Explain the relationship between the solubility of ionic and molecular compounds in aqueous and nonaqueous solvents, and the intermolecular interactions between particles.
The whole of EK 3.10.A.1 is one sentence: substances with similar intermolecular interactions tend to be miscible or soluble in one another. "Like dissolves like" is the slogan; the reasoning behind it is an energy comparison.
Dissolving requires three things to happen:
If the new solute–solvent interactions are comparable in strength to the ones broken, dissolving is favorable. A polar solute in a polar solvent works because the strong solute–solute and solvent–solvent interactions are replaced by equally strong solute–solvent ones. A polar solute in a nonpolar solvent fails because breaking the strong solute–solute interactions buys you only weak dispersion interactions in return.
Structural reasoning. Look at what fraction of a molecule is polar. Methanol (CH₃OH) is fully water-miscible: a tiny nonpolar tail and a hydrogen-bonding –OH. As the chain lengthens the nonpolar portion dominates and water solubility collapses — 1-octanol is essentially insoluble. Soaps and detergents work precisely because they have both: a polar/ionic head and a long nonpolar tail.
Ionic solutes in water dissolve via ion–dipole interactions strong enough to compete with the lattice attractions. Whether a given salt dissolves depends on the balance between lattice energy and hydration energy — high charges and small ions make the lattice hard to break, which is why MgO and Al₂O₃ are insoluble while NaCl and KNO₃ are freely soluble.
For predicting precipitation, the CED requires only that you know all sodium, potassium, ammonium, and nitrate salts are soluble (see 4.7). Any further solubility rules will be supplied.
Explain why ethanol (CH₃CH₂OH) is completely miscible with water while hexane (C₆H₁₄) is not.
Ethanol and water: ethanol has an –OH group, so it can both donate and accept hydrogen bonds. When ethanol and water mix, the ethanol–ethanol and water–water hydrogen bonds that are broken are replaced by ethanol–water hydrogen bonds of comparable strength. The energy cost of separating the particles is essentially repaid by the new interactions, so the two are miscible in all proportions.
Hexane and water: hexane is nonpolar and experiences only London dispersion forces. To dissolve hexane, water's extensive hydrogen-bonding network would have to be broken apart, which costs a great deal of energy. The only new interactions available in return are weak dispersion forces between hexane and water. The energy released cannot compensate for what was spent, and additionally the water network must reorganize unfavorably around the hexane, so the two form separate layers.