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Understanding Chemical Equilibrium and Le Chatelier’s Principle

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prentice
23 diapositives · 353 Views · Sep 16, 2014

This chapter delves into the concept of chemical equilibrium, highlighting the differences between reactions that go to completion and reversible reactions. It explains how to express equilibrium using the equilibrium constant (K_eq) and the significance of this value in predicting the direction of a reaction. Key concepts include homogeneous and heterogeneous equilibria, the reaction quotient (Q), and the applications of Le Chatelier's principle to understand the effects of changes in concentration, pressure, and temperature on equilibrium systems.

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  1. Chapter 16 Chemical Equilibrium

  2. Before we look at chemical equilibrium, let’s look at the past • Many reactions in chemistry go to 100% completion 1. Example: burning methane. CH4 + O2 CO2 + H2O 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 product remains constant.

  5. a. aA + bB  cC + dD II. How to express chemical equilibrium Keq: Equilibrium Constant [ ]=Concentration, • keq= [products]coefficient • [reactants] coefficient Molarity [C]c [D]d c. keq= [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. Question of the Day 3/13 a. rates of forward/ reverse reactions are equal b. the Concentrations of reactants and products are constant. • What is chemical equilibrium? • What’s the significance of Keq? • Write the keq expression for: Br2(g) + 2NO(g)  2NOBr(g) • Calculate Keq for the reaction in #3 [.214]=Br2 [.0677]=NOBr [0.428]=NO 5. What does the reaction mainly have at equilibrium? The value tells you at Equilibrium, what you mostly have……reactants OR products OR equal amounts of both Keq= [NOBr]2 [Br2][NO]2 Keq= [.0677]2 [.214][.428]2 <1 Keq=0.117 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. 1. Proposed by a Chemistry Professor, Henry Chatelier What will happen to a system that is already at equilibrium if I change conditions?? 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.

  16. Concentration Possible changes (stressors) Pressure Temperature

  17. 1. Change in Concentration 2NO2(g) N2O4(g) Example:  • More NO2 is added: • More N2O4 is added: • N2O4 is taking away: • NO2 is taken away: Shift Right Shift Left Shift Right Shift Left

  18. Applies only to gases 2. Change in Pressure: 2NO2(g) N2O4(g) P When pressure is applied the shift goes from more moles of gas to less moles of gas. a. What will happen if the pressure is increased?? Shift to the Right NO2 NO2 N2O4 N2O4 N2O4 N2O4 N2O4

  19. LeChatelier Principle ----->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 (N2O4). 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 #1Le Chatelier's principle can be used to predict the effect of stress (change in pressure, volume, concentration, and temperature) on a system at equilibrium.TrueFalse Question #2In the reaction,N2(g) + 2O2(g) <->2NO2(g), when a pressure increases, what happens to [N2(g)] and [NO2(g)]?[A] [N2(g)]  increases and [NO2(g)] increases.[B] [N2(g)] decreases and [NO2(g)] decreases.[C] [N2(g)]  increases and [NO2(g)] decreases.[D] [N2(g)] decreases [NO2(g)] increases. Question #3N2O4(g) <-> 2NO2(g)From the reaction above, describe the change in concentration as the pressure decreases.[A] [N2O4(g)] increases and [NO2(g)] increases.[B] [N2O4(g)] decreases and [NO2(g)] decreases.[C] [N2O4(g)] decreases and [NO2(g)] increases.[D] [N2O4(g)] increases and [NO2(g)] decreases. Question #4If 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.TrueFalse Question #5In the reaction 2CO(g) + O2(g) <-> 2CO2(g) ,when [CO2(g)] increases, [O2(g)][A] increases.[B] decreases.[C] remains the same.[D] is unpredictable. Pressure Animation Question #7If the temperature increases the reaction releasing heat (exothermic) is favored.TrueFalse Question #8N2(g) + O2(g) <-> 2NO(g), change in heat = +182.6KJdetermine whether this reaction is endothermic or exothermic, and predict what will happen to [N2(g)] if the temperature increases.[A] endothermic; [N2(g)] will increase.[B] endothermic;[N2(g)] will decrease.[C] exothermic;[N2(g)] will increase.[D] exothermic;[N2(g)] will decrease. Question #9Given the reaction at equilibrium : 2C(s) + H2(g) + energy <-> C2H2(g) What change will result in an increase in the amount of C2H2(g)?[A] decreasing the pressure[B] increasing the concentration of H2[C] decreasing the concentration of C[D] decreasing the temperature

  20. b. What will happen if the pressure is decreased? 2NO2(g) N2O4(g) P Pressure will shift to the left. Bottom line: The shift will always be toward the deficiency and away from the excess.

  21. 3. Change in: Temperature IF HEAT IS INCREASED 1. Heat + NH4Cl(s)NH3(g) + HCl(g) 2. H2O2(l)H2O(l) + O2(g) + 190KJ IF HEAT IS DECREASED 3. Heat + H2O(s)H2O(l) 4. H2(g) + Cl2(g) 2 HCl(g) + Heat

  22. Equilibrium video

  23. Warm up 3/__/13 http://www.sciencegeek.net/Chemistry/taters/LeChatelier.htm