A.P. Chemistry. Unit #11. Chemical Equilibrium

Transcription

1 A.P. Chemistry Unit #11 Chemical Equilibrium

2 I. Chemical Equilibrium the point in a reaction at which the concentrations of products and reactants remain constant Dynamic Equilibrium the equilibrium condition does not indicate that the reaction has stopped (is static) but rather that the forward and reverse rates are equal to one another The Haber Process the process of making industrial ammonia N 2 (g) + 3 H 2 (g) 2 NH 3 (g) II. The Equilibrium Constant (K) A. The Equilibrium Expression uses the law of mass action to describe the equilibrium condition Consider the equation: ja (aq) + kb (aq) lc (aq) + md (aq) l m [ C] [ D] [ A] j [ B] k K= = K or K c = equilibrium constant (in terms of concentration) B. Sample Exercise 15.1 Write the equilibrium expression for K c for the following reactions: a) 2 O 3 (g) 3 O 2 (g) b) 2 NO (g) + Cl 2 (g) 2NOCl (g) C. Expressing Equilibrium Constant in terms of Pressure, K p Ex) N 2 O 4 (g) 2 NO 2 (g) D. Magnitude of K c - K >>> 1 = eq. posn. favors products; lies far to right K <<< 1 = eq. posn. favors reactants; lies far to left 2

3 E. Heterogeneous Equilibria if a pure solid or a pure liquid is involved in a heterogeneous equilibrium, its concentration is not included in the equilibrium expression, but they must be present for the reaction to reach equilibrium. Ex) CaCO 3 (s) CaO (s) + CO 2 (g) K c = F. Sample Exercise 15.5 Write the equilibrium expression for K c and K p for each of the following reactions: A) CO 2 (g) + H 2 (g) CO (g) + H 2 O (l) B) SnO 2 (s) + 2 CO (g) Sn (s) + 2 CO 2 (g) III. Calculating equilibrium constants (K) A. The 4 Step Procedure for solving for equilibrium constants (K) 1. Tabulate the known initial and equilibrium concentrations of all species involved in the equilibrium. 2. For those species for which both the initial and equilibrium concentrations are known, calculate the change in concentration that occurs as the system reaches equilibrium. 3. Use the stoichiometry of the reaction to calculate the changes in concentrations for all the other species in the equilibrium. 4. From the initial concentration and the changes in concentration, calculate the equilibrium concentration. These are then used to calculate the equilibrium constant (K). B. Sample Exercises: 1. In one of their experiments, Haber and co-workers introduced a mixture of hydrogen and nitrogen into a reaction vessel and allowed the system to attain chemical equilibrium at 472 C. The equilibrium mixture of gases was analyzed and found to contain M H 2, M N 2, and M NH 3. From these data, calculate the equilibrium constant, K c, for the Haber process. 3

4 2. A mixture of 5.00 x 10 3 mol of H 2 and 1.00 x 10 2 mol of I 2 is placed in a 5.00 L container at 448 C and allowed to come to equilibrium. Analysis of the equilibrium mixture shows that the concentration of HI is 1.87 x 10 3 M. Calculate K c at 448 C for the reaction H 2 (g) + I 2 (g) 2 HI (g) C. Relating K c and K p - Example- Sample Exercise 15.8 the K c for the following reaction is 0.105, calculate the K p at 472 C: N 2 (g) + 3 H 2 (g) 2 NH 3 (g) 4

5 IV. Applications of Equilibrium Constants A. Predicting the direction of Reaction 1. The Reaction Quotient (Q) 2. Sample Exercise 15.9 At 448 C the equilibrium constant, K c, for the reaction H 2 (g) + I 2 (g) 2 HI (g) Is Predict how the reaction will proceed to reach equilibrium at 448 C if we start with 2.0 x 10 2 mol of HI, 1.0 x 10 2 mol of H 2, and 3.0 x 10 2 mol of I 2 in a 2.0 L container. B. Calculating the Equilibrium Constant 1. Sample Exercise solving for an unknown quantity For the Haber process, K p = 1.45 x 10 5 at 500 C. In an equilibrium mixture of the 3 gases at 500 C, the partial pressure of H 2 is atm and that of N 2 is atm. What is the partial pressure of NH 3 in this equilibrium mixture? 5

