Chapter 12 Section 2

Section 2 The Solution Process

Factors Affecting the Rate of Dissolution

• Because the dissolution process occurs at the surface of the solute, it can be sped up if the surface area of the solute is increased


• Stirring or shaking helps to disperse solute particles and increase contact between the solute surface. This speeds up the dissolving process


• At higher temperatures, collisions between solvent molecules and solvent are more frequent and are of higher energy than at lower temperature. This helps to disperse solvent molecules among the solvent molecules, and speeds up the dissolving process

Solubility

• If you add spoonful after spoonful of sugar to tea, eventually no more sugar will dissolve
• This illustrates the fact that for every combination of solvent with a solid solute at a given temperature there is a limit to the amount of solid that can be dissolved
• The point at which this limit is reached for any solvent-solute combination is difficult to predict precisely and depends on the nature of the solute, the nature of the solvent and the temperature.
• When a solute is first added to a solvent-solute molecules leave the solid surface and more about at random in the solvent
• As more solute is added, more collisions occur between dissolved solute particles. Some of the solute molecules return to the crystal
• When maximum solubility is reached molecules are returning to the solid form at the same rate at which they are going into the solution
• Solution equilibrium is the physical state in which the opposing processes of dissolution and crystallization of a solute occur at which the same rates

Saturated vs. Unsaturated Solutions

• A solution that contains the maximum amount of dissolved solute is described as a saturated solution
• If more solute is added to a saturated solution it falls to the bottom of that container and doesn't dissolve
• A solution that contains less solute than a saturated solution under the existing conditions is called unsaturated solution
• When a saturated solution is cooled the excess solute usually comes out of solution, leaving the solution at the lower temperature
• But sometimes the excess solute doesn't separate a supersaturated solution is produced which is a solution that contains more dissolved solution that contains a more dissolved solute than a saturated solution under the same conditions
• The solubility of a sub solution is the amount of that sub solution required to form a saturated solution with a specific amount of solvent at a specific temperature
• Solubility's vary widely and must be determined experimentally
• They can be found in chemical hand/books and are usually given as gimmes of solute per 100g of solvent at a given temperature

Solute-Solvent Interactions

• Solubility varies greatly with the type of compound involved
• "Like" dissolves like it is a rough but useful rule for predicting whether one substance will dissolve in another
• What makes substances similar depends on type of bonding, polarity and non polarity of molecules, and intermolecular forces between solute and solvent

Dissolving Ionic Compounds in Aqueous Solutions

• The polarity of water molecules plays an important role in the formation of solutions of ionic compounds in water
• The slightly charged parts of water molecules attract ions in the ionic compounds and surround them to keep them separated from other ions in the solution
• This solution process with water as the solvent is referred to as hydration

Non polar Solvents

• Ionic compounds are generally not soluble in non polar solvents such as carbon tetrachloride and toluene
• The non polar solvent molecules do not attract the ions of the crystal strongly enough to overcome the forces holding the crystal together
• Ionic and non polar substances differ widely in bonding, polarity, and intermolecular forces

Liquid Solutes and Solvents

• Liquids that are not soluble in each other are immiscible
• Liquids that dissolve freely in one another in any proportion are said to be miscible