1. General Chemistry II CHEM 152 Unit 2 Week 6

2. Week 6 Reading Assignment Chapter 14 – Sections 14.6 through 14.9 (calculations - Le Châtelier)

11. Shifting Equilibrium The equilibrium state for a chemical process can be affected by changes in the concentration of reactants or products, or by varying the temperature and pressure of the system. The direction in which the equilibrium shifts (increasing the concentration of reactants or products) can be predicted.

12. Butane-Isobutane Equilibrium butane isobutane Let us consider the following equilibrium Shifting Equilibrium

15. Le Châtelier’s Principle The outcome is governed by the: Le CHÂTELIER’S PRINCIPLE “ ...if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.” Changes in concentration , pressure , and temperature affect the position of chemical equilibrium.

16. Let’s Analyze It N 2 O 4  2NO 2 What would you expect to happen if we double the concentration of NO 2 ? How does K=[NO 2 ] 2 eq / [N 2 O 4 ] eq change?

17. Consider the reaction CaCO 3 (s) + CO 2 (aq) + H 2 O(l)  Ca 2+ (aq) + 2 HCO 3 - (aq) Predict the effect on the equilibrium of: -Removing CO 2 -Adding more CaCO 3 to the system -Adding CaCl 2 to the solution -Adding more water -HCO 3 - is added

19. Another Reaction H 2 + I 2  2HI What would happen in this case if similar “stresses” are applied to this equilibrium?

20. V  (P  ) – Reaction shifts to the side with fewer moles of gas V  (P  ) – Reaction shifts to the side with greater moles of gas The value of K DOES NOT CHANGE. Summary

21. Temperature Effects 2NO 2  N 2 O 4 Let’s now analyze the effect of changing temperature. Evaluate the value of K before and after the change.

22. Based on the shifts in the equilibrium with changing temperature, decide whether this is an endothermic or an exothermic reaction. PCl 5 (g)  PCl 3 (g) + Cl 2 (g) Temperature Effects

23. Increase T  More reactants (K decreases)  H = - (exothermic) Temperature Effects Decrease T  More products (K increases) 2NO 2  N 2 O 4 + Heat Treat the Heat as if it were a product

24. Increase T  More products (K increases)  H = + (endothermic) Temperature Effects Decrease T  More reactants (K decreases) Treat the Heat as if it were a reactant Heat + PCl 5 (g)  PCl 3 (g) + Cl 2 (g)

26. Equilibrium Calculations Let ’ s now learn how to calculate equilibrium constants or the concentrations of reactant and products at equilibrium.

27. All [Products] eq known: H 2 ( g ) + I 2 ( g )  2 HI ( g ) @ 445°C Initial Equilibrium Equilibrium Constant [H 2 ] [I 2 ] [HI] [H 2 ] [I 2 ] [HI] 0.50 0.50 0.0 0.11 0.11 0.78 0.0 0.0 0.50 0.055 0.055 0.39 0.50 0.50 0.50 0.165 0.165 1.17 1.0 0.5 0.0 0.53 0.033 0.934

29. Find the value of K for the reaction 2 CH 4 ( g )  C 2 H 2 ( g ) + 3 H 2 ( g ) at 1700°C if the initial [CH 4 ] = 0.115 M and the equilibrium [C 2 H 2 ] eq = 0.035 M Your Turn +  Construct an ICE table for the reaction For the substance whose equilibrium concentration is known, calculate the change in concentration 2CH 4 C 2 H 2 3H 2 initial 0.115 0.000 0.000 change equilibrium 0.035

30. Find the value of K for the reaction 2 CH 4 ( g )  C 2 H 2 ( g ) + 3 H 2 ( g ) at 1700°C if the initial [CH 4 ] = 0.115 M and the equilibrium [C 2 H 2 ] eq = 0.035 M Answer +3x +x -2x 0.105 0.045 K = (0.035*0.105 3 )/(0.045 2 ) = 0.020 K = [C 2 H 2 ][H 2 ] 3 [CH 4 ] 2 +  2CH 4 C 2 H 2 3H 2 initial 0.115 0.000 0.000 change equilibrium 0.035

34. Nitrogen Dioxide Equilibrium N 2 O 4 (g)  2 NO 2 (g) Your Turn Find the equilibrium concentrations for NO 2 and N 2 O 4 at 298 K if [N 2 O 4 ] o =0.50 M.  

39. For the reaction I 2 (g)  2 I (g) the value of K = 3.76 x 10 -5 at 1000 K. If 1.00 moles of I 2 is placed into a 2.00 L flask and heated, what will be the equilibrium concentrations of [I 2 ] and [I] ? Equilibrium Concentrations

40. For the reaction I 2 (g)  2 I(g) the value of K = 3.76 x 10 -5 at 1000 K. If 1.00 moles of I 2 is placed into a 2.00 L flask and heated, what will be the equilibrium concentrations of [I 2 ] and [I]? Equilibrium Concentrations  [I 2 ] 2[I] initial 0.500 0 change - x +2 x equilibrium 0.500- x 2 x

41. The approximation is valid!! Equilibrium Concentrations  [I 2 ] 2[I] initial 0.500 0 change - x +2 x equilibrium 0.500- x 2 x

42. For the reaction I 2 (g)  2 I (g) the value of K = 3.76 x 10 -5 at 1000 K. If 1.00 moles of I 2 is placed into a 2.00 L flask and heated, what will be the equilibrium concentrations of [I 2 ] and [I]? x = 0.00217 0.500  0.00217 = 0.498 [I 2 ] = 0.498 M 2(0.00217) = 0.00434 [I] = 0.00434 M  [I 2 ] 2[I] initial 0.500 0 change - x +2 x equilibrium 0.500- x 2 x  . Approximation is valid

43. Summary Activity 1. Consider the reduction of carbon dioxide by hydrogen to give water vapor and carbon monoxide: H 2 (g) + CO 2 (g)  H 2 O(g) + CO(g) K c =0.10 (at 420 o C) Suppose the initial concentrations of CO 2 and H 2 are the same: 0.050 M. What are the equilibrium concentrations of all the species at 420 o C?

44. Summary Activity 2. If the same reaction as we looked at in Summary Activity 1 was endothermic, what would be the effect on the equilibrium by each of the following: Decrease the Volume Decrease the Temperature

45. Summary Activity 3. Suppose that a mixture of 1.00 mol of HI(g) and 1.00 mol of H 2 (g) is sealed into a 10.0 L flask at 745 K. What will be the concentration of all species at equilibrium? 2HI(g)  H 2 (g) + I 2 (g) K c =0.0200