P107 LECTURE 5

terms only, for mulcho

AgCl

white curdy precipitate

precipitate

insoluble ionic solid

precipitation reactions

occur when cations and anions in aqueous solution combine to form a precipitate

ion-product constant (Qsp)

the product of the molar concentrations of the ions in solution raised to their stoichiometric coefficients

solubility-product constant (Ksp)

the ion-product constant when the solution becomes saturated

less soluble

lower Ksp implies that a salt is _________

unsaturated solution

more solute dissolves

saturated solution

no more solute dissolves

supersaturated solution

crystals may grow

precipitation will occur

Qsp > Ksp

precipitation will not occur

Qsp < Ksp

precipitation will not occur yet

Qsp = Ksp

common-ion effect

solubility of a slightly soluble ionic compound is lowered when a second solute that furnishes a common ion is added to the solution

99.9%

precipitation is considered complete if about ____ of the target ion is precipitated

• a very small value of Ksp

• high initial concentration of the target ion

• concentration of common ion that greatly exceeds that of the target ion

conditions that generally favor the completeness of precipitation:

somewhat dissolve

if an anion of a precipitate is that of a weak acid (e.g. F, anion of HF, weak acid), the precipitate will _________ when the pH is lowered

have no effect, will not dissolve

if an anion of a precipitate is that of a strong acid (e.g. Cl, anion of HCl, strong acid), the precipitate will __________ when the pH is lowered

• silver nitrate

• potassium thiocyanate

common titrants used in precipitation titration:

silver nitrate

• “argentometry”

• forms insoluble precipitates with halides and pseudohalides

• reaction ratio is 1:1

potassium thiocyanate

used as back titrant

• appearance of a turbidity

• formation of a colored complex ion

• formation of a colored secondary precipitate

• formation of a colored adsorption compound

methods of endpoint detection for precipitation titrations:

potassium chromate

in Mohr’s method, __________ is used as an indicator in the titration of halogens

silver nitrate

in Mohr’s method, ___________ is the titrant

Mohr’s method

after all the chloride has been precipitated, the first excess of silver results in the formation of a silver chromate precipitate

neutral or slightly basic medium

in Mohr’s method, the titrations are performed in ______________

silver hydroxides

(Mohr’s) in strongly basic conditions, __________ is formed

chromic acid

(Mohr’s) in acidic conditions, ___________ is formed

Volhard method

suitable for the determination of chlorides, bromides, and iodides in acidic solutions

potassium thiocyanate

(Volhard) used in residual titrations where ____________ is used as a back titrant

FeSCN²⁺

(Volhard) when all silver ions are consumed, a blood red complex is formed. What is the said complex?

1M in nitric acid, ferric hydroxide

(Volhard) the solution must be acidic, with a concentration of about _________ to prevent the precipitation of __________

all iodide is precipitated with Ag

(Volhard) in case of Br & I as analyte, indicator should be added when ___________

Fajan’s method

uses adsorption indicators such as dichlorofluorescein and eosin

precipitate

(Fajan’s) the color change is observed in the ___________