AP Chemistry Chemical Equilibrium
Changing Reaction Conditions
LeChatelier's Principle
How can we maximize product formation of a gaseous reaction that has reached equilibrium?
LeChatelier's Principle:
A disturbed equilibrium will shift to counteract the change in conditions
First: change concentrations by adding/removing products/reactants!
LeChatelier's Principle
1200 K, 10.00 L vessel
CO (g) + 3 H2 (g) CH4 (g) + H2O (g)
Initial: 1.000 mol 3.000 mol
Equil: .613 mol 1.839 mol
0 mol
.387 mol
0 mol
.387 mol
Cool the reaction vessel, remove the H2O...
The equilibrium reacts to restore the original concentrations (shifts right)
CO (g) + 3 H2 (g) CH4 (g) + H2O (g)
After
.509 mol .122 mol
H2O .491 mol 1.473 mol
removed
Same effect if CH4 was removed, or if
CO or H2 was added...
14.7­14.9
AP Chemistry Chemical Equilibrium
LeChatelier's Principle
CO (g) + 3 H2 (g) CH4 (g) + H2O (g)
Q =
[CH4] [H2O]
[CO] [H2]3
@ equilibrium: Q = Kc
Remove H2O or CH4: Q < Kc
reaction goes right (products)
Add H2 or CO: Q < Kc
reaction goes right (products)
Add H2O or CH4 Q > K
c
Remove H2 or CO: reaction goes left (reactants)
*If Kc is so small (like 10­100), equilibrium is basically all reactants, so adding more reactants will not really have an effect.
Changing Reaction Conditions
Second: change pressure conditions
If an equilibrium involves a change in total moles of gas, and pressure is increased by
reducing the volume of the reaction mixture...
The reaction will shift in the direction of fewer moles of gas, and vice­versa
Pressure decreased, shift towards
more moles of gas
14.7­14.9
AP Chemistry Chemical Equilibrium
CO (g) + 3 H2 (g) CH4 (g) + H2O (g)
4 moles gas
2 moles gas
If the reaction volume is cut in half, the pressure is doubled (therefore, concentration is doubled). Which way will the reaction shift?
Towards Products (forward)
where there's less moles of gas...less pressure!
Trying to restore equilibrium pressure
Again...ignore liquids and solids:
CO2 (g) + C (s) 2 CO (g)
2 mol gas
1 mol gas
Increase pressure, reaction goes left
Decrease pressure, reaction goes right
Changing Reaction Conditions
Third: change temperature conditions
Increased temperature typically increases reaction rate, so reactions reach equilibrium quicker.
Many gaseous reactions are sluggish at
room temperature but commercially feasible
at higher temperatures
Temperature effects: look at H
Exothermic: Increase temperature, go left
Decrease temperature, go right
Endothermic: Increase temperature, go right
Decrease temperature, go left
14.7­14.9
AP Chemistry Chemical Equilibrium
CO (g) + 3 H2 (g) CH4 (g) + H2O (g)
H = ­206.2 kJ
*Remember, Kc varies with temperature 1200 K = 3.92
298 K = 4.9 x 1027; 800 K = 1.38 x 105; 1000 K = 254
Exothermic: Larger Kc at lower temperature
Smaller Kc at higher temperature
Endothermic: Larger Kc at higher temperature
Smaller Kc at lower temperature
Optimum Reaction Conditions Haber Process
Fe
N2 (g) + 3 H2 (g) 2 NH3 (g)
H = ­91.8 kJ
Exothermic, so...
Lower temperatures, NOT room
Experimentally found to be 450 C
Less moles of gas produced, so...
Higher pressure
Experimentally found to be 600 atm
Also...
Add reactants; remove products:
Cool reactor, NH3 liquefies, remove it
N2 and H2 left to continue reaction!
Iron catalyst, too
14.7­14.9
AP Chemistry Chemical Equilibrium
Effect of a Catalyst
SO2 (g) + O2 (g) 2 SO3 (g)
Kc = 1.7 x 1026
Should be mostly SO3, but it is found that when sulfur burns in O2, it makes mostly SO2 (oxidation to SO3 is just too slow).
However, a Platinum catalyst speeds it up!
A catalyst has NO EFFECT on equilibrium COMPOSITION of a reaction mixture!
Speeds up reaching equilibrium!
Catalysts are useful for reactions with
large Kc that are normally slow!
A catalyst is little help to a reaction with a small Kc
Effect of a Catalyst
Catalysts CAN affect WHAT PRODUCT will form as it can affect the rate of one reaction over another when several are possible:
Ostwald Process for making HNO3
4 NO (g) + 3 O2 (g) + 2 H2O (l) 4 HNO3 (aq)
NO is prepared by the oxidation of NH3:
2 possible reactions!
Platinum Catalyst
4 NH3 (g) + 5 O2 (g) 4 NO (g) + 6 H2O (g)
Copper Catalyst
4 NH3 (g) + 3 O2 (g) 2 N2 (g) + 6 H2O (g)
14.7­14.9