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HOW ARE HOW ARE EQUILIBRIUMEQUILIBRIUM

EXPRESSIONS WRITTEN?EXPRESSIONS WRITTEN?

Q VS. K

“Systems”: two reactions that differ only in direction

• Any reversible reaction

H2

+ I2

↔ 2HI

noted by the double arrow; ↔

Reversible Reactions

H2 + I2 ↔ 2HI • the products may react back to original

reactants.

• “closed system”: ONLY if all reactant are present

• If one piece is completely gone it has ”gone to competition” and no longer reversible

Equilibrium

•The state in which a chemical reaction and its reverse reaction occur at the same rate.

Equilibrium = No change in amount over time

6Properties of an Properties of an Equilibrium Equilibrium

Equilibrium systems are

• DYNAMIC (in constant motion)

• REVERSIBLE

• can be approached from either direction

Equilibrium systems are

• DYNAMIC (in constant motion)

• REVERSIBLE

• can be approached from either direction

E + Co(H2O)6Cl2 Co(H2O)4Cl2 + 2 H2 H22OO

Le Chatelier’s Principle

•If a system at equilibrium is stressed, it will react to undo the

stress.

Can we explain WHY?

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The Reacton The Reacton Quotient, QQuotient, Q

In general, all reacting chemical systems are

characterized by their REACTION QUOTIENT

at equilibrium, Q = K

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Reaction QuotientReaction QuotientAt any point in the reaction

H2 + I2 --->---> 2 HI

At any point in the reaction

H2 + I2 --->---> 2 HI

Q reaction quotient = [HI]2

[H2 ][I2 ]

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Equilibrium ConstantEquilibrium Constant

[HI]2

[H2 ][I2 ] = 55.3 = K

K = equilibrium constant

Equilibrium achieved

In the equilibrium region

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THE EQUILIBRIUM CONSTANTTHE EQUILIBRIUM CONSTANTFor any type of chemical equilibrium of the

type

a A + b B ---> c C + d Dthe following is a CONSTANT (at a given T)

K =[C]c [D]d

[A]a [B]b

conc. of products

conc. of reactantsequilibrium constant

If K is known, then we can predict concs. of products or

reactants..

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Writing and Manipulating Writing and Manipulating K ExpressionsK Expressions

Writing and Manipulating Writing and Manipulating K ExpressionsK Expressions

Solids NEVER appear in equilibrium expressions.

S(s) + O2(g) ---> SO2(g)

K [SO2 ][O2 ]

K [SO2 ][O2 ]

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Writing and Manipulating Writing and Manipulating K ExpressionsK Expressions

Writing and Manipulating Writing and Manipulating K ExpressionsK Expressions

Liquids NEVER appear in equilibrium expressions.

NH3(aq) + H2O(liq) ---> NH4+(aq) + OH-(aq)

K [NH4

+][OH- ][NH3 ]

K [NH4

+][OH- ][NH3 ]

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Writing Equilibrium Expressions

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Product- or Reactant Product- or Reactant FavoredFavored

Product-favored Reactant-favored

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For: N2(g) + 3 H2(g) ---> 2 NH3(g)Using K: Is the reaction product-favored

or reactant-favored?

Kc = [NH3 ]2

[N2 ][H2]3 = 3.5 x 108Kc =

[NH3 ]2

[N2 ][H2]3 = 3.5 x 108

When K is much greater than 1the reaction is strongly

product-favored.

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For

AgCl(s) Ag+(aq) + Cl-(aq)

Kc = [Ag+] [Cl-] = 1.8 x 10-5

If K is If K is much lessmuch less than 1 than 1

The reaction is strongly The reaction is strongly

reactant-reactant-favoredfavored..

AgAg++(aq) + Cl(aq) + Cl--(aq)(aq) AgCl(s) AgCl(s)is product-favored.is product-favored.

AgAg++(aq) + Cl(aq) + Cl--(aq)(aq) AgCl(s) AgCl(s)is product-favored.is product-favored.

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calculating Equilibrium constants:

product or reactant favored?

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Using K: Using K: Can determine if the reaction is

at equilibrium.

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

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If [iso] = 0.35 M and [n] = 0.15 M,

are you at equilibrium?

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

If not, which way does the reaction “shift” to approach equilibrium?

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REACTION QUOTIENT, Q

Characterize all chemical systems

Q = product concentrationsreactant concentrations

Q = product concentrationsreactant concentrations

Q = conc. of isoconc. of n

= 0.350.15

= 2.3Q = conc. of isoconc. of n

= 0.350.15

= 2.3

Q (2.33) < K (2.5) Reaction is NOT at equilibrium, [iso] must ________ and [n] must ____________._.

If Q = K, then system is at equilibrium..

25Experimental Experimental Determination of Determination of

Equilibrium Constant, K Equilibrium Constant, K

Experimental Experimental Determination of Determination of

Equilibrium Constant, K Equilibrium Constant, K 2 NOCl(g) 2 NOCl(g) --->---> 2 NO(g) + Cl2 NO(g) + Cl22(g)(g)

Place 2.00 mol of NOCl is a 1.00 L flask. At equilibrium you find 0.66 mol/L of NO.

Calculate K.

Set of an “ICE” table of concentrations

[NOCl] [NO] [Cl2]

Initial 2.00 0 0

Change

Equilibrium 0.66

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Determining KDetermining KDetermining KDetermining K2 NOCl(g) ---> 2 NO(g) + Cl2(g)

[NOCl] [NO] [Cl2]

Initial 2.00 0 0

Change -0.66 +0.66 +0.33

Equilibrium 1.34 0.66 0.33

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2 NOCl(g) ---> 2 NO(g) + Cl2(g)

[NOCl] [NO] [Cl2]

Initial 2.00 0 0

Change -0.66 +0.66 +0.33

Equilibrium 1.34 0.66 0.33

K [NO]2[Cl2 ]

[NOCl]2 =

(0.66)2(0.33)

(1.34)2 = 0.080K

[NO]2[Cl2 ]

[NOCl]2 =

(0.66)2(0.33)

(1.34)2 = 0.080