EQUILIBRIUM CHEMICAL. Chemical Equilibrium Reversible Reactions: A chemical reaction in which the...
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Transcript of EQUILIBRIUM CHEMICAL. Chemical Equilibrium Reversible Reactions: A chemical reaction in which the...
Chemical EquilibriumChemical Equilibrium
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
Law of Mass ActionLaw of Mass Action
[ ] [ ]
[ ] [ ]
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
Writing an Equilibrium ExpressionWriting an Equilibrium Expression
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:
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
Equilibrium Expressions Involving PressureEquilibrium Expressions Involving Pressure
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
( )( ) gasnpK K RT
Conclusions about Equilibrium ExpressionsConclusions about Equilibrium Expressions
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) + O 2NO(g) + O22(g(g))
2NO(g) + O2NO(g) + O22(g) (g) 2NO2NO22(g)(g) 2
22
2
[ ]1'
[ ] [ ]
NOK
K NO O
Conclusions about Equilibrium ExpressionsConclusions about Equilibrium Expressions
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) + O 2NO(g) + O22(g(g))
NONO22(g) (g) NO(g) + ½O NO(g) + ½O22(g(g))1
1 222
2
[ ][ ]'
[ ]
NO OK K
NO
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 K = Q, the system is at equilibrium
If K < Q, the system shifts to the left, consuming products and forming reactants until equilibrium is achieved
If K > Q, the system shifts to the right, consuming reactants and forming products until equilibrium is achieved
Solving for Equilibrium ConcentrationSolving for Equilibrium Concentration
Consider this reaction at some temperature: H2O(g) + CO(g) H2(g) + CO2(g) K = 2.0Assume you start with 8 mol/L of H2O and 6 mol/L of CO. Wha is concentration of H2O, CO, H2, and CO2 at equilibrium?
Here, we learn about “ICE”“ICE” – the most important problem solving technique.
Solving for Equilibrium ConcentrationSolving for Equilibrium Concentration
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 ConcentrationSolving for Equilibrium Concentration
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 ConcentrationSolving for Equilibrium Concentration
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 ConcentrationSolving for Equilibrium Concentration
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 placed When a system at equilibrium is placed under stress, the system will undergo a under stress, the system will undergo a change in such a way as to relieve that change in such a way as to relieve that stress.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