Ch. 13: Chemical Equilibrium

Ch. 13: Chemical Ch. 13: Chemical EquilibriumEquilibrium

13.1 The Equilibrium 13.1 The Equilibrium Condition Condition

EquilibriumEquilibrium dynamic equilibriumdynamic equilibrium

may seem like no changes are occurring may seem like no changes are occurring but there are changes but there are changes

no NET changesno NET changes When did this reaction reach it?When did this reaction reach it?

reactants

products

H2O(g) + CO(g) H2(g) + CO2(g)

At equilibrium, forward and reverse At equilibrium, forward and reverse reaction rates are ____________reaction rates are ____________

EquilibriumEquilibrium

equilibrium position of a reaction is equilibrium position of a reaction is determined by determined by initial _________________initial _________________ ____________ of reactants and products____________ of reactants and products degree of __________________ of reactants degree of __________________ of reactants

and productsand products GOAL: GOAL:

__ __

Equilibrium Position Equilibrium Position

lies “to the left”lies “to the left” more _________more _________ less less

______________________ lies “to the lies “to the

right”right” less __________less __________ more _________more _________

If reactants are If reactants are mixed and mixed and concentrations do concentrations do not changenot change could already be could already be

at equilibriumat equilibrium reaction rates are reaction rates are

so ________ that so ________ that change is too change is too difficult to detectdifficult to detect


13.2 Equilibrium Constant 13.2 Equilibrium Constant

Law of Mass ActionLaw of Mass Action

created in 1864 by Guldberg and created in 1864 by Guldberg and Waage (Norweigen)Waage (Norweigen)

For a reaction: jA + kB For a reaction: jA + kB ⇄ ⇄ lC + mDlC + mD equilibrium constant: Kequilibrium constant: K

K

Law of Mass ActionLaw of Mass Action

becausebecause RateRateforwardforward = = RateRatereversereverse = = so if Rateso if Rateff = Rate = Rater r

kkff[A][A]jj[B][B]k k == kkrr[C][C]ll[D][D]mm

kj

ml

BA

DCK

][][

][][

k

k

r

f

ExampleExample

Write the equilibrium expression for:Write the equilibrium expression for:

4NH4NH33(g) + 7O(g) + 7O22(g) (g) 4NO 4NO22(g) + 6H(g) + 6H22O(g)O(g)

What would it be for the reverse What would it be for the reverse reaction?reaction?

K

Equilibrium Constant Equilibrium Constant

will always have the same value at a will always have the same value at a certain temperaturecertain temperature no matter what amounts are added no matter what amounts are added ratio at equilibrium will always be sameratio at equilibrium will always be same

Equilibrium PositionEquilibrium Position each set of equilibrium each set of equilibrium

concentrationsconcentrations depends on initial concentrationsdepends on initial concentrations


13.3 Equilibrium Expressions 13.3 Equilibrium Expressions with Pressurewith Pressure

Equilibrium with GasesEquilibrium with Gases

equilibria involving gases can be equilibria involving gases can be described using __________ instead of described using __________ instead of ______________________________

NN22(g) + 3H(g) + 3H22(g) (g) 2NH 2NH33(g)(g)

RT

PCCRTPRT

V

nPnRTPV

C

HN

NH KCC

C

HN

NHK

3

2

32

12

23

22

3

][][

][

Equilibrium with GasesEquilibrium with Gases

NN22(g) + 3H(g) + 3H22(g) (g) 2NH 2NH33(g)(g)

3

2

3

2

3

2

11

1

22

3

22

3

RTRT

RT

PP

P

RT

P

RT

P

RT

P

KHN

NH

HN

NH

C

2)(RTKK PC

Calculating K from KCalculating K from KPP

KKPP = K = KCC : RT can cancel out if total of : RT can cancel out if total of coefficients are same on each sidecoefficients are same on each side

where ∆n is the difference in moles where ∆n is the difference in moles of gas on either side of the equationof gas on either side of the equation