6 2. Sample Exercise solving for the equilibrium concentration A L flask is filled with mol of H 2 and mol of I 2 at 448 C. The value of the equilibrium constant, K c, for the reaction H 2 (g) + I 2 (g) 2 HI (g) At 448 C is What are the concentrations of H 2, I 2, and HI in the flask at equilibrium? 3. Sample Exercise solving for the equilibrium concentrations The equilibrium constant for the Haber process at 472 C is K c = A 2.00 L flask is filled with mol of NH 3 and is allowed to reach equilibrium at 472 C. What are the equilibrium concentrations of NH 3, N 2, and H 2? 6

7 V. Factors Affecting Equilibrium LeChatelier s Principle If a system at equilibrium is disturbed by a change in temperature, pressure, of the concentration of one component, the system will shift its equilibrium position so as to counteract the effect of the disturbance. A. in Reactant of Product Concentration: B. Effects of Volume and Pressure Changes: C. Effect of Temperature : D. Effects of a Catalyst: E. Example 1. Manufacture of ammonia N 2(g) + 3H 2(g) 2NH 3(g) H = - 92 kjmol -1 Change in external factor Increase in pressure Increase in temperature Increase in concentration of nitrogen and hydrogen Increase in concentration of ammonia Shift in position of equilibrium Shifts to RHS Shifts to LHS Shifts to RHS Shifts to LHS Reason Less moles of gas on RHS so pressure is reduced Backward reaction is endothermic so removes heat by shifting to the LHS Less nitrogen and hydrogen on RHS Less ammonia on LHS 7

8 2. Oxidation of ammonia in the manufacture of nitric acid 4NH 3(g) + 5O 2(g) 4NO (g) + 6H 2 O (g) H = kjmol -1 Change in external factor Increase in pressure Increase in temperature Increase in concentration of ammonia and oxygen Increase in concentration of nitrogen monoxide and water Shift in position of equilibrium Shifts to LHS Shifts to LHS Shifts to RHS Shifts to LHS Reason Less moles of gas on LHS so pressure is reduced Backward reaction is endothermic so removes heat by shifting to the LHS Less ammonia and oxygen on RHS Less nitrogen monoxide and water on LHS Both of these reactions can be catalyzed, the first with iron, the second with a platinum and rhodium catalyst. It is very important to note that the catalyst will not effect the position of equilibrium and hence not affect Kc, it only allows the equilibrium position to be achieved more quickly. F. Sample Exercises Applying LeChatelier s Principle: Given the following equilibrium: N 2 O 4 (g) 2 NO 2 (g) H = 58.0 kj In what direction will the equilibrium shift when each of the following changes is made to a system at equilibrium? (a) Add N 2 O 4 ; (b) remove NO 2 ; (c) increase the total pressure by adding N 2 (g) ; (d) increase the volume ; (e) decrease the temperature? Determine the standard enthalpy change for the reaction (use Appendix C) : N 2 (g) + 3 H 2 (g) 2 NH 3 (g) Determine how the equilibrium constant for this reaction should change with temperature. 8

9 VI. The Common Ion Effect A. Definition B. Sample Exercise VII. Solubility Equilibrium (K sp ) - A. The Solubility-Product Constant (K sp ) Sample Exercise 17.9 Write the expression for the solubility-product constant for Ca 3 (PO 4 ) 2. B. Solubility and K sp Molar Solubility Sample Exercise Solid silver chloride is added to pure water at 25 C. Some of the solid remains undissolved at the bottom of the flask. The mixture is stirred for several days to ensure that equilibrium is achieved between the undissolved AgCl (s) and the solution. Analysis of the equilibrated solution shows that its silver-ion concentration is 1.34 x 10 5 M. What is K sp for AgCl? 9

10 Sample Exercise The K sp for CaF 2 is 3.9 x at 25 C. What is the solubility of CaF 2 in water in grams per liter? VIII. Criteria for Precipitation or Dissolution A. Ion Product (Q) B. Sample Exercise Calculate the molar solubility of CaF 2 at 25 C in a solution containing (a) M Ca(NO 3 ) 2 (b) M NaF 10

11 C. Sample Exercise Will a precipitate form when L of 3.0 x 10 3 M Pb(NO 3 ) 2 is added to L of 5.0 x 10 3 M Na 2 SO 4? D. Solubility and ph E. Complex Ion Formation and Solubility 11