Unit 2 Flashcards
equilibrium constant
- ratio concentrations of products and reactants when there is NO net charge in the system
Write out the equation for the equilibrium constant of:
a A + b B -> c C + d D
K=[C]^c[D]^d/[A]^a[B]^b
Write out equilibrium constant equation for:
2 Na+ (aq) + Co(H2O)6 ^(2+) (aq) + 6 Cl- (aq) -> CoCl4 ^(2-) (aq) + 2 NaCl (s) + 6 H2O (l)
K= [Na+]^(2)[Co(H2O)6^(2+)][Cl-]^(6) / [CoCl4^(2-)]
- don’t include water and solids
- raise concentrations to power of molar coefficients
Define Bronstead-Lowry acid and base
- acids donate proton to water -> H3O+ ions
- bases accept protons from water -> OH- ions
Define strong base versus weak base
- SB ionize completely in water, weak base only ionize partially in water
Write out Ka for:
HCl + H2O -> H30+ + Cl-
Trick question - HCl is a strong acid that ionizes completely so no Ka equation
Write out Ka equation for weak acid:
HA + H2O -> H3O+ + A-
Ka= [H3O+][A-]/[HA]
Write out Kb equation for weak base:
B + H2O -> BH+ + OH-
Kb = [BH-][OH-]/[B]
Equation for Kw?
Kw = KaKb = 10^(-14)
Equation for Kb with Kw and Ka?
Kb = Kw /Ka
Equation for pH (from concentration of H+ ions)?
pH = - log [H+]
Equation for pH (from pKa)
pH = pKa + log [A-]/[HA]
What is the pH of a 0.010 M solution of acetic acid in water? (Ka= 1.80 x 10^-5)
pH = 3.37
See Equilibrium #1 slides, slide 9 for worked soln
What does a charge balance equation do?
Represents the electroneutrality of the solution. Sum of the positive charges should equal negative charges
True or false: for charge balance equations, there should be a charge balance for every reaction that is in equilibrium in the solution
False. There should only be one charge balance equation regardless of how many equilibria there are
True or false: in charge balance equations, charged spectator ions should always be shown
True
Write out the charge balance equation for:
HCl (aq) + NaOH (aq) -> H2O (l) + NaCl (aq)
[H3O+] + [Na+] = [OH-] + [Cl-]
What are polypeptide acids?
Acids which two or more ionizable protons. Will have multiple Ka corresponding to each proton
For polypeptide acids, does the second Ka (corresponding to second H that can be removed) increase, decrease or stay the same compared to the first Ka?
Decrease
Write out the acid equilbria for carbonic acid (H2CO3) and the corresponding Ka expressions.
Ask Braydon lol
What is amphiprotic?
Species that can act as either a base or an acid.
Will an aqueous solution of sodium bicarbonate be acidic or alkaline?
Alkaline (pH will be greater than 7). Calculate values for Ka and Kb. If Ka is greater than Kb, it’ll be acidic. If Kb larger, it’ll be basic.
What does alpha (a) represent
a represents the fraction of a given species that is present
If there are four species (a1, a2, a3, a4) in a solution, what should the total sum of the alpha values equal?
a1 + a2 + a3 + a4 = 1
What does a high Ksp value mean?
The higher Ksp is, the more soluble the compound is
What is the equation for relative supersaturation? Define all terms.
RSS = (Q - S)/S
Q= actual concentration of solute
S=eqm concentration of solute
What does a high RSS and low RSS mean?
Low RSS= precipitate crystals tend to nucleate, rapid nucleation of particles
High RSS= precipitate crystals grow larger, slow steady growth
Do you want a high or low RSS?
Low RSS is better analytically (large particles, slow formation)
What is a chelating agent?
binds to metal ions to form chelates
What is EDTA?
a chelating agent that binds to one metal ion (regardless of charge) per molecule of EDTA
For a sparingly soluble salt, what happens to solubility when pH decreases?
@pH less than pKa= solubility increases as pH decreases d
@pH greater than pKa= solubility stays the same
Assuming NH3+ can form a complex with the cation of the precipitate, what does adding more NH3+ do to solubility?