∆ ∆n = (l+m) – (j+k)n = (l+m) – (j+k)

NN22(g) + 3H(g) + 3H22(g) (g) 2NH 2NH33(g)(g) ∆ ∆n =n =

ExampleExample

Setup the expression for KSetup the expression for KPP in terms in terms of Kof KCC, R and T, R and T

2NO(g) + Cl2NO(g) + Cl22(g) (g) 2NOCl(g) 2NOCl(g)


13.4: Heterogeneous 13.4: Heterogeneous EquilibriaEquilibria

Heterogeneous EquilibriaHeterogeneous Equilibria

involve more than one phaseinvolve more than one phase

position of heterogeneous equilibria position of heterogeneous equilibria does NOT depend on amounts of:does NOT depend on amounts of: __ __

because their concentrations stay because their concentrations stay constant (since they are PURE)constant (since they are PURE)

Heterogeneous EquilibriaHeterogeneous Equilibria do not include liquids or solids in do not include liquids or solids in

equilibrium expressionequilibrium expression only include ________ and ______________only include ________ and ______________

Example 1Example 1

2H2H22O(l) O(l) ⇄ ⇄ 2H2H22(g) + O(g) + O22(g)(g)

2H2H22O(g) O(g) ⇄ ⇄ 2H2H22(g) + O(g) + O22(g)(g)


13.5/6: Applications of 13.5/6: Applications of Equilibrium Constant (K) Equilibrium Constant (K)

Equilibrium ConstantEquilibrium Constant

if we know the value of K, we can if we know the value of K, we can predict:predict: tendency of a reaction to occurtendency of a reaction to occur if a set of concentrations could be at if a set of concentrations could be at

equilibriumequilibrium equilibrium position, given initial equilibrium position, given initial

concentrations concentrations

Equilibrium ConstantEquilibrium Constant

If you start a reaction with only If you start a reaction with only reactants:reactants: concentration of reactants will decrease concentration of reactants will decrease

by a certain amountby a certain amount concentration of products will increase concentration of products will increase

by a same amountby a same amount

Example 2Example 2

The following reaction has a K of 16. The following reaction has a K of 16. You are starting reaction with 9 OYou are starting reaction with 9 O33 molecules and 12 CO molecules.molecules and 12 CO molecules.

Find the amount of each species at Find the amount of each species at equilibrium.equilibrium.

OO33(g) + CO(g) (g) + CO(g) CO CO22(g) + O(g) + O22(g)(g)

Example 2Example 2

K

OO33(g) + CO(g) (g) + CO(g) O O22(g) + (g) + COCO22(g)(g)

InitialInitial II

ChangeChange CC

EquilibriumEquilibrium EE

Example 2Example 2

Example 2Example 2

OO33(g) + CO(g) (g) + CO(g) O O22(g) + (g) + COCO22(g)(g)

II 99 1212 00 00

CC -x-x -x-x +x+x +x+x

EE

Extent of a ReactionExtent of a Reaction If _________If _________

mostly productsmostly products goes essentially to goes essentially to

completioncompletion lies far to rightlies far to right

If _________If _________ mostly reactantsmostly reactants reaction is reaction is

negligiblenegligible lies far to leftlies far to left

size of K and size of K and time needed to time needed to reach reach equilibrium are equilibrium are NOT relatedNOT related

time required is time required is determined by determined by reaction rate (Ereaction rate (Eaa))

Reaction QuotientReaction Quotient Q: equal to equilibrium expression but Q: equal to equilibrium expression but

___________ have to be at equilibrium___________ have to be at equilibrium used to tell if a reaction is at equilibrium or used to tell if a reaction is at equilibrium or

notnot relationship between Q and K tells which relationship between Q and K tells which

way the reaction will shiftway the reaction will shift _______: at equilibrium, no shift_______: at equilibrium, no shift _______: too large, forms reactants, shift to left_______: too large, forms reactants, shift to left ______: too small, forms products, shift to right______: too small, forms products, shift to right

