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3/28/2011
Definitions:
Equilibrium
The state of a reaction in which the rates of the forward and reverse reactions are the same.
Conc. Concentration Temp. Temperature [A] = Concentration of A in mol/L or M Keq or Kc Equilibrium Constant
3Fe( s ) 4 H 2 O( g )
Initially, there is only Fe 3O4 and H2 in the reaction vessel. As it is heated, Fe and H 2O begin to form and the rate of the fwd rxn begins to decrease (Figure 1 – next page) When there is some Fe and H 2O, they begin to reform Fe3O4 and H2, so the rate for the rev rxn begins to increase. (Figure 1 – next page)
Figure 1 Rate of Reaction
Fe3 O4 ( s ) 4 H 2 ( g )
Eq. = Equilibrium Fwd. Forward Rev.(Rvs) Reverse Rxn. Reaction
Forward Rate
Reverse Rate
Reaction Coordinate (time)
Eventually, the forward rate decreases and the reverse rate increases so that they are equal. At this point, they are at equilibrium. (Figure 2 – next page)
Rate of Reaction
Figure 2
Forward Rate
Reverse Rate
Reaction Coordinate (time)
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Keq – The Eq. Constant
Equilibrium Concentrations
At equilibrium, for a specific reaction, at a given temperature, there is a set concentration of each reactant and product. Since these concentrations are the same for each reaction, at a set temperature, we can write an equilibrium constant, Keq
mA nB
Pure liquids and solids are removed from the equilibrium constant expression because they have a constant concentration. So, for: Fe3 O4 ( s ) 4 H 2 ( g )
3Fe( s ) 4 H 2 O( g )
The eq. constant is:
K eq
[ Fe]3 [ H 2 O]4 [ Fe3O4 ][H 2 ]4
pC
qD
For this reaction,
K eq
HOWEVER!
Consider a generalized reaction:
[C ] p [ D]q [ A]m [ B]n
Notice that the coefficients become superscripts—they raise the concentration to that power.
What is Keq for this equation?
H 2 (g)
I 2 (g)
K eq
2 HI ( g )
[ HI ]2 [ H 2 ][I 2 ]
[ H 2 O] 4 [ H 2 ]4
What does Keq mean??
If Keq = ~1, then there are about the same amount of reactants and products at equilibrium. If Keq < 1, then there are more reactants than products at equilibrium.
Le Chatelier’s Principle Or How to Get Away from Stress
Therefore, the REVERSE rxn is favored.
If Keq > 1, then there are more products than reactants at equilibrium.
Therefore, the FORWARD rxn is favored.
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What are the effects of each stress?
Le Chatelier’s Principle
When a system at equilibrium is subjected to a stress, the equilibrium will tend to shift in the direction to relieve the stress. What kind of ―stress‖ could a chemical reaction be subjected to??
Adding a catalyst? NO! Not a stress.
Change in temperature – system shifts eq. so that heat is equalized.
2 SO3 ( g )
Add NH 3?
Take out NH 3?
HEAT
Raise the temperature
Eq. shifts to absorb the heat Eq. shifts to produce more heat
Eq. shifts . End with more NH3.
Raise the pressure
Eq. shifts Shift is towards smallest total number of moles of gas, so the eq. shifts away from 4 moles (3 +1) towards 2 moles.
Lower the pres s ure
Lower the temperature
Eq. shifts to the left (). End with more N2 and H2.
Change in Pressure – System shifts to equalize the number of moles of gas. 3H 2 ( g ) N 2 ( g ) 2 NH 3 ( g )
2 NH 3 ( g )
Equilibrium shifts to the right (). End with more NH3
Change in pressure affects which state of matter?
2 SO2 ( g ) O2 ( g )
N 2 ( g ) 3H 2 ( g ) Add more N 2?
Change in concentration – system shifts to get back to the same ratio of reactants and products.
Change in concentration Change in temperature Change in pressure
Eq. shifts Shift equalizes pressure.
NOTE: This is a famous reaction – the Haber process. It is extremely important in the agricultural industry to make fertilizers.
Change amount of Catalyst?
NO! This does not affect the position of the equilibrium. It speeds the reaction in BOTH directions, so that it reaches equilibrium faster.
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