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LL ee cc tt uu rr ee 2 2

Precipitation equilibrium

Associate profAssociate prof . L.V. Vronska. L.V. VronskaAssociate profAssociate prof . M.M. Mykhalkiv

OutlineOutline

Precipitation equilibrium it Precipitation equilibrium it heterogeneous equilibriumequilibrium

Calculation of solubility and solubility Calculation of solubility and solubility product Kproduct Kspsp

Influence of chemical factors is on Influence of chemical factors is on solubility of precipitatesolubility of precipitate

Completeness of precipitation and factors Completeness of precipitation and factors which influence on herwhich influence on her

Conditions of dissolution of precipitationConditions of dissolution of precipitation

Precipitation equilibriumPrecipitation equilibrium

precipitate

An insoluble solid that forms when two or more soluble reagents are combined.

The most common precipitation reactionis a metathesis reaction, in which two soluble

ionic compounds exchange parts.

Thus, the precipitation of PbCl2 is written as

Pb2+(aq) + 2Cl–(aq) = PbCl2(s) In the equilibrium treatment of precipitation,

however, the reverse reaction describing the dissolution of the precipitate is more frequently encountered.

PbCl2(s) = Pb2+(aq) + 2Cl–(aq)

Precipitation equilibriumPrecipitation equilibriumThe equilibrium constant for this reaction is

called the solubility product, Ksp, and is given as

Ksp = [Pb2+][Cl–]2 = 1.7 ·10–5

and for all and for all electrolyteselectrolytes AAmmBBnn

Ksp = [A]m[B]n

solubility product Ksp - the equilibrium constant for a reaction in which a solid dissociates into its ions

ConcentrationalConcentrational ( (realreal) ) constantconstant solubility product KR

sp for all for all electrolyteselectrolytes AAmmBBn n ((use, use,

when we have real conditions (influence of when we have real conditions (influence of ionic strength)ionic strength)))

KRsp = [A]m[B]n

Thermodynamic constant solubility product KT

sp for all for all electrolyteselectrolytes AAmmBBnn

n

B

m

A

R

sp

n

B

m

A

nmn

B

m

A

T

sp

ffK

ffBAaaK

][][

Thermodynamic constant solubility product KT

sp depends on:depends on:

TemperatureTemperature PressurePressure Nature of solventNature of solvent Nature of precipitateNature of precipitate

Thermodynamic constants solubility product

KTsp are adduction in reference books

ConditionalConditional constant constant solubility product KCsp

for all for all electrolyteselectrolytes AAmmBBnn

)/(

)/(][][n

B

m

A

R

sp

n

B

m

A

nmn

B

m

A

C

sp

K

BACCK

)/(

)/(n

B

m

A

n

B

m

A

T

sp

C

sp

n

B

m

A

n

B

m

A

n

B

m

A

C

sp

ffKK

ffaaK

We use , when we have the We use , when we have the following real conditions:following real conditions:

TemperatureTemperature PressurePressure Influence of ionic strengthInfluence of ionic strength Influence of competitive reactionsInfluence of competitive reactions

C

spK

Calculation of solubility and Calculation of solubility and solubility product Ksolubility product Kspsp

Solubility is a property of matter to give/ to form a solution with a certain solvent at certain conditions

We determine Solubility as:

Coefficient of Solubility (ks)

Molar Solubility (S)


It is mass of matter which dissolves at this It is mass of matter which dissolves at this temperature in 100 g or 100 mL of solventtemperature in 100 g or 100 mL of solvent

100)(

solvent

matters Vm

mk

Molar Solubility (S)

It is a molar concentration of matter in the It is a molar concentration of matter in the saturated solution saturated solution

ААmmВВnn mА + nВ mА + nВ

KKRRspsp = [A] = [A]mm [B] [B]nn

[A] = m[A[A] = m[AmmBBnn] = mS] = mS

[B] = n[A[B] = n[AmmBBnn] = nS] = nS

KKRRsp=sp=(mS)(mS)mm(nS)(nS)nn = m = mmmnnnnSSm+nm+n

.nmnm

sp

nm

KS

A rule of solubility product:A rule of solubility product:in saturated solution above sediment product of ions in saturated solution above sediment product of ions

concentrations is permanent at a stationaryconcentrations is permanent at a stationary temperaturetemperature

In unsaturated solutionIn unsaturated solution

[[AA]]mm[[BB]]nn KKspsp

In saturatedIn saturated solutionsolution[[AA]]mm[[BB]]nn =K=Kspsp

• In In supersaturated solution solution

[[AA]]mm[[BB]]nn >> KKspsp

If ionic strength can be adopted even a zero If ionic strength can be adopted even a zero and to neglect of competitive reactions, and to neglect of competitive reactions, solubility of precipitate is expected on the size solubility of precipitate is expected on the size of Kof KTT

sp sp (if (if μ→μ→0 then f0 then f→→1; 1; αα=1=1))

