Today is Thursday, May 31 st, 2018

Transcription

1 In This Lesson: Unit 6: Solutions Solutions and Solubility (Lesson 1 of 4) Today is Thursday, May 31 st, 2018 Stuff You Need: Calculator Periodic Table Polyatomic Ion List Pre-Class: Where have you seen solutions in everyday life? (I mean the chemistry kind of solution, FYI) Solution!

2 Today s Agenda Solutions Solubility Curves Why I get so angry at TV sometimes. Where is this in my book? P. 518 and following

3 By the end of this lesson You should be able to describe solutions and colloids in terms of their properties. You should be able to identify solubility trends based on pressure, agitation, and temperature.

4 Wait what s a solution? Recall that a solution is a homogeneous mixture. If you ve ever stirred sugar into tea or coffee, you ve made a solution. A solute (usually either a gas or a solid, but sometimes a liquid) is what s dissolved into a liquid, called the solvent. Example: Salt water Solute: Salt Solvent: Water Example: Seltzer water Solute: CO 2 Solvent: Water

5 Solutions Up Close Water doesn t really understand personal boundaries. On a molecular level, when something dissolves, water (or a different solvent) completely surrounds the solute particles. It forms a hydration shell. As a result, we get a very easy-to-remember solubility rule (next slide).

6 Like Dissolves Like Video Here s an introduction to an important solubility concept

7 Like Dissolves Like Non-polar solutes dissolve best in non-polar solvents: Fats, steroids, waxes into benzene, hexane, toluene. Polar (and ionic solutes) dissolve best in polar solvents: Inorganic salts and sugars into water, small alcohols, acetic acid.

8 Hydrophobic/Hydrophilic Hydrophilic (polar) substances can dissolve easily in water. Hydrophobic (non-polar) substances cannot. Oil, for example, doesn t. That s why you see this:

9 Hydrophobic/Hydrophilic

10 Hydrophobic/Hydrophilic

11 Hydrophilic/Hydrophobic

12 Hydrophobicity in Practice TED: Mark Shaw One Very Dry Demo

13 Solutions Also remember that solutions are considered aqueous, but the solute does not actually change state. For example, sugar stays a solid when dissolved into tea; carbon dioxide stays a gas in soda. Lastly, solubility is the word chemists use to describe the maximum amount of solute that can dissolve in a certain quantity of solvent.

14 IMPORTANT Recall that ionic compounds will dissociate (break down) into their component ions when they dissolve. Example: NaCl becomes Na + and Cl - in water. PbCl 2 becomes Pb 2+ and 2Cl - in water. Note that there is a coefficient of 2 in front of Cl -, since there are twice as many chloride ions as lead ions in solution.

15 Reminder: Dissociation Bound ions in component ions out. Pb NO 3 NO 3 Pb 2+ NO - 3 NO 3 -

16 Dissociation Practice Into what does H 2 SO 4 dissociate? What s the compound made of, and how many of each? 2H and 1SO 4 Write their charges in: 2H + and 1SO 4 2- Ta-da!

17 Dissociation Practice Into what does CaCl 2 dissociate? Ca 2+ and 2Cl - Into what does Na 2 S dissociate? 2Na + and S 2- Into what does MgO dissociate? Mg 2+ and O 2- Into what does Mg(NO 3 ) 2 dissociate? Mg 2+ and 2NO 3 -

18 Solubility Rate As you may have guessed, not all solutions are created equally. Some solvents can hold a lot of a certain substance, some can t hold any.

19 Solubility Trends Solids dissolve best when: Heated (more heat = more solubility) Stirred Ground into small particles (more surface area = more solubility) Gases dissolve best when: Chilled Under high pressure This explains why many beer commercials describe their drinks as being frost-brewed it s because they have to be!

20 Gas Dissolution in Soda When a soda bottle is capped, vapor pressure above the liquid keeps the carbon dioxide dissolved. Opening the bottle decreases the pressure allowing CO 2 to escape. Hence, bubbling.

21 Aside: Henry s Law Not on your exam. The concentration of a dissolved gas in a solution is directly proportional to the pressure of the gas above the solution: S S S Solubility (g/l) 1 2 P Pressure Applies most accurately for dilute solutions of gases that do not dissociate or react with the solvent: Good for CO 2, N 2, O 2 Bad for HCl, HI P 1 P 2

22 Aside: The Bends The bends, also known officially as decompression sickness, occurs when divers go to extreme depths. At these depths, water pressure force-dissolves nitrogen gas into blood vessels. If those divers then rise to the surface too quickly, the nitrogen gas bubbles out of the blood (like CO 2 from a freshly-opened soda bottle). These N 2 bubbles restrict oxygen flow and cellular function. The result? Crippling pain forcing people to bend over, resulting in permanent damage or even death.

23 Gas Solutions Interestingly, biochemists have been playing around with the possibility of creating a liquid that has so much oxygen dissolved in it that it s possible to breathe. That s right, breathing a liquid. Like that scene in The Abyss. Cue the video!

24 Terms to Describe a Stadium If a stadium has 50,000 seats and there are 50,000 people there, how do we describe it? At capacity. If a stadium has 50,000 seats and there are 49,999 people there, how do we describe it? Under capacity. If a stadium has 50,000 seats and there are 60,000 people there, how do we describe it? Over capacity.

25 Terms to Describe a Solution Saturated Full of solute. Unsaturated Not full of solute. It can have some solute, just not the maximum. Oversaturated More solute than can dissolve (some not dissolved). Supersaturated More solute than can dissolve (all dissolved). Typically, a supersaturated solution has been heated.

26 Supersaturated Solutions Making a supersaturated solution is like putting 60,000 fans in a 50,000 seat stadium and getting them all in seats. It doesn t make sense, I know, but it works in chemistry. Supersaturated solutions aren t very stable. The solute will actually fall out of solution (precipitate) if disturbed. AKA those extra 10,000 fans stand up. Video!

