Chapter 16 – Chapter 16 – Chemical Chemical EquilibriaEquilibria

Objectives:1. Determine equilibrium constant expressions.2. Know the factors which affect equilibrium.3. Calculate Q and K and predict effects on equilibrium.4. Perform equilibrium calculations.5. Recall the Le Chatelier’s principle and predict effects on

equilibrium.

Chemical EquilibriumChemical EquilibriumEquilibrium systems are:• DYNAMIC (in constant motion)• REVERSIBLE (can be approached from either direction).

Pink to blueCo(H2O)6Cl2 ---> Co(H2O)4Cl2 + 2 H2O

Blue to pinkCo(H2O)4Cl2 + 2 H2O ---> Co(H2O)6Cl2

Dynamic EquilibriumDynamic EquilibriumFeFe3+3+ + SCN + SCN-- FeSCN FeSCN2+2+

• After a period of time, the concentrations of After a period of time, the concentrations of reactants and products are _____________. reactants and products are _____________.

• The forward and reverse reactions The forward and reverse reactions __________ after equilibrium is attained.__________ after equilibrium is attained.

+

Chemical EquilibriumChemical EquilibriumIn Phase Changes:

In the formation of stalactites and stalagmites:

CaCO3(s) + H2O(liq) + CO2(g) Ca2+(aq) + 2 HCO3-(aq)

Reaction Quotient & Reaction Quotient & Equilibrium ConstantEquilibrium Constant

Equilibrium achieved

Product concentration increases; it becomes constant at equilibrium

Reactants concentration decrease; then become constant at equilibrium

Reaction Quotient & Reaction Quotient & Equilibrium ConstantEquilibrium Constant

At any point in the reaction:

Q = [HI]2

[H2][I2]

At Equilibrium:

K = [HI]2 = 55.3 [H2][I2]

ALL reacting chemical systems

are characterized by their

REACTION QUOTIENT, Q.

When Q = K, the system is at

equilibrium.

In general, for a reaction: aA + bB cC + dD

Q = [C]c[D]d

[A]a[B]b

Product concentrations

Reactant concentrations

Reaction Quotient & Reaction Quotient & Equilibrium ConstantEquilibrium Constant

• For any time of chemical equilibrium of the type:

• The following is a constant:

• If K is known, it is possible to predict concentrations of reactants and products.

aA + bB cC + dD

K =

Equilibrium ExpressionsEquilibrium Expressions

• Solids and liquids never appear in equilibrium expressions.

S (s) + O2 (g) SO2 (g) K =

NH3 (g) + H2O (l) NH4 (aq) + OH- (aq) K =

Meaning of KMeaning of K

1. Can tell if a reaction if product-favored or reactant-favored.

N2 (g) + 3 H2 (g) 2 NH3 (g)

The concentration of products is ________________ than the

reactants; the reaction is _____________ favored.

AgCl (s) Ag+(aq) + Cl –

(aq) K =

The concentration of products is ________________ than the

reactants; the reaction is _____________ favored.

K =

Product or Reactant FavoredProduct or Reactant Favored

Meaning of KMeaning of K

K comes from Thermodynamics.

In chapter 19:If Go < 0 ; reaction is product favored.If Go > O ; reaction is reactant favored.

If K is really big; lnK > 0 Go is __________.If K is really small; lnK < O Go is __________.

Go = -RT lnK

Meaning of KMeaning of K

2. Can tell if a reaction is at equilibrium, or predict with way it will go to reach it.

H

H

H

H

H

H

H

H

H H H

CH

HHH H

H—C—C—C—C—H H—C—C—C—H

K =

n-butane iso-butane

[iso]

[n] = 2.5

H

H

H

H

H

H

H

H

H H H

CH

HHH H

H—C—C—C—C—H H—C—C—C—H

K =

n-butane iso-butane

[iso]

[n] = 2.5

PracticePractice• For the isomerization of butane ( ) into isobutene ( ), the Kc = 2.50

at 298K. Determine if the following systems are at equilibrium and if not, in which direction will the reaction proceed to achieve equilibrium.

• Is the system at equilibrium when [butane] = 0.75 M and [isobutene] = 2.60 M? If it is not, in which direction will the reaction proceed to achieve equilibrium?

a) b) c)

An aqueous solution of ethanol and acetic acid, each at an initial concentration of 0.810 M, is heated to 100oC. At equilibrium, the acetic

acid concentration is 0.748 M. Calculate K for the reaction.

CH3CH2OH + CH3COOH CH3COOCH2CH3 + H2O

A 1.000 L flask is filled with 1.00 mol of H2 and 1.00 mol of I2 at 448oC. The value of the equilibrium constant Kc for the reaction at 448oC is 50.5. What are the equilibrium concentrations of H2, I2 and HI in moles per liter?

H2 + I2 HI2

Manipulating KManipulating K

When two reactions are added to give a third net, the third K is the ____________ of the first two.

S(s) + OS(s) + O22(g) (g) SOSO22(g)(g)

SOSO22(g) + 1/2 O(g) + 1/2 O22(g) (g) SO SO33(g)(g)

Find K for: Find K for: S(s) + 3/2 OS(s) + 3/2 O22(g) (g) SO SO33(g)(g)

Manipulating KManipulating K

When a reaction is multiplied by a number to change its coefficients, the K must __________________.

