Chemical Equilibrium Part I What does it mean

Chemical Equilibrium Part I: What does it mean? How does Le Châtelier's Principle explain it? Jespersen Chap. 15 Sec 1, 2 & 6 Dr. C. Yau Fall 2014 1

Decay of I-131 is 1 st order Recall the radioactive decay of I-131 Xe-131 + e. Reactant is eventually all used up. This is typical of an irreversible rxn. 2

N 2 O 4 (g) N 2 O 4 NO 2 2 NO 2 (g) Not anywhere near zero At a certain time t concentrations don’t change any more. Conc. [N 2 O 4] and [NO 2] become constant. System has reached a dynamic equilibrium. Note: [N 2 O 4] and [NO 2] are not necessarily equal. 3

How is this reaction different? This is also a reversible reaction. A and B reaches an equilibrium, but [B]> [A]. 4

Chemical Equilibrium An equilibrium has been reached when the rate of the forward rxn = rate of the reverse rxn. rate (forward) = rate (reverse) Reactant Product It does NOT mean [reactant] = [product]. [reactant] can be greater or less than [product]. [reactant] > [product] Equilibrium favors reactant. [reactant] < [product] Equilibrium favors product. [reactant] = [product] Equilibrium favors both equally. 5

You can reach the same equilibrium mixture from either direction at a specified temperature. Fig. 15. 2 p. 698 N 2 O 4 (g) 0. 0350 M N 2 O 4 colorless 2 NO 2 (g) 0. 0292 M N 2 O 4 0. 0116 M NO 2 amber 0. 0700 M NO 2 brown 6

The Equilibrium Constant, K K is the equilibrium constant. It is the molar concentration of the product divided by the molar concentration of reactants, brought to the power of the corresponding stoichiometric coefficients. x Reactant y Product 7

Keq, Kc, Kp, and Q For N 2 O 4 (g) 2 NO 2 (g) Mass Action Expression Kc = K in molar concentration Kp = K in partial pressures (for gases) Q = reaction quotient = numerical value of K at any point regardless of whether at equilibrium At equilibrium, Q = Keq 8

Le Châtelier’s Principle When an equilibrium system comes under stress, the equilibrium will shift in the direction that will relieve the stress. What kind of stresses? . . . • Changes in concentration of reactants/products • Changes in temperature • Changes in pressure • Addition of chemicals that affect the concentration of reactants or products 9

Le Châtelier’s Principle Cu(H 2 O)42+ (aq) + 4 Cl (aq) Cu. Cl 42 (aq) + 4 H 2 O (l) blue yellow After the system reaches equilibrium, the color is green. If HCl is added… what changes in color would you see? What is the equilibrium expression? How does the reaction quotient compare with K? What does Q tell you the system will do? 10

Le Châtelier’s Principle Cu(H 2 O)42+(aq) + 4 Cl (aq) Cu. Cl 42 (aq) + 4 H 2 O(l) yellow blue A B C Fig. 15. 7 p. 711 Tube B = system at equilibrium = mixture of Cu(H 2 O)42+ and Cu. Cl 42 (blue/green) Tube C = after HCl is added & equilibrium re-established (becomes green. . . more yellow mixed in) Tube A = after water is added & equilibrium re-established (becomes bluer. . . more blue mixed in) 11

Le Châtelier’s Principle If a reaction is exothermic, it is written as… Reactant Product + heat Heat can be considered a “product” in analyzing with Le Châtelier’s Principle. If this system is placed in hot water, would the equilibrium shift to the left or the right? Unlike changing concentrations, heat WILL change the K value. In this case, would K become larger or smaller? 12

Cu(H 2 O)42+ (aq) + 4 Cl (aq) blue Cu. Cl 42 (aq) + 4 H 2 O (l) yellow ICE WATER A B C Tube B = mixture of complexes at equilibrium Fig. 15. 8 p. 713 HOT WATER Tube A = mixture in ice bath Is this reaction exothermic or endothermic? Tube C = mixture placed in hot water Is this consistent with an endothermic reaction? 13

What does increase of T do to K? Don't try to memorize this! Learn to figure this out. There are too many things to memorize and you will get it all mixed up. Consider a generic endothermic rxn: heat + R P What is its mass action expression? If T increases, which way will the equilibrium shift? How will that affect K? 14

Effect of Pressure on Equilibrium Pressure affects only gases. Why? Solids and liquids are essential noncompressible. Increasing P does not affect them. P can be increased by decreasing the V. (PV=n. RT, so P and V are inversely proportional. ) When V decreases, P increases. 15

