AP Chemistry Lunch Review Equilibrium AP Chemistry Lunch Review

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

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)

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

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

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

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)

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

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

Solubility Equilibrium Ksp = [Pb2+][Br-]2 for PbBr2 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

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