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Equilbrium Constant and Equilbrium Constant and EXTERNAL EFFECTSEXTERNAL EFFECTSEquilbrium Constant and Equilbrium Constant and EXTERNAL EFFECTSEXTERNAL EFFECTS

Formation of stalactites and

stalagmites

CaCO3(s) + H2O(liq) + CO2(g)

Ca2+(aq) + 2 HCO3-(aq)

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EQUILIBRIUMEQUILIBRIUM

• Temperature, catalysts, and changes in concentration/ pressure affect

equilibria.

• The outcome is governed by LE CHATELIER’S PRINCIPLE

• “...if a system at equilibrium is disturbed, the system tends to shift its equilibrium

position to counter the effect of the disturbance.”

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Equilibrium constant and Equilibrium constant and ConcentrationConcentration

Concentration changes

–no change in K

–only the position of equilibrium changes.

4

Butane-Butane-Isobutane Isobutane EquilibriuEquilibriu

mm

Butane-Butane-Isobutane Isobutane EquilibriuEquilibriu

mmK =

[isobutane][butane]

2.5K = [isobutane]

[butane] 2.5

butanebutane

isobutaneisobutane

5ButaneButane IsobutaneIsobutane

butanebutane

isobutaneisobutane

• At equilibrium with [iso] = 1.25 M and [butane] = 0.50 M. K = 2.5.

• Add 1.50 M butane.

• When the system comes to equilibrium again, what are [iso] and [butane]?

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Solution

Calculate Q immediately after adding more butane and compare with K.

Q = [isobutane]

[butane]

1.250.50 + 1.50

= 0.63Q = [isobutane]

[butane]

1.250.50 + 1.50

= 0.63

Q is LESS THAN K. Therefore, the Q is LESS THAN K. Therefore, the reaction will shift to the ____________.reaction will shift to the ____________.Q is LESS THAN K. Therefore, the Q is LESS THAN K. Therefore, the reaction will shift to the ____________.reaction will shift to the ____________.

ButaneButane IsobutaneIsobutane

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Q is less than K, shifts right

toward isobutane.Set up ICE table

[butane] [isobutane]

Initial

Change

Equilibrium

0.50 + 1.50 1.25

- X + X

2.00 – x 1.25 + x

ButaneButane IsobutaneIsobutane

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K = 2.50 = [isobutane]

[butane]

1.25 + x2.00 - x

K = 2.50 = [isobutane]

[butane]

1.25 + x2.00 - x

x = 1.07 M

At the new equilibrium position, [butane] = 0.93 M and [isobutane] = 2.32 M.

Equilibrium has shifted toward isobutane..

ButaneButane IsobutaneIsobutane

9Equilibrium Constant and Equilibrium Constant and

CatalystCatalyst• Add catalyst: NO change in K• A catalyst only affects the RATE it

approach equilibrium.

Catalytic exhaust systemCatalytic exhaust system

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Pressure and EquilibriumPressure and EquilibriumNN22OO44(g) (g) 2 NO 2 NO22(g)(g)

Increase P in the system by reducing the volume (at constant

Temp).

Kc = [NO2 ]2

[N2O4 ] = 0.0059 at 298 KKc =

[NO2 ]2

[N2O4 ] = 0.0059 at 298 K

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

Increase P in the system by reducing the volume.

In gaseous system the equilibrium will shift to the side with fewer molecules (in

order to reduce the P).

Therefore, reaction shifts LEFT and P of NO2 decreases and P of N2O4 increases.

Kc = [NO2 ]2

[N2O4 ] = 0.0059 at 298 KKc =

[NO2 ]2

[N2O4 ] = 0.0059 at 298 K

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Temperature Effects on Temperature Effects on EquilibriumEquilibrium

Figure 16.6Figure 16.6

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Temperature Effects on Temperature Effects on EquilibriumEquilibrium

N2O4 (colorless) + heat

2 NO2 (brown)

∆Ho = + 57.2 kJ (endo)

Kc [NO2 ]2

[N2O4 ]Kc

[NO2 ]2

[N2O4 ]

Kc (273 K) = 0.00077

Kc (298 K) = 0.0059

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Every T has a unique KEvery T has a unique KEvery T has a unique KEvery T has a unique KTemperature change = change in K

Consider the fizz in a soft drink

CO2(aq) + HEAT CO2(g) + H2O(l)

K = P (CO2) / [CO2]

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

• K increases as T goes up because P(CO2) increases and [CO2] decreases.

• Decrease T. Now what?

• Equilibrium shifts left and K decreases.

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

K = 3.5 x 108 at 298 K

NHNH33 ProductioProductio

nn

NHNH33 ProductioProductio

nn

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Le Chatelier’s PrincipleLe Chatelier’s Principle

• Change T - changes K – causes change in P or concentrations at

equilibrium

• Use a catalyst: K not changed.Reaction comes more quickly to equilibrium.

• Add or take away reactant or product: K does not change

Reaction adjusts to new equilibrium “position”

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Examples of Examples of Chemical EquilibriaChemical Equilibria

Phase changes such as H2O(s) H2O(liq)