Equilibrium

Chapter 15 Chemical Equilibrium

Equilibrium

Hb + O2 HbO2

Hb = Hemoglobin

( lungs) (blood )

What is a chemical equilibrium?

Hb + O2 HbO2(muscles) (blood )

The reaction of hemoglobin withoxygen is a reversible reaction.

Equilibrium

If the forward and reverse rxns move at the same speed we combine both rxns and represent it like this:

Hb + O2 HbO2

Hb = Hemoglobin

The double arrows indicate there is a forward reaction and a reverse reaction moving at the same speed or rate.

Equilibrium

High Altitude Training IncreasesThe Concentration of Hemoglobin

An Altitude or Hypoxic Tent

Equilibrium

Equilibrium

An Altitude Training Tent for Dogs

Equilibrium

What is chemical equilibrium?

Equilibrium

Assuming the forward and reverse rxns are elementary, then the hemoglobin reaction can be defined in terms of

chemical kinetics.

Hb + O2 HbO2

kf

kr

Equilibrium

We can show for a reversible rxn having equal rates in both directions that….

Equilibrium

What is a chemical equilibrium?

A chemical equilibrium is a

reversible reaction where the

ratefwd = rate rev

Equilibrium

N2O4(g) 2NO2 (g)

Colorless Brown

Equilibrium

N2O4(g) 2NO2 (g)

Colorless Brown

Equilibrium

N2O4(g) 2NO2 (g)

Colorless Brown

Equilibrium

N2O4(g) 2NO2 (g)

Equilibrium

Note: The reactant and product concentrations do not change

when a reaction is at equilibrium.

0

3.0

1.5

Con

cent

rati

on

What is the equilibrium concentration of the product?What is the equilibrium concentration of the reactant?

N2O4(g) 2NO2 (g)

Colorless Brown

Equilibrium

Note: The reactant and product concentrations are not equal

when a reaction is at equilibrium but ….. but … but

two or more reactants or two or more products can have equal

but constant equilibrium concentrations.

Equilibrium

Writing Equilibrium Constant-Expressions

Equilibrium

The equilibrium constant expression is the ratio

of product concentrations to reactant concentrations.

Kc = [C]c[D]d

[A]a[B]b

aA + bB cC + dD

Equilibrium

Kc = [C]c[D]d

[A]a[B]bKc =

[C]c[D]d

[A]a[B]b[C]c[D]d

[A]a[B]b

This is the equilibrium- constant expression

This is the equilibrium constant

Equilibrium

Kc = [C]c[D]d

[A]a[B]bKc =

[C]c[D]d

[A]a[B]b[C]c[D]d

[A]a[B]b

These are equilibrium reactant concentrations

These are equilibrium product concentrations

Equilibrium

Kc = [C]c[D]d

[A]a[B]bKc =

[C]c[D]d

[A]a[B]b[C]c[D]d

[A]a[B]b

What do a, b, c, and d represent?

Equilibrium

The Equilibrium Constant

The equilibrium expression can also be written in terms

of pressure when the expression contain only gases.

Kc = [C]c[D]d

[A]a[B]bKc =

[C]c[D]d

[A]a[B]b[C]c[D]d

[A]a[B]b Kp =(PC)c (PD)d

(PA)a (PB)bKp =(PC)c (PD)d

(PA)a (PB)b(PC)c (PD)d

(PA)a (PB)b

becomes

Equilibrium

Hb + O2 HbO2

kf

kr

Recall how Kc is derived

Equilibrium

Equilibrium Constant Expression Workshop

Equilibrium

Relationship between Kc and Kp

Using the Ideal Gas Law (PV = nRT) we can show derive and equation relating Kp and Kc and Kp.

where

Kp = Kc (RT)n

n = (moles of gaseous product) − (moles of gaseous reactant)

Equilibrium

Calculating Equilibrium Concentrations

(Using the equilibrium expression to determine [ ]eq

Equilibrium

Equilibrium Calculation Workshop

Calculate the [ ]eqs for this reaction.

2 ICl(g) I2 (g) + Cl2 (g)

The Kc is 0.110 and 0.500 mol of ICl was placed in a 5.00-L flask.

Equilibrium

Organize the process from beginning to end.

Step I. Write the equilibrium expression

Step II. Express the unknown [ ]eqs in terms of x andthe respective initial concentrations ( constructing a table is helpful)

Step III. Substitute the unknown [ ]eqs and Kc in the equilibrium expression and solve for x

Equilibrium

Practice Problem 15-6

Calculate the [ ]eqs for this reaction.

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

The Kc is 5.10 at 700K and 1.000 mol of each compound was placed in a 1.000-L flask.

Equilibrium

Equilibrium Calculations

A system initially contains 1.000 x 10−3 M H2 and 2.000 x 10−3 M I2 and is allowed to reach equilibrium. The concentration of HI in the equilibrium mixture is 1.87 x 10−3 M .

What is Kc at 448C for the reaction shown below.

H2 (g) + I2 (g) 2 HI (g)

Equilibrium

What Do We Know?

