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LL ee cc tt uu rr ee 2 2
Precipitation equilibrium
Associate prof . L.V. VronskaAssociate prof . L.V. VronskaAssociate prof . M.M. MykhalkivAssociate prof . M.M. Mykhalkiv
OutlineOutline
1.1. Precipitation equilibrium as Precipitation equilibrium as heterogeneousheterogeneous equilibriumequilibrium
2.2. Calculation of solubility and solubility Calculation of solubility and solubility product Kproduct Kspsp
3.3. Influence of chemical factors is on Influence of chemical factors is on solubility of precipitatesolubility of precipitate
4.4. Completeness of precipitation and factors Completeness of precipitation and factors which influence on itswhich influence on its
5.5. Conditions of dissolution of precipitationConditions of dissolution of precipitation
1. PRECIPITATION EQUILIBRIUM AS 1. PRECIPITATION EQUILIBRIUM AS HETEROGENEOUSHETEROGENEOUS EQUILIBRIUMEQUILIBRIUM
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
2. CALCULATION OF SOLUBILITY AND 2. 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)
Coefficient of Solubility (ks)
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
Coefficient of Solubility (ks)
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 supersaturated solutionIn supersaturated 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))
3. INFLUENCE OF CHEMICAL FACTORS IS 3. INFLUENCE OF CHEMICAL FACTORS IS ON SOLUBILITY OF PRECIPITATEON SOLUBILITY OF PRECIPITATE
The Common-Ion The Common-Ion effecteffect The activity effectThe activity effect Acid-base reactionsAcid-base reactions Complexation reactionsComplexation reactions Red-ox reactionsRed-ox reactions
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.
4. COMPLETENESS OF 4. COMPLETENESS OF PRECIPITATION AND FACTORS PRECIPITATION AND FACTORS
WHICH INFLUENCE ON ITSWHICH INFLUENCE ON ITS
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 precipitate does not exceed a precipitate does not exceed a 1010-6-6 mol/L mol/L
Factors which influence on Factors which influence on completeness of precipitationcompleteness of precipitation
1.1. Excess of precipitation reagent (50 %) Excess of precipitation reagent (50 %)
2.2. Strength of electrolyte-Strength of electrolyte-precipitatorprecipitator
3.3. pH of solutionpH of solution
4.4. Fractional precipitationFractional precipitation
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
5. CONDITIONS OF DISSOLUTION OF 5. CONDITIONS OF DISSOLUTION OF PRECIPITATIONPRECIPITATION
It is necessary for dissolution of sediment, that It is necessary for dissolution of sediment, that its ionic product became more small its 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.
Decrease of ions concentration it can be carried Decrease of ions concentration it can be carried out the followings methods:out the followings methods:
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
Decrease of ions concentration it can be carried Decrease of ions concentration it can be carried out the followings methods:out the followings methods:
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!