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Chemical Equilibrium Chemical Equilibrium Reactions Don’t Just Stop, Reactions Don’t Just Stop, They find Balance They find Balance

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Page 1: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Chemical EquilibriumChemical Equilibrium Reactions Don’t Just Stop, They find Reactions Don’t Just Stop, They find

BalanceBalance

Page 2: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Equilibrium is Attained When the Rates of the Equilibrium is Attained When the Rates of the Forward and Reverse Reactions are the Same.Forward and Reverse Reactions are the Same.

Forward Rate = kf [A]Reverse Rate = kr [B]

kf [A] = kr [B]

Page 3: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Equilibrium : The Haber Process andEquilibrium : The Haber Process andNitrogen FixationNitrogen Fixation

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

Chemistry at work, p. 521

Page 4: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Equilibrium : The Haber Process andEquilibrium : The Haber Process andNitrogen FixationNitrogen Fixation

Note that equilibrium can be reached from either the forwardor reverse direction

Page 5: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

jA + kB pR + q S The LAW OF MASS ACTIONLAW OF MASS ACTION allows us to express the relative concentrations of reactants and products at equilibrium in terms of a quantity called the equilibrium constant, ‘K’ such that:

[R]p [S]q

[A]j [B]kK =

Page 6: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

The Haber Process and the Law of Mass ActionThe Haber Process and the Law of Mass Action

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

[NH3]2

[N2][H2]3K =

The equilibrium constant expression depends only on the stoichiometry of the reaction, not on its mechanism.

Page 7: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Suppose we discover that the equilibrium concen-trations of NO2 and N2O4 are 0.0172 M and 0.00140 M, respectively

[NO2]2

[N2O4]Kc =

[0.0172]2

[0.00140]= = 0.221

N2O4 2 NO2

Page 8: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Heterogeneous EquilibriaHeterogeneous Equilibriaa reaction which may possess reactants orproducts which are in different phases.

CaCO3(s) CaO(s) + CO2(g)

Page 9: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

The density of a pure liquid or solid is a constant at any given temperature and changes very little with temperature. Thus the effective concentration of a pure liquid or solid is constant regardless of how much pure liquid or solid is present.

K = [CaO(s)][CO2]

[CaCO3(s)]

Given: CaCO3(s) CaO(s) + CO2(g)

so K = [CO2]

Even though they do not appear in the equilibrium constant expression,pure solids and liquids must be present for equilibrium to be established

Page 10: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

The Magnitude of Equilibrium The Magnitude of Equilibrium ConstantsConstants

What does it mean when the constant equals

K = 1 x 1040

orK = 1 x 10-40

Page 11: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

In one of their experiments, Haber and co-workers introduced a mixture of hydrogen and nitrogen into a reaction vessel and allowed the system to

attain chemical equilibrium at 472 °C. The equilibrium mixture of gases was analyzed and

found to contain 0.01207 M H2, 0.0402 M N2, and 0.00272 M NH3. From these data, calculate the

equilibrium constant for:

N2(g) + 3H2(g) 2NH3(g) (.00272)2

(.0402)(.01207)3= 104.7

Page 12: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Converting from Kc to Kp

Solutions are Understood in terms of Molarities Gas Pressure is Understood

in Terms of AtmospheresKp = Kc(RT)n, n = (mol. of prod – mol. of react)

Using the value of Kc = 0.105 for the reaction: N2(g) + 3H2(g) 2NH3at 472 °C Convert to Kp

Kp = 0.105 (0.0821 L-atm/mol-K)(745 K)-2

Kp= (0.0821 L-atm/mol-K)(745 K)2

0.105

Kp= 2.81 x 10-5 atm

Page 13: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Le Châteliers principle : If a system at Equilibrium is disturbed

by a change in temperature, pressure or the concentration of a component, the system will shift its equilibrium position so as to

counteract the effect of the disturbance.

