Dissolving

Bond Polarity In order for polarity to occur, there must be a significant difference in electronegativity between bonding atoms. Results in… Polar Covalent Bond: Uneven e- sharing, causing partial charges to form on each atom in the bond. Ionic Bond: Complete donation of e-, causing full charges to form on each atom in the bond.

Molecular Polarity In order for a molecule to be polar, it must have two distinct regions of positive and negative charges on the ends of the molecule. There must be: 1) Bonds causing charges to form on the molecule (polar covalent or ionic) 2) Uneven distribution of charges, causing one end to be (+) and one (-)

Polar or Non-Polar? Acetone (CH3COCH3)

Acetone is….Polar!

Polar or Non-Polar? Cyclohexane (C6H12)

Cyclohexane is… Non-Polar!

Polar or Non-Polar? Calcium Bromide (CaBr2)

Calcium Bromide is… Polar! Full charges form across the molecule

Dissolving How a substance is “dissolved” When a substance (solute) is dissolved, it goes from a state of being surrounded by other molecules of itself to being surrounded by molecules of the solvent.

Polarity and Dissolving In our lab, we determined that “like dissolves like” Polar solvents dissolve polar solutes Non-polar solvents dissolve non-polar solutes Like dissolves like video Polarity is important because it determines whether or not the solute and solvent molecules will “like” to interact In other words, it determines whether the two molecules will mix. Water dissolving salt animation

Dissolving Like dissolves like because: The charges on polar solvent attract other polar molecules of solute, allowing them to mix.

Why Polar / Non-Polar Don’t Mix This same charge attraction occurs between identical molecules of a polar solute. Meaning that if they are mixed with a non-polar solvent, the two will not mix because all of the polar molecules will clump together and separate out due to differences in density.

Non-Polar Dissolving Non-Polar Non-Polar / Non-Polar dissolving occurs because the lack of charges on the solvent and solute molecules allow the two to freely interact, once physically mixed. Think about how the iodine dissolved in hexane, did it dissolve immediately or did you need to stir it first?

Solubility The measure of a solvent’s ability/capacity to dissolve a solute. We’ve seen polarity determines whether or not two substances will mix (dissolve), can you think of other factors? Mixing (stirring) Temperature Surface Area

Factors Influencing Solubility To understand the factors influencing solubility, or dissolving, think about what happens during dissolving… Molecules mix by coming into contact with one another! So, think about what you do to sugar (solute) to dissolve more quickly in your tea/coffee… Stir it Heat it up Put in small grains rather than large chunks

Factors Influencing Solubility Stirring physically increases the rate of molecular collision Temperature is a measure of molecular kinetic energy (how fast molecules move), so increasing temperature increases molecular speed and therefore rate of collision Surface Area determines the number of solute molecules exposed to collision with the solvent. Greater SA = greater solubility