Example 3Example 3

For the synthesis of ammonia at For the synthesis of ammonia at 500°C, the equilibrium constant is 6.0 500°C, the equilibrium constant is 6.0 x 10x 10-2-2. Predict the direction the system . Predict the direction the system will shift to reach equilibrium in the will shift to reach equilibrium in the following case:following case:

NN22(g) + 3H(g) + 3H22(g) (g) 2NH 2NH33(g)(g)

K

Example 3Example 3

[NH[NH33]]00 = 1.0x10 = 1.0x10-3-3 M, M,

[N[N22]]00=1.0x10=1.0x10-5-5 M M

[H[H22]]00=2.0x10=2.0x10-3-3 M M

Q __ K so forms _________, shifts to _____Q __ K so forms _________, shifts to _____

Q

Example 4Example 4

In the gas phase, dinitrogen tetroxide In the gas phase, dinitrogen tetroxide decomposes to gaseous nitrogen dioxide:decomposes to gaseous nitrogen dioxide:

NN22OO44(g) (g) ⇄ ⇄ 2NO2NO22(g)(g) Consider an experiment in which gaseous Consider an experiment in which gaseous

NN22OO44 was placed in a flask and allowed to was placed in a flask and allowed to reach equilibrium at a T where Kreach equilibrium at a T where KPP = 0.133. = 0.133. At equilibrium, the pressure of NAt equilibrium, the pressure of N22OO44 was was found to be 2.71 atm. found to be 2.71 atm.

Calculate the equilibrium pressure of NOCalculate the equilibrium pressure of NO22..

Example 4Example 4

PK

22NOP

Example 5Example 5 At a certain temperature a 1.00 L At a certain temperature a 1.00 L

flask initially contained 0.298 mol flask initially contained 0.298 mol PClPCl33(g) and 8.70x10(g) and 8.70x10-3-3 mol PCl5(g). mol PCl5(g). After the system had reached After the system had reached equilibrium, 2.00x10equilibrium, 2.00x10-3-3 mol Cl mol Cl22(g) was (g) was found in the flask. found in the flask.

PClPCl55(g) (g) PCl PCl33(g) + Cl(g) + Cl22(g)(g) Calculate the equilibrium Calculate the equilibrium

concentrations of all the species and concentrations of all the species and the value of K.the value of K.

Example 5Example 5 PClPCl55(g) (g) PCl PCl33(g) + Cl(g) + Cl22(g)(g)

II

CC

EE

K

ApproximationsApproximations

If K is very small, we can assume that If K is very small, we can assume that the change (x) is going to be negligiblethe change (x) is going to be negligible

can be used to cancel out when adding can be used to cancel out when adding or subtracting from a “normal” sized or subtracting from a “normal” sized numbernumber

to simplify algebrato simplify algebra

32

2

2

2

4)0.1(

)2)((

)20.1(

)2)((K x

xx

x

xx

0

Example 6Example 6

At 35°C, K=1.6x10At 35°C, K=1.6x10-5-5 for the reaction for the reaction

2NOCl(g) 2NOCl(g) ⇄⇄ 2NO(g) + Cl 2NO(g) + Cl22(g)(g)

Calculate the concentration of all species at Calculate the concentration of all species at equilibrium for the following mixturesequilibrium for the following mixtures

2.0 mol NOCl in 2.0 L flask2.0 mol NOCl in 2.0 L flask 1.0 mol NOCl and 1.0 mol NO in 1.0 L flask1.0 mol NOCl and 1.0 mol NO in 1.0 L flask 2.0 mol NOCl and 1.0 mol Cl2.0 mol NOCl and 1.0 mol Cl22 in 1.0 L flask in 1.0 L flask

