1. Chapter 16 Chemical Equilibrium

2. Before we look at chemical equilibrium, let’s look at the past A.Many reactions in chemistry go to 100% completion 1.Example: burning methane. CH 4 + O 2  CO 2 + H 2 O 100% of the products are made

3. Here is where equilibrium comes in……… B.Some reactions don’t go to completion because they are reversible - the products collide and react to produce the original reactants A B Does NOT make 100% of the products, because products regenerate reactants

4. I.Chemical Equilibrium: 1. The RATE At equilibrium, both the forward and reverse reactions are occurring at the same rate, equal. 2. Concentration At equilibrium the amount of each reactant and constant product remains constant.

5. II.How to express chemical equilibrium  Keq: Equilibrium Constant a.aA+bB  cC+dD b. keq= [products] coefficient [reactants] coefficient [ ]=Concentration, Molarity c.keq= [C] c [D] d [A] a [B] b

6. III.Homogeneous and Heterogeneous Equilibria Homogeneous: All states of matter the same Heterogeneous: Different states of matter in the chemical reaction Only use the gases (g) when writing the Keq equation!!! Problems on overhead. IV.Calculating Keq On Overhead

7. What Does the Value of Keq Mean? If Keq >> 1, the reaction is product-favored; product predominates at equilibrium. If Keq << 1, the reaction is reactant-favored; reactant predominates at equilibrium. Stop here

8. Warm Up 1.What is chemical equilibrium? 2.What’s the significance of Keq? 3.Write the keq expression for: Br 2(g) +2NO (g)  2NOBr (g) 4.Calculate Keq for the reaction in #3 [.214]=Br 2 [.0677]=NOBr [0.428]=NO 5.What does the reaction mainly have at equilibrium? a.rates of forward/ reverse reactions are equal b.the Concentrations of reactants and products are constant. The value tells you at Equilibrium, what you mostly have……reactants OR products OR equal amounts of both Keq= [NOBr] 2 [Br 2 ][NO] 2 Keq= [.0677] 2 [.214][.428] 2 Keq=0.117 <1 At equilibrium, the system has more reactants

9. The Reaction Quotient (Q) 1.Q gives the same ratio the equilibrium expression gives, but for a system that may or may not be at equilibrium. 2.Indicates if a system is at equilibrium and if not, which position does it need to shift to reach equilibrium.

10. K is @ Equilibrium Q is @ Any Time (eq. or not)

11. If Q = K, the system is at equilibrium.

12. If Q > K, there is too much product and the equilibrium shifts to the left.

13. If Q < K, there is too much reactant, and the equilibrium shifts to the right.

14. Remember the Q uic K way of figuring the shift out.

15. 0.035 moles of SO 2, 0.500 moles of SO 2 Cl 2, and 0.080 moles of Cl 2 are combined in an evacuated 5.00 L flask and heated to 100 o C. What is Q before the reaction begins? Which direction will the reaction proceed in order to establish equilibrium? 1. Write the expression to find the reaction quotient, Q 2. Determine the concentrations of each gas. 3. Calculate Q by substituting the concentrations. 4. Determine how the reaction proceeds. SO 2 Cl 2 (g)  SO 2 (g) + Cl 2 (g) Keq = 0.078 at 100 o C

16. 0.500 mole SO 2 Cl 2 /5.00 L = 0.100 M SO 2 Cl 2 0.035 mole SO 2 /5.00 L = 0.0070 M SO 2 0.080 mole Cl 2 /5.00 L = 0.016 M Cl 2 0.078 (K) > 0.0011 (Q) Since K >Q, the reaction will proceed in the forward direction in order to increase the concentrations of both SO2 and Cl2 and decrease that of SO2Cl2 until Q = K. Q = 0.0011

17. 1.Proposed by a Chemistry Professor, Henry Chatelier 2.Came up with this (LeChatelier’s) principle in 1884 3.If a change in condition occurs in a system, the system will change to offset the change. What will happen to a system that is already at equilibrium if I change conditions??

18. Possible changes (stressors) Temperature Pressure Concentration

19. 1.Change in Concentration 2NO 2(g) a.More NO 2 is added: b.More N 2 O 4 is added: c.N 2 O 4 is taking away: d.NO 2 is taken away: Shift Right Shift Left N 2 O 4(g)  Example:

20. N2O4N2O4 2. Change in Pressure: When pressure is applied the shift goes from more moles of gas to less moles of gas. 2NO 2(g)  N 2 O 4(g) NO 2 a.What will happen if the pressure is increased?? Shift to the Right Applies only to gases N2O4N2O4 N2O4N2O4 N2O4N2O4 N2O4N2O4 P

