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REVISION OF NAMES AND FORMULAS
SOLUBILITY EQUILIBRIA
Do all the exercises in your study guide.
REVISION OF IONS IN SOLUTION AND SOLUBILITY GIUDELINES
Revise the ions in solution and solubility.
Include the solubility table given to you in CMY 117. Some of such tables are shown below.
PRECIPITATION REACTION CONT. SOLUBILITY TABLE:
Generally soluble in water
Chlorides, ClBromides, BrIodides, IFluorides, FSulphates, SO42-
Exceptions, Insoluble
AgCl, Hg2Cl2,PbCl2
AgBr, Hg2Br2,PbBr2
AgI, Hg2I2, PbI2
SrF2, CaF2 BaF2, PbF2 MgF2
Ag2SO4, HgSO4, PbSO4,
SrSO4, CaSO4,, BaSO4
Study the table of names and formulas.
Examples are:
C2O42‐ ‐‐‐‐ Oxalate ion
(NH4)2C2O4‐Ammonium oxalate PRECIPITATION REACTION CONT. SOLUBILITY TABLE:
• Solubility Table
Generally soluble in water
Na+, K+, NH4+
Nitrates, NO3-, Nitrites,
NO2Chlorates, ClO3Perchlorates, ClO4-,
Permanganates,MnO4Acetate ion, CH3COO-
Exceptions
No common exception
No common exception
No common exception
No common exception
PRECIPITATION REACTION CONT. SOLUBILITY TABLE:
Generally insoluble in water Exceptions
Na+, K+, NH4+, Mg2+, Ca2+
Sulphides, S-2
Oxides, O-2, Hydroxide, OH- Na+, K+, NH4+, Li+, Ba2+,
Sr2+
Carbonate, CO32Na+, K+, NH4+
Phosphate, PO42-, Arsnates,
AsO43Oxalates, C2O42Na+, K+, NH4+
Chromates, CrO42Na+, K+, NH4+
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EXAMPLES OF THE COMPOUNDS THAT ARE NOT SOLUBLE IN WATER
AlPO4 Cu3(PO4)2
AgCN CuSCN
CdS
Bi2S3
BaSO4 Zn3(ASO4)3 SrCO3
Mn(lO3)
PbCrO4 Ag2CrO4
Fe(OH)3
PbBr2
Hg2Br2
THE SOLUBILITY PRODUCT CONSTANT
• The solubility product constant , is the equilibrium constant for the extent of the solubility of insoluble solid substance in aqueous solution. That is, it represent the extent to which insoluble salt is dissolved in aqueous solution.
• It is indicated by ksp
• It differ from the solubility because the solubility is a certain amount of solute in a solvent or in a solution.
• Example are molality, molarity, etc.
THE SOLUBITLITY PRODUCT CONSTANT
As in the case of AgCl, the solubility of AgBr is also less than 0.01M.
If some AgBr is placed in pure water, a smallest amount of this salt dissolves, and equilibrium is established. THE SOLUBILITY PRODUCT CONSTANT
Unlike in the previous sections where we worked with the homogeneous mixtures (acid‐base soln), In this section we are going to work with the heterogeneous mixture, viz precipitation reactions.
Precipitation reaction is an exchange reaction in which one product is soluble in water while another one is not soluble in water.
The compound having the water solubility of less than 0.01M, is said to be insoluble in water.
SOLUBITLITY OF SALTS
Example of such reactions is:
AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
In this case AgCl(s is a precipitate.
That is its solubility is less than 0.01M (mol/L).
Although this is not soluble in water, it can dissolve to a certain extent in water to form:
AgCl(s) ) → Ag+ (aq) + Cl‐(aq)
SOLUBITLITY OF SALTS
• AgBr(s)↔ Ag+(aq, 7.35 x 10‐7M) + Br‐(aq, 7.35 x 10‐7M)
• When AgBr has dissolved and equilibrium is attained, the solution is said to be saturated.
• The solubility equilibrium constant Ksp:
•
ksp = [Ag+][Br‐]
•
=(7.35 x 10‐7) (7.35 x 10‐7)
•
=5.4 x 10‐13
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THE SOLUBITLITY PRODUCT CONSTANT, ksp
The solubility and the solubility product constant of a compound should not be confused:
The solubility of a compound is the quantity of solute present in a certain volume of solvent or solution, expressed in moles per litre, moles per kilograms, grams per 100mL, etc.
