midterm (9, 10, 12, 13)

chapter 9

  • chemical equilibrium → [reac]:[prod] is constant (they aren’t necessarily equal)

  • mass-action effect: is the shift in position of an equilibrium caused by adding one of the reactants or products to a system (LCP shows how to reverse it)

  • kw = [h3o+][oh-]

  • pKw = pH + pOH

  • common ion effect decreases solubility

  • amount that dissolves in a specific volume = SVM

  • if u have double common ions, use limiting and excess then find the excess left over then use that as the common ion

  • Ka = [H+]= [H+][A-]/HA[A-]/HA

    • HA ⇌ H+ + A- or with water and hydronium ykyk

  • Kb = [HB+][OH-]/B

    • B + H2O ⇌ HB+ + OH-

  • Kw = (Ka)(Kb)

  • in a weak acid solution: [H3O+] = sqrt ( Ka [HA] )

  • in a weak base solution: [OH-] = sqrt ( Kb [B] )

  • buffers !!!

    • a mixture of either weak acid & conj base or weak base & conj acid that resists pH changes in a solution

    • pH = pKa + log ( [A-]/[HA] )

      • conj base / weak acid

    • pOH = pKb + log ( [HB+]/[B] )

      • conj acid / weak base


chapter 10 - effect of electrolytes

  • the position of solute equilibria can be affected by the addition of electrolytes (!!! not neutral species), even when the added electrolyte contains no ion in common with those involved in the equilibrium

  • concentration based equilibrium constant = Kx’

  • thermodynamic equilibrium constant = Kx

    • at low electrolyte conc, Kx’ = Kx

  • ionic participants increase the magnitude of the electrolyte effect with charge

  • ionic strength (μ) = ½ Σ[ (ci)(zi2) ]

    • molar species conc of ions = ci

    • ion charge = zi

    • ionic strength is measured in M

  • the electrolyte effect results from the electrostatic attractive and repulsive forces that exist between the ions of an electrolyte and the ions involved in an equilibrium

    • this is because if u put an electrolyte in the dissolution of slightly soluble AB for example, the anions of the electrolyte coat A+ with a negative layer and the cations of the electrolyte B- with a positive layer, slightly making A+ less positive and B+ slightly less negative

    • this decreases the overall attraction between A ions and B ions so less of them will go back to AB which increases solubility which will keep increasing the more you put electrolytes in

  • increasing ionic strength = increasing ion dissociation / solubility

  • activity (ax) = yx[X]

    • a is the activity of X

    • y is the activity coefficient

    • x is the molar conc of X

  • if XmYm is a precipitate, we can find its Ksp by:

    • Ksp = (yxm)(yyn)[X]m[Y]n(Ksp’)

      • yx & yy are activity coefficients

      • don’t forget to raise the concentrations to their powers

  • in solutions that aren’t so concentrated, the activity coefficient for a given species is independent of the electrolyte and dependent only on the ionic strength

  • -log yx = [ 0.51 (Zx)2 sqrt (μ) ] / [ 1 + 3.3(ax) sqrt(μ) ]

    • yx is activity coeff - this eq to calc activity coeff aslan

    • z is charge of X

    • mew is ionic strength of the whole solution

    • a is effective diameter of hydrated x in nanometers (10-9m)

  • when u have a strong electrolyte + weak acid / base, omit the weak one from the ionic strength calculation

  • same way Ka = [H+][A-]/[HA], it is also = aH+aA-/aHA


chapter 12 - gravimetric methods

  • gravimetric analysis is accurate, fast, and doesn’t require calibration

    • based on the measurement of mass

  • precipitation gravimetry is when the analyte is separated as a precipitate

  • volatilization gravimetry is when the sample is heated / chemically decomposed to remove any volatile compounds - change in mass is measured

    • like determining the water compound in a sample by evaporating it all

  • specific reagent: reacts with only one chemical species

  • selective reagent: reacts with a limited number of species

  • ideal precipitates:

    • readily filtered and washed

    • low solubility/Ksp so no loss of solid occurs

    • unreactive with the atmosphere

    • known composition when it dries

  • particle types:

    • colloidal suspensions: 10-7 to 10-4 cm in diameter

      • pass thru filter paper (so small)

      • don’t settle

      • hard to filter

    • crystalline suspensions: >10-4 cm in diameter

      • more favorable, opposite of above

  • higher particle size = more pure substance

  • coprecipitation: the process of precipitation of some substances alone with the principal precipitate - these substances are soluble under normal conditions

  • relative supersaturation (RS) = (Q - S)/S

    • Q = solute concentration

    • S = solubility at equilibrium

    • RS is how much a dissolved substance will exceed its normal solubility limit

  • large RS = colloidal

  • small RS = crystalline

  • PS ∝ 1 / RS

  • to minimize RS (favorable):

    • high temp (bigger S)

    • more dilute solution (smaller Q)

    • slow addition with stirring (smaller Q)

    • acidic pH (bigger S)

  • precipitates form by nucleation and particle growth (opposites!)

    • nucleation is when the precipitate has a large number of small particles

    • particle growth is when the precipitate has a small number of large particles

  • RS increases with increate of nucleation

  • coagulation: converting a colloid suspension into a filterable solid by making them lose charge and sink to the bottom

    • can be accelerated by - heating, stirring, and adding an electrolyte

      • heating decreases the number of adsorbed ions and gives KE to overcome repulsion barrier

      • adding an electrolyte - volume of counter ions decreases, counter ion layer shrinks, particles can come closer together

  • peptization: a process by which coagulated colloids return to their dispersed state

    • done by washing it with water (removal of electrolyte which increases the counter ion layer which means they start to push each other)

    • washing precipitation with a volatile electrolyte prevents peptization

  • adsorption: when a substance is held on the surface of a solid

  • electrical double layer model (unfavorable for coagulation):

    • A+ + B- → AB (s)

      • here, there is excess AC (aq)

    • AB colloidal layer

    • the electric double layer consists of two main layers - adsorption layer and counter ion layer

      • primary adsorbed layer consists of lattice ions in excess - so these ions r the ones that are both in the precipitate and excess (in this case, A+). this is the surface of the coagulated colloidal particles

        • this is held by a chemical bond

      • counter-ion layer consists of the opposite charge of the primary adsorbed ion, so the other ion in excess (here, C-)

        • this is held by an electrostatic attraction

  • digestion a process in which a precipitate is heated for an hour ++ in the solution from which it was formed

  • crystalline precipitates are easier to deal with in general

  • coprecipitation (CONTAMINATION) types:

    • surface adsorption (contamination in colloidal precipitate)

    • mixed-crystal contamination (when a contaminant ion replaces an ion of the same charge in a crystal lattice)

    • occlusion (when a compound is trapped or occluded within a pocket formed during rapid crystal growth, crystalline usually)

    • mechanical entrapment (several tiny crystals grow together and trap a portion of the solution in a tiny pocket)

  • contamination results in positive error

  • precipitation from a homogenous solution - slow chemical rxn

  • weighing form - a precipitate after heating

  • gravimetric analysis use inorganic precipitating agent

  • reducing agent

  • organic precipitating agent


chapter 13 (until HClO4)

  • volumetry - the volume of reagent needed to react with analyte is measured

  • titrimetry - increments of the titrant are added to the analyte until the endpoint is reached

  • titrant = top