Equilibrium. Definition. Definition. Definition

Transcription

1 Definition Class Demo with hall Chemical Equilibrium when the rate of the forward and reverse reactions are equal Dynamic Reactions at eq never stop Equilibrium DOES NOT mean that the amount of reactants and products are equal. They have reached an unchanging ratio Equilibrium Equilibrium GI Definition Cis Trans Definition 1

2 N 2 O 4 (g) 2NO 2 (g) Clear Brown Cold Hot 1. Starting with all reactants (N 2 O 4 ) N 2 O 4 (g) 2NO 2 (g) Graphs 2. Starting with all products (NO 2 ) 1. K c = Eq. Constant involving molarity a. Molarity = [ ] b. Example = [0.50 M] 2. K p = Eq. Constant involving pressure a. Atmospheres b. We live at about 1 atm 3. Generic Example aa + bb cc + dd Eq. Constants 2O 3 (g) 3O 2 (g) 2NO(g) + Cl 2 (g) 2NOCl(g) H 2 (g) + I 2 (g) 2HI(g) Eq. Constants Heterogeneous Equilibrium 1. More than one state is present 2. Exclude solids and liquids from K. (not considered to have a molarity or pressure) SnO 2 (s) + 2CO(g) Heterogeneous Eq. Sn(s) + 2CO 2 (g) Pb(NO 3 ) 2 (aq)+na 2 SO 4 (aq) PbSO 4 (s) +2NaNO 3 (aq) Ba 2+ (aq) + SO 4 2- (aq) BaSO 4 (s) 2

3 1. Exclude liquid water (often the solvent) 2. Keep gaseous water 3. Examples CO 2 (g) + H 2 (g) 3Fe(s) + 4H 2 O(g) A Note About Water CO(g) + H 2 O(l) Fe 3 O 4 (s) + 4H 2 (g) 1. An example CO(g) + Cl 2 (g) COCl 2 (g) K c = 4.6 X Rules K>>1 Favors the products K<<1 Favors the reactants K~1 Reactants ~ Products 1. Does the following reaction favor the products or reactants? N 2 (g) + O 2 (g) 2NO(g) K c = 1 X For the following reaction, K c = 794 at 298 K and K c = 54 at 700 K. Should you heat or cool the mixture to promote the formation of HI? H 2 (g) + I 2 (g) 2HI(g) 1. K c = 2.5 X for N 2 (g) + O 2 (g) 2NO(g) calculate K c for: 2NO(g) N 2 (g) + O 2 (g) 2. Calculate K c for ½ N 2 (g) + ½ O 2 (g) NO(g) 3. The K c for N 2 (g) + 3H 2 (g) Calculate K c for: is 4.43 X N 2 (g) + 6H 2 (g) 4NH 3 (g) Converting Between K c and K p K p = K c (RT) n R = L atm/mol K T = Kelvin Temperature n = change in number of moles of gas Calculate K p for the following reaction at 300 o C: N 2 (g) + 3H 2 (g) K c = 9.60 ANS: (4.34 X 10-3 ) 3

4 Calculate K p for the following reaction at 1000 K: 1. A mixture is allowed to reach eq.. At eq., the vessel contained M H 2, M N 2, and M NH 3. Calculate the equilibrium constant. N 2 (g) + 3H 2 (g) 2 SO 3 (g) 2SO 2 (g) + O 2 (g) K c = 4.08 X 10-3 ANS: (Ans: 0.105) 2. At eq., a vessel contained M NO 2 Cl, M NO 2, and M Cl 2. Calculate the equilibrium constant. NO 2 Cl(g) NO 2 (g) + Cl 2 (g) 3. A mixture of mol of H 2 and mol of I 2 is placed in a 5.00 L flask and allowed to reach eq.. At eq., the mixture is found to be [HI] = M. Calculate K c. H 2 (g) + I 2 (g) HI(g) (Ans: 0.558) (Ans: 51) 4. A vessel is charged with M SO 3. At eq., the SO 3 concentration had dropped to M SO 3. What is the value of K c? SO 3 (g) SO 2 (g) + O 2 (g) mol of HI was placed in a 5.00 L flask and allowed to decompose. At eq. It was found that the vessel contained mol of I 2. What is the value of K c? HI(g) H 2 (g) + I 2 (g) (Ans: 0.020) (Ans: ) 4

5 6. An eq. mixture of gases is analyzed. The partial pressure of nitrogen is atm and the partial pressure of hydrogen is atm. If K p is 1.45 X 10-5, what is the partial pressure of ammonia? N 2 (g) + 3H 2 (g) 7. Consider the following equilibrium: PCl 5 (g) PCl 3 (g) + Cl 2 (g) a. At equilibrium, the partial pressure of PCl 5 and PCl 3 are measured to be atm and atm, respectively. If K p = 0.497, what is the partial pressure of Cl 2? (1.22 atm) b. Suppose at equilibrium the partial pressure of PCl 5 is 2.00 atm. Calculate the partial pressure of PCl 3 and Cl 2. Assume K p is still and only PCl 5 was initially in the flask. (0.997 atm) 1. 0 = ax 2 + bx + c 2. x = -b + \/ b 2 4ac 2a 1. A gas cylinder is charged with 1.66 atm of PCl 5 and allowed to reach eq.. If the K p = 0.497, what are the pressures of all the gases at equilibrium? 3. 2x 2 + 4x = 1 PCl 5 (g) PCl 3 (g) + Cl 2 (g) (Ans: 0.97 atm, atm) Do I always need the quadratic, or can I cheat? 2. A L flask is filled with mol of H 2 and mol of I 2. The K c = What are the concentrations of all the gases at equilibrium? H 2 (g) + I 2 (g) 2HI(g) (Ans: M, M, 1.87 M) The equilibrium constant for the following reaction is NO(g) N 2 (g) + O 2 (g) If the initial concentration of NO is M, calculate the equilibrium concentrations of NO, N 2 and O 2. (Ans: M, M, M) 5