Example 6Example 6 2.0 mol NOCl in 2.0 L flask2.0 mol NOCl in 2.0 L flask

[NOCl]=[NOCl]= [NO]=[NO]= [Cl[Cl22]=]=

2NOCl(g) 2NOCl(g) 2NO(g) + Cl 2NO(g) + Cl22(g)(g)

II

CC

EE

Example 6Example 6 1.0 mol NOCl and 1.0 mol NO in 1.0 L flask1.0 mol NOCl and 1.0 mol NO in 1.0 L flask

[NOCl]=[NOCl]= [NO]=[NO]= [Cl [Cl22]=]=


II

CC

EE

Example 6Example 6 2.0 mol NOCl and 1.0 mol Cl2.0 mol NOCl and 1.0 mol Cl22 in 1.0 L flask in 1.0 L flask

[NOCl]=[NOCl]= [Cl[Cl22]=]= [NO]=[NO]=


II

CC

EE


13.7: Le’ Chatlier’s Principle 13.7: Le’ Chatlier’s Principle

Le Châtlier’s PrincipleLe Châtlier’s Principle

can predict how certain changes in a can predict how certain changes in a reaction will affect the position of reaction will affect the position of equilibriumequilibrium

Changing ConcentrationChanging Concentration

system will shift away from the system will shift away from the added component or towards a added component or towards a removed componentremoved component

Ex: NEx: N22 + 3H + 3H22 2NH 2NH33

if more Nif more N22 is added, then equilibrium is added, then equilibrium position shifts to rightposition shifts to right

if some NHif some NH33 is removed, then is removed, then equilibrium position shifts to rightequilibrium position shifts to right

Change in PressureChange in Pressure adding or removing gaseous reactant adding or removing gaseous reactant

or product is same as changing conc.or product is same as changing conc. adding inert or uninvolved gasadding inert or uninvolved gas

increase the ___________________increase the ___________________ ___________effect the equilibrium position___________effect the equilibrium position

Change in PressureChange in Pressure

changing the volumechanging the volume decrease Vdecrease V

•decrease in # gas moleculesdecrease in # gas molecules•shifts towards the side of the reaction shifts towards the side of the reaction

with _____ gas moleculeswith _____ gas molecules increase Vincrease V

•increase in # of gas moleculesincrease in # of gas molecules•shifts towards the side of the reaction shifts towards the side of the reaction

with _____ gas moleculeswith _____ gas molecules

Change in TemperatureChange in Temperature

all other changes alter the all other changes alter the concentration at equilibrium position concentration at equilibrium position but don’t actually change value of Kbut don’t actually change value of K

value of K does change with value of K does change with temperaturetemperature

Change in TemperatureChange in Temperature

if energy is added, the reaction will if energy is added, the reaction will shift in direction that consumes shift in direction that consumes energyenergy

treat energy as a treat energy as a __________: for endothermic reactions__________: for endothermic reactions __________: for exothermic reactions__________: for exothermic reactions

AsAs44OO66(s) + 6C(s) (s) + 6C(s) ⇄ ⇄ AsAs44(g) + 6CO(g)(g) + 6CO(g)

add COadd CO

add Cadd C

remove Cremove C

add Asadd As44OO66

remove Asremove As44OO66

remove Asremove As44

decrease volumedecrease volume

add Ne gasadd Ne gas

PP44(s) + 6Cl(s) + 6Cl22(g) (g) ⇄⇄ 4PCl 4PCl33(l)(l)

decrease volumedecrease volume

increase volumeincrease volume

add Padd P44

remove Clremove Cl22

add Kr gasadd Kr gas

add PCladd PCl33

energy + Nenergy + N22(g) + O(g) + O22(g) (g) ⇄⇄ 2NO(g)2NO(g)

endo or exo?endo or exo?

increase tempincrease temp

increase volumeincrease volume

decrease tempdecrease temp

NN22(g) + 3H(g) + 3H22(g) (g) 2NH 2NH33(g)(g)

Ch. 13: Chemical Equilibrium

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Transcript of Ch. 13: Chemical Equilibrium