Topic 7: Equilibrium SL Le Chatelier’s Principle 7.2.3 Apply Le Chatelier’s principle to predict qualitative effects of changes of temperature, 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 process Suitable examples include the Haber and Contact processes.
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Topic 7: Equilibrium SL Le Chatelier’s Principle 7.2.3 Apply Le Chatelier’s principle to predict qualitative effects of changes of temperature, pressure.
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Topic 7: Equilibrium SLLe Chatelier’s Principle
7.2.3 Apply Le Chatelier’s principle to predict qualitative effects of changes of temperature, 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 process
Suitable examples include the Haber and Contact processes.
LeChatelier’s PrincipleLeChatelier’s Principle
• When a system at equilibrium is placed When a system at equilibrium is placed under stress, the system will undergo a under stress, the system will undergo a change in such a way as to relieve that change in such a way as to relieve that stress.stress.
Le Chatelier Translated:
• When you take something away from a When you take something away from a system at equilibrium, the system system at equilibrium, the system shifts in such a way as to replace what shifts in such a way as to replace what you’ve taken away.you’ve taken away.
• When you add something to a system When you add something to a system at equilibrium, the system shifts in at equilibrium, the system shifts in such a way as to use up what you’ve such a way as to use up what you’ve added.added.
Le Chatelier Example #1Le Chatelier Example #1
A closed container of ice and water at equilibrium. The temperature is raised.
Ice + Energy <-- > Water
The equilibrium of the system shifts to the _______ to use up the added energy.
right
Le Chatelier Example #2Le Chatelier Example #2
A closed container of N2O4 and NO2 at equilibrium. NO2 is added to the container.
N2O4 (g) + Energy < - - > 2 NO2 (g)
The equilibrium of the system shifts to the _______ to use up the added NO2.
left
Le Chatelier Example #3Le Chatelier Example #3
A closed container of water and its vapor at equilibrium. Vapor is removed from the system.
water + Energy vapor
The equilibrium of the system shifts to the _______ to replace the vapor.
right
Le Chatelier Example #4Le Chatelier Example #4
A closed container of N2O4 and NO2 at equilibrium. The pressure is increased.
N2O4 (g) + Energy < - - > 2 NO2 (g)
The equilibrium of the system shifts to the _______ to lower the pressure, because there are fewer moles of gas on that side of the equation.
left
Pressure Changes to system
If the volume decreases, the concentration increases, and there will be a shift to the side with the less amount of moles.
If the volume increases, the concentration decreases, and there will be a shift to the side with the more amount of moles.
Example;
If I increase the pressure, where is the shift?(right)
If I decrease the pressure, where is the shift? (left)
2SO2 + O2 <--> 2SO3
(3moles) (2moles)
Effect of Concentration
1. If you add more reactant, it shifts to the right 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 the right, increasing the formation of product.
4. If you remove reactant, it shifts to the left
Effect of temperature
• Energy is treated as a reactant if endothermic equation, and as a product if exothermic equation.
• If cooling a system, then it shifts so more heat is produced.
• If heating a system, then it shifts so extra heat is used up.
Example for temp. changes for Endothermic Reaction
Heating the below reaction causes the system to shift to the right = more products, because you treat energy like a reactant.
2NaCl +H2SO4 + energy < -- > 2HCl + Na2SO4
Cooling the above reaction causes the system to shift to the left = less reactants, so need to make up more
Effect of temp change on exothermic reactions
• Heating the below reaction causes the system to shift to the left, to use up the extra heat.
2SO2 + O2 <--> 2SO3 + energy
• Cooling the above reaction causes the system to shift to the right, to make up for the lost heat.
The effect of a catalyst on equilibrium
• Adding a catalyst speeds up a reaction by providing an alternative mechanism with a lower activation 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 more quickly.
Haber Process
• N2(g) + 3H2(g) < - - > 2NH3(g) ΔH= -92 kJ/mol
– Mixture’s 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 causes a shift to the left, favouring product formation.
• This is expensive to due and most production plants will resist compressing gases in terms of operating costs. Compromise will be met.
Compromise
• This is an exothermic reaction, so low temperatures would be favourable to produce product.– Low temps mean low reaction rates, so we
may get a higher yield but it will take a long time to get it. Not good for business.
– A compromise temp, as well as the use of a catalyst will aid in speeding up the reaction to a more acceptable standard.
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 we approach equilibrium, and typically only 20% of N2 and H2 is converted.
• The gases are cooled and NH3 is condensed and removed, leaving unused N2 and H2 available for further production.