Chemical Equilibrium

Chapter 18

Consider a glass of water…

Evaporation

Consider a glass of water…

Now, put a lid on it….

Consider a glass of water…

Evaporation continues, but condensation also occurs...

Consider a glass of water…

The rates equalize, and the system reaches equilibrium.

Chemical Equilibrium

Consider the following reaction(s):

H2O (liquid) H20 (gas)

H2O (gas) H2O (liquid)

H2O (liquid) H2O (gas)

Equilibrium Symbol

Chemical equilibrium occurs when opposing reactions are proceeding at equal rates. The rate at which the products are formed from the reactants equals the rate at which the reactants are formed from the products.

For equilibrium to occur, neither reactant nor products can escape from the system.

N2 + 3H2 2NH3 + 22 KCal

Forward Reaction

N2 + 3H2 2NH3 + 22 KCal

Reverse Reaction

Reversible Reactions

REVERSIBLE REACTIONS do not go to completion and can occur in either direction:

aA + bB cC + dD

CHEMICAL EQUILIBRIUM exists when the forward & reverse reactions occur at exactly the same rate

Reactant

Product

Time (reaction progress )

EQUILIBRIUM

conc

entr

atio

n

At equilibrium:

If there are more products than reactants, the products are said to be favored.

If there are more reactants than products, the reactants are said to be favored.

The Equilibrium Constant, Kc

For the reaction: aA + bB cC + dD at equilibrium, the constant, kc or keq:

Kc is a measure of the extent to which a reaction occurs; it varies with temperature.

ba

dc

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

][reactants[products]

K

A few notes about Kc…Kc is completely unrelated to rate constant, k

“k” is generically used for “constant” in science

In Kc, the “c” is for “concentration”Note that we plug in molarities, or

concentrations to solve for Kc

There are other equilibrium constants that are studied in IB Chemistry (Ka, Kb, Kw, Kp,Ksp)

Example (a): Write the equilibrium expression for…

PCl5 PCl3 + Cl2

][PCl]][Cl[PCl

K5

23c

Example (b): Write the equilibrium expression for…

4NH3 + 5O2 4NO + 6H2O

52

43

62

4

c ][O][NHO][H[NO]

K

Ex: One liter of the equilibrium mixture from example (a) was found to contain 0.172 mol PCl3, 0.086 mol Cl2 and 0.028 mol PCl5. Calculate Kc.

M) (0.028M)(0.086M) (0.172

Kc

PCl5 PCl3 + Cl2 ][PCl]][Cl[PCl

K5

23c

53.0

What does Kc=0.53 mean to me???

When Kc >> 1, most reactants will be converted to products.

When Kc << 1, most reactants will remain unreacted.

The equilibrium constant allows us to ….

Predict the direction in which a reaction mixture will proceed to achieve equilibrium.

Calculate the concentrations of reactants and products once

equilibrium has been reached.

Reaction Quotient (Q)

Reaction Quotient (Q) is calculated the same as Kc, but the concentrations are not necessarily equilibrium concentrations.

Comparing Q with Kc enables us to predict the direction in which a rxn will occur to a greater extent when a rxn is NOT at equilibrium.

Comparing Q to Kc

When Q < Kc:

When Q = Kc:

When Q > Kc:

Forward rxn predominates – “reaction proceeds to the right”(until equil. is reached)

System is at equilibrium

Reverse reaction predominates – “reaction proceeds to the left” (until equilibrium is reached)

Ex: H2(g) + I2(g) 2HI(g) Kc for this reaction at 450 C is 49. If 0.22 mol I2, 0.22 mol H2, and 0.66 mol HI are put into a 1.00-L container, would the system be at equilibrium? If not, what must occur to establish equilibrium.

]][I[H

[HI]Q

22

2

M) M)(0.22 (0.22M) (0.66 2

0.9Q < Kc; therefore forward reaction predominates until equilibrium is reached.

Ex: PCl3(g) + Cl2(g) PCl5(g) Kc=1.9

In a system at equilibrium in a 1.00 L container, we find 0.25 mol PCl5, and 0.16 mol PCl3. What equilibrium concentration of Cl2 must be present?

]][Cl[PCl][PCl

K23

5c

]M)[Cl (0.16M) (0.25

9.12

M 0.82 ][Cl2