CHEMICAL EQUILIBRIUM

CHEMICAL CHEMICAL EQUILIBRIUMEQUILIBRIUM

Chemical Chemical EquilibriumEquilibrium

Reversible Reactions:

A chemical reaction in which the products

can react to re-form the reactantsChemical Equilibrium:

When the rate of the forward reaction

equals the rate of the reverse reaction

and the concentration of products and

reactants remains unchanged

2HgO(s) 2Hg(l) + O2(g)

Arrows going both directions ( ) indicates equilibrium in a chemical equation

2NO2NO22(g) (g) 2NO(g) + O 2NO(g) + O22(g)(g)

Remember this?Why was it so important to measure reaction rate at the start of the reaction(method of initial rates?)

Law of Mass Law of Mass ActionAction

[ ] [ ]

[ ] [ ]

l m

j k

C DK

A B

For the reactionFor the reaction:

Where Where KK is the equilibrium is the equilibrium constant, and is unitlessconstant, and is unitless

jA + kB lC + mD

Product Favored EquilibriumProduct Favored EquilibriumLarge values for K signify the reaction

is “product favored” k > 1

When equilibrium is achieved, most reactant has been converted to product

Reactant Favored EquilibriumReactant Favored EquilibriumSmall values for K signify the reaction

is “reactant favored” k < 1

When equilibrium is achieved, very little reactant has been converted to product

Writing an Equilibrium Writing an Equilibrium ExpressionExpression

2NO2NO22(g) (g) 2NO(g) + O 2NO(g) + O22(g)(g)

K = ???2

22

2

[ ] [ ]

[ ]

NO OK

NO

Write the equilibrium expression for the reaction:

Conclusions about Equilibrium Conclusions about Equilibrium ExpressionsExpressions

The equilibrium expression for a The equilibrium expression for a reaction is the reciprocal for a reaction reaction is the reciprocal for a reaction written in reversewritten in reverse

222

2

[ ] [ ]

[ ]

NO OK

NO

2NO2NO22(g) (g) 2NO(g) + 2NO(g) +

OO22(g(g))

2NO(g) + O2NO(g) + O22(g) (g) 2NO2NO22(g)(g) 2

22

2

[ ]1'

[ ] [ ]

NOK

K NO O

Conclusions about Equilibrium Conclusions about Equilibrium ExpressionsExpressions

When the balanced equation for a When the balanced equation for a reaction is multiplied by a factor reaction is multiplied by a factor nn, the , the equilibrium expression for the new equilibrium expression for the new reaction is the original expression, reaction is the original expression, raised to the raised to the nthnth power. power.

222

2

[ ] [ ]

[ ]

NO OK

NO

2NO2NO22(g) (g) 2NO(g) + 2NO(g) +

OO22(g(g))

NONO22(g) (g) NO(g) + ½O NO(g) + ½O22(g(g))1

1 222

2

[ ][ ]'

[ ]

NO OK K

NO

Equilibrium Expressions Involving Equilibrium Expressions Involving PressurePressure

For the gas phase reaction: 3H2(g) + N2(g) 2NH3(g)

3

2 2

2

3( )( )NH

pN H

PK

P P

3 2 2, ,NH N HP P P areequilibrium partial pressures

( ) npK K RT K and Kp are NOT interchangeable!

Heterogeneous EquilibriaHeterogeneous Equilibria

The position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present

Write the equilibrium expression for the reaction: PCl5(s) PCl3(l) + Cl2(g)

Pure solid

Pure liquid

2[ ]K Cl

2p ClK P

CAN you…..CAN you…..

• Write an Keq expression?Write an Keq expression?• Tell how Keq changes as Tell how Keq changes as

the stoichiometry changes?the stoichiometry changes?• Convert Keq in terms of Kc Convert Keq in terms of Kc

vs Kp?vs Kp?• Explain what K means for Explain what K means for

the rxn?the rxn?

The Reaction QuotientThe Reaction Quotient

When the reaction is NOT at equilibrium for a given time, the reaction quotient, Q takes the place of K, the equilibrium constant, in the law of mass action.

[ ] [ ]

[ ] [ ]

l m

j k

C DQ

A B

jA + kB lC + mD

Useful in determining what will happen under special conditions.

Significance of the Reaction Significance of the Reaction QuotientQuotient

If Q = K, the system is at equilibrium If Q > K, the system shifts to the left, consuming products and forming reactants until equilibrium is achieved

If Q < K, the system shifts to the right, consuming reactants and forming products until equilibrium is achieved

Solving for Equilibrium Solving for Equilibrium ConcentrationConcentration

Consider this reaction at some temperature:H2O(g) + CO(g) H2(g) + CO2(g) K = 2.0Assume you start with 8 molecules of H2O and 6 molecules of CO. How many molecules of H2O, CO, H2, and CO2 are present at equilibrium?

Here, we learn about “RICE”“RICE” – the most important problem solving technique in the second semester. You will use it for the next 4 chapters!


H2O(g) + CO(g) H2(g) + CO2(g) K = 2.0

Step #1: We write the law of mass action for the reaction:

2 2

2

[ ][ ]2.0

[ ][ ]

H CO

H O CO


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

IInitial:

CChange:hange:

EEquilibriuquilibrium:m:

Step #2: We “RICE” the problem, beginning with the R for balanced reaction

8 6 0 0

-x -x +x +x

8-x 6-x x x

Reaction



8-x8-x 6-x6-x xx xx

Step #3: We plug equilibrium concentrations into our equilibrium expression, and solve for x

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

( )( )2.0

(8 )(6 )

x x

x x

4x


Step #4: Substitute x into our equilibrium concentrations to find the actual concentrations

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

m:m:8-x8-x 6-x6-x xx xx


8-4=48-4=4 6-4=26-4=2 44 44

4x

LeChatelier’s PrincipleLeChatelier’s PrincipleWhen a system at equilibrium is placedWhen a system at equilibrium is placedunder stress, the system will undergo aunder stress, the system will undergo achange in such a way as to relieve thatchange in such a way as to relieve thatstress.stress.

Translated:Translated: The system undergoes a The system undergoes a temporary shift in order to restore temporary shift in order to restore equilibrium.equilibrium.

LeChatelier Example #1LeChatelier Example #1

A closed container of ice and water is at equilibrium. Then, the temperature is raised.

Ice + Energy Water

The system temporarily shifts to the _______ to restore equilibrium.right


A closed container of N2O4 and NO2 is at equilibrium. NO2 is added to the container.

N2O4 (g) + Energy 2 NO2 (g)

The system temporarily shifts to the _______ to restore equilibrium.left


A closed container of water and its vapor is at equilibrium. Vapor is removed from the system.

water + Energy vapor

The system temporarily shifts to the _______ to restore equilibrium.right


A closed container of N2O4 and NO2 is at equilibrium. The pressure is increased.

N2O4 (g) + Energy 2 NO2 (g)

The system temporarily shifts to the _______ to restore equilibrium, because there are fewer moles of gas on that side of the equation.

left

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CHEMICAL EQUILIBRIUM