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| Chem 110 |
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Fall 2003 |
| Lecture Notes::Lec 5_5 September |
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| PREVIOUS |
pH Effects on Ionic Solubility: pH can affect solubility dramatically if the anion is a weak acid salt. This is because reaction of the anion with hydrogen ion can convert it to another species and thus remove it from the equilibria. (Remember salts are strong!)
Example: What is the solubility of nickel sulfide in a solution of 0.1 M hydrogen sulfide at pH 6.0? Ksp = 1 x 10-19
Ksp = [Ni2+] [S2-] and Ka = {[H+]2 [S2-]}/[H2S] = 1 x 10-22 So the concentration of sulfide ion, and therefore nickel ion, is dependent on pH.
From the pH given, [H+] = 1 x 10-6M. Substituting and rearranging, [S2-] = ([H2S]/[H+]2)(1 x 10-22) = 0.1(1 x 10-22)/(1 x 10-6)2 = 1 x 10-11 And solubility = [Ni2+] = (Ksp)/(1 x 10-11) [Ni2+] = (1 x 10-19)(1 x 10-11)-1 = 1 x 10-8M The effect of pH can be further seen by trying another calculation at pH 2.0:
[S2-] = 0.1(1 x 10-2)-2(1 x 10-22) = 1 x 10-19 [Ni2+] = (1 x 10-19)(1 x 10-19)-1 = 1 M
Complex ions consist of metal ions surrounded by ligands.
A ligand is an electron pair donor (a Lewis base) that can bind to a metal ion via a weak covalent bond. Of course formation of a covalent bond requires not only an available electron pair on the ligand, but also an empty orbital on the metal ion where they can be shared. As a result complex ions most commonly involve transition metal ions with their available d- and p-orbitals. We will look at the bonding in complex ions in more depth later when we study the transition metals.
Note that for all of these metal ions the hydrated (aquo) ion is also a complex ion with water as the ligand.
The number of ligands around a complex ion is referred to as the coordination number. Some examples as listed below:
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Each of these ions is formed via stepwise equilibrium processes, with the tabulated dissociation constant the products of the individual step constants.
We can look at these processes as formations and get Formation constants (or stability constants), which are just the inverse of the dissociation constants. For example, if one adds ammonia to a solution of silver ions, the silver ammonia complex ion is formed stepwise as shown below:
Ag(H2O)2+ + NH3 ´ Ag(H2O)(NH3)+ K1 = 2.1 x 103 Ag(H2O)(NH3)+ + NH3 ´ Ag(NH3)2+ K2 = 8.2 x 103 combined Ag(H2O)2+ + 2NH3 ´ Ag(NH3)2+ K = 1.7 x 107
The First Law of Thermodynamics: This is simply a restatement of the Law of Conservation of Energy - Energy can neither be created nor destroyed. In other words, the energy of the Universe is constant. (Recall also that in a strict sense only the mass-energy of the Universe is conserved, since mass and energy are interconvertable by Einstein's famous equation, E=mc2. For our purposes now we can consider energy to be conserved as energy in chemical processes since the amount of mass-energy conversion is so slight. At the end of the semester when we look at nuclear chemistry we will need to take into account Einstein's equation.)
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© R A Paselk
Last modified 8 September 2003