[H2], M [I2], M [HI], M

Initially 1.000 x 10-3 2.000 x 10-3 0

Change

At equilibrium

1.87 x 10-3

Equilibrium

[HI] Increases by 1.87 x 10-3 M

[H2], M [I2], M [HI], M

Initially 1.000 x 10-3 2.000 x 10-3 0

Change +1.87 x 10-3

At equilibrium

1.87 x 10-3

Equilibrium

Stoichiometry tells us [H2] and [I2]decrease by half as much

[H2], M [I2], M [HI], M

Initially 1.000 x 10-3 2.000 x 10-3 0

Change -9.35 x 10-4 -9.35 x 10-4 +1.87 x 10-3

At equilibrium

1.87 x 10-3

Equilibrium

We can now calculate the equilibrium concentrations of all three compounds…

[H2], M [I2], M [HI], M

Initially 1.000 x 10-3 2.000 x 10-3 0

Change -9.35 x 10-4 -9.35 x 10-4 +1.87 x 10-3

At equilibrium

6.5 x 10-5 1.065 x 10-3 1.87 x 10-3

Equilibrium

…and, therefore, the equilibrium constant

Kc =[HI]2

[H2] [I2]

= 51

=(1.87 x 10-3)2

(6.5 x 10-5)(1.065 x 10-3)

Equilibrium

Equilibrium Can Be Reached from Either Direction

As you can see, the ratio of [NO2]2 to [N2O4] remains constant at this temperature no matter what the initial concentrations of NO2 and N2O4 are.

Equilibrium

Equilibrium Can Be Reached from Either Direction

This is the data from the last two trials from the table on the previous slide.

Equilibrium

Equilibrium Can Be Reached from Either Direction

It does not matter whether we start with N2 and H2 or whether we start with NH3. We will have the same proportions of all three substances at equilibrium.

Equilibrium

Equilibrium Position

Equilibrium

The size of K is an indication of the equilibrium position.

Equilibrium position is the side of the reaction the equilibrium

favors terms of reactants and products.

• If K >> 1, the reaction is product-favored; or the equilibrium lies to the right.

Equilibrium

• If K << 1, the reaction is reactant-favored; the equilibrium lies to the left.

Equilibrium

Heterogeneous Equilibria

Equilibrium

Pure solids and pure liquids are not included in Kc expressions for heterogeneous equilibria

What is the equilibrium-constant expression for this reaction?

Kc = [Pb2+] [Cl−]2

Equilibrium

Equilibrium

The Reaction Quotient, Qc

Equilibrium

The Reaction Quotient (Q)

The reaction quotient is a ratio of nonequilibrium or

initial reactant and product concentrations.

Q is used to predict the direction a reaction will take

in order to reach equilibrium.

Equilibrium

The Reaction Quotient (Q)

To do this we must:

(1) calculate Q from the initial concentrations and

(2) compare Q to K .

There are three possibilities:

Q = K, Q < K, or Q > K

Equilibrium

If Q = K means the reaction is at equilibrium and Change is required in [product] or [reactant]

Equilibrium

Q < K means that [products] must be increased forthe reaction to attain equilibrium

Q < K , equilibrium shifts right

Equilibrium

Q > K , equilibrium shifts left

Q > K means that [products] must be decreased forthe reaction to attain equilibrium.

Equilibrium

Qc = [C]c[D]d

[A]a[B]b

a A + b B c C + d D

Q is calculated the same as K except only initial concentrations can be used in the reaction quotient expression.

Equilibrium

Le Châtelier’s Principle

Equilibrium

Le Châtelier’s Principle

Le Châtelier’s principle states:

a chemical equilibrium will shift left or shift right to counteract a change that disturbs the equilibrium.

Equilibrium

What Happens When More of a Reactant Is Added to a System?

Equilibrium

The Haber Process

The transformation of nitrogen and hydrogen into ammonia (NH3) is of tremendous significance in agriculture, where ammonia-based fertilizers are of utmost importance.

Equilibrium

Effect of Pressure

Equilibrium

The Haber Process

Equilibrium

Effect of Temperature

pink blue

∆H > 0

Reaction is placedin a hot water bath

Equilibrium

Effect of Temperature

pink blue

∆H > 0

Reaction is placedin a ice-water bath

Equilibrium

A catalyst does not affect

the equilibrium position

Effect of Catalysts

Equilibrium

Equilibrium

Equilibrium

Equilibrium

Equilibrium

Equilibrium

Which reaction does not represent a chemical equilibrium?

H2 + I2 2HI

H2 + I2 2HI

H2 + I2 2HI

H2 + I2 2HI

a.

b.

c.

d.

e. None of these reactions represent an equilibrium

Equilibrium

Manipulating Equilibrium Constants

The equilibrium constant of a reaction in the reverse reaction is the reciprocal of the equilibrium constant of the forward reaction.

10.212

=

Kc = = 0.212 at 100C[NO2]2

[N2O4]N2O4 (g) 2 NO2 (g)

Kc = = 4.72 at 100C

[N2O4][NO2]2

N2O4 (g)2 NO2 (g)

Equilibrium

Manipulating Equilibrium ConstantsThe equilibrium constant of a reaction that has been multiplied by a number is the equilibrium constant raised to a power that is equal to that number.

Kc = = 0.212 at 100C[NO2]2

[N2O4]N2O4 (g) 2 NO2 (g)

Kc = = (0.212)2 at 100C[NO2]4

[N2O4]22 N2O4 (g) 4 NO2 (g)

Equilibrium

Manipulating Equilibrium Constants

The equilibrium constant for a net reaction made up of two or more steps is the product of the equilibrium constants for the individual steps.