Equilibrium: reactants ® products and products ® reactants, simultaneously,
same rate (dynamic equilibrium)
Most reactions are reversible as long as the product(s) do not escaped. (gas, ppt)
Equilibrium constant for jA + kB « IC + mD (Sample 15.1 on page 532)
Kc = [C]l [D]m
[A]j [B]k
Kp = (PC)l (PD)m
(PA)j (PB)k
Homework: Read Chapter 15. Pg 660, #13
Equilibrium constants are usually written without units – but they have them. Don’t write units unless specifically asked for them.
Kreverse = 1/K
If equation is multiplied by some factor: njA + nkB « nIC + nmD, then Knew = Kn
Relationship of Kc and Kp: Kp = = Kc(RT)Dn
Sample and practice 15.2 on page 634
Homework: Pg 660, #17
Kc(overall) = (Kc1)(Kc2) The product of each step Kc
So while we can calculate thermodynamic functions for a reaction in steps, we can also calculate the equilibrium constant. (Don’t get rules mixed up)
Chart K thermo
reverse 1/K change sign
coefficient factor raise to power multiply
steps multiply add
Sample and Practice 15.5 (page 638) Homework: Page 660, #23
Pure solids and liquids are not included in equilibrium constants
(Thermodynamic activity of 1) That’s why Ksp didn’t have a demoninator!
See example 15.6 on page 640. (So solids, liquids are not. Gases and aqueous solutions are.)
Calculating K from know equilibrium concentrations or pressures
Sample and Practice 15.8. Make sure you can use your calculator efficiently.
How to solve for K when some initial and equilibrium information is provided.
See steps on page 642
Other things to remember.
IF some initial concentration is zero, the reactions will produce some of that.
IF there is no zero concentration in the given information, a Q test is need to determine which direction the reaction will proceed.
Q < K, the forward reaction will proceed (not enough products – numerator is small)
Q = K, at equilibrium already
Q > K, the reverse reaction will proceed (too much product – numerator is big)
Sample and Practice 15.9 on page 643. Homework: Page 661, #31 & 33
Sample and Practice 15.10 on page 645. Homework: Page 661, #37
Sample and Practice 15.11 on page 646. Homework: Page 661, #39 & 41a
Sample and Practice 15.12 on page 646. Homework: page 661, #43-49 odd
Homework: Worksheet and Prelab (handout)
Le Chatelier's Principle (Section 15.7 on page 648)
1. Change in reactant or product concentration
(Solids and liquids don’t matter as long as you have some.)
Add a component, reaction will use it. Take a component, reaction will make it.
2. Reactions with gases: Increase pressure (add an inert gas or reduced volume) favors fewer gases, if nothing is added or removed from system.
3. van't Hoff law: An increase in temperature favors the endothermic reaction.
To predict effect of temperature change you need to know which reaction (forward or reverse) is endothermic. Add DH to reaction and treat it like a reactant or product.
4. Effect of catalyst: none on the equilibrium, just get there faster.
Note: Only a change in temperature will change K. See note on bottom of 652.
Samples 15.13 on page 653 and Integrative exercise on page 655
Homework: Pg 662, #51, 53, 55 and complete lab report.
Back to thermodynamics.
1. See section 19.7 (page 826). Can calculate thermodynamic state functions at non-standard states.
DG = DGo + RT ln(Q) R = 8.3145 J/K mol, T in Kelvin
At standard condition: Read paragraph right above Sample 19.10
Sample and Practice 19.11 on page 828
2. See Section 16.8 (p 819) "The equilibrium point occurs at the lowest value of free energy available to the reaction system." DG = 0 So: DGo = -RT ln(K)
Solving for K = e-(DG/RT)
DG K
DGo = 0 K = 1
DGo < 0 K > 1
DGo > 0 K < 1
Sample and Practice 19.12 on page 829
Homework: Pg 838, #73, 75, 77, 79, 106bcd
Assignment: AP Problem Set
Equilibrium Review List
1. Raoult’s Law: P = C Po
Partial pressure of component = mole fraction in liquid solution x pure vapor pressure
2. Dalton’s Law: PT = P1 + P2 + P3 ….. Total pressure = sum of partial pressures
Cvapor = P/PT Mole fraction in vapor = partial pressure / total pressure
3. How to write equilibrium expressions for Kc and Kp
4. Kp = Kc (RT)Dn where Dn = (l+m) – (j+k)
5. for solutions: AaBb(s) « a A+ + bB-: Ksp = [A]a[B]b
6. LeChatelier’s principle: concentration, temperature, pressure, catalyst
7. Relationship of thermodynamic state function and equilibrium constant
DGfo, DH fo. DSfo data (free elements: DGfo and DH fo = 0)
DG = DH - TDS
DG = DGo + RT ln Q
at equilibrium: DG = 0 = DGo + RT ln K
DGo = -RT ln K
K = e-(DGo / RT)
Review Worksheet Answers
1C
2D
3B
4D
5b) i) 0.595M ii) 0.0205M
5c) 0.251
5d) 1.18 atm
5e) 2.00
7a) 0.800 atm
7b) SO2 = Cl2 = 0.63 atm; SO2Cl2 = 0.17 atm
7c) 2.3 atm
8a) 5.4 x 10-6 g AgBr
8b) 0.22 g AgBr
9a) 3.14 kJ/mol
9b) DH = 3.08 x 104 J/mol, DS = 92.8J k-1 mol-1
9c) 332 K
9d) 0.281 atm