Equilibrium The condition of a system in which competing influences are balanced. The condition of a system in which competing influences are balanced. (Chemistry) The state of a reaction in which the rates of the forward and reverse reactions are the same. (Chemistry) The state of a reaction in which the rates of the forward and reverse reactions are the same.

First, some definitions: Eq. = Equilibrium Eq. = Equilibrium Fwd., Rev., Rxn., Conc., Temp. Fwd., Rev., Rxn., Conc., Temp. [A] = Concentration of A in mol/L or M [A] = Concentration of A in mol/L or M

What do we know? We know every rxn has a certain rate. We know every rxn has a certain rate. Some rxns are reversible – they spontaneously proceed from products to reactants. Some rxns are reversible – they spontaneously proceed from products to reactants. Fwd rxn  Fwd rxn  Rev rxn  Rev rxn 

Initially, there is only Fe 3 O 4 and H 2 in the reaction vessel. Initially, there is only Fe 3 O 4 and H 2 in the reaction vessel. As it is heated, Fe and H 2 O begin to form and the rate of the fwd rxn begins to decrease (Figure 1 – next page) As it is heated, Fe and H 2 O begin to form and the rate of the fwd rxn begins to decrease (Figure 1 – next page) When there is some Fe and H 2 O, they begin to reform Fe 3 O 4 and H 2, so the rate for the rev rxn begins to increase. (Figure 1 – next page) When there is some Fe and H 2 O, they begin to reform Fe 3 O 4 and H 2, so the rate for the rev rxn begins to increase. (Figure 1 – next page)

Figure 1 Reaction Coordinate (time) Rate of Reaction Forward Rate Reverse Rate

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) 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)

Figure 2 Reaction Coordinate (time) Rate of Reaction Forward Rate Reverse Rate

Equilibrium Concentrations At equilibrium, for a specific reaction, at a given temperature, there is a set concentration of each reactant and product. 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, K eq Since these concentrations are the same for each reaction, at a set temperature, we can write an equilibrium constant, K eq

K eq – The Eq. Constant Consider a generalized reaction: Consider a generalized reaction: For this reaction, For this reaction, Notice that the coefficients become superscripts—they raise the concentration to that power. Notice that the coefficients become superscripts—they raise the concentration to that power.

HOWEVER! Pure liquids and solids are removed from the equilibrium constant expression because they have a constant concentration. Pure liquids and solids are removed from the equilibrium constant expression because they have a constant concentration. So, for: So, for: The eq. constant is: The eq. constant is:

What is K eq for this equation?

What does K eq mean?? If K eq = ~1, then there are about the same amount of reactants and products at equilibrium. If K eq = ~1, then there are about the same amount of reactants and products at equilibrium. If K eq < 1, then there are more reactants than products at equilibrium. If K eq < 1, then there are more reactants than products at equilibrium. Therefore, the REVERSE rxn is favored. Therefore, the REVERSE rxn is favored. If K eq > 1, then there are more products than reactants at equilibrium. If K eq > 1, then there are more products than reactants at equilibrium. Therefore, the FORWARD rxn is favored. Therefore, the FORWARD rxn is favored.

Review/Tutorial We’ll review with an online tutorial, Equilibrium. We’ll review with an online tutorial, Equilibrium. Equilibrium

Le Chatelier’s Principle Or How to Get Away from 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. 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?? What kind of “stress” could a chemical reaction be subjected to?? Change in concentration Change in concentration Change in temperature Change in temperature Change in pressure Change in pressure Change in pressure affects which state of matter? Change in pressure affects which state of matter? Adding a catalyst? NO! Not a stress. Adding a catalyst? NO! Not a stress.

What are the effects of each stress? Change in concentration – system shifts to get back to the same ratio of reactants and products. Change in concentration – system shifts to get back to the same ratio of reactants and products. Add more N 2 ? Add more N 2 ? Equilibrium shifts to the right (  ). End with more NH 3 Equilibrium shifts to the right (  ). End with more NH 3 Add NH 3 ? Add NH 3 ? Eq. shifts to the left (  ). End with more N 2 and H 2. Eq. shifts to the left (  ). End with more N 2 and H 2. Take out NH 3 ? Take out NH 3 ? Eq. shifts . End with more NH 3. Eq. shifts . End with more NH 3.

Change in temperature – system shifts eq. so that heat is equalized. Change in temperature – system shifts eq. so that heat is equalized. Raise the temperature Raise the temperature Eq. shifts  to absorb the heat Eq. shifts  to absorb the heat Lower the temperature Lower the temperature Eq. shifts  to produce more heat Eq. shifts  to produce more heat NOTE: Lowering the temp. may shift the eq. in the direction of the products, but if the temp. is lowered TOO much, what happens to the rxn. rate? NOTE: Lowering the temp. may shift the eq. in the direction of the products, but if the temp. is lowered TOO much, what happens to the rxn. rate? SLOOOOOOOW!!!! SLOOOOOOOW!!!!

Change in Pressure – System shifts to equalize the number of moles of gas. Change in Pressure – System shifts to equalize the number of moles of gas. Raise the pressure Raise the pressure Eq. shifts  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. 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 pressure Lower the pressure Eq. shifts  Eq. shifts  Shift equalizes pressure. Shift equalizes pressure. NOTE: This is a famous reaction – the Haber process. It is extremely important in the agricultural industry to make fertilizers. NOTE: This is a famous reaction – the Haber process. It is extremely important in the agricultural industry to make fertilizers.

Change amount of Catalyst? 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. NO! This does not affect the position of the equilibrium. It speeds the reaction in BOTH directions, so that it reaches equilibrium faster.