equilibrium
DESCRIPTION
Equilibrium. Chapter 13. The Concept of Equilibrium. Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate. The Concept of Equilibrium. As a system approaches equilibrium, both the forward and reverse reactions are occurring. - PowerPoint PPT PresentationTRANSCRIPT
EquilibriumEquilibrium
Chapter 13Chapter 13
The Concept of EquilibriumThe Concept of Equilibrium
Chemical equilibrium occurs when a Chemical equilibrium occurs when a reaction and its reverse reaction reaction and its reverse reaction proceed at the same rate.proceed at the same rate.
The Concept of EquilibriumThe Concept of Equilibrium As a system As a system
approaches approaches equilibrium, both the equilibrium, both the forward and reverse forward and reverse reactions are reactions are occurring.occurring.
At equilibrium, the At equilibrium, the forward and reverse forward and reverse reactions are reactions are proceeding proceeding at the at the same ratesame rate..
A System at EquilibriumA System at Equilibrium
Once equilibrium Once equilibrium is achieved, the is achieved, the amountamount of each of each reactant and reactant and product remains product remains constant.constant.
Depicting EquilibriumDepicting Equilibrium
Since, in a system at equilibrium, both Since, in a system at equilibrium, both the forward and reverse reactions are the forward and reverse reactions are being carried out, we write its being carried out, we write its equation with a double arrow.equation with a double arrow.
N2O4 (g) 2 NO2 (g)
The Equilibrium ConstantThe Equilibrium Constant
Forward reaction:Forward reaction:NN22OO4 (g)4 (g) 2 NO 2 NO2 (g)2 (g)
Rate Law:Rate Law:Rate = Rate = kkff [N [N22OO44]]
The Equilibrium ConstantThe Equilibrium Constant
Reverse reaction:Reverse reaction:2 NO2 NO2 (g)2 (g) N N22OO4 (g)4 (g)
Rate Law:Rate Law:Rate = Rate = kkrr [NO [NO22]]22
The Equilibrium ConstantThe Equilibrium Constant
Therefore, at equilibriumTherefore, at equilibrium
RateRateff = Rate = Raterr
kkff [N [N22OO44] = ] = kkrr [NO [NO22]]22
Rewriting this, it becomesRewriting this, it becomes
kfkr
[NO2]2
[N2O4]=
The Equilibrium ConstantThe Equilibrium Constant
The ratio of the rate constants is a The ratio of the rate constants is a constant at that temperature, and constant at that temperature, and the expression becomesthe expression becomes
Keq =kfkr
[NO2]2
[N2O4]=
The Equilibrium ConstantThe Equilibrium Constant
Consider the generalized reactionConsider the generalized reaction
The equilibrium expression for this The equilibrium expression for this reaction would bereaction would be
Kc = [C]c[D]d
[A]a[B]b
aA + bB cC + dD
The Equilibrium ConstantThe Equilibrium Constant
Since pressure is proportional to Since pressure is proportional to concentration for gases in a closed concentration for gases in a closed system, the equilibrium expression system, the equilibrium expression can also be writtencan also be written
Kp =(PC
c) (PDd)
(PAa) (PB
b)
Relationship Between Relationship Between KKcc and and KKpp
From the Ideal Gas Law we know thatFrom the Ideal Gas Law we know that
Rearranging it, we getRearranging it, we get
PV = nRT
P = RTnV
Relationship Between Relationship Between KKcc and and KKpp
Plugging this into the expression for Plugging this into the expression for KKpp for each substance, the relationship for each substance, the relationship between between KKcc and and KKpp becomes becomes
wherewhere
Kp = Kc (RT)n
n = (moles of gaseous product) - (moles of gaseous reactant)
Equilibrium Can Be Reached Equilibrium Can Be Reached from Either Directionfrom Either Direction
As you can see, the ratio of [NOAs you can see, the ratio of [NO22]]22 to [N to [N22OO44] ] remains constant at this temperature no matter remains constant at this temperature no matter what the initial concentrations of NOwhat the initial concentrations of NO22 and N and N22OO44 are. are.
Equilibrium Can Be Reached Equilibrium Can Be Reached from Either Directionfrom Either Direction
This is the data This is the data from the last two from the last two trials from the table trials from the table on the previous on the previous slide.slide.
Equilibrium Can Be Reached Equilibrium Can Be Reached from Either Directionfrom Either Direction
It doesn’t matter whether we start with NIt doesn’t matter whether we start with N22 and Hand H22 or whether we start with NH or whether we start with NH33: we : we will have the same proportions of all three will have the same proportions of all three substances at equilibrium.substances at equilibrium.
What Does the Value of What Does the Value of KK Mean?Mean?
If If KK>>1, the reaction >>1, the reaction is is product-favoredproduct-favored; ; product predominates product predominates at equilibrium.at equilibrium.
What Does the Value of What Does the Value of KK Mean?Mean?
If If KK>>1, the reaction >>1, the reaction is is product-favoredproduct-favored; ; product predominates product predominates at equilibrium.at equilibrium.
If If KK<<1, the reaction is <<1, the reaction is reactant-favoredreactant-favored; ; reactant predominates reactant predominates at equilibrium.at equilibrium.
Manipulating Equilibrium Manipulating Equilibrium ConstantsConstants
The equilibrium constant of a The equilibrium constant of a reaction in the reverse reaction is reaction in the reverse reaction is the reciprocal of the equilibrium the reciprocal of the equilibrium constant of the forward reaction.constant of the forward reaction.
Kc = = 0.212 at 100 C[NO2]2
[N2O4]N2O4 (g)
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture. 2 NO2 (g)
Kc = = 4.72 at 100 C[N2O4][NO2]2
N2O4 (g)
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture.2 NO2 (g)
Manipulating Equilibrium Manipulating Equilibrium ConstantsConstants
The equilibrium constant of a reaction The equilibrium constant of a reaction that has been multiplied by a number is that has been multiplied by a number is the equilibrium constant raised to a the equilibrium constant raised to a power that is equal to that number.power that is equal to that number.
Kc = = 0.212 at 100 C[NO2]2
[N2O4]N2O4(g)
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture. 2 NO2(g)
Kc = = (0.212)2 at 100 C[NO2]4
[N2O4]22 N2O4(g)
QuickTime™ and aPhoto - JPEG decompressor
are needed to see this picture. 4 NO2(g)
Manipulating Equilibrium Manipulating Equilibrium ConstantsConstants
The equilibrium constant for a net The equilibrium constant for a net reaction made up of two or more reaction made up of two or more steps is the product of the steps is the product of the equilibrium constants for the equilibrium constants for the individual steps.individual steps.