If to take into account ionic strength, but to If to take into account ionic strength, but to neglect of competitive reactions, solubility is neglect of competitive reactions, solubility is expected after the size expected after the size of Kof KRR

sp sp (if (if μ≠μ≠0 then 0 then

ff≠≠1; 1; αα=1=1)) If we cannot neglect by competitive reactions, If we cannot neglect by competitive reactions,

then solubility is expected on the size then solubility is expected on the size of Kof KCCspsp

(if (if μ≠μ≠0 then f0 then f≠≠1; 1; α ≠α ≠ 1 1))

Influence of chemical factors is on solubility of Influence of chemical factors is on solubility of precipitateprecipitate

The Common-Ion effect The activity effectThe activity effect Acid-base reactionsAcid-base reactions Complexation reactionsComplexation reactions Red-ox reactionsRed-ox reactions

Completeness of precipitation and Completeness of precipitation and factors which influence on herfactors which influence on her

The precipitation The precipitation is considered is considered practically practically completecomplete, if the concentration of the , if the concentration of the precipitate’s ions in solution above precipitate’s ions in solution above sediment does not exceed a sediment does not exceed a 1010-6-6 mol/L mol/L

The Common-Ion Effect

The common-ion effect. The solubility of at 25°C decreases markedly on addition of ions. Note that the calculated solubility is plotted on a logarithmic scale.

The solubility of precipitate decreases in the presence of a solution that already contains one of its ions. This is known as the common ion effect.

The activity effectThe activity effect

Clearly the equilibrium position for the reaction

AgIO3(s) Ag+(aq) + IO3–(aq)

depends on the composition of the solution. When the solubility product for AgIO3 is calculated using the equilibrium concentrations of Ag+ and IO3

–

Ksp = [Ag+][IO3–]

its apparent value increases when an inert electrolyte such as KNO3 is added.

The true thermodynamic equilibrium constant, Ksp, for the solubility of AgIO3, therefore, is

KTsp = a(Ag+) a(IO3

–)

KTsp=[Ag+][IO3

-]f(Ag+)f(IO3–)

KTsp= KR

sp f(Ag+)f(IO3–)

KRsp= KT

sp/ f(Ag+)f(IO3–)

To accurately calculate the solubility of AgIO3, we must know the activity coefficients for Ag+ and IO3

–.

Mention !!!Mention !!! First, as the ionic strength approaches zero, the activity

coefficient approaches a value of one. Thus, in a solution where the ionic strength is zero, an ion’s activity and concentration are identical. We can take advantage of this fact to determine a reaction’s thermodynamic equilibrium constant. The equilibrium constant based on concentrations is measured for several increasingly smaller ionic strengths and the results extrapolated back to zero ionic strength to give the thermodynamic equilibrium constant.

Second, activity coefficients are smaller, and thus activity effects are more important, for ions with higher charges and smaller effective diameters. Finally, the extended Debye–Hückel equation provides reasonable activity coefficients for ionic strengths of less than 0.1. Modifications to the extended Debye–Hückel equation, which extend the calculation of activity coefficients to higher ionic strength, have been proposed.

The pH of the Solution An ionic compound that contains a basic anion

becomes more soluble as the acidity of the solution increases. The solubility of CaCO3, for example, increases with decreasing pH because the CO3

2- ions combine with protons to give HCO3

- ions. As CO32- ions are removed from the

solution, the solubility equilibrium shifts to the right, as predicted by Le Châtelier’s principle. The net reaction is dissolution of CaCO3, in acidic solution to give Ca2+ ions and HCO3

- ions:

Formation of Complex IonsThe solubility of an ionic compound increases

dramatically if the solution contains a Lewis base that can form a coordinate covalent bond to the metal cation. Silver chloride, for example, is insoluble in water and in acid, but it dissolves in an excess of aqueous ammonia, forming the complex ion [Ag(NH3)2]+. A complex ion is an ion that contains a metal cation bonded to one or more small molecules or ions, such as NH3, CN- or OH-. In accord with Le Châtelier’s principle, ammonia shifts the solubility equilibrium to the right by tying up the Ag+ ion in the form of the complex ion:

Formation of Complex Ions

Silver chloride is insoluble in water (left) but dissolves on addition of an excess of aqueous ammonia (right).

The solubility of AgCl in aqueous ammonia at 25°C increases with increasing ammonia concentration owing to formation of the complex ion [Ag(NH3)2]+. Note that the solubility is plotted on a logarithmic scale.

