Equilibrium

1. Equilibrium • Stoichiometry • We assume that the reaction goes to completion • all the reactants are converted to products • In reality • many reactions do not go to completion • Systems reach chemical equilibrium • the state where the concentration of reactants and products remain constant over time

2. Equilibrium • Equilibrium • How does a system reach equilibrium? • Why does a system reach equilibrium? • How do we calculate the concentration of reactants and products at equilibrium?

3. Equilibrium • Equilibrium is not static, it is dynamic • while there appear to be no changes (the concentrations of reactants and products remain constant), there is always something going on. • Example:

4. Equilibrium • Ex: H2O(g) + CO(g) <==> H2(g) + CO2(g) • Graph of Concentration vs. Time: • At equilibrium, the concentration of reactants never go to zero • At equilibrium, the rate of the reactant becoming products is the same as the rate of products becoming reactants

5. Equilibrium • Why does equilibrium occur? • Molecules react when they collide with each other • Initially, there are many reactant molecules, so the rate of the forward reaction is high. • As the reactant molecules disappear, the rate of the forward reaction decreases. • Meanwhile, the concentration of product molecules increases.

6. Equilibrium • With higher concentration of products, the rate of the reverse reaction increases. • Eventually, the rate of the forward reaction will be equal to the rate of the reverse reaction • The system has reached equilibrium

7. Equilibrium • Equilibrium position • we say that a reaction favors • the forward reaction (more products are made) or • the reverse reaction (few products are made) • depends on • the relative energies of the reactants and products (I.e. DH) • entropy of the system

8. Equilibrium • The equilibrium constant - Keq • Cato Maximilian Guldberg and Peter Waage • first proposed the law of mass action to describe the equilibrium condition • for a reaction aA + bB <==> cC + dD, the law of mass action is given by the equilibrium expression: Keq = [C]c[D]d / [A]a[B]b

9. Equilibrium • Calculate the value of Keq if you know the equilibrium concentrations of the products and reactants. • We can manipulate Keq to find the Keq for related reactions. • Ex: N2 + 3 H2 <==> 2 NH3 Keq = 6.02 x 10-2

10. Equilibrium • Reverse a reaction: K/eq = 1/Keq • for 2NH3 <==> N2 + 3 H2 • Keq = 1/6.02 x 10-2 = 1.66 x 101 • Multiply a reaction by some factor, n,: • K//eq = (Keq)n • for NH3 <==> 1/2 N2 + 3/2 H2 • Keq = (1.66 x 101)1/2 = 4.07 • Traditionally, Keq is written without units.

11. Equilibrium • The Law of Mass Action describes all type of chemical equilibria • Keq is the same regardless of the initial concentration of reactants • Keq is the same even though the individual sets of concentrations are different, Keq is a ratio that remains constant. • Keq changes when temperature changes

12. Equilibrium • For equilibria involving gases, we can write a Kc or a Kp for a reaction: • aA(g) + bB(g) <==> cC(g) + dD(g) • Kp = (pC)c(pD)d/ (pA)a(pB)b • How is Kp related to Kc (I.e. Keq)?

13. Equilibrium • Heterogeneous Equilibrium • Sometimes different phases exist in the course of a reaction • Ex: CaCO3(s) <==> CaO(s) + CO2(g) • Experimental results show that the position of equilibrium does not depend on the amounts of pure liquids or solids, i.e. don’t include terms for solids or liquids in the equilibrium expression • Keq = [CO2] in this case

14. Equilibrium • The extent of a reaction…or the tendency for a reaction to form products • large Keq • favors the formation of products at equilibrium • the equilibrium lies to the right • the reaction goes essentially to completion

15. Equilibrium • The extent of a reaction…or the tendency for a reaction to form products • small Keq • system at equilibrium will consist mostly of reactants • very few products will be made • favors the reactants • equilibrium lies to the left • the reaction does not occur to any significant extent

16. Equilibrium • Is a reaction at equilibrium? • If the reaction is not at equilibrium, which direction will it go? Will it go towards the products or towards the reactants? • Look at Q, the reaction quotient, and compare it to Keq • Q = [Co]c[Do]d/ [Ao]a[Bo]b • Instead of equilibrium concentrations, plug in the concentrations in question. • If Q = Keq , then the system is at equilibrium. • If Q < Keq , then there aren’t enough products, so the system will go forward (shift to the right) to make more products. • If Q > Keq, then there are too many products, so the system will go in the reverse direction (shift to the left) to get rid of the excess product.

17. Equilibrium • Le Chatelier’s Principle • The position of equilibrium can be manipulated. • This is important in industry where it would be good to produce a lot of products, thus, the forward reaction would need to be favored. • Apply Le Chatelier’s Principle to alter equilibrium positions

18. Equilibrium • Le Chatelier’s Principle • When stress is applied to a system at equilibrium, the system will respond to counteract the stress and regain (a new) equilibrium…or… • If a change is imposed on a system at equilibrium, the position of equilibrium will shift in a direction that tends to reduce the change

19. Equilibrium • Stress • Changes in concentrations • add reactants • the system tries to get rid of the extra reactants, it will favor the __________ reaction • remove reactants • the system tries to make more reactants, it will favor the ___________ reaction

20. Equilibrium • Stress • Changes in concentration • remove products • the system will try to make more products, it will favor the _____________ reaction

21. Equilibrium • Stress • Changes in Pressure • for systems involving gases • Add or remove a gaseous reactant or product • Add or remove an inert gas (one not involved in the reaction • Change the volume of the system

22. Equilibrium • Add or remove a gaseous reactant or product • same results as adding or removing a reactant or product • Add or remove an inert gas • there is no effect on the equilibrium position • the inert gas does not affect the concentrations or the partial pressures of the relevant gases even though the total pressure changes. • The inert gas does not participate in the reaction

23. Equilibrium • Stress • Change the volume of the container (consider the pressure of the system) • Compress the system (Decrease the volume, means the pressure of the system (i.e. the partial pressure of each of the gases) is greater) • the system will respond to decrease the pressure. How? • If the system could reduce the number of molecules, the pressure would decrease. • How can the system reduce the number of molecules? It would favor the reaction that results in a net decrease of molecules.

24. Equilibrium • Stress • Changes in Temperature • changes the equilibrium position • also changes the Keq • Note: the other stresses, changes in concentrations, changes in pressure, and changes in volume do not change the Keq when they change the equilibrium position

25. Equilibrium • Stress • Changes in Temperature • Treat heat as a reactant or product when applying Le Chatelier’s Principle