Predicting the DirectionPredicting the Direction of of EquilibriumEquilibrium

Page 14: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Effects of Pressure and Volume ChangesIf a system is at equilibrium and the total

pressure is increased by the application of an external pressure by a change in volume, the system will respond by a

shift in equilibrium in the direction that reduces the pressure by shifting to the

side with less moles.

2NO2(g) N2O2(g)

Page 15: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

What would happen if 1 atm of argon gas were added to

the following reaction already at equilibrium.

N2 + 3H2 2 NH3

Page 16: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Effects of Temperature ChangesWhen heat is added to a system, the equilibrium shifts in the

direction that absorbs heat. In an endothermic reaction reactants are converted to products, and K increases. In an exothermic reaction, the opposite occurs.

Co(HCo(H22O)O)662+2+

(aq)+ 4Cl-(aq)CoClCoCl44

2-2-(aq)+6H2O(l) H > 0

heating coolingRoom temperature

Page 17: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Predicting the Direction of Equilibrium

Given, N2(g) + 3H2(g) 2NH3(g)

[NH3]2

[N2][H2]3K = and Kc = 0.105

If the equilibrium concentrations were to start at:

[2.00]2

[1.00][2,00]3Q = = 0.500

What must happen in order for the 0.500 value, lets now call it the reaction quotient ‘Q’, to return to Kc= 0.105?

Page 18: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

[NH3]2

[N2][H2]3

Q = = 0.500 and Kc = 0.105

Equilibrium will re-emerge if the concentration of NH3 decreases and the concentrations of N2 and H2 increase. In a closed system, this would require that the reaction favor the formation of reactants

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

Predicting the Direction of Equilibrium

If Q > K, the reaction will shift to the reactantsIf Q > K, the reaction will shift to the reactants

If Q < K, the reaction will shift to the productsIf Q = K, the reaction is at equilibriumIf Q = K, the reaction is at equilibrium

Page 19: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Kc = [HI]2

[H2] [ I2]

ICEBOX A mixture of 5.00 x 10-3 mol of H2 and 1.00 x 10-2 mol of I2 is placed in a 5.00 L container

at 448°C and allowed to come to equilibrium. Analysis of the equilibrium mixture shows that the concentration of HI is 1.87 x 10-3 M. Calculate Kc

at 448 °C for the reaction: H2(g) + I2(g) 2HI

H2(g) + I2(g) 2HIInitial

Change

Equilib.

1.00 x 10-3 M 2.00 x 10-3 M O M

1.87 x 10-3 M

+-- 1.87 x 10-3 M.935 x 10-3.935 x 10-3

1.065 x 10-3.065 x10-3

= (1.87 x 10-3 )2

(1.065 x 10-3)(.065 x10-3)

Page 20: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Initial

Change

Equilibrium

A 1.000-L flask is filled with 1.000 mol of H2 and 2.000 mol I2 at 448°C. The value of the equilibrium constant for the following reaction is 50.5. What are the equilibrium concentrations of H2, I2, and HI? H2(g) + I2(g) 2HI

H2(g) + I2(g) 2HI1.000 M 2.000 M 0 M

Page 21: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Initial

Change

Equilibrium

H2(g) + I2(g) 2HI

1.000 M 2.000 M 0 M

- x M - x M + 2x M

(1.000 - x) (2.000 - x) 2x

50.5 = [HI]2

[H2][I]=

(2x)2

(1.000 - x)(2.000 - x)

4x2 = 50.5(x2 - 3.00 x + 2.00)

46.5x2 - 151.5x +101.0 = 0

x =-(-151.1) + (-151.4)2 - 4(46.5)(101.0)

2(46.5)x= 2.323 or 0.935

[H2] = 0.065 M[I2 ] = 1.065 M[HI] = 1.870 M

Page 22: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Haber Process

Page 23: Chemical Equilibrium Reactions Dont Just Stop, They find Balance Reactions Dont Just Stop, They find Balance

Haber Process