27 IMPORTANT Supersaturated is not as simple as more solute than saturated. Typically, supersaturated solutions are heated to allow more solute to dissolve, then allowed to cool gently to a lower temperature. Any disturbance, however, and the solute will precipitate. If you don t heat it and simply dump in so much solute that it sits at the bottom and doesn t dissolve, that s an oversaturated solution.

28 Concentration Concentration is the relative amount of solute and solvent. High concentration means more solute, less solvent. Low concentration means less solute, more solvent. When you concentrate something, you make it a higher concentration. When you dilute something, you make it a lower concentration. Concentrations are indicated with [brackets]. Example: [H + ] means concentration of hydrogen ions.

29 Solubility Curves So, since there are so many different things that affect solubility, not to mention how much solvent you have in the first place, chemists created the solubility curve. Solubility curves are lines on a graph that represent the point at which a given quantity of a solvent is saturated at a given temperature. Often, these are graphed with grams of solute per 100 grams of water on the Y-axis.

30 Solubility Curves How much potassium bromide can be dissolved in 100 g of H 2 O at 80 C? About 100 grams.

31 Solubility Curves How much potassium bromide can be dissolved in 150 g of H 2 O at 80 C? About 150 grams.

32 Solubility Curve Saturation Before we practice, remember: On the line is saturated. Above is oversaturated. Below is unsaturated. Oversaturated Saturated Unsaturated

33 Practice Solubility Curves Packet Page 1 (hints on next slides) Page 2 (#10 is difficult) Page 4 (#3 A and E are difficult)

34 Solubility Curves Packet #2 How many grams of potassium chloride can be dissolved in 200 g of water at 80 C? Let s look at the graph at 80 C, the curve for KCl corresponds to 50 g on the Y-axis. What does that mean? That means you can fit 50 g of solute (KCl) in 100 g water. So you can fit 100 g KCl in 200 g H 2 O. 200 g H 2 O 50 g KCl 100 g H 2 O = 100 g KCl

35 Solubility Curves Packet #5 At 30 C, 90 g of sodium nitrate is dissolved in 100 g of water. Is this solution saturated, unsaturated, or supersaturated? Let s look at the graph at 30 C, the curve for NaNO 3 corresponds to about 92 g per 100 g H 2 O on the Y-axis. Since we only added 90 g NaNO 3 to 100 g of water, this solution is unsaturated.

36 Solubility Curves Packet #6 A saturated solution of potassium chlorate is formed from one hundred grams of water. If the saturated solution is cooled from 80 C to 50 C, how many grams of precipitate are formed? Let s look at the graph at 80 C, the curve for KClO 3 corresponds to 40 g on the Y-axis. At 50 C, the curve corresponds to 20 g on the Y-axis. That means that as the solution cools, 20 g of KClO 3 precipitate are formed. 40 g 20 g = 20 g

37 Electrolytes An electrolyte is a substance whose aqueous solution conducts an electric current. Usually an ionic compound. A non-electrolyte is one that, well, doesn t.

38 Electrolyte or Non? Pure Water No, since H 2 O is a molecular compound. Tap Water Yes, but only because there are dissolved solutes in there (fluoride ions and other stuff). Sugar solution No, sugar is a molecular compound and doesn t dissociate. Sodium chloride solution Yes, NaCl is an ionic compound.

39 Electrolyte or Non? Hydrochloric acid solution Yes, it s a covalently-bonded compound but one that dissociates in water (more in two lessons). Lactic acid solution Yes, though weak. Ethanol solution No, it s not ionic. Solid sodium chloride Nope, electrolytes must be aqueous.

40 Suspensions and Colloids Two things that aren t solutions are: Suspensions: Particles are so large they will settle if not constantly stirred. Colloids: Particles are between suspension and solution size.

41 Tyndall Effect There s one weird property that allows for detecting a solution versus a suspension, and that s called the Tyndall Effect. If light is shined into a colloid/solution, colloids will scatter light, solutions will not. Solution Colloid Which is the solution?

42 Types of Colloids Example Dispersing Medium Dispersed Substance Colloid Type Fog, aerosol sprays Gas Liquid Aerosol Smoke, airborne germs Whipped cream, soap suds Gas Solid Aerosol Liquid Gas Foam Milk, mayonnaise Liquid Liquid Emulsion Paint, clays, gelatin Liquid Solid Solid Marshmallow, Styrofoam Solid Gas Solid Foam Butter, cheese Solid Liquid Solid Emulsion Ruby, glass Solid Solid Solid

43 Aside: The Fog of Colloid? You may have noticed that fog is a colloid. That means it scatters light. That s why when you re driving you shouldn t use your high beams (the brights ) as it will simply scatter more light and blind you or an oncoming driver. You wouldn t want to blind them right into your car.

44 Closure (1 of 4) Which is the best drawing for magnesium chloride in water? Magnesium Chlorine A B C D

45 Closure (2 of 4) How would that drawing have changed if it were magnesium oxide instead of magnesium chloride? a) The ion ratio would be the same as for MgCl 2. b) The ratio would be 1 Mg : 1 O. c) The ratio would be 2 Mg : 1 O. d) You would need different colors.

46 Closure (3 of 4) Which image best shows a saturated solution? A B C D

47 Closure (4 of 4) and Homework You can dissolve approximately 135 grams of KI in 100 grams of water at 10 C. If you fill a beaker with 100 grams of H 2 O and dissolve 134 grams of KI in it, how would you describe the solution? (assume it is at 10 C) Unsaturated. Tonight s homework: Finish the solubility curve worksheets. These will be checked at the beginning of class tomorrow.