Chemical Equilibrium Part III: Equilibrium Concentrations

1. Chemical Equilibrium Part III: Equilibrium Concentrations Jespersen 6th Ed Chap. 15 Sec 7 & 8 C. Yau Fall 2014 1

2. Experimental Determination of K Value N2O4(g) 2NO2(g) How do we determine the value of K? If we can measure the concentration of N2O4 and of NO2 at equilibrium, we can just plug in the values in the equilibrium expression, which is..... ? Note: Since these are concentrations at equilibrium, the value you get is K, not Q. 2

3. Determination ofConcentrations at Equilibrium How do we determine the concentrations at equilibrium? This involves something called the “ICE” table: Initial Concentration Changes in Concentration Equilibrium Concentration 3

4. Example 15.6 p. 717 H2(g) + I2(g) 2HI (g) At a certain temp, a mixture H2 and I2 was prepared by placing 0.200 mol H2 and 0.200 mol I2 into a 2.00 liter flask. After a period of time the equilibrium was established. The purple color of the I2 vapor was used to monitor the rxn, and from the decreased intensity of the purple color it was determined that, at equilibrium, the I2 concentration had dropped to 0.020 mol L-1. What is the value of Kcfor this rxn at this temp? We will set up an ICE table for this problem. Do Pract Exer 15, 16, 17 p.719

5. Example 15.7 p.719 The reversible rxn below has been used as a commercial source of hydrogen. CH4 (g) + H2O (g) CO (g) + 3 H2 (g) At 1500 oC, an equilibrium mixture of the gases was found to have the following concentrations: [CO] = 0.300 M, [H2] = 0.800 M, [CH4] = 0.400 M. At 1500 oC, Kc = 5.67 for the rxn. What was the equilibrium concentration of H2O (g) in the mixture? Do Pract Exer 18, 19 p. 720 5

6. Example 15.8 p. 720 CO (g) + H2O (g) CO2 (g) + H2 (g) The rxn has Kc = 4.06 at 500 oC. If 0.100 mol CO and 0.100 mol H2O are placed in a 1.00 liter rxn vessel at this temp, what are the conc of the reactants and products when the system reaches equilibrium? 6

7. Example 5.9 p. 722 In the preceding example... CO (g) + H2O (g) CO2(g) + H2(g) has Kc = 4.06 at 500oC. Suppose 0.0600 mol each of CO and H2O are mixed with 0.100 mol each of CO2 and H2 in a 1.00 L rxn vessel. What will the conc of all the substances be when the mixture reaches equilibrium at that temperature?

8. Example 15.10 p. 724 At a certain temp, Kc = 4.50 for the rxn N2O4(g) 2NO2(g) If 0.300 mol of N2O4 is placed into a 2.00 L container at that temperature, what will be the equilibrium conc of both gases? Do Pract Exer 20, 21 on p. 726

9. Example 15.11 p. 726 Hydrogen, a potential fuel, is found in great abundance in water. Before the hydrogen can be used as a fuel, however, it must be separated from the oxygen; the water must be split into H2 and O2. One possibility is thermal decomposition, but this requires very high temperatures. Even 1000 oC, Kc = 7.3x10-18 for the reaction 2H2O (g) 2H2(g) + O2(g) If at 1000 oC the H2O conc in a rxn vessel is set initially at 0.100 M, what will the H2 conc be when the rxn reaches equilibrium? Do Pract Exer 22 & 23 p. 728

10. When can you assume x to be negligible? The x value is negligible if you can leave it out and it makes no difference in your calculations. You can assume x to be negligible if K is very small and only if you are considering... number + x or number – x but not x(number) or x (number- x) WHY? Let say number = 50 and x = 0.1 50 + 0.1 is about the same as 50 50-0.1 is about the same as 50 But 0.1(50) =5 which is VERY different from 50. And 0.1(50-0.1) = around 5, also very diff from 50.

11. Practice Exercise 5.23 p. 728 In air at 25oC and 1 atm, the N2 concentration is 0.033 M and O2 conc is 0.00810 M. The rxn N2(g) + O2(g) 2 NO (g) has Kc = 4.8x10-31 at 25oC. Taking the N2 and O2 conc given above as initial values, calc the equilibrium NO concentration that should exist in our atmosphere from this rxn at 25 oC.