Topic 7 Equilibrium

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# Topic 7 Equilibrium - PowerPoint PPT Presentation

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

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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
• Equilibrium doesn’t mean “equal”
7.2 Position of equilibrium

A + B C + D

Forward 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 Bc 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’sPrinciple

If a chemical system at

equilibrium experiences

a change in concentration,

temperatureor volume/

pressure, then the

equilibrium shifts to

counteract the imposed

change and a new

equilibrium is established.

Change in concentrationofreactants

Co(H2O)62+ + 4 Cl- + energyCoCl42- + 6 H2O

The sizeof the denominatorincreaseswhichcauses a change in

the quotientwhichwillinduce a change in the position of the

Equilibrium.

To keepKcconstant the concentrationof the productshave to increase

Thiswillshift the equilibrium to the right and the solution willturnblue

Kc= [CoCl4][H2O]6=

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

Change in concentrationofproducts

Co(H2O)62+ + 4 Cl- + energyCoCl42- + 6 H2O

If youaddmore H2O the concentrationof H2O increases

The sizeof the nominatorincreaseswhichcauses a change in

the quotient, whichwillinduce a change in the position of the

equilibrium

To keepKcconstant the concentrationof the reactantshave to increase

Thiswillshift the equilibrium to the left and the solution willturnpink

Kc= [CoCl4][H2O]6=

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

Change in temperature- endothermicreaction

A change in temperatureaffects the valueofKc

If the Kcincreases or decreasesdepends on the DH of the reaction

Youcantreat the temperature as a reactant/product and reason

accordingly:

If the temperature is increased the equilibrium has to change position

so the extra heat is consumed. When the reaction is endothermic, as

above, thiswillshift the equilibrium to the right and the solution will

turnblue

The shift to the right willincrease the concentrationofproducts

(nominator)

whichwillincrease the valueofKc

Co(H2O)62+ + 4 Cl- + energyCoCl42- + 6 H2O

Kc= [CoCl4][H2O]6=

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

Change in temperature- exothermicreaction

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

If the temperature is increased the equilibrium has to change position

so the extra heat is consumed. When the reaction is exothermic

thiswillshift the equilibrium to the left

The shift to the leftwillincrease the concentrationofreactants

(the denominator)

Kc= [HI]2

[H2][I2]

whichwilldecrease the valueofKc

Change in pressure

Changes in pressuremay be seen as a change in

concentrationofbothreactants and products.

If the reaction has the same numberofgaseousparticles

as reactants and products a change in pressurewillaffect

them in the same fashion.

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

Changing the pressure for thisreactionaffects the rate

ofreactionbut not the position of the equilibrium

Change in pressure

If the reaction has different numberofgaseousparticles

as reactants and products a change in pressurewillaffect

them like this:

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

Increasing the pressure for thisreaction is like increasing

the concentrationsof all the species. In the expression:

thiswillresult in that the denominatorincreases

morethan the nominatorbecause it has

more terms. Thus the ratiowill be decreasedand

the equilibrium has to shift to the right and producemore

NH3 to maintain the valueofKc.

Kc= [NH3]2

[N2][H2]3

Le Chatelier’sPrincipleChanges in Temperature, Pressure and Concentration will affect the equilibrium. A new equilibrium must be found.

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