CHEMICAL EQUILIBRIUM Chapter 16

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CHEMICAL EQUILIBRIUMChapter 16

• equilibrium vs. completed reactions
• equilibrium constant expressions
• Reaction quotient
• computing positions of equilibria examples
• Le Chateliers principle - effect on equilibria
  of
• addition of reactant or product
• pressure
• temperature

YOU ARE NOT RESPONSIBLE for section 16.7
(relation to kinetics)
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THE EQUILIBRIUM CONSTANT

• For any type of chemical equilibrium of the type

the following is a CONSTANT (at a given T)
If K is known, then we can predict
concentrations of products or reactants.
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Q - the reaction quotient

• All reacting chemical systems can be
  characterized by their REACTION QUOTIENT, Q.

Q has the same form as K, . . . but uses
existing concentrations
If Q K, then system is at equilibrium.




Since K 2.5, system NOT AT EQUIL.
To reach EQUILIBRIUM Iso must INCREASE and n
must DECREASE.
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Typical EQUILIBRIUM Calculations
2 general types a. Given set of
concentrations, is system at
equilibrium ?
Calculate Q compare to K
IF Q gt K or Q/K gt 1 ?
REACTANTS Q lt K or Q/K lt 1 ?
PRODUCTS
QK at EQUILIBRIUM
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H2(g) I2(g) 2 HI(g) Kc 55.3
Step 1 Define equilibrium condition in terms of
initial condition and a change variable H2
I2 HI At equilibrium 1.00-x 1.00-x 2x

• Step 2
• Put equilibrium
• into Kc .

Step 3. Solve for x. 55.3
(2x)2/(1-x)2 Square root of both sides solve
gives x 0.79
H2 I2 1.00 - x 0.21 M HI 2x
1.58 M
At equilibrium
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EQUILIBRIUM AND EXTERNAL EFFECTS

• The position of equilibrium is changed when there
  is a change in
• pressure
• changes in concentration
• temperature
• The outcome is governed by
• LE CHATELIERS PRINCIPLE

Henri Le Chatelier 1850-1936 - Studied mining
engineering - specialized in glass and ceramics.
...if a system at equilibrium is disturbed, the
system tends to shift its equilibrium position to
counter the effect of the disturbance.
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Shifts in EQUILIBRIUM Concentration

• If concentration of one species changes,
• concentrations of other species CHANGES
• to keep the value of K the same (at constant T)
• no change in K - only position of equilibrium
  changes.

ADDING PRODUCTS - equilibrium shifts to REACTANTS
ADDING REACTANTS - equilibrium shifts to PRODUCTS
- GAS-FORMING PRECIPITATION
REMOVING PRODUCTS - often used to DRIVE
REACTION TO COMPLETION
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Effect of changed on an equilibrium
INITIALLY n 0.50 M iso 1.25
M CHANGE ADD 1.50 M n-butane What happens ?

• Solution
• A. Calculate Q with extra 1.50 M n-butane.

16_butane.mov (16m13an1.mov)
Q iso / n 1.25 / (0.50 1.50) 0.63
Q lt K . Therefore, reaction shifts to PRODUCT
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Butane/Isobutane

• Solution

B. Solve for NEW EQUILIBRIUM - set up
concentration table n-butane isobutane Ini
tial 0.50 1.50 1.25 Change - x
x Equilibrium 2.00 - x 1.25 x
x 1.07 M. At new equilibrium position,
n-butane 0.93 M isobutane 2.32 M.
Equilibrium has shifted toward isobutane.
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Effect of Pressure (gas equilibrium)

• Increase P in the system by reducing the volume.

Increasing P shifts equilibrium to side with
fewer molecules (to try to reduce P). Here,
reaction shifts LEFT PN2O4 increases
16_NO2.mov (16m14an1.mov)
PNO2 decreases
See Ass2 - question 6
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EQUILIBRIUM AND EXTERNAL EFFECTS

• Temperature change ? change in K
• Consider the fizz in a soft drink

Kc CO2(aq)/CO2(g)

• Change T New equilib. position? New value of
  K?

• Increase T
• Equilibrium shifts left CO2(g) ? CO2
  (aq) ?
• K decreases as T goes up.
• Decrease T
• CO2 (aq) increases and CO2(g) decreases.
• K increases as T goes down

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Temperature Effects on Chemical Equilibrium

• Kc 0.00077 at 273 K
• Kc 0.00590 at 298 K

?Horxn 57.2 kJ
Increasing T changes K so as to shift equilibrium
in ENDOTHERMIC direction
16_NO2RX.mov (16m14an1.mov)
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EQUILIBRIUM AND EXTERNAL EFFECTS
Catalytic exhaust system

• Add catalyst ---gt no change in K
• A catalyst only affects the RATE of approach to
  equilibrium.

