Topic 7 Equilibrium

• Dynamic equilibrium• The position of equilibrium• Equilibrium constants• Le Chateliers principle• Equilibrium in chemical processes

• When A and B mixes the product C starts to form• C reacts “back” to form A and B• The reaction A +B C and C A + B goes on at the same time• At equilibrium the rate for both

reactions is the same• http://www.chm.davidson.edu/

ronutt/che115/K/K_Solutions.htm

7.1 Dynamic equilibrium

• Chemical equilibrium is a dynamic equilibrium. It may look like nothing is happening but reactions occur all the time.

A + B C + D• means a reversible reaction, but

sometimes ↔ or is used

• In an equilibrium all of the species, A, B, C and D, is present.

• Equilibrium doesn’t mean “equal”

Physical equilibrium

H2O(l) H2O(g)

7.2 Position of equilibrium

A + B C + DForward rate = kf[A][B] Reverse rate = kr[C][D]

• At equilibrium: Forward rate = Reverse rate• kf/kr = Kc =[C][D] / [A][B]

Kc :The Equilibrium constant

Kc: temperature dependent

a A + b B c C + d DKc = [C]c[D]d / [A]a[B]b

• H2 + I2 2 HI Kc = [HI]2 / [H2][ I2]

• 4 NH3 + 5 O2 4 NO + 6 H2O

Kc =[ H2O]6[NO]4 / [NH3]4[O2]5

Units

• Unit: Kc has not a fixed unit. Sometimes Kc can be without unit.

• E.g. Kc =[ H2O]6[NO]4 / [NH3]4[O2]5

M6M4/M4M5= M

• When Kc >>1, the reaction goes almost to completion

• When Kc <<1, the reaction hardly proceeds.

Le Chatelier’s Principle If a chemical system at

equilibrium experiences a change in concentration, temperature or volume/pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.

Co(H2O)62+ + 4 Cl- + energy   CoCl42- + 6 H2O

Kc= [CoCl4][H2O]6=

[Co(H2O)6][Cl-]4

If you add more HCl the concentration of Cl- increasesThe size of the denominator increases which causes a change in the quotient which will induce a change in the position of the Equilibrium.To keep Kc constant the concentration of the products have to increaseThis will shift the equilibrium to the right and the solution will turn blue

Change in concentration of reactants

Co(H2O)62+ + 4 Cl- + energy   CoCl42- +

6 H2O

If you add more H2O the concentration of H2O increasesThe size of the nominator increases which causes a change in the quotient, which will induce a change in the position of the equilibriumTo keep Kc constant the concentration of the reactants have to increaseThis will shift the equilibrium to the left and the solution will turn pink

Change in concentration of products

Kc= [CoCl4][H2O]6=

[Co(H2O)6][Cl-]4

A change in temperature affects the value of Kc

If the Kc increases or decreases depends on the DH of the reactionYou can treat the temperature as a reactant/product and reason accordingly:

If the temperature is increased the equilibrium has to change positionso the extra heat is consumed. When the reaction is endothermic, as above, this will shift the equilibrium to the right and the solution will turn blueThe shift to the right will increase the concentration of products(nominator)which will increase the value of Kc

Co(H2O)62+ + 4 Cl- + energy   CoCl42- +

6 H2O

Change in temperature- endothermic reaction

Kc= [CoCl4][H2O]6=

[Co(H2O)6][Cl-]4

H2(g) + I2(g) 2HI(g) + energy DH = -51.0 kJ/mol

If the temperature is increased the equilibrium has to change positionso the extra heat is consumed. When the reaction is exothermic this will shift the equilibrium to the left The shift to the left will increase the concentration of reactants (the denominator)Kc= [HI]2

[H2][I2]which will decrease the value of Kc

Change in temperature- exothermic reaction

Changes in pressure may be seen as a change in concentration of both reactants and products.If the reaction has the same number of gaseous particles as reactants and products a change in pressure will affect them in the same fashion.

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

Changing the pressure for this reaction affects the rate of reaction but not the position of the equilibrium

Change in pressure

If the reaction has different number of gaseous particles as reactants and products a change in pressure will affect them like this:

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

Increasing the pressure for this reaction is like increasing the concentrations of all the species. In the expression: this will result in that the denominator increases more than the nominator because it has more terms. Thus the ratio will be decreased and the equilibrium has to shift to the right and produce more NH3 to maintain the value of Kc.

Change in pressure

Kc= [NH3]2

[N2][H2]3

Le Chatelier’s Principle Changes in Temperature, Pressure and Concentration will affect the

equilibrium. A new equilibrium must be found.Change Effect on Equilibrium Change in Kc ?

Increase concentration Shift to the opposite side No

Decrease concentration Shift to that side No

Increase pressure Shift to side with least moles of gas

No

Decrease pressure Shift to side with most moles of gas

No

Increase temperature Shift in endothermic direction

Yes

Decrease temperature Shift in exothermic direction

Yes

Add a catalyst No change No

• Observe that a catalyst doesn’t change the equilibrium but the equilibrium may be reached more rapidly.

The Haber process

N2(g) + 3 H2(g) 2 NH3(g) DH = -92 kJ/mol

With Iron catalyst.How to build an ammonia factory ?

The Contact process

2SO2(g) + O2(g) 2SO3(g) DH= -196kJ/mol

With V2O5 catalyst