Why does AgCl dissolve in NH3 but AgBr and AgI don't?
KSP of AgCl = 1.8 x 10-10
KSP of AgBr = 5.0×10–13
KSP of AgI = 1.5 x 10-16
AgCl, AgBr, and AgI are all silver halide compounds, and their solubility in a given solvent depends on the strength of the solvent's interaction with the halide ions.
In the case of AgCl, it dissolves in NH3 due to the formation of a complex ion called the diammine silver(I) ion, [Ag(NH3)2]+. This complex ion forms because NH3 is a Lewis base, meaning it can donate an electron pair to form a coordinate bond with the silver ion (Ag+). The interaction between Ag+ and NH3 is strong enough to overpower the forces holding AgCl together, leading to its dissolution.
On the other hand, AgBr and AgI do not dissolve in NH3 because the solvation energies of Br- and I- are not strong enough to form stable complexes with Ag+. In other words, the forces of attraction between the halide ions and the NH3 molecules are weaker than the forces of attraction between the halide ions and Ag+. Consequently, AgBr and AgI remain insoluble in NH3.
The solubilities of AgCl, AgBr, and AgI can be explained by comparing their solubility product constants (KSP). The KSP values indicate the equilibrium constant for the dissolution of these compounds, with lower KSP values indicating lower solubility. AgCl has the highest KSP value among the three, suggesting that it is the most soluble in water. AgBr and AgI have lower KSP values, which indicate much lower solubilities in water. However, KSP values alone do not explain the solubility in NH3, as it is a different solvent with different solvation energies.
The solubility of ionic compounds in a solvent is determined by the relative strength of the forces acting between the ions in the compound and the solvent molecules. In this case, we will compare the solubility of AgCl, AgBr, and AgI in ammonia (NH3).
Ammonia (NH3) is a polar molecule with a partial negative charge on the nitrogen atom and partial positive charges on the hydrogen atoms. It can form hydrogen bonds with other polar molecules or ions.
When AgCl is added to ammonia, the Cl- ions are attracted to the partial positive hydrogen atoms in NH3 through ion-dipole interactions. These interactions can overcome the strong forces holding the Ag+ and Cl- ions together in the solid crystal lattice, causing AgCl to dissolve in NH3. The ammonia molecules surround the individual Ag+ and Cl- ions, effectively solvating them.
On the other hand, AgBr and AgI are less soluble in NH3 compared to AgCl due to two main reasons:
1. Size of the anion: The solubility of ionic compounds generally decreases as the size of the anion increases. In this case, Br- and I- ions are larger than Cl- ions. The larger anions experience weaker ion-dipole interactions with the ammonia molecules, making it more difficult for them to dissolve.
2. Lattice energy: The lattice energy is the energy required to separate the ions in a solid ionic compound. It is directly related to the strength of the forces holding the ions together in the crystal lattice. The lattice energy increases as the charges on the ions increase and as the radii of the ions decrease. In this case, AgBr and AgI have larger lattice energies compared to AgCl. The higher lattice energies make it more difficult for the interactions between the ammonia molecules and the Ag+ and Br- or I- ions to overcome the lattice energy and dissolve the compounds.
In summary, AgCl dissolves in NH3 due to the strong ion-dipole interactions between the Cl- ions and the ammonia molecules. However, AgBr and AgI do not dissolve as readily in NH3 due to the larger size of the anions and the higher lattice energies.
AgCl + 2 NH3 --> Ag[(NH3)2]^+ + Cl^-
K for the rxn shown is Kf*Ksp
Kf*Ksp for AgCl is about 2.9E-3
Kf*Ksp for AgBr is about 8E-6
Kr*Ksp for AgI is about 2.4E-9
As we proceed from AgCl to AgI the Ksp and Krxn shifts smaller and smaller to the solubility of the AgBr is about 1000 less and that of AgI is about 1 million times smaller. Actually, AgBr is slightly soluble in dilute (6M NH3) but AgI is virtually insoluble.
K for AgCl at about 1E-3 is not that small; compare with H3PO4 with k1 - about that number. But AgI complex is so small that compares with Ka of HCN.