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Equilibrium: The Extent of Chemical Reactions

Equilibrium: The Extent of Chemical Reactions. N 2 O 4 ( g ) 2NO 2 ( g ). Figure 17.1. Reaching equilibrium on the macroscopic and molecular levels. Equilibrium - the condition in which the concentrations of all the reactants and products in a closed system cease to change with time.

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Equilibrium: The Extent of Chemical Reactions

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  1. Equilibrium: The Extent of Chemical Reactions

  2. N2O4(g)2NO2(g) Figure 17.1 Reaching equilibrium on the macroscopic and molecular levels.

  3. Equilibrium - the condition in which the concentrations of all the reactants and products in a closed system cease to change with time At equilibrium: rateforward= ratereverse no further net change is observed because changes in one direction are balanced by changes in the other but it doesn’t mean that the reaction had stopped. The amount of reactants and products are constant but they are not necessarily equal.

  4. kforward [products]n kreverse [reactants]m If rateforward = ratereverse then kforward[reactants]m = kreverse[products]n = K = the equilibrium constant The values of m and n are those of the coefficients in the balanced chemical equation. The rates of the forward and reverse reactions are equal, NOT the concentrations of reactants and products. K is dependent only of the temperature. Note:The terms for pure solids or pure liquids do not appear in the equilibrium constant expression

  5. N2O4(g)2NO2(g) Equilibrium constant expression for K =

  6. Practice Problem Write the Equilibrium Constant for the combustion of Propane gas C3H8(g) + O2(g)  CO2(g) + H2O(g) 1. Balance the Equation C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) The subscript “c” in Kc indicates the equilibrium constant is based on reactant and product concentrations The value of “K” is usually shown as a unitless number, BUT IT ACTUALLY DOES HAVE A UNIT EXPRESSION

  7. small K large K intermediate K The range of equilibrium constants Figure 17.2

  8. Thus, a mixture of N2 and O2 will react to a very small extent to produce NO at equilibrium. Thus, a mixture of N2 and H2 will almost completely be converted to NH3 at equilibrium.

  9. Sample Problem 15.2 • In a 500 ml stainless steel reaction vessel at 900⁰C, carbon monoxide and water vapor react to produce carbon dioxide and hydrogen. Evidence indicates that this reaction establishes an equilibrium with only partial conversion of reactants to products. Initially 2.00 mol of each reactant is placed in the vessel. Kc foe this reactionis 4.20 at 900 ⁰C. What amount concentration of each substance will be present at equilibrium?

  10. Cont… • VT=500 ml nCO(g)=nH2O(g)=2.00 molKc=4.20 T=900⁰C • [CO(g)]=[H2O(g)]=2.00 mol/0.500L = 4.00 M • Solve for x  Kc=4.20= [CO2(g)] [H2(g)] [CO(g)] [H2O(g)]

  11. Homework • P. 688 # 1, 3, 4, 6, 8, 10

  12. LeChatelier’s Principle When a chemical system at equilibrium is subjected to a stress, the system will return to equilibrium by shifting to reduce the stress. If the concentration increases, the system reacts to consume some of it. If the concentration decreases, the system reacts to produce some of it.

  13. Factors Affecting Equilibrium • Concentration Changes • Pressure Changes • Temperature Changes • Addition of a Catalyst

  14. I. Concentration Changes If the conc. of a substance is increased, the equilibrium will shift in a way that will decrease the conc. of the substance that was added. Where will the reaction shift? forward Decrease HI - Decrease I2 - backward H2 HI forward Increase H2 - I2 backward Increase HI -

  15. Note: adding/removing a solid/liquid to an equilibrium system will not cause any shift in the position of equilibrium. addition of an inert gas such as He, Ar, Kr, etc. at constant volume, pressure and temperature does not affect the equilibrium. Increase CO2 : Increase CaO Decrease CO2 Adding Kr - backward - No effect - forward - No effect

  16. PROBLEM: To improve air quality and obtain a useful product, chemists often remove sulfur from coal and natural gas by treating the fuel contaminant hydrogen sulfide with O2; 2H2S(g) + O2(g) 2S(s) + 2H2O(g) Practice Exercise: Predicting the Effect of a Change in Concentration on the Equilibrium Position In what direction will the rection shift if (b)H2O is added? (a) O2 is added? (d) sulfur is added? (c) H2S is removed?

  17. + lower P (higher V) more moles of gas higher P (lower V) fewer moles of gas I. Pressure Changes

  18. The effect of pressure (volume) on an equilibrium system. An increase in pressure (decrease in volume) shifts the position of the equilibrium in such a way as to decrease the number of moles of gaseous component. When the volume is increased (pressure decreased), a net reaction occurs in the direction that produces more moles of gaseous component Decrease in pressure : Decrease in volume: backward forward

  19. PROBLEM: For the following reactions, predict the direction of the reaction if the pressure is increased: (a) CaCO3(s) CaO(s) + CO2(g) (b) S(s) + 3F2(g) SF6(g) (c) Cl2(g) + I2(g) 2ICl(g) Sample Problem 17.12 Predicting the Effect of a Change in Volume (Pressure) on the Equilibrium Position (a) CO2 is the only gas present. The equilibrium will shift to the direction with less moles of gas. Answer: backward SOLUTION: • There are more moles of gaseous reactants than products. • Answer: forward (c) There are an equal number of moles of gases on both sides of the reaction. Answer: no effect

  20. The Effect of a Change in Temperature on an Equilibrium Consider heat as a product or a reactant. In an exothermic reaction, heat is a product, H0rxn = negative In an endothermic reaction, heat is a reactant, H0rxn = positive Increase temperature: backward CO2 92.4 kJ

  21. PROBLEM: In what direction will the reaction shift if there is a decrease in temperature (a) CaO(s) + H2O(l) Ca(OH)2(aq) DH0 = -82kJ (b) CaCO3(s) CaO(s) + CO2(g) DH0 = 178kJ (c) SO2(g) S(s) + O2(g) DH0 = 297kJ (a) CaO(s) + H2O(l) Ca(OH)2(aq) heat (b) CaCO3(s) + heat CaO(s) + CO2(g) (c) SO2(g) + heat S(s) + O2(g) Sample Problem 17.13 Predicting the Effect of a Change in Temperature on the Equilibrium Position SOLUTION: A decrease in temperature will shift the reaction to the right The reaction will shift to the left The reaction will shift to the left

  22. Addition of a Catalyst The presence of a catalyst has no effect on the position of the chemical equilibrium, since a catalyst affects the rates of the forward and reverse reactions equally.

  23. Homework • Le Chatelier’s Principle • P. 695 # 1-3 • P. 699 # 1-6 • Visual Dictionary Due

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