- ratio concentrations of products and reactants when there is NO net charge in the system

Unit 2 Flashcards by Braydon Huynh

equilibrium constant

ratio concentrations of products and reactants when there is NO net charge in the system

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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

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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

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Define Bronstead-Lowry acid and base

acids donate proton to water -> H3O+ ions

- bases accept protons from water -> OH- ions

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Define strong base versus weak base

SB ionize completely in water, weak base only ionize partially in water

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Write out Ka for:

HCl + H2O -> H30+ + Cl-

Trick question - HCl is a strong acid that ionizes completely so no Ka equation

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Write out Ka equation for weak acid:

HA + H2O -> H3O+ + A-

Ka= [H3O+][A-]/[HA]

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Write out Kb equation for weak base:

B + H2O -> BH+ + OH-

Kb = [BH-][OH-]/[B]

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Equation for Kw?

Kw = KaKb = 10^(-14)

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Equation for Kb with Kw and Ka?

Kb = Kw /Ka

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Equation for pH (from concentration of H+ ions)?

pH = - log [H+]

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Equation for pH (from pKa)

pH = pKa + log [A-]/[HA]

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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

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What does a charge balance equation do?

Represents the electroneutrality of the solution. Sum of the positive charges should equal negative charges

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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

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True or false: in charge balance equations, charged spectator ions should always be shown

True

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Write out the charge balance equation for:

HCl (aq) + NaOH (aq) -> H2O (l) + NaCl (aq)

[H3O+] + [Na+] = [OH-] + [Cl-]

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What are polypeptide acids?

Acids which two or more ionizable protons. Will have multiple Ka corresponding to each proton

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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

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Write out the acid equilbria for carbonic acid (H2CO3) and the corresponding Ka expressions.

Ask Braydon lol

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What is amphiprotic?

Species that can act as either a base or an acid.

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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.

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What does alpha (a) represent

a represents the fraction of a given species that is present

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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

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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?

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)