21. Pressure AnimationAnimation LeChatelier Principle ----->Tutorial<-----Tutorial Le Chatelier's principle can be used to predict the effect of a change in conditions on a chemical equilibrium This principle basically states that if stress is applied to a system at equilibrium, the position of the equilibrium will shift in the direction that reduces the stress to reinstate equilibrium. For example, if more reactants are added to the system, the reaction will shift in the forward direction, and if more products are added, the reaction will shift in the reverse direction. If heat is added to the system and the reaction is exothermic, heat should be thought of as a product and the reaction will shift to the left; if the reaction is endothermic and heat is added, the reaction will shift to the right. Pressure Changes- The addition of pressure will cause a shift in the direction that results in the fewer number of moles of a gas, while if pressure is relieved, the reaction will shift in the direction that produces more moles of a gas. Start the animation and see for yourself. The pressure increase shifts the equilibrium to the side with less gases (N 2 O 4 ). What if the number of reacting gases are equal to the number of product gases? The equilibrium will not shift. Start the animation and see for yourself. Question #1 Le Chatelier's principle can be used to predict the effect of stress (change in pressure, volume, concentration, and temperature) on a system at equilibrium. True False ANSWER-->True Question #2 In the reaction,N 2(g) + 2O 2(g) 2NO 2(g), when a pressure increases, what happens to [N 2(g) ] and [NO 2(g) ]? [A] [N 2(g) ] increases and [NO 2(g) ] increases. [B] [N 2(g) ] decreases and [NO 2(g) ] decreases. [C] [N 2(g) ] increases and [NO 2(g) ] decreases. [D] [N 2(g) ] decreases [NO 2(g) ] increases. ANSWER-->ANSWER--> Choice D, equilibrium shifts to the rightChoice D, equilibrium shifts to the right Question #3 N 2 O 4(g) 2NO 2(g) From the reaction above, describe the change in concentration as the pressure decreases. [A] [N 2 O 4(g) ] increases and [NO 2(g) ] increases. [B] [N 2 O 4(g) ] decreases and [NO 2(g) ] decreases. [C] [N 2 O 4(g) ] decreases and [NO 2(g) ] increases. [D] [N 2 O4 (g) ] increases and [NO 2(g) ] decreases. ANSWER-->ANSWER--> Choice C, equilibrium shifts to the rightChoice C, equilibrium shifts to the right Question #4 If the concentration of a substance in the system at equilibrium changes the system will relieve the stress by adjusting it to reach a new equilibrium. True False ANSWER-->ANSWER--> True True Question #5 In the reaction 2CO (g) + O 2(g) 2CO 2(g),when [CO 2(g) ] increases, [O 2(g) ] [A] increases. [B] decreases. [C] remains the same. [D] is unpredictable. ANSWER-->ANSWER--> Choice B, equilibrium shift to the rightChoice B, equilibrium shift to the right ANSWER-->ANSWER--> Choice A, equilibrium shift to the leftChoice A, equilibrium shift to the left Question #7 If the temperature increases the reaction releasing heat (exothermic) is favored. True False ANSWER-->ANSWER--> FalseFalse Question #8 N 2(g) + O 2(g) 2NO (g), change in heat = +182.6KJ determine whether this reaction is endothermic or exothermic, and predict what will happen to [N2(g)] if the temperature increases. [A] endothermic; [N 2(g) ] will increase. [B] endothermic;[N 2(g) ] will decrease. [C] exothermic;[N 2(g) ] will increase. [D] exothermic;[N 2(g) ] will decrease. ANSWER-->ANSWER--> Choice B, equilibrium shift to the rightChoice B, equilibrium shift to the right Question #9 Given the reaction at equilibrium : 2C (s) + H 2(g) + energy C 2 H 2(g) What change will result in an increase in the amount of C 2 H 2(g) ? [A] decreasing the pressure [B] increasing the concentration of H 2 [C] decreasing the concentration of C [D] decreasing the temperature ANSWER-->ANSWER--> Choice B, increasing Carbon has no affect because it is a solid.Choice B, increasing Carbon has no affect because it is a solid.

22. b.What will happen if the pressure is decreased? 2NO 2(g)  N 2 O 4(g) Pressure will shift to the left. Bottom line: The shift will always be toward the deficiency and away from the excess. P

23. 3.Change in: Temperature 1.Heat + NH 4 Cl (s)  NH 3(g) + HCl (g) 2.H 2 O 2(l)  H 2 O (l) + O 2(g) + 190KJ 3.Heat + H 2 O (s)  H 2 O (l) 4.H 2(g) + Cl 2(g)  2 HCl (g) + Heat IF HEAT IS INCREASED IF HEAT IS DECREASED

24. Equilibrium video

25. http://www.sciencegeek.net/Chemistry/taters/Le Chatelier.htm Warm up 3/__/13