Exam 2 Flashcards by Samantha Boyer

advantages of gravimetric

not many measurements
mass is easy to measure
fairly accurate
inexpensive

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disadvantages of gravimetric

prone to error
meticulous
time consuming
not suited to many trials

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3 types of gravimetric

precipitation
combustion analysis
thermogravimetric analysis

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TGA curve

percent original mass as a function of temperature (gravimetric)

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methods of precipitating non-water-soluble analytes for gravimetric

in concentrated acids
wet ashing
dry ashing
fusion

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aqua regia

HCl + HNO3

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wet ashing

heat + acid

metals converted to metallic cations

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dry ashing

heating to extremely high temps in air

metals converted to oxides soluble in acid

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fusion

used for rocks and minerals

mix with Na2CO3 and heat to high temp

Na2SiO3 is formed, soluble in water

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_________ agent is added in excess in gravimetric precipitation

precipitating

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characteristics of good precipitates for gravimetric

stable at high temp
not colloidal - settle out
filterable - large particles
purity
very insoluble
known composition

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explain primary adsorbed layer

outer layer of precipitate particle

usually made of ion in excess in solution (precipitating agent)

gives a surface charge, and opposite charged spectators are attracted

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explain counter-ion layer

layer of spectator ions attracted to primary adsorbed layer of precipitate

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______ held counter-ion layer produces a colloidal suspension

loosely

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explain saturation during precipitation

requires supersaturation, but not too much, because supersaturation favors nucleation but not crystal growth

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2 steps of precipitate formation

nucleation - formation of small nuclei of prec.
crystal growth - combination of nuclei upon collision

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how to achieve correct supersaturation in precipitate formation

high temp
don’t use overexcess of prec agent
stir with addition of prec agent
use large volumes of faily dilute solutions
homogenous precipitation

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explain homogenous precipitation

precipitating agent is formed by a slow reaction in situ, and released slowly

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explain digestion (Ostwald ripening)

product is allowed to sit in mother liquor for long period at high temp

increases purity and crystal size

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2 methods of precipitate filtration

filter paper

porous crucibles

washing precipitate with a volatile electrolyte solution (HCl or HNO3) does what?

prevents peptization (conversion to colloid)

electrolytes replace primary adsorbed layer, and heat then drives them off

main problem with gravimetric analysis

coprecipitation

3 types of coprecipitation

occlusion
inclusion
adsorption

explain occlusion

type of error

mechanical entrapment of impurities within crystal

(+) error

explain inclusion type of error

contaminant ions of similar size/charge as desired ions are incorporated into crystal (+) or (-) error

explain adsorption (coprecipitation) type of error

contaminants are attracted to surface of crystal (+) error

most difficult type of coprecipitation to correct for

inclusion must redissolve and try again

characteristics of rxn suitable for titration

- instantaneous - equilibrium lies far toward products (high Keq) - need indicator of completion

titration error

difference between endpoint and eq point

how is titration error estimated?

with a blank titration

2 types of indicators

visual graphical/instrumental

First derivative curve

𝚫pH/𝚫vol as a function of Vavg

Gran plot

V10^-pH as a function of V titrant

the end point on a gran plot is where...

slope changes

good qualities of a primary standard

pure stable low cost soluble large MW (smaller error)

explain back titration

analyte + excess reagent --> product + unreacted reagent unreacted reagent + titrant --> product

moles reacted reagent in backtitration =

moles total reagent - moles unreacted reagent

general category of precipitation titrations

argentometric methods

3 argentometric methods

Volhard Mohr Fajan's

Volhard method _____ is indicator, _____ is titrant, ______ is analyte

Fe3+ is indicator (blood red) SCN- is titrant cation is analyte

Mohr method _____ is indicator, ______ is titrant, _______ is analyte

CrO4 2- (yellowish) is indicator Ag+ is titrant bromide, chloride and cyanide are analytes

Fajan's method _______ is indicator, _______ is titrant, ______ is analyte

anionic organic dye (adsorptive) is indicator Ag+ is titrant anions are analytes

in Fajan's method we want colloidal particles because...

more surface area for dye means more vibrant endpoint

acid-base reaction with no H2O involved

neutralization

acid-base reaction related to Arrhenius concept, making water and salt

dissociation

strong acids (6)

HCl H2SO4 HI HBr HNO3 HClO4 (sort of)

strong bases (2)

hydroxides of Group 1 & 2 metals oxides of group 1 & 2 metals

Kw = (all)

1.0 x 10^-14 @25°C [H3O][OH] KaKb

when is x negligible in ICE table?

when 500Ka is < the concentration of HA when 500Kb is < the concentration of B-