Effect of Pressure on Equilibrium For rxn: 3 H 2(g) + N 2(g) 2 NH 3(g) At equilibrium, what is present? If P is increased, how is the system stressed? How would the system relieve the stress? If the system shifts to the right, would it become more or less crowded? Would that relieve the stress? Ans. Yes, so if P , it will shift to the right. 16

Effect of Pressure on Equilibrium If V , what will that do to the equilibrium? a) 4 NH 3(g) + 3 O 2(g) 2 N 2(g) + 6 H 2 O(g) b) C 3 H 8(g) + 5 O 2(g) 3 CO 2(g) + 4 H 2 O(l) c) Fe 3 O 4(s) + 4 H 2(g) 3 Fe(s) + 4 H 2 O(l) d) H 2(g) + I 2 (g) 2 HI (g) 17

Effect of Pressure on Equilibrium What if we increase the pressure by adding an inert gas (without changing the volume)? The total pressure is increased, but the concentration of the reactants and products remains the same. Frequency of collision of reactants is not increased. Collision with the inert gas does not lead to product & therefore does not affect the equilibrium. Adding an inert gas has no effect on the equilibrium. 18

Four Experiments Using H 2 + I 2 2 HI • Fig. 15. 3 p. 699 shows us how different amts of reactants and product are placed in a 10. 0 L rxn vessel at 440 o. C. • When equilibrium is reached, different amounts of reactants and product are found. • Table 15. 1 p. 700 shows us at equilibrium, concentrations of reactants and products are different but K is the same. • KEEP THIS IN MIND! The concentrations are not the same, only K’s are the same. 19

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At equilibrium, K is a constant, BUT molar concentrations do not have to be the same to give the same K. Remember that K varies with T. The above K value of 49. 5 is only for T = 440 o. C. 21

How can we determine K and Q from the graph of Conc vs. Time of reversible reactions? We can det. K if the stoichiom. ratio is known. 3. 0 M 2. 0 M Can we say that K = 3. 0 M/2. 0 M = 1. 5? What if the rxn is 2 A B? 22

Reactant Product Figure below depicts 3 different equilibria. A B C In system A, how does K compare with 1? K >1 In system B. . . In system C. . . 23

• The relationship between K and Q: When is Q > K? When is Q<K? When is Q=K? • Remember • n. Reactant m. Product • If we begin with only Reactant present, [prod] = 0. Q would be zero and will slowly increase as the rxn proceeds (Q<K) until Q = K. • What will the system do in order for Q to equal K? • It will shift to the right to form more product. • When is Q > K? • If we add more product to equilibrium system. • What will the system do? 24

REMINDER: • How do we write the mass action expression (MAE) from a given chemical equation? • 3 A 4 B + 2 C • How is writing the MAE different from writing the rate law? 25

Reaction is first allowed to reach equilibrium. If additional H 2 is added, the system will selfadjust to relieve the stress to remove the excess H 2 until equilibrium is reestablished. [NH 3] has increased, & [N 2] has decreased at the new equilibrium. (K is the same, but conc are different. ) 26

Effect of Catalyst on Equilibrium • A catalyst speeds up a reaction. • In a reversible reaction, it speeds up both the forward and reverse reaction. • A catalyst does not affect the equilibrium. • It speeds up the process of REACHING equilibrium. 27

PE Diagram of Reversible vs. Irreversible Rxns Diagram A Diagram B Which PE diagram more closely corresponds to a reversible reaction? Why? 28

Equilibrium is based on the relative stability of the reactants and products Diagram A Diagram B How do you suppose K is different for the two reactions depicted above? Which has K < 1? K>1? What would the diagram look like for K = 1? 29

Potential E PE diagram and k Lower Ea means smaller k faster rate. 1) What is the title of the x-axis? 2) For the rxn depicted, at equilibrium at a given temperature, which rate is faster? the rate for the forward rxn or the reverse rxn? 3) At equilibrium, which is larger kf or kr? 4) How can the rates be the same then? 30

PE Diagram: k versus K Remember this! For kinetics we look at the Ea. R P For equilibrium, we look at ΔH. What do we look at when we want to talk about k? What do we look at when we talk about K? ΔH shows the relative thermodynamic stability of R and P. 31

Suppose a mixture contained equal conc of H 2, Br 2 and HBr. Given that the rxn below has Kc = 1. 4 x 10 -21, H 2(g) + Br 2(g) 2 HBr(g) will the rxn proceed to the L or R in order to reach equilibrium? 1) What might the PE diagram look like? 2) Which part of the PE diagram can't we be sure of? Why? 3) Which is thermodynamically more stable? the reactants or the product? 32