Le Chatelier's Principle IB

7/31/2019 Le Chatelier's Principle IB

1/27

Topic 7: Equilibrium SL

Le Chateliers Principle7.2.3 Apply Le Chateliers principle to predict qualitative effects of changes oftemperature, pressure and concentration on the position of equilibrium and on

the value of the equilibrium constant.

7.2.4 State and explain the effect of a catalyst on an equilibrium reaction.7.2.5 Apply the concepts of kinetics and equilibrium to industrial processSuitable examples include the Haber and Contact processes.


2/27

LeChateliers Principle

When a system at equilibrium is placedunder stress, the system will undergo achange in such a way as to relieve that

stress.


3/27

Le Chatelier Translated:

When you take something away from asystem at equilibrium, the systemshifts in such a way as to replace what

youve taken away.

When you add something to a systemat equilibrium, the system shifts in

such a way as to use up what youveadded.


4/27

Le Chatelier Example #1

A closed container of ice and water atequilibrium. The temperature is raised.

Ice + Energy Water

The equilibrium of the system shifts tothe _______ to use up the added energy.right


5/27


A closed container of N2O4 and NO2 atequilibrium. NO2 is added to the container.

N2O4 (g) + Energy < - - > 2 NO2(g)

The equilibrium of the system shifts tothe _______ to use up the added NO2.left


6/27

Le Chatelier Example #3A closed container of water and its vapor atequilibrium. Vapor is removed from the system.

water + Energy vapor

The equilibrium of the system shifts tothe _______ to replace the vapor.right


7/27


A closed container of N2O4 and NO2 atequilibrium. The pressure is increased.

N2O4 (g) + Energy < - - > 2 NO2(g)

The equilibrium of the system shifts tothe _______ to lower the pressure,because there are fewer moles of gason that side of the equation.

left


8/27

Pressure Changes to system

If the volume decreases, the concentrationincreases, and there will be a shift to theside with the less amount of moles.

If the volume increases, the concentrationdecreases, and there will be a shift to theside with the more amount of moles.


9/27

Example;

If I increase the pressure, where is the shift?

(right)

If I decrease the pressure, where is theshift? (left)

2SO2 + O2 2SO3

(3moles) (2moles)


10/27

Effect of Concentration

1. If you add more reactant, it shifts to theright increasing the formation of product,using up the reactants.

2. If you add product, it shifts to the left

3. If you remove product, it shifts to theright, increasing the formation of product.

4. If you remove reactant, it shifts to the left


11/27

Effect of temperature

Energy is treated as a reactant ifendothermic equation, and as a product ifexothermic equation.

If cooling a system, then it shifts so moreheat is produced.

If heating a system, then it shifts so extraheat is used up.


12/27

Example for temp. changes forEndothermic Reaction

Heating the below reaction causes the system toshift to the right = more products, because youtreat energy like a reactant.

2NaCl +H2SO4 + energy < -- > 2HCl + Na2SO4

Cooling the above reaction causes the system toshift to the left = less reactants, so need to makeup more


13/27

Effect of temp change onexothermic reactions

Heating the below reaction causes thesystem to shift to the left, to use up theextra heat.

2SO2 + O2 2SO3 + energy

Cooling the above reaction causes thesystem to shift to the right, to make up forthe lost heat.


14/27

The effect of a catalyst onequilibrium

Adding a catalyst speeds up a reaction byproviding an alternative mechanism with a loweractivation energy, thus speeding up both the

forward and backward reaction rate. It shortens the time needed to attain equilibrium

concentrations

It has no effect on the position of equilibrium,however equilibrium will be attained morequickly.


15/27

Haber Process

N2(g) + 3H2(g) < - - > 2NH3(g) H= -92 kJ/mol

Mixtures volume is compressed and passed

over a heated iron catalyst.

Conditions for his equilibrium is critical. High pressure is favourable due to 4 moles on left

and 2 moles on right. Increased pressure causesa shift to the left, favouring product formation.

This is expensive to due and most productionplants will resist compressing gases in terms ofoperating costs. Compromise will be met.


16/27

Compromise

This is an exothermic reaction, so lowtemperatures would be favourable toproduce product.

Low temps mean low reaction rates, so wemay get a higher yield but it will take a longtime to get it. Not good for business.

A compromise temp, as well as the use of acatalyst will aid in speeding up the reaction toa more acceptable standard.


17/27

Typical conditions

Pressure between 20-100 MPa (200-1000 atm)

Temperatures around 700 K

The reaction is not allowed to reach equilibrium,

because reaction rate decreases as weapproach equilibrium, and typically only 20% ofN2 and H2 is converted.

The gases are cooled and NH3 is condensedand removed, leaving unused N2 and H2available for further production.


18/27


19/27

http://www.absorblearning.com/media/item.action?quick=128#

Animation of Haber process
http://www.absorblearning.com/media/item.action?quick=128http://www.absorblearning.com/media/item.action?quick=128http://www.absorblearning.com/media/item.action?quick=128http://www.absorblearning.com/media/item.action?quick=128


20/27

Ammonias Uses

Manufacture of fertilizers (ammonia saltsand urea)

Manufacturing nitrogen used in polymersfor the fabrication of nylon

Used in the production of explosives (TNT,dynamite)


21/27

Contact Process

Production of sulfuric acid by the oxidation ofsulfur.

1. Sulfur is burnt in air to form sulfur dioxide

S(s) + O2(g) < - - > SO2(g)

2. Sulfur dioxide is mixed with air and passedover vanadium(V)oxide catalyst to producesulfur trioxide.

2SO2(g) + O2(g) < - - > 2SO3(g) H= -196 kJ/mol3. Sulfur trioxide is reacted with water to produce

sulfuric acid.

SO3(g) + H2O(l)H2SO4(l)


22/27

High pressure would favour the formation of SO3in the 2nd step, however its too expensive.

Reactants are compressed to 2 atm to achievethe desired flow rate in the reactor.

Pure O2 would drive the equilibrium to the right,however its an unnecessary expense.

Low temperatures, because its exothermic,would be best, but it slows the rate too much.

More money, more SO3


23/27

Compromised conditions

Temp between 700-800 K (fast initialreaction rate)

The use of a finely divided V2

O5

catalyst

Oxidation is done in converters at lowertemperatures (slows reaction rate)

Overall conversion is 90% to SO3


24/27


25/27

Uses of H2SO4

Fertilizers (converting insoluble phosphate rockinto soluble phosphates)

Polymers

Detergents Paints

Pigments

Petrochemical industry

Processing of metals

Electrolyte in car batteries


26/27

Le Chateliers principle is a memory aid, it

doesnt explain why these changes occur.

Listen carefully and read over text pagesto help you develop further understandingof explanation.

http://www.mhhe.com/physsci/chemistry/e

ssentialchemistry/flash/lechv17.swf
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/lechv17.swfhttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/lechv17.swfhttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/lechv17.swfhttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/lechv17.swf


27/27

Haber process notes

http://www.chemguide.co.uk/physical/equilibria/haber.html
http://www.chemguide.co.uk/physical/equilibria/haber.htmlhttp://www.chemguide.co.uk/physical/equilibria/haber.htmlhttp://www.chemguide.co.uk/physical/equilibria/haber.htmlhttp://www.chemguide.co.uk/physical/equilibria/haber.html

Documents

Le Chatelier's Principle IB