1

Chemical Reaction Equilibrium Until now we assume reaction A + B C + D goes to complete

Meaning a reaction only stops when either A or B is consumed completely

Experimental observations Some reactions will ‘cease’ without complete consumption of limiting reactant Without altering reaction conditions (T, P, [ ] etc.) the ratio of conc. remains constant After a change (T, P, [ ] etc.), the ratio of conc’s changes to another constant value

Example 1: NH3(aq) + H2O(l) NH4+(l) + OH-(aq)

Follow concentrations of each component with time at constant T and P,

at t

After changing T, a new constant ratio is established

If [NH3] is reduced the amount [NH4] decrease accordingly

while the ratio above remains constant

We say the reaction has reached equilibrium state

Chemical Reactions

constantO]][H[NH

]][OH[NH

23

4

t

[NH4]1

[NH3]1

[NH3]2

[NH4]2

2

Chemical Reaction Equilibrium Experimental observations

Example 2: 2NO(g) + O2(g) 2NO2(g)

Again at constant T and P, when t

Reaction equilibrium is achieved when t

We say the reaction has reached equilibrium state

The ratio of product concentration to reactants, with the stoichiometry coefficient as the power index, is called reaction quotient

When reaction quotient = constant value reaction reaches equilibrium

The value of reaction quotient at equilibrium, is called equilibrium constant, Keq

At equilibrium, reactants may or may not be consumed completely

e.g. A feed gas mixture: NO=500ppm, O2=10%, N2=89.95%, achieves equilibria at the following T’s

Temperature / °C 50 325 500

NO remaining at equil / ppm 0 96.5 390

NO conversion at equil / % 100 80.3 22

Chemical Reactions

t

NO2

NO

O2

constant2

2

2

2

ONO

NO

PP

P

3

Chemical Reaction Equilibrium Important concepts of reaction equilibrium 2NO(g) + O2(g) 2NO2(g)

Is the reaction between reactants still going on?

YES. Reaction goes forward as well as reverses.

At equilibrium: Rforward = Rreverse

though there is no NET change of all conc.’s

The equilibrium constant, Keq, has a meaning of

Keq=Rforward / Rreverse

Changing T causes both Rforward & Rreverse to change, leading to a new Keq.

If Keq >>1, which means Rforward >> Rreverse, the reaction tends to go forward

If Keq <<1, which means Rforward << Rreverse, the reaction tends to go backward

Will there be an equilibrium constant for reactions that go complete?

YES. There is Keq for all reactions (at a constant P) once T and conc’s are fixed.

For a reaction that tends to go complete, Rforward >> Rreverse (Keq >>1).

Chemical Reactions

2

2

2

2

ONO

NOp PP

PK

t

NO2

NO

O2

4

Chemical Reaction Equilibrium Equilibrium constant and Gibbs Free Energy

For reaction vAA + vBB vCC + vDD

Remember: The value of G° determines the direction of reaction

No more change possible reaction in equilibrium G°= 0

Is the G value related to the equilibrium constant?

YES. G° and Keq are related by the equation below (calculate one from the other)

G°T= - RTln(Keq)

Chemical Reactions

reaction is spontaneous

reaction at equilibrium (no further change possible)

reverse reaction is spontaneous

0

0

0

T

T

T

G

G

Gfor a reaction at constant T, P,

5

Chemical Reaction Equilibrium Equilibrium constant - for different type of rxns

General form: vAA + vBB vCC + vDD

gas phase 2NO(g) + O2(g) 2NO2(g)

gas-solid phase CaCO3(s) CaO (s)+CO2(g)

liquid phase NH3(aq)+H2O(l) NH4+(l)+OH-(aq)

liquid-solid Cu(OH)2(s) Cu2+(aq)+2OH- (aq)

gas-liquid NH3(g)+H2O(l) NH4OH(aq)

Chemical Reactions

)(][[A]

[D][C]Tf

PP

PP

BK

BA

DC

BA

DC

vB

vA

vD

vC

vv

vv

eq

for liquid phase rxnfor gas phase rxn

2

2

2

2

ONO

NOp PP

PK

O]][H[NH

]][OH[NH

23

4

cK

2COp PK

22 ]][OH[Cu cK

31 NHp P/K

For reactions that have gas components, we normally use pressure to represent the conc’s

For reactions involves gas+liquid or gas+solid, only gas terms appear in the Keq expression

6

Le Chatelier’s Principle

“When a system in equilibrium is subjected to an external stress, the system will establish a new equilibrium, when possible, so as to minimise the external stress”

Stresses: Changes in [ ], temperature or pressure

Example: N2(g) + 3H2(g) 2NH3(g) + heat (exothermic)

a) Effect of ([ ]). Increasing [ ] of substance shifts equil. in the direction of the long arrow

N2 + 3H2 2NH3 + heat

N2 + 3H2 2NH3 + heat

N2 + 3H2 2NH3 + heat

b) Effect of heat. Addition or removal of heat at constant temperature

Addition of heat: N2 + 3H2 2NH3 + heat

c) Effect of Pressure. (only affects reactions that have volume change before & after). Increase in pressure: N2 + 3H2 2NH3 + heat

4 volumes 2 volumes

Factors Affecting Reaction EquilibriumChemical Reactions

&

or const

223

322

22

3

HNNH

NHHN

3HN

2NH

PPP

PPP

PP

PK p

7

Chemical Reaction Equilibrium Write the equilibrium expression for the following reactions and determine the

units for Keq:

1) 2O3(g) 3O2(g)

2) Ag+(aq) + 2NH3(aq) Ag(NH3)2+(aq)

3) 2NaN3(s) 3Na(s) + 3N2(g)

4) 2Na(s) + Cl2(g) 2NaCl(s)

5) 2NaCl(s) 2Na(s) + Cl2(g)

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

Note 1. The Keq expression depends on how the rxn equation is written (compare rxns 4&5).

2). The unit of Keq depends on the way how the rxn eqn is written & the unit used of each.

Chemical Reactions

][atm][[atm]

[atm]

2

3

2

3

3

2 ,P

PK

O

Op

]][atm[ 33

2 ,PK Np

][1/atm][ 1

2

,P

KCl

p

][atm][ 2

,PK Clp

]][1/atm[][atm][atm

][atm][ 2

3

2

3HN

2NH

22

3 ,PP

PK p

]/l[mol ][ ]/ll[mol/l][mo

[mol/l]][

]][NH[Ag

])[Ag(NH 22222

3

23

,Kc

8

Chemical Reaction Equilibrium1. Analysis shows that a mixture of N2 (2.46 atm), H2 (7.38 atm) and NH3 (0.116 atm) at

472°C in reaction (N2(g) + 3H2(g) 2NH3(g)) is in equilibrium state.

Calculate: 1) Keq; 2) G°; 3). The total pressure. 4) Will the rxn be push to the product by decreasing the reaction pressure? Give reason why? 5) Will the removal of NH3 from reaction mixture promote the product formation? Explain why.

1)

2) G°T= - RTln(Keq)=-8.314x(472+273)ln(2.79x10-5)=65 kJ/mol

3) Ptotal= PN2+PH2+ PNH3=2.46+7.38+0.116=9.956 atm

4) A decrease reaction P favours the reverse rxn because Volreactant > Volproduct.

5) Yes. As Keq=P2NH3/(PN2xP3

H2)=constant, the removal of NH3 will reduce PNH3, to

compensate the change, more N2 and H2 will be converted to NH3 in order to keep the

same Keq (reaction quotient).

Chemical Reactions

253

2

3HN

2NH /atm10792

.38)7.46)(2(

)1660(

22

3 ..

PP

PK p