Lecture 15. Polar vs Non-Polar Substances. Professor Hicks Inorganic Chemistry II (CHE151)

Transcription

1 Lecture 15 Professor icks Inorganic hemistry II (E151) Polar vs on-polar Substances Ionic compounds Molecules with significant dipole moments (from polar bonds) Molecules with little or no dipole moment, often lots - bonds Polar water itself and water soluble compounds on-polar fats, oils, grease 1

2 Like Dissolves Like Fats, oils, grease, contain lots of non-polar - bonds non polar oil Polar substances will dissolve in other polar substances on-polar substances will dissolve in other non-polar substances Polar and non-polar substances are not soluble in each other water Surfactants soaps, detergents, phospholipids Polar head group P S 3 - non-polar chain - different ways to represent the same structure - soaps have polar and non-polar regions 2

3 Micelles Form when using soap with water Polar head groups point out facing solvent water on-polar tails point in ils (dirt) dissolve in the non-polar interior micelle Liposomes Double layer formed Polar faces inside and out Forms spontaneously Resemble cell membrane liposome Emulsions Emulsions are mixtures of oil and water that are temporarily stable They can be formed using soaps or detergents Egg whites can also facilitate the formation of emulsions like mayonaise (oil + vinegar + egg white) 3

4 Why does cleaning your skin with soaps dry it out? Soaps allow oil to be dissolved in micelles Removing the oil allow moisture underneath to evaporate resulting in the dryness. Similarly using a lotion that contains oily molecules creates a thin barrier layer that water cannot pass through Plants use oils in the same way to create a protective barrier on the leaf surface to prevent water loss by evaporation 4

5 Intermolecular Forces (IMF) ause molecules to stick to each other 3 types 1) ydrogen bonds a special type of dipole-dipole force 2) ther dipole-dipole forces 3) London forces stronger forces higher boiling point dipole-dipole forces = attraction of oppositely charged ends of DM ydrogen bonds + Strongest intermolecular force Positive end of dipole must be a bonded to,, or F egative end of the dipole must be,, or F with a lone pair Most substances that can bond are polar IMF s shown with dashed lines hydrogen bonds T - or just any bond to in a Lewis structure you must memorize this 5

6 ther dipole-dipole forces - + S Attractions between oppositely charge parts of dipole moment Typically about 10 weaker than hydrogen bonds Substances with substantial dipole forces tend to be polar similar to a hydrogen bond but does not fit the definition b/c negative end is S London Forces Electrons form a cloud around nucleus Electrons move cloud fluctuates momentary dipole moments are created ~ 10 weaker than dipole-dipole forces Present in all molecules/ions two helium atoms no attraction momentary fluctuations cause temporary dipole moments attraction between to atoms e e momentary fluctuations fade attraction fades 6

7 London Forces Increase with total number of electrons Bigger electron clouds fluctuate more Exists in all substances Substances that have London forces as their only IMF tend to be non-polar weakest forces strongest forces e e Ar Kr Lowest boiling point ighest boiling point Fritz London IMF and boiling point Liquid Gas overcome intermolecular attraction In liquid phase molecules are in contact In gas phase molecules are far apart Energy (heat) must be added to break the hydrogen bonds (or other IMF s) + - igher melting points also reflect stronger IMFs 7

8 Vapor pressure Pressure above all liquids reated by evaporating molecules Stronger IMF lower vapor pressure VP increases with temperature 2 Vapor pressure = Applied pressure liquid boils vapor pressure bonding is special # electrons increases London Forces increase boiling points increase -bonding strongest IMF boiling point water much higher

9 d. Ar or e 9

10 What are the strongest IMF s in pure samples of each of the following species? What is the strongest IMF present in pure samples of each of the following species? F 10

11 What is the strongest IMF present between each of the following pairs of species? F F F + What is the strongest IMF present between each of the following pairs of species? F 11

12