  molecules
• the position of equilibrium favors reactants

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A Large Equilibrium Constant
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A Small Equilibrium Constant
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Equilibrium Constants for Heterogeneous
Equilibria

• pure substances in the solid and liquid state
  have constant concentrations
• adding or removing some does not change the
  concentration because they do not expand to fill
  the container or spread throughout a solution
• therefore these substances are not included in
  the equilibrium constant expression

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Calculating Keq

• the value of the equilibrium constant may be
  determined by measuring the concentrations of all
  the reactants and products in the mixture after
  the reaction reaches equilibrium, then
  substituting in the expression for Keq
• even though you may have different amounts of
  reactants and products in the equilibrium
  mixture, the value of Keq will always be the same
• the value of Keq depends only on the temperature
• the value of Keq does not depend on the amounts
  of reactants or products you start with

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Initial and Equilibrium Concentrations forH2(g)
I2(g) ? 2HI(g)
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• Example
• A mixture of CH4, C2H2 and H2 is allowed to come
  to equilibrium at 1700C. The measured
  equilibrium concentrations are CH4 0.0203 M,
  C2H2 0.0451 M, and H2 0.112 M. What is
  the value of the equilibrium constant at this
  temperature?
• 2 CH4(g) ? C2H2(g) 3 H2(g)

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• Example
• In an equilibrium mixture the concentrations of
  H2 and I2 are both 0.020 M. What is the value
  of the equilibrium concentration of HI?
• H2(g) I2(g) ? 2 HI(g) Keq 69 at 340C

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Disturbing and Re-establishingEquilibrium

• once a reaction is at equilibrium, the
  concentrations of all the reactants and products
  remain the same
• however if the conditions are changed, the
  concentrations of all the chemicals will change
  until equilibrium is re-established
• the new concentrations will be different, but the
  equilibrium constant will be the same
• unless you change the temperature

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Le Châteliers Principle

• Le Châteliers Principle guides us in predicting
  the effect on the position of equilibrium when
  conditions change
• When a chemical system at equilibrium is
  disturbed, the system shifts in a direction that
  will minimize the disturbance

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An Analogy Population Changes
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The Effect of Concentration Changes on Equilibrium

• Adding a reactant will decrease the amounts of
  the other reactants and increase the amount of
  the products until a new position of equilibrium
  is found
• that has the same Keq
• Removing a product will increase the amounts of
  the other products and decrease the amounts of
  the reactants.
• you can use to this to drive a reaction to
  completion!
• Remember, adding more of a solid or liquid does
  not change its concentration and therefore has
  no effect on the equilibrium

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The Effect of Concentration Changes on Equilibrium
When NO2 is added, some of it combines to make
more N2O4
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The Effect of Concentration Changes on Equilibrium
When N2O4 is added, some of it decomposes to make
more NO2
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Practice - Predict the Effect on the Equilibrium
when the Underlined Substance is Added to the
Following Systems

• 2 CO2(g) Û 2 CO(g) O2(g)
• BaSO4(s) Û Ba2(aq) SO42-(aq)
• CH4(g) 2 O2(g) Û CO2(g) 2 H2O(l)

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Practice - Predict the Effect on the Equilibrium
when the Underlined Substance is Added to the
Following Systems

• 2 CO2(g) Û 2 CO(g) O2(g)
• BaSO4(s) Û Ba2(aq) SO42-(aq)
• CH4(g) 2 O2(g) Û CO2(g) 2 H2O(l)

shift right, removing some of the added CO2 and
increasing the concentrations of CO and O2
shift left, removing some of the added Ba2 and
reducing the concentration of SO42-
shift right, removing some of the added CO2 and
decreasing the O2, while increasing the
concentration of CO2
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Effect of Volume Changeon Equilibrium

• for solids, liquids or solutions, changing the
  size of the container has no effect on the
  concentration
• changing the volume of a container does change
  the concentration of a gas
• same number of moles, but different number of
  liters, resulting in a different molarity

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Effect of Volume Changeon Equilibrium

• decreasing the size of the container increases
  the concentration of all the gases in the
  container
• increases their partial pressures
• if their partial pressures increase, then the
  total pressure in the container will increase
• according to Le Châteliers Principle, the
  equilibrium should shift to remove that pressure
• the way to reduce the pressure is to reduce the
  number of molecules in the container
• when the volume decreases, the equilibrium shifts
  to the side with fewer molecules

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The Effect of Volume Changes on Equilibrium
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Practice - Predict the Effect on the Equilibrium
when the Volume is Reduced

• 2 CO2(g) Û 2 CO(g) O2(g)
• BaSO4(s) Û Ba2(aq) SO42-(aq)
• CH4(g) 2 O2(g) Û CO2(g) 2 H2O(l)

