1. AP Chemistry Lunch Review
Equilibrium
AP Chemistry Lunch Review

2. Le Chatelier’s Principle
If you stress a system that is in equilibrium, the system will adjust to reduce the stress (creating a new equilibrium)
Increase in reactant or decrease in product shifts to right (forward is favored)
Decrease in reactant or increase in product shifts to left (reverse is favored)
Increase in temperature favors the endothermic reaction – shifts to use up the heat (put heat in reaction and treat as a material)
Pressure : (no shift if gas moles are the same on both sides)
Increase causes a shift to less gas moles
Decrease causes a shift to more gas moles

3. Writing EquilibriumExpressions
K is an equilibrium constant
Use a subscript to show type : eq, p, c, a, b, w, sp
Coefficients are written as exponents
Products are in the numerator
Reactants are in the denominator
Liquids and solids are not included (their concentrations don’t change if you add more)

4. Acid/Base Equilibrium
Ka = [H+][ B-] HB (aq) ↔ B- (aq) + H+(aq)
[HB]
Kb = [OH-][HB] B-(aq) + H2O ↔ HB(aq) + OH-(aq)
[B-]
Kw = [H+][OH-] = 1 x 10-14
Relationship between Ka and Kb: Ka ∙ Kb = Kw
so Kw = Ka Kb

5. pH and Ka Use Ka or Kb to determine pH of a weak acid or weak base:
Find [H+] or [OH-]
Take –log to get pH or pOH (then 14-pOH = pH)
Use pH or pOH to determine Ka or Kb:
Find [H+] or [OH-] by 10-pH or 10-pOH
Plug into Ka or Kb expression

6. Buffer Solutions [H+] = Ka ∙ mol HB mol B- pH = pKa + log mol B-
pH of buffer should be ≈ pKa so that the mole ratio of HB/B- can be about 1
Ka = [H+] when the ratio is 1

7. Titrations Equivalence point is where moles of
H+ and OH- are equivalent
Endpoint is where indicator changes color
Choose the indicator so the endpoint
occurs at equivalence point
Standardize solution – react with known amount of material to find exact concentration of solution (often done to standardize an acid or base before an acid-base titration
Neutralization – stoichiometric amounts of acid and base have been added (does NOT mean solution is neutral – weak acid anions make solution basic, weak base cations make solution acidic)

8. Titration Curves Weak Acid/strong base
 Strong acid/strong base Weak Base/strong acid
Neutral at eq. point Acidic at eq. point
Weak Acid/strong base
Basic at
eq. point

9. Gaseous Equilibrium KP pressure (atm) KC concentration (M)
Use K to determine partial pressure or concentration:
Use coefficients to identify change in each species (make an ICE table  with +x, -2x, etc.)
Plug into KP or KC
Solve for x and apply to table
Use pressure or concentration to find K:
Use ∆P or ∆C to get Peq
Reaction Quotient Q can be used to determine shift by comparing to K

10. Solubility Equilibrium
Ksp = [Pb2+][Br-]2 for PbBr Pb2+ + 2Br-
Ion concentrations are raised to power of the coefficient
Reaction Quotient: Q
Calculate for any conditions
Used to predict if precipitate will form (if Q > Ksp)
Make a table when solving problems
Use solubility to determine Ksp:
Plug in solubility as concentration for ions (use s)
Solve: For PbBr2 the Ksp = s ∙ (2s)2 = 4s3

11. Solubility Use Ksp to determine solubility: Common Ion Effect:
Use s as concentration in the Ksp
Solve for s
Common Ion Effect:
Less soluble if solution already contains one of the ions in the ionic solid that is trying to dissolve
This is because the product constant can’t exceed the Ksp