Le Chateliers Principle When a chemical system at equilibrium is disturbed by a change in a property of the system the system always appears to react in the direction that opposes the change until a new equilibrium is reached ID: 629282
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Slide1
Le Chatelier’s PrincipleSlide2
Le Chatelier’s Principle
When a chemical system at equilibrium is disturbed by a change in a property of the system, the system always appears to react in the direction that
opposes the change (until a new equilibrium is reached)Slide3
In my own words:Slide4
Concentration Change If you increase the concentration on one side, the shift will be in the opposite directionSlide5
That is, if you add reactant, the equilibrium will shift towards the products If you add products, the equilibrium will shift towards the reactantsSlide6
This is because when you add concentration, more molecules are available to react, creating an increased forward reactionSlide7
ExampleThe production of freon-12 (a CFC refrigerant) involves the following equilibrium reaction:CCl4(l) + 2HF(g)
--> CCl2F2(g) + 2HCl(g)
To improve the yield of freon-12 (CCL2F2), more hydrogen fluoride is added to the initial equilibrium system, shifting it to the right.Slide8
Temperature ChangeWhether energy is added or removed, the equilibrium shifts to minimize the change in energy
Depends on if the reaction is exothermic or endothermic Slide9
ExampleIn the salt-sulfuric acid process, used to produce HCl
, the system is heated in order to increase the percent yield of hydrogen chloride gas:
2NaCl(s) + H2SO4(l) + energy --> 2HCl(g) + Na2SO4
Adding energy shifts the equilibrium to the right to absorb some of the energySlide10
Example 2In the production of sulfuric acid, the key reaction step is the equilibrium represented by the following:
2SO2(g) + O2(g) -->
2SO3(g) + energyProducts are increased by removing energy, causing the system to replace the energy lost and shift to the rightSlide11
Pressure and Volume Change
According to Boyle’s Law, the concentration of a gas is directly proportional to its pressureSlide12
If the volume is decreased, the concentration increases and the number of molecules will decrease (when possible)Slide13
ExampleIn the equilibrium reaction of sulfur dioxide and oxygen, three moles of gaseous reactants produce 2 moles of gaseous products:
2SO2(g) + O2(g) -->
2SO3(g)If the volume is decreased, the overall pressure increases and this causes the reaction to shift right, which decreases the number of gas moleculesSlide14
Example 2A system with equal numbers of gas molecules on each side (i.e. H2 + I2 --> 2HI) is not affected by change in volumeSlide15
NOTE on Gases Adding or removing gas not involved in the equilibrium will not influence the equilibriumSlide16
Catalyst Reactions Catalysts decrease the time required to reach an equilibrium position, but does not affect the final position of equilibrium. Slide17
Catalysts affect both forward and reverse reactions at the same rate Does not influence the equilibriumSlide18
Graphing Changes Changes result in a quick spike
Le Chatelier’s allows for the gradual return to equilibrium by shifting in the opposite directionSlide19
ExampleThe Haber-Bosch process produces ammonia from nitrogen and hydrogen gas. It is an important process for adding nitrates to fertilizers and was used in the manufacture of explosives during the Second World War. Graph the effects of the following changes:
N2(g) + 3H2(g) -->
2NH3(g) + heatSlide20
N2(g) + 3H2(g) --> 2NH3(g) + heatIncrease N2
Response = decrease N2 (forward reaction)Slide21
N2(g) + 3H2(g) --> 2NH3(g) + heatCool reaction
Response = Increase temp (forward reaction)Slide22
N2(g) + 3H2(g) --> 2NH3(g) + heatDecrease NH3
Response = increase NH3 (forward reaction) Slide23
N2(g) + 3H2(g) --> 2NH3(g) + heatAdd Catalyst
Response = increase rxn rate – no changeSlide24
N2(g) + 3H2(g) --> 2NH3(g) + heatDecrease pressure
Response = increase pressure to more moles (reverse reaction)Slide25
N2(g) + 3H2(g) --> 2NH3(g) + heatDecrease the volume
Response = increase pressure, shift towards less moles (forward reaction)