Explaining Solutions SCH3U

The Dissolving Process The process of dissolving is related to the forces of attraction between the solute and solvent particles. Solute-solvent attractions must overcome attractions within both the solute and solvent for dissolving to occur.

The Process at a Molecular Level: Energy is required to break the forces of attraction between particles in the solid. Ionic compound: forces between ions Molecular compound: forces between molecules Energy is also required to break some of the intermolecular forces between particles in the liquid. Energy is given off by an attraction between solvent and solute particles.

Solubility and Intermolecular Forces: Dipole-Dipole attraction: Is the attraction between opposite charges of two different polar molecules. In water, this is called hydrogen bonding. This is the strongest type of intermolecular force. Hydrogen bonding: the attraction between a hydrogen atom on one molecule and a small, very electronegative atom on another molecule.

Solubility and Intermolecular Forces:

Solubility of Ionic Compounds in Water: Ion-Dipole attraction: Is an attractive force between an ion and a polar molecule. Two attractions: The negative end of water is attracted to the positive surface of the ionic crystal. The positive end of water is attracted to the negative surface of the ionic crystal.

When ions leave a crystal, they become surrounded by a sphere of water molecules (called hydration). Disassociation.

Most ionic compounds are soluble in water, however, the ionic compounds that have very strong forces between their ions are less soluble. Predicting whether an ionic compound is soluble: Ion charge: The force of attraction is directly related to the amount of charge on each ion. Greater the charge on each, the less soluble it will be. Oxygen in oxides, O2-, makes most oxides insoluble. Ion size: As ion size increases, so does solubility. The force of attraction between opposite charges decreases as the distance between the charges increases. AgNO3 very soluble, AgCl insoluble.

The Solubility of Molecular Compounds Dipole-dipole attraction occurs. Weaker than ionic bonds, dipole-dipole attraction is often strong enough to hold polar molecules together as a solid crystal. Most polar compounds dissolve in water. Dipole-dipole attraction between the molecules is generally much weaker than hydrogen bonds between the solute molecules and the water molecules.

The greater the non-polar part of a polar molecule, the less chance there is that the compound is soluble in water. The forces of attraction that exist between a non-polar solvent and a non-polar solute is usually London dispersion forces.