The solubility product constant of a compound is the solubility constant (it has no units).
THE SOLUBITLITY PRODUCT CONSTANT, ksp
The solubility product constants (ksp) are determined by careful laboratory measurement of the concentration the ions.
RELATING SOLUBITLITY AND ksp
The ksp value cannot directly be used to compare the solubility of two or more salts.
For an example, the ksp of BaSO4 = 1.1 x 10‐10
and that of MgF2 = 5.2 x 10‐11
One may think that BaSO4 is more soluble than MgF2 .
When comparing their solubility's as calculated above
For
BaSO4 = 0.0024 g/L
and that of MgF2 = 0.015 g/L
This shows that MgF2 is more soluble than BaSO4.
THE SOLUBITLITY PRODUCT CONSTANT, ksp
That is, it is the product of the concentrations of the ions formed from the insoluble salts raised to the power of their numerical coefficients. Although the solubility and the solubility constant differ from each other, the knowledge of one can be used to calculate the other one.
RELATING SOLUBITLITY AND ksp
• The ksp of insoluble salts can be used to estimate solubility of solid salt and determine weather a solid will precipitate when the solution of its anion and cation are mixed RELATING SOLUBITLITY AND ksp
The reason for this is that the cation : anion ratio of BaSO4
MgF2 is 1 : 1
while that of
is 1 : 2.
Ksp values can be used to compare the solubilities of salts only if they have the same cation : anion
ratios.
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RELATING SOLUBITLITY AND ksp
For example:
The cation : anion ratios of the following salts is 1:1 and their ksp values can therefore be used to predict their solubilities. RELATING SOLUBITLITY AND ksp
The cation : anion ratios of the following salts is 1:2
and their ksp values can therefore be used to predict their solubilities.
PbI2(Ksp=8.5x10‐9)<PbBr2(Ksp=5.4x10‐6)<PbCl2(Ksp=1.8 x 10‐5)
Increasing Ksp and increasing solubility
AgI(Ksp=8.5x10‐17)<AgBr(Ksp=5.4x10‐13)<AgCl(Ksp=1.8 x 10‐10)
Increasing Ksp and increasing solubility
FACTORS THAT AFFECT THE SOLUBILITY OF INSOLUBLE SALTS
SOLUBILITY AND THE COMMON ION EFFECT
There are three factors that affect the solubility of insoluble salt:
* Common Ion Effect
* pH Effect
This is the effect of the ion that is similar to at least one of the ions formed from the insoluble salts.
For an example; when adding NaCl to the solution of AgCl that is in equilibrium; the equilibrium will shift to the left.
That is more AgCl will form because of the presence of extra Cl‐ ions.
The common ion decrease the ksp value of the insoluble salt.
SOLUBILITY AND THE COMMON ION EFFECT
The solubility of AgCl in pure water is 1.3 x 10‐5M.
But the solubility of AgCl in the presence of Cl‐ is 3.3 x 10‐10M.
This confirms Le chatelier’s principle and clearly shows that the equilibrium is disturbed by adding Cl‐ and the equilibrium shift to the left (to the reactant) to counteract the disturbance in order to establish the new equilibrium.
This results into the formation of more AgCl and hence less solubility of AgCl.
SOLUBILITY AND pH
This shows the way in which the basicity and/ or the acidity of an insoluble salt or part of the insoluble salt affect the solubility of that salt.
We are going to look at the EFFECT OF THE BASIC ANION ON SALT SOLUBILITY.
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EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
This is the effect of a strong base formed from an insoluble salt on the solubility of that salt.
For an example: If you have:
AgI(s) ↔ Ag+(aq) + I‐(aq)
In this case the effect of I‐(aq) on the solubility of AgI(s)
is determined.
EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
These basic anions can undergo hydrolysis.
That is, they can accept a proton from the water molecule.
For example:
EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
An acid is a proton donor.
A base is a proton acceptor.
A basic anion is an anion that can accept a proton more easily.
S2‐
CO32‐
PO43‐
CH3COOCN‐
All these are strong bases. Examples of basic anions are:
EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
Most of the salts of these are insoluble in water.