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CHEMICAL EQUILIBRIUMChapter 16

• equilibrium vs. completed reactions
• equilibrium constant expressions
• Reaction quotient
• computing positions of equilibria examples
• Le Chateliers principle - effect on equilibria
  of
• addition of reactant or product
• pressure
• temperature

YOU ARE NOT RESPONSIBLE for section 16.7
(relation to kinetics)
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Entropy and Free Energy (Kotz Ch 20)

• Spontaneous vs. non-spontaneous
• thermodynamics vs. kinetics
• entropy randomness (So)
• Gibbs free energy (?Go)
• ?Go for reactions - predicting spontaneous
  direction
• ?Grxn versus ?Gorxn
• predicting equilibrium constants from ?Gorxn

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Entropy and Free Energy ( Kotz Ch 20 )

• some processes are spontaneous others never
  occur. WHY ?

• How can we predict if a reaction can occur, given
  enough time?

THERMODYNAMICS
9-paper.mov 20m02vd1.mov

• Note Thermodynamics DOES NOT say how quickly (or
  slowly) a reaction will occur.
• To predict if a reaction can occur at a
  reasonable rate, one needs to consider

KINETICS
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Thermodynamics

• state of a chemical system (P, T, composition)
• From a given state, would a chemical reaction
  decrease the energy of the system?
• If yes, system is favored to react a
  product-favored system which will have a
  spontaneous reaction.
• Most product-favored reactions are exothermic.
• Spontaneous does not imply anything about time
  for reaction to occur. (kinetics)

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Thermodynamics versus Kinetics

• Diamond to Graphite
• spontaneous from thermodynamics
• but not kinetically favored.

• Paper burns.
• - product - favored reaction.
• - Also kinetically favored once reaction is
  begun.

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Product-Favored Reactions
In general, product-favored reactions are
exothermic.

• E.g. thermite reaction
• Fe2O3(s) 2 Al(s) ? 2 Fe(s) Al2O3(s)
• DH - 848 kJ

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Non-exothermic spontaneous reactions

• But many spontaneous reactions or processes are
  endothermic . . .

NH4NO3(s) heat ? NH4 (aq) NO3-
(aq) ?Hsol 25.7 kJ/mol
or have ?H 0 . . .
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Entropy, S

• One property common to product-favored processes
  is that the final state is more DISORDERED or
  RANDOM than the original.
• Spontaneity is related to an increase in
  randomness.
• The thermodynamic property related to randomness
  is ENTROPY, S.

Reaction of K with water
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PROBABILITY - predictor of most stable state
WHY DO PROCESSES with ? H 0 occur ? Consider
expansion of gases to equal pressure
This is spontaneous because the final state, with
equal molecules in each flask, is much more
probable than the initial state, with all
molecules in flask 1, none in flask 2
SYSTEM CHANGES to state of HIGHER
PROBABILITY THIS IS USUALLY the more RANDOM state.
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Gas expansion - spontaneity from greater
probability
Consider distribution of 4 molecules in 2 flasks
P1 lt P2
P1 gt P2
P1 P2
With more molecules (gt1020) P1P2 is most
probable by far
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WHAT about EXOTHERMIC REACTIONS ?
Consider 2 H2 (g) O2 (g) ? 2 H2O (l)

• HIGHLY EXOTHERMIC
• final state (liquid) is MUCH MORE ORDERED
• (less random arrangement) than initial state (2
  gases)

• BUT PROBABILITY of final state is higher when
  one
• considers change in the surroundings. WHY ?
• Heat evolved increases motion of molecules in
  the surroundings

Increased disorder of surroundings
decreased disorder of system
gt
MUST consider change in disorder in BOTH SYSTEM
and SURROUNDINGS to predict DIRECTION of
SPONTANEITY
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Directionality of Reactions

• How probable is it that reactant molecules will
  react?
• PROBABILITY suggests that a product-favored
  reaction will result in the dispersal of energy
  or dispersal of matter or both.

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Directionality of Reactions

• Probability suggests that a product-favored
  reaction will result in the dispersal of energy
  or of matter or both.

Energy Dispersal
Matter Dispersal
9_gasmix.mov 20m03an1.mov
9_exorxn.mov 20m03an2.mov
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Directionality of ReactionsEnergy Dispersal

• Exothermic reactions involve a release of stored
  chemical potential energy to the surroundings.
• The stored potential energy starts out in a few
  molecules but is finally dispersed over a great
  many molecules.
• The final statewith energy dispersedis more
  probable and makes a reaction product-favored.

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Standard Entropies, So

• Every substance at a given temperature and in a
  specific phase has a well-defined Entropy
• At 298o the entropy of a substance is called
• So - with UNITS of J.K-1.mol-1
• The larger the value of So, the greater the
  degree of disorder or randomness
• e.g. So (in J.K-1mol-1) Br2 (liq)
  152.2
• Br2 (gas) 245.5
• For any process

?So ? So(final) - ? So(initial)
?So(vap., Br2) (245.5-152.2) 93.3 J.K-1mol-1
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Entropy, S
So (J/Kmol) H2O(gas) 188.8 H2O(liq) 69.9 H2O
(s) 47.9

• S (gases) gt S (liquids) gt S (solids)