6 Equilibrium Q: Reaction Quotient 1. Reaction Quotient 2. Calculated the same as K, but using initial concentrations 3. Q < K shifts to products Q = K at equilibrium Q > K shifts to reactants 1. If you introduce mol of HI, mol of H 2 and mol of I 2 in a 2.00 L flask, which way will the reaction proceed to reach equilibrium? 2. Predict which way the following reaction will proceed as it reaches eq. Assume that you start with [SO 3 ] = M, [SO 2 ] = M and [O 2 ] = 0.03M. H 2 (g) + I 2 (g) 2HI(g) K c = 51 2SO 3 (g) 2SO 2 (g) + O 2 (g) K c = (Ans: Q = 1.3) (Ans: Q = 0.2) 3. Predict which way the following reaction will proceed as it reaches eq. Assume that you start with [NH 3 ] = M, [N 2 ] = M and no H 2. K c = N 2 (g) + 3H 2 (g) Blue Bottle Demo 5 grams KOH 3 grams Dextrose 250 ml of water 1 drop methylene blue LeChatelier s Principle 6

7 Le Chatelier s Principle If a system at eq. Definition If a system is at disturbed, eq. Is it will shift to relieve that will shift to disturbance relieve that disturbance LeChatelier s Principle Disturbing and K N 2 (g) + 3H 2 (g) Add N 2 Add NH 3 Remove NH 3 as it forms Remove H 2 N.B. Does NOT apply to solids and liquids. They do not appear in the K. LiCl(s) Li + (aq) + Cl - (aq) Adding products = K increases (TEMPORARILY) Adding reactants = K decreases (TEMPORARILY) K c = [NH 3 ] 2 N 2 (g) + 3H 2 (g) [N 2 ][H 2 ] 3 7

8 N 2 (g) + 3H 2 (g) Identify the # of moles of gas on either side. Show piston drawing 1. Increase the volume of the container 2. Decrease the volume of the container N 2 (g) + 3H 2 (g) 1. Increase the pressure of the system 2. Decrease the pressure of the system Soda example (CO 2 (aq) CO 2 (g)) N.B. Adding a noble or inert gas has no effect on the eq. Pressure change without a volume change. Endothermic Reactions absorb heat from the surroundings Heat is added (reactants) Cooking is an example H + Exothermic Reactions Release heat Give off heat (products) Fire is an example H - CO(g) + 3H 2 (g) 1. Heat the system 2. Cool the system CH 4 (g) + H 2 O(g) H = -206 kj/mol 1. Catalyst 2. Examples a. Enzymes b. Vitamins c. Catalytic convertor 3. No effect on the position of equilibrium 8

9 Example 1 N 2 O 4 (g) 2NO 2 (g) H = 58 kj/mol Example 2. 2PbS(s) + 3O 2 (g) 2PbO(s) + 2SO 2 (g) H = -37 kj/mol a. Add N 2 O 4 b. Remove NO 2 as it forms c. Increase the total pressure d. Increase the total volume e. Cool the solution f. Add a catalyst a. Add PbS b. Remove SO 2 as it forms c. Add O 2 d. Increase volume e. Decrease the pressure f. Heat the flask Example 3. Given the following eqn., how could you promote the formation of PCl 3 and Cl 2? PCl 5 (g) PCl 3 (g) + Cl 2 (g) H = 88 kj/mol Example 4. How could you promote the formation of CH 4? CO(g) + 3H 2 (g) CH 4 (g) + H 2 O(g) H = -206 kj/mol SINGLE ARROW Reaction goes to completion Double ARROW Reaction goes to equilibrium (all species present) HCl + NaOH NaCl + H 2 O I C End Na + Cl - H 2 O N 2 + 3H 2 2NH 3 I C Equilibrium N 2 H 2 K = [NH 3 ] 2 ratio NH 3 [N 2 ][H 2 ] 3 9

10 HC 2 H 3 O 2 (aq) + H 2 O(l) H 3 O + (aq) + C 2 H 3 O 2- (aq) Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) Co(H 2 O) 2+ 6 (aq) + 4Cl - (aq) + 50 kj/mol CoCl 2-4 (aq) + 6H 2 O(l) (pink) (blue) NaCl(s) Na + (aq) + Cl - (aq) 14.[O 2 ] 3 /[O 3 ] 2 1/[Cl 2 ] 2 [C 2 H 6 ] 2 [O 2 ]/[C 2 H 4 ] 2 [H 2 O] 2 [CH 4 ]/[H 2 ] 2 [Cl 2 ] 2 /[HCl] 4 [O 2 ] 16. a) Products b) Reactants 18. K c = a) 1.35 X 10 5 b) H 2 S favored 22a) 13.3 b) c) K p = 1/P SO2 Na 2 O is a solid, no molarity or pressure 28. K c = , Products favored 32. a) 0.14 M, M, 0.40 M b) a) (R) b) Reactants c) 1.1 X10-5 (P) atm 42a g I 2 b) g SO M M 48a M PH 3 and BCl 3 52 a) increase(p) b) decrease(r) c) decrease(r) d) decrease(r) e) no change f) decrease(r) 54 a) Endothermic b) more moles of gas in product 56 a) kj b) exothermic c) Increase pressure 59. K p = 24.7 K c =

11 Take-Home Pretest g NaOH Products 6. Q = 0.637, moves to products 7. [H 2 ] = M, [Cl 2 ] = M, [HCl] = M Equilibrium 1.82 = 2 m log 1.82 = log 2 m log 1.82 = m log = m m =