For example:
PbS
CaCO3
These salts have the solubility of less than 0.01M. S2‐(aq) + H2O (ℓ) ↔ HS‐ (aq) + OH‐ (aq)
EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
Example 7:
When dissolving PbS in water you get: PbS(s) ↔ Pb2+ (aq) + S2‐(aq)
This is followed by the hydrolysis of S2‐ which is a strong base. S2‐
(aq)
+ H2O (ℓ)
↔ HS‐
(aq)
+ OH‐
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(aq) kb=1x 10 .
Which decreases the presence of S2‐ and force the equilibrium to shift to the right. EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
PbS becomes more soluble than it is expected.
The concentration of Pb2+ increases.
This lead to the general conclusion that:
“Any salt containing an anion that is a conjugate base of a weak acid will dissolve in water to a greater extent than the given ksp”
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EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
This shows that the salts of the following basic anions are affected by this factor.
S2‐
CO32‐
PO43‐
CH3COOCN‐
EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
In general:
X‐(aq) + H2O (ℓ) ↔ HX (aq) + OH‐ (aq)
Do all the exercises in your study guide.
This shows that: “Insoluble salts in which the anion is the conjugate base of a weak acid, is soluble in strong acids”
It is generally known that if an acid is added to a water; insoluble salt, like the salts of the above‐
mentioned anion, will dissolves.
EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
Remember:
The ka of the reverse reaction is equal to the reciprocal of the former equation: e.g
ka2= 1/ka1
When adding two or more equations, the ka of the net equation is equal to the product of the added equations : e.g
knet= k1x k2x k3x etc
Example 8: The dissolution of CaCO3 in the absence of a strong acid, gives:
CaCO3(s) ↔ Ca2+(aq) + CO32‐(aq) ksp= 3.4 x 10‐9
When adding a strong acid like H3O+ to a solution of CaCO3, this salt will dissolve. CaCO3(aq) + 2H3O+(aq) →Ca2+(aq) + H2CO3(aq) + 3H2O(ℓ) EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
EFFECT OF THE BASIC ANION ON SALT SOLUBILITY
In contrast:
The addition of a strong acid (e.g. H3O+) to an insoluble salt containing an anion that is not a basic anion, does not make that salt to dissolve to the extent that is greater than ksp.
Example:
The solubility of AgCl is not affected by the addition of a strong acid because Cl‐ is not a strong basic anion. Cl‐ is a conjugate base of a strong acid.
AgCl(s) ↔ Ag+(aq) + Cl‐(aq) ksp = 1.8 x 10‐10
Cl‐(aq) + H3O+(aq) ↔HCl(aq) + H2O(ℓ) k<< 1
This shows that this salt is not affected by the addition of H3O+(aq).
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PRECIPITATION REACTION
Precipitation reaction is the reaction in which precipitate is formed.
How should one know that the precipitate is going to form in a particular reaction.
For an example: How do you know that the precipitate will form in the following reaction?
Ksp AND REACTION QUOTIENT, Q
Q which is the reaction quotient have to be calculated and compared with Ksp to decide whether the precipitation will occur or not:
Q and Ksp are both reaction constants, but the different is that Ksp works on the equilibrium reaction only, while Q is used for any reaction including the reaction that is in equilibrium.
AgCl(s) ↔ Ag+(aq) + Cl‐(aq) ksp = 1.8 x 10‐10
In addition to the value of ksp one have to calculate the value of Q and compare them.
Ksp AND REACTION QUOTIENT, Q
For the reaction:
AgCl(s) ↔ Ag+(aq) + Cl‐(aq) ksp = 1.8 x 10‐10
ksp = [Ag+][Cl‐] and Q = [Ag+][Cl‐]
ksp may be given but Q have to be calculated Ksp AND REACTION QUOTIENT, Q
If Q < ksp, then the reaction is not yet at equilibrium.
The solution is not yet saturated and no precipitation will be formed.
If Q = ksp, then the reaction is at equilibrium The solution is saturated and no precipitation will be form. If Q > ksp, then the reaction is at equilibrium.
The solution is supersaturated and the precipitation will be formed. Ksp , THE REACTION QUOTIENT AND PRECIPITATION REACTION
In this section, if one knows the Ksp or Q and the concentration of one ion, the concentration of another ion can be calculated.
1. One can tell whether the precipitation will be formed .
2. How much of one or another ion is needed for the precipitation to form.
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