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Practice - Predict the Effect on the Equilibrium
when the Volume is Reduced

• 2 CO2(g) Û 2 CO(g) O2(g)
• BaSO4(s) Û Ba2(aq) SO42-(aq)
• CH4(g) 2 O2(g) Û CO2(g) 2 H2O(l)

shift left, because there are fewer gas
molecules on the reactant side than on the
product side
no effect since none of the substances are gases
shift right, because there are fewer gas
molecules on the product side than on the
reactant side
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The Effect of Temperature Changes on Equilibrium

• exothermic reactions release energy and
  endothermic reactions absorb energy
• if we write Heat as a product in an exothermic
  reaction or as a reactant in an endothermic
  reaction, it will help us use Le Châteliers
  Principle to predict the effect of temperature
  changes
• even though heat is not matter and not written in
  a proper equation

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The Effect of Temperature Changes on Equilibrium
for Exothermic Reactions

• for an exothermic reaction, heat is a product
• increasing the temperature is like adding heat
• according to Le Châteliers Principle, the
  equilibrium will shift away from the added heat
• the concentrations of C and D will decrease and
  the concentrations of A and B will increase
• the value of Keq will decrease
• how will decreasing the temperature effect the
  system?

aA bB ? cC dD Heat
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The Effect of Temperature Changes on Equilibrium
for Endothermic Reactions

• for an endothermic reaction, heat is a reactant
• increasing the temperature is like adding heat
• according to Le Châteliers Principle, the
  equilibrium will shift away from the added heat
• the concentrations of C and D will increase and
  the concentrations of A and B will decrease
• the value of Keq will increase
• how will decreasing the temperature effect the
  system?

Heat aA bB ? cC dD
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The Effect of Temperature Changes on Equilibrium
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Practice - Predict the Effect on the Equilibrium
when the Temperature is Reduced

• Heat 2 CO2(g) Û 2 CO(g) O2(g)
• BaSO4(s) Û Ba2(aq) SO42-(aq) endothermic
• CH4(g) 2 O2(g) Û CO2(g) 2 H2O(l) exothermic

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Practice - Predict the Effect on the Equilibrium
when the Temperature is Reduced

• Heat 2 CO2(g) Û 2 CO(g) O2(g)
• Heat BaSO4(s) Û Ba2(aq) SO42-(aq)
• CH4(g) 2 O2(g) Û CO2(g) 2 H2O(l) Heat

shift left, reducing the value of Keq
shift left, reducing the value of Keq
shift right, increasing the value of Keq
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Solubility Solubility Product

• even insoluble salts dissolve somewhat in water
• insoluble less than 0.1 g per 100 g H2O
• the solubility of insoluble salts is described in
  terms of equilibrium between undissolved solid
  and aqueous ions produced
• AnYm(s) Û n A(aq) m Y-(aq)
• equilibrium constant for this process is called
  the solubility product constant, Ksp
• Ksp AnY-m
• if there is undissolved solid in equilibrium with
  the solution, the solution is saturated
• larger Ksp more soluble
• for salts that produce the same number of ions

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Example - Determine the Ksp of PbBr2 if its
solubility is 1.44 x 10-2 M
PbBr2(s) Û Pb2(aq) 2 Br(aq) init -- 0
0 equil -- 0.0144 0.0288
Ksp Pb2Br2 (0.0144)(0.0288)2 1.19 x
10-5
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• Example
• Calculate the molar solubility of BaSO4. Ksp
  1.07 x 10-10 at 25C

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Activation Energy

• the energy barrier that prevents any collision
  between molecules from being an effective
  collision is called the activation energy
• the larger the activation energy of a reaction,
  the slower it will be
• at a given temperature

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Exothermic Reaction
Activation Energy, large
Activation Energy, small
Reactants
Relative Potential Energy
DHreaction
Products
Progress of Reaction
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Endothermic Reaction
Activation Energy
Products
Relative Potential Energy
DHreaction
Reactants
Progress of Reaction
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Catalysts

• a catalyst is a substance that increases the rate
  of a reaction, but is not consumed in the
  reaction
• catalysts lower the activation energy of a
  reaction
• catalysts work by providing a easier pathway for
  the reaction
• Link to catalyzed reaction of H2S and SO2 above

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Catalyst Effecton Activation Energy
Link above is to surface Catalylzed hydrogenation
reaction
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Catalyst Effect on Activation Energy
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Enzymes

• enzymes are protein molecules produced by living
  organisms that catalyze chemical reactions
• the enzyme molecules have an active site that
  organic molecules bind to
• when the organic molecule is bound to the active
  site, certain bonds are weakened
• this allows a particular chemical change to occur
  easier and quicker
• i.e. the activation energy is lowered

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Sucrase