Equilibrium Constant (KEquilibrium Constant (Keqeq))

A constant which can tell you which A constant which can tell you which side of an equilibrium is favored side of an equilibrium is favored under certain conditions.under certain conditions.

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Equilibrium Constant (KEquilibrium Constant (Keqeq))

When KWhen Keqeq

is large, is large, reaction reaction favors favors the the products.products.

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Equilibrium Constant (KEquilibrium Constant (Keqeq))

When KWhen Keqeq

is small, is small, reaction reaction favors the favors the reactants.reactants.

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Which has the largest Keq?

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Equilibrium CalculationsEquilibrium Calculations

The equilibrium constant can be The equilibrium constant can be used to calculate the used to calculate the concentrations (Molarity!) when a concentrations (Molarity!) when a reaction reaches equilibrium.reaction reaches equilibrium.

][reactants

[products]Keq

Equilibrium CalculationsEquilibrium Calculations

Example Reaction:Example Reaction: A + B C + D⇆A + B C + D⇆

[A][B][C][D]

Keq

““[ ]” means molarity values!!![ ]” means molarity values!!!

Equilibrium CalculationsEquilibrium Calculations

Reactions with coefficients:Reactions with coefficients: 2 NO2 NO22 (g) (g) N⇆ N⇆ 22OO44 (g) (g)

22

42eq ][NO

]O[NK

Coefficient becomes a

power!

Equilibrium CalculationsEquilibrium Calculations

Include only gases and aqueous Include only gases and aqueous solutions (aq):solutions (aq):

C (s) + HC (s) + H22O (g) O (g) CO (g) + H⇆ CO (g) + H⇆ 22 (g) (g)

O][H

][CO][HK

2

2eq

Solids not included!

Equilibrium CalculationsEquilibrium Calculations

Example:Example: HH22 (g) + I (g) + I22 (g) 2 HI (g)⇆ (g) 2 HI (g)⇆ If the concentrations are:If the concentrations are:

– HH22=0.10 M=0.10 M

– II22=0.10 M=0.10 M

– HI=0.20 MHI=0.20 M

– What is KWhat is Keqeq??

]][I[H[HI]

K22

2

eq

[0.1][0.1][0.2]

K2

eq

4.0Keq

Reaction Quotient (Q)Reaction Quotient (Q)

Calculation for the reaction and Calculation for the reaction and compare to Kcompare to Keqeq

to see if a reaction to see if a reaction

has reached equilibrium.has reached equilibrium.

][reactants

[products]Q

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Reaction Quotient (Q)Reaction Quotient (Q)

Example:Example: 2 NO2 NO22 (g) N⇆ (g) N⇆ 22OO44 (g) (g) If the concentrations are:If the concentrations are:

– NONO2 2 = 0.150 M= 0.150 M– NN22OO4 4 = 0.300 M= 0.300 M– What is Q?What is Q?– If KIf Keqeq = 4.73, is the reaction at = 4.73, is the reaction at

equilibrium?equilibrium?

22

42

][NO

]O[NQ

13.3 Q

2[0.150]

[0.300]Q

Reaction Quotient (Q)Reaction Quotient (Q)

Reaction is not at equilibrium…Reaction is not at equilibrium… Q (13.3) > KQ (13.3) > Keqeq (4.73), so reaction is (4.73), so reaction is

shifting towards the reactants.shifting towards the reactants.