S(s) + 3/2 OS(s) + 3/2 O22(g) (g) SO SO33(g)(g)

X 2 : 2 S(s) + 3 O2 S(s) + 3 O22(g) (g) 2 SO 2 SO33(g)(g)

Manipulating KManipulating K

• When a reaction is reversed in direction, the new K if found by calculating _____________.

S(s) + OS(s) + O22(g) (g) SOSO22(g)(g)

SOSO22(g)(g) S(s) + O S(s) + O22(g) (g)

Manipulating KManipulating KConcentration UnitsWe have been writing K in terms of mol/L. These are designated by Kc

But with gases, P = (n/V)•RT = conc • RTP is proportional to concentration, so we can write K in terms of

P. These are designated by Kp. Kc and Kp may or may not be the same.

K using concentration and pressure unitsK using concentration and pressure unitsKp = Kc (RT)Kp = Kc (RT)∆n∆n ** Change in moles** Change in moles

For For S(s) + OS(s) + O22(g) (g) SOSO22(g)(g)∆∆n = 0 and Kp = Kcn = 0 and Kp = Kc

For SOFor SO22(g) + 1/2 O(g) + 1/2 O22(g) (g) SO SO33(g)(g)∆∆n = –1/2 and Kp = Kc(RT)n = –1/2 and Kp = Kc(RT)-1/2 -1/2

Practice – Determine KPractice – Determine KGiven the following equilibria,

a) MgCO3(s) Mg2+(aq) + CO32-(aq) K1 = 6.8 10-6

b) MgF2(s) Mg2+(aq) + 2 F-(aq) K2 = 5.2 10-11

Determine the equilibrium constant for the following reaction.c) MgCO3(s) + 2 F-(aq) MgF2(s) + CO3

2-(aq) K3 =?

Equilibrium and External Equilibrium and External EffectsEffects

• Temperature, catalysts, and changes in Temperature, catalysts, and changes in concentration affect equilibria.concentration affect equilibria.

• The outcome is governed by The outcome is governed by ____________________________________________________________

• ““...if a system at equilibrium is disturbed, the ...if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to system tends to shift its equilibrium position to counter the effect of the disturbance.”counter the effect of the disturbance.”

Henri Le ChatelierHenri Le Chatelier

1850-19361850-1936

Studied mining engineering.Studied mining engineering.

Interested in glass and Interested in glass and ceramics.ceramics.

Equilibrium and External Equilibrium and External EffectsEffects

• Temperature changeTemperature change

• Consider the fizz in a soft drinkConsider the fizz in a soft drink

COCO22(aq) (aq) COCO22(g) + H(g) + H22O(liq) O(liq)

• Increase T. What happens to equilibrium position? Increase T. What happens to equilibrium position? To value of K? To value of K?

• Decrease T. Now what happens to equilibrium Decrease T. Now what happens to equilibrium position?position?

Equilibrium and External Equilibrium and External EffectsEffects

• Add catalyst Add catalyst • A catalyst only affects the RATE of A catalyst only affects the RATE of

approach to equilibrium.approach to equilibrium.• A catalyst affects both reactions forward A catalyst affects both reactions forward

and reverse.and reverse.

Catalytic exhaust systemCatalytic exhaust system

Equilibrium and External Equilibrium and External EffectsEffects

• Concentration changes Concentration changes

– Only the equilibrium composition Only the equilibrium composition changes.changes.

– Reaction adjusts to new equilibrium “position”

Le Chatelier’s PrincipleLe Chatelier’s PrincipleAdding a “reactant” to a chemical system.Adding a “reactant” to a chemical system.

Removing a “reactant” from a chemical system.

Equilibrium shifts___________ .

Equilibrium shifts___________ .

Le Chatelier’s PrincipleLe Chatelier’s PrincipleAdding a “product” to a chemical system.

Removing a “product” from a chemical system.

Equilibrium shifts___________ .

Equilibrium shifts___________ .

Predict effect on KPredict effect on K

Nitrogen Dioxide Nitrogen Dioxide EquilibriumEquilibrium

Increase P in the system by reducing the Increase P in the system by reducing the volume (at constant T). volume (at constant T).

NN22OO44(g) (g) 2 NO 2 NO22(g)(g)

K = [NO2]2

N2O4

= 0.0058 at 298 K

In gaseous system the equilibrium will shift to the side with fewer molecules (in order to reduce the P). Therefore, reaction shifts ________ and P of NO2 _____________ and P of N2O4 _____________.

Cobalt Complexes Cobalt Complexes EquilibriumEquilibrium

• Is the following reaction endothermic, or exothermic? In hot water, the solution looks blue, in ice-water bath the solution looks pink.

Co(H2O)62+ (aq) + 4 Cl- CoCl42- + 6 H2O

pink blue

RememberRemember

• Go over all the contents of your textbook.

• Practice with examples and with problems at the end of the chapter.

• Practice with OWL tutor.• Practice with the quiz on CD of

Chemistry Now.• Work on your OWL assignment for

Chapter 16.