Increases solubility. Complexing agent helps pull precipitate into solution.
How do you increase the solubility of Ni(OH)2 (s)?
A) decrease pH B) increase pH C) add complexing agent D) A and C E) B and C F) none of the above
D (both A and C)
Why do you perform titrations?
to determine the quanitity of a substance A by adding measured increments of B
Titrant
solution of known composition and concentration, dispensed from buret
Titrand
unknown solution in beaker, contains unknown amount of analyte
Equivalence point
point when the moles of titrant equals moles of titrand
True or false: equivalence point is when the concentration of titrant and titrand are equal
False. It is number of moles are equal
Endpoint
an observable change that approximately signals the equivalence point
True or false: the pH of the equivalence point of strong acids increases as pKa decreases
False. SA always have pH = 7 at eqv pt
True or false: the pH of the equivalence point of weak acids increases as pKa decreases
True.
For weak acids, how do you determine the value of the pH at the equivalence point?
pH = pKa at eqv pt
Describe procedure of a back titration
1) “A” is reacted with a known and excess amount of B
2) Remaining amount of B is titrated with C (which provides a rapid reaction and an observable endpoint)
3) Difference between amount of B measured by 2nd titration and amount originally added can be used to find amount of A
Buffer
Mixture of WA and conjugate base - resists pH changes with small additions of acid or base
When do you use a back titration?
1) when titration reaction too slow
2) no suitable indicator
3) no useful direct titration rxn actually occurs
What is the transition range for an indicator dye usually around in terms of pKa?
Usually around pKa= + or - 1 (but visual range sometimes within 0.5)
True or false: When choosing an appropriate indicator dye, always choose one that has a pKa that is sufficiently larger than the pKa of the weak acid
True!
Displacement titration?
Basically a back titration
Gravimetric analysis?
Analyzing the amount of an analyte based on mass
Argentiometric titration
Titration to determine the amount of halide and halide-like ions (SCN-, CN-, CNO-, etc) and other inorganic anions
Inclusions? And how do you remove them?
Type of precipitate impurity. Interfering ions gets substituted inside lattice (mixed in with entire precipitate).
Removed by recrystallization (but risk losing analyte)
Occlusions? And how do you remove them?
Type of precipitate impurity. Interfering ions trapped inside middle of precipitate.
Get rid of by aging at high temperature (dissolve small particles to form large particles so less impurities stuck inside)
Adsorbates? And how do you remove them?
Type of precipitate impurity. Interferents stuck on on surface of crystals.
Make larger crystals so there is less surface area.
Three main types of precipitate impurities
Inclusions, occlusions, adsorbates
Fluorescin
Indicator used in titrations with halides
Volhard method
Method used to titration halide solution with an excess of silver nitrate. Add Fe3+ as indicator and back titrate with thiocyanate.
Why do you use EDTA?
For back titrations with metal ions
How many metal ion molecules can each molecule of EDTA?
1
True or false: EDTA can bind with metal ions with less charge (ex: +/- 1) than those with higher charge (ex: +/- 1)
False. Only one metal ion per, regardless of charge
Which one of these is EDTA? A) X-4 B) X-3 C) X-2 D) Y-4 E) Y-3 F) Y-2
D (Y-4)
What pH do you do EDTA titrations at?
pH 10 (to maximize amount of Y-4)
In an EDTA titration, what happens to the concentration of metal ion when the pH increases
Decrease as pH increases
Sketch an EDTA titration curve
See slides Eqm #4 slide 13
Auxiliary complexing agent
Something you add to an EDTA titration to prevent the formation of metal hydroxides (which aren’t really soluble)
Name a common auxiliary complexing agent
Ammonia
Molecular formula of ammonia
NH3
In an EDTA titration, what happens to the concentration of unbound metal ions when the formation constant increase?
Decreases
Eriochrome black T
EDTA titration indicator for low concentrations (micromolar) and pH greater than 7 but only works with a few metal ions
Xylenol orange
EDTA titration indicator for pH 5-6 (turns orange to red/violet when there are metal ions)
