chemical equilibrium. chemical equilibrium reversible reactions: a chemical reaction in which the...
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CHEMICAL CHEMICAL EQUILIBRIUMEQUILIBRIUM
Chemical Chemical EquilibriumEquilibrium
Reversible Reactions:
A chemical reaction in which the products
can react to re-form the reactantsChemical Equilibrium:
When the rate of the forward reaction
equals the rate of the reverse reaction
and the concentration of products and
reactants remains unchanged
2HgO(s) 2Hg(l) + O2(g)
Arrows going both directions ( ) indicates equilibrium in a chemical equation
2NO2NO22(g) (g) 2NO(g) + O 2NO(g) + O22(g)(g)
Remember this from Chapter 12?Why was it so important to measure reaction rate at the start of the reaction(method of initial rates?)
2NO2NO22(g) (g) 2NO(g) + O 2NO(g) + O22(g)(g)
Law of Mass Law of Mass ActionAction
[ ] [ ]
[ ] [ ]
l m
j k
C DK
A B
For the reactionFor the reaction:
Where Where KK is the equilibrium is the equilibrium constant, and is unitlessconstant, and is unitless
jA + kB lC + mD
Product Favored EquilibriumProduct Favored EquilibriumLarge values for K signify the reaction is “product favored”
When equilibrium is achieved, most reactant has been converted to product
Reactant Favored EquilibriumReactant Favored EquilibriumSmall values for K signify the reaction is “reactant favored”
When equilibrium is achieved, very little reactant has been converted to product
Writing an Equilibrium Writing an Equilibrium ExpressionExpression
2NO2NO22(g) (g) 2NO(g) + O 2NO(g) + O22(g)(g)
K = ???2
22
2
[ ] [ ]
[ ]
NO OK
NO
Write the equilibrium expression for the reaction:
Conclusions about Equilibrium Conclusions about Equilibrium ExpressionsExpressions
The equilibrium expression for a The equilibrium expression for a reaction is the reciprocal for a reaction reaction is the reciprocal for a reaction written in reversewritten in reverse
222
2
[ ] [ ]
[ ]
NO OK
NO
2NO2NO22(g) (g) 2NO(g) + 2NO(g) +
OO22(g(g))
2NO(g) + O2NO(g) + O22(g) (g) 2NO2NO22(g)(g) 2
22
2
[ ]1'
[ ] [ ]
NOK
K NO O
Conclusions about Equilibrium Conclusions about Equilibrium ExpressionsExpressions
When the balanced equation for a When the balanced equation for a reaction is multiplied by a factor reaction is multiplied by a factor nn, the , the equilibrium expression for the new equilibrium expression for the new reaction is the original expression, reaction is the original expression, raised to the raised to the nthnth power. power.
222
2
[ ] [ ]
[ ]
NO OK
NO
2NO2NO22(g) (g) 2NO(g) + 2NO(g) +
OO22(g(g))
NONO22(g) (g) NO(g) + ½O NO(g) + ½O22(g(g))1
1 222
2
[ ][ ]'
[ ]
NO OK K
NO
Equilibrium Expressions Involving Equilibrium Expressions Involving PressurePressure
For the gas phase reaction: 3H2(g) + N2(g) 2NH3(g)
3
2 2
2
3( )( )NH
pN H
PK
P P
3 2 2, ,NH N HP P P areequilibrium partial pressures
( ) npK K RT
Heterogeneous EquilibriaHeterogeneous Equilibria
The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present
Write the equilibrium expression for the reaction: PCl5(s) PCl3(l) + Cl2(g)
Pure solid
Pure liquid
2[ ]K Cl
2p ClK P
The Reaction QuotientThe Reaction Quotient
For some time, t, when the system is not at equilibrium, the reaction quotient, Q takes the place of K, the equilibrium constant, in the law of mass action.
[ ] [ ]
[ ] [ ]
l m
j k
C DQ
A B
jA + kB lC + mD
Significance of the Reaction Significance of the Reaction QuotientQuotient
If Q = K, the system is at equilibrium If Q > K, the system shifts to the left, consuming products and forming reactants until equilibrium is achieved
If Q < K, the system shifts to the right, consuming reactants and forming products until equilibrium is achieved
Solving for Equilibrium Solving for Equilibrium ConcentrationConcentration
Consider this reaction at some temperature:H2O(g) + CO(g) H2(g) + CO2(g) K = 2.0Assume you start with 8 molecules of H2O and 6 molecules of CO. How many molecules of H2O, CO, H2, and CO2 are present at equilibrium?
Here, we learn about “ICE”“ICE” – the most important problem solving technique in the second semester. You will use it for the next several chapters!
Solving for Equilibrium Solving for Equilibrium ConcentrationConcentration
H2O(g) + CO(g) H2(g) + CO2(g) K = 2.0
Step #1: We write the law of mass action for the reaction:
2 2
2
[ ][ ]2.0
[ ][ ]
H CO
H O CO
Solving for Equilibrium Solving for Equilibrium ConcentrationConcentration
H2O(g) + CO(g) H2(g) + CO2(g)IInitial:
CChange:hange:
EEquilibriuquilibrium:m:
Step #2: We “ICE” the problem, beginning with the Initial concentrations
8 6 0 0
-x -x +x +x
8-x 6-x x x
Solving for Equilibrium Solving for Equilibrium ConcentrationConcentration
EEquilibriuquilibrium:m:
8-x8-x 6-x6-x xx xx
Step #3: We plug equilibrium concentrations into our equilibrium expression, and solve for x
H2O(g) + CO(g) H2(g) + CO2(g)
( )( )2.0
(8 )(6 )
x x
x x
4x
Solving for Equilibrium Solving for Equilibrium ConcentrationConcentration
Step #4: Substitute x into our equilibrium concentrations to find the actual concentrations
H2O(g) + CO(g) H2(g) + CO2(g)EEquilibriuquilibriu
m:m:8-x8-x 6-x6-x xx xx
EEquilibriuquilibrium:m:
8-4=48-4=4 6-4=26-4=2 44 44
4x
LeChatelier’s PrincipleLeChatelier’s PrincipleWhen a system at equilibrium is placedWhen a system at equilibrium is placedunder stress, the system will undergo aunder stress, the system will undergo achange in such a way as to relieve thatchange in such a way as to relieve thatstress.stress.
Translated:Translated: The system undergoes a The system undergoes a temporary shift in order to restore temporary shift in order to restore equilibrium.equilibrium.
LeChatelier Example #1LeChatelier Example #1
A closed container of ice and water is at equilibrium. Then, the temperature is raised.
Ice + Energy Water
The system temporarily shifts to the _______ to restore equilibrium.right
LeChatelier Example #2LeChatelier Example #2
A closed container of N2O4 and NO2 is at equilibrium. NO2 is added to the container.
N2O4 (g) + Energy 2 NO2 (g) The system temporarily shifts to
the _______ to restore equilibrium.left
LeChatelier Example #3LeChatelier Example #3
A closed container of water and its vapor is at equilibrium. Vapor is removed from the system.
water + Energy vapor
The system temporarily shifts to the _______ to restore equilibrium.right
LeChatelier Example #4LeChatelier Example #4
A closed container of N2O4 and NO2 is at equilibrium. The pressure is increased.
N2O4 (g) + Energy 2 NO2 (g)
The system temporarily shifts to the _______ to restore equilibrium, because there are fewer moles of gas on that side of the equation.
left