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Name: Dennis
Status: educator
Grade: 9-12
Location: Australia
Country: N/A
Date: August 2006

Question:
We all know that alkali metal carbonates are soluble forming hydrated ions in solution.Why is it that most other metal carbonates are generally insoluble? Why is calcium carbonate for example essentially insoluble? Is it a steric thing caused by the bulky carbonate ion or does the bonding have some covalent character as well as ionic bonding?



Replies:
Dennis,

Solubility occurs in the first place because of entropy and thermodynamics. Entropy is what drives diffusion and aids the transformation into a homogeneous mixture. This will occur as long as the total energy of the system is lower as a mixture than as two unique solids, AND entropy is a great enough force to over come the activation barrier to get it there. Think of this equation: dG = dH - SdT if G is negative then it will spontaneously dissolve because there is either heat released from the system or the entropic gain from the system is positive enough to overcome the heat that must be absorbed into the system. Chemically speaking, if the solute-solvent interactions are more stabilizing than the solute-solute and solvent-solvent interactions, then it will dissolve. If not, then there is no driving force for dissolution.

Given these facts, the column II elements tend to form stronger salts because they have two bonding interactions (Ca+2) as opposed to just one with the column I elements (Li+1). Because the salts have stronger bonds, they tend to be less soluble because they are already in a more stable state (compared to the column I salts). Carbonate is not the issue in the case of dissolution because sodium carbonate (Na2CO3) is soluble in water, whereas calcium carbonate (CaCO3) has little solubility in water. If the carbonate ion had issues dissolving, then sodium carbonate wouldn't dissolve either. The calcium carbonate ionic bond is a stronger ionic bond than sodium carbonate because it is more covalent in character. If you remember, the bond strength is directly proportional to the differences in electronegativity between the two bonded atoms (at least in a two atom system). Since the Column I atoms have are more electropositive than their column II counterparts, the resulting ionic bonds are weaker.

Matt Voss



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