Equilibrium

1. Equilibrium 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.

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

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

4. Initially, there is only Fe3O4 and H2 in the reaction vessel. • As it is heated, Fe and H2O begin to form and the rate of the fwd rxn begins to decrease (Figure 1 – next page) • When there is some Fe and H2O, they begin to reform Fe3O4 and H2, so the rate for the rev rxn begins to increase. (Figure 1 – next page)

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

6. 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)

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

8. 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

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

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

11. What is Keq for this equation?

12. 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. • Therefore, the REVERSE rxn is favored. • If Keq > 1, then there are more products than reactants at equilibrium. • Therefore, the FORWARD rxn is favored.

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

14. Le Chatelier’s Principle Or How to Get Away from Stress

15. 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?? • Change in concentration • Change in temperature • Change in pressure • Change in pressure affects which state of matter? • Adding a catalyst? NO! Not a stress.

16. What are the effects of each stress? • Change in concentration – system shifts to get back to the same ratio of reactants and products. • Add more N2? • Equilibrium shifts to the right (). End with more NH3 • Add NH3? • Eq. shifts to the left (). End with more N2 and H2. • Take out NH3? • Eq. shifts . End with more NH3.

17. Change in temperature – system shifts eq. so that heat is equalized. • Raise the temperature • Eq. shifts  to absorb the heat • Lower the temperature • 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? • SLOOOOOOOW!!!!

18. Change in Pressure – System shifts to equalize the number of moles of gas. 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 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.

19. 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.