Separation of Ions by Selective Precipitation A convenient method for separating a mixture of

ions is to add a solution that will precipitate some of the ions but not others. The anions SO4

2- and Cl- for example, can be separated by addition of a solution of Ba(NO3)2. Insoluble BaSO4 precipitates, but Cl- remains in solution because BaCl2 is soluble.

Similarly, the cations Ag+ and Zn2+ can be separated by addition of dilute HCl. Silver chloride, AgCl, precipitates, but Zn2+ stays in solution because ZnCl2 is soluble.

Separation of Ions by Fractional Precipitation

Ions BaIons Ba2+2+ and Ca and Ca2+2+ can be separates if can be separates if concentration of SOconcentration of SO44

2-2- ions is controlled. ions is controlled.

BaSOBaSO44 has Khas Kspsp = 1 = 11010–10–10 andand CaSOCaSO44 has has KKspsp = =

2,32,31010–5–5

That precipitate of BaSOThat precipitate of BaSO44 have been removed have been removed

out, his ionic product must be greater Kout, his ionic product must be greater Kspsp, but, , but,

that precipitate of CaSOthat precipitate of CaSO44 did not removed out it did not removed out it

is simultaneously necessary, that ionic product is simultaneously necessary, that ionic product

[Ca[Ca2+2+][SO][SO442-2-] ] KKspspCaSOCaSO44, ,

butbut [Ba [Ba2+2+][SO][SO442-2-]] KKspspBaSOBaSO44

Therefore, if concentrations both ions are 10Therefore, if concentrations both ions are 10 -2-2 mol/L, concentration SOmol/L, concentration SO44

2-2- ions must be ions must be

betweenbetween

andand

./101][

][ 8

2

2

44 лмоль

Ba

KSO BaSOsp

./103,2][

][ 3

2

2

44 лмоль

Ca

KSO CaSOsp

Conditions of dissolution of precipitationConditions of dissolution of precipitation

It is necessary for dissolution of sediment, that It is necessary for dissolution of sediment, that his ionic product became more small his ionic product became more small constants of solubility product:constants of solubility product:

[[KatKat++][An][An––] ] KKsp_sp_KatAnKatAn

Decrease of ions concentration it can be carried Decrease of ions concentration it can be carried out the followings methods:out the followings methods:

1. 1. strong dilution of solutionstrong dilution of solution Descriptive term Descriptive term Approximate volume of solvent in Approximate volume of solvent in

milliliters per gram of solute milliliters per gram of solute

very soluble very soluble

less than 1 less than 1

freely soluble freely soluble from 1 to 10 from 1 to 10

soluble soluble from 10 to 30 from 10 to 30

sparingly solublesparingly soluble from 30 to 100from 30 to 100

slightly soluble slightly soluble from 100 to 1000from 100 to 1000

very slightly soluble very slightly soluble from 1000 to 10 000 from 1000 to 10 000

practically insolublepractically insoluble more than 10 000more than 10 000

The term 'partly soluble' is used to The term 'partly soluble' is used to describe a mixture of which only some of describe a mixture of which only some of the components dissolve. the components dissolve.


2. 2. The ions of precipitate are connected in The ions of precipitate are connected in compounds which well water-solublecompounds which well water-soluble

Co(OH)ClCo(OH)Cl + HCl = CoCl + HCl = CoCl22 + H + H22OO

3. 3. The ions of precipitate are connected in The ions of precipitate are connected in compounds which give gascompounds which give gas

ZnSZnS + 2HCl = ZnCl + 2HCl = ZnCl22 + H + H22SS

Formation and dissolution of Cr(OH)Formation and dissolution of Cr(OH)33


3. 3. The ions of precipitate are connected in The ions of precipitate are connected in compounds which are complexcompounds which are complex

AgClAgCl + 2NH + 2NH33 = [Ag(NH = [Ag(NH33))22]Cl ]Cl

4. 4. Oxidation and reduction of ions of Oxidation and reduction of ions of precipitateprecipitate in others compoundsin others compounds

MnO(OH)MnO(OH)22 + H + H22CC22OO44 + H + H22SOSO44 = =

MnSOMnSO44 + 2CO + 2CO22 + 3H + 3H22OO

Dissolution of sulfatic-precipitate Dissolution of sulfatic-precipitate

sodium carbonate extraction sodium carbonate extraction is a translation of is a translation of sulfates of second analytical group in sulfates of second analytical group in carbonates carbonates

BaSOBaSO44 KKspsp=1,1 =1,1 10 10-10-10

BaCOBaCO3 3 KKspsp=5,1 =5,1 10 10-9-9

.50][SO

][CO

24

23

Aplication of Aplication of Precipitation equilibrium

Gravimertic analysis:Gravimertic analysis:

- - Particulate gravimetryParticulate gravimetry

- Precipitation gravimetry- Precipitation gravimetry

Thanks for your attention!

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