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Definitions:

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Equilibrium

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The state of a reaction in which the rates of the forward and reverse reactions are the same.

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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)

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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 

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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

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mA nB 

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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 )

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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?? 

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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. 

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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 

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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??   

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Adding a catalyst? NO! Not a stress.

Change in temperature – system shifts eq. so that heat is equalized.

2 SO3 ( g )

Add NH 3?

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Take out NH 3? 

HEAT

Raise the temperature 

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Eq. shifts  to absorb the heat Eq. shifts  to produce more heat

Eq. shifts . End with more NH3.

Raise the pressure 

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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 )

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2 NH 3 ( g )

Equilibrium shifts to the right (). End with more NH3

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Change in pressure affects which state of matter?

2 SO2 ( g ) O2 ( g )

N 2 ( g ) 3H 2 ( g ) Add more N 2? 

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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 

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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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