Chapter 16 “Solutions & Mixtures” Stephen L. Cotton 1

Classification of Matter Review 2

Heterogeneous Mixtures: Suspensions and Colloids Suspensions and colloids LOOK like solutions BUT are NOT solutions. Suspensions: The particles are so large that they settle out of the solvent if not constantly stirred. EX.Chocolate in milk, OJ, Schuylkill water Colloids: The particle is intermediate in size between those of a suspension and those of a solution, they cloud the water and do NOT settle out. 3

Smog – A Gaseous Suspension 4

The Tyndall Effect Colloids scatter light, making a beam visible. Solutions do not scatter light. Which glass contains a colloid? solution colloid 5

Solute A solute is the dissolved substance in a solution.(usually less) EX: A solvent is the dissolving medium in a solution. (usually more) EX: Solvent Salt in salt water Sugar & Carbon dioxide in soda drinks Oxygen in air Water in salt water Water in soda Iron in steel Carbon in steel Nitrogen in air 6

Liquids l Miscible means that two liquids can dissolve in each other – water and antifreeze – water and ethanol l Partially miscible- slightly – water and ether l Immiscible means they can’t – oil and vinegar 7

When all else fails, draw the lewis structure, then the molecular geometry, then dipole vectors! l Not everything dissolves in everything! The simple guideline is: “ Like Dissolves Like ” l This means a non-polar solvent will usually only dissolve a non-polar solute. l Polar and ionic solutes usually dissolve best in polar solvents Solvent VS Solute: ‘Like dissolves Like’ Some guidelines: Organic compounds/hydrocarbons are often nonpolar due to carbon’s 4 bonds. Ex, methane, butane, glucose Ionic compounds are VERY polar: salts, acids, water (even though it’s covalent) 8 Like dissolves like WS

An electrolyte is: A substance whose aqueous solution conducts an electric current. Most Ionic compounds (aq) since they form ions and dissolve in water. A nonelectrolyte is: A substance whose aqueous solution does not conduct an electric current. Most covalent compounds (aq). Electricity and Electrolytes ? 9 Electricity is a flow of electrons. Electrons ‘flows’ if they are free to transfer. Electrons ‘flows’ if they are free to transfer. Ions is solution and metals allow easy transfer.

Ammeter: measures the flow of electrons (current) through the circuit. (We used this in our ionic vs. covalent lab) If the ammeter measures a current, and the bulb glows, then the solution conducts and is an electrolyte. If the ammeter fails to measure a current, and the bulb does not glow, the solution is nonconducting and is a non-electrolyte. Electrolytes vs. Nonelectrolytes 10

1.Pure water 2.Tap water 3.Sugar solution (covalent) 4.NaCl solution 5.HCl acid solution 6.Lactic acid solution 7.Ethyl alcohol solution (covalent) 8.Pure NaCl crystals (not in water) 1.Pure water 2.Tap water 3.Sugar solution (covalent) 4.NaCl solution 5.HCl acid solution 6.Lactic acid solution 7.Ethyl alcohol solution (covalent) 8.Pure NaCl crystals (not in water) Try to classify the following substances as electrolytes or nonelectrolytes… ELECTROLYTES:NONELECTROLYTES: Page 1 Nature of solutions 11

Solution Formation l The “nature” (polarity, or composition) of the solute and the solvent will determine… 1. Whether a substance will dissolve 2. How much will dissolve l Factors determining rate of dissolving stirring (agitation) 2. surface area the dissolving particles 3. Temperature 4. For gases : air pressure 12

Making Solutions l In order to dissolve, the solvent molecules must come in contact with the solute. 1. Stirring (agitation) moves fresh solvent into contact with the solute. 2. Smaller pieces increase the amount of surface area of the solute. - think of how fast a breath mint dissolves after you chew it vs. before 13

Temperature and Solutions 3. Higher temperature makes the molecules of the solvent move faster and contact the solute harder and more often and Speeds up dissolving. l Higher Temperature ALSO Usually increases the amount that will dissolve; l *an exception is gases, which dissolve better in cold temps!. l Think of boiling water losing gas, bubbling! 14

How Much? l Solubility- is the maximum amount of substance that will dissolve at a specific temperature. The units for solubility are: grams of solute/100 grams solvent 1) Saturated solution- Contains the maximum amount of solute dissolved. NaCl = 36.0 g/100 mL water 2) Unsaturated solution- Can still dissolve more solute (for example 28.0 grams of NaCl/100 mL) 3) Supersaturated- solution that is holding (or dissolving) more than it theoretically can; cooling it, and/or a “seed crystal” for nucleation will make it precipitate; 15 Super saturated Cool/add crystal

Solubility Chart Reading l Solubility curves l Solubility curves are used to show how the solubility of a substance changes with temperature. temperature l The temperature of the solution affects how much of the solute is dissolved by the solvent. l Experimentally dozens of salts (ionic compounds) & gases have been determined for you. does not always l Increasing the temperature does not always increase the solubility. 16

17 To Read the Graph: Find the line for the your substance Find your temp. The amount that dissolves is on the y-axis.

How much KNO 3 dissolves in 100g H 2 O at 50 o C? 1. Find the line (red) 2. Find the temperature and follow up to the line. (green) 3. Read across to the y- axis and this is the answer. (blue) g on your chart 18

To do calculations using a Solubility Chart: l This point on the line 40C is a saturated solution. l This point above the line 40C is supersaturated. This point below the line 40C is unsaturated. l To calculate how much extra has been dissolved (supersat 120 g) subtract the line value at that temperature. Here? l To calculate how much more can be dissolved,(unsat 90g) subtract the value given from the line value at that temperature. Here? 19 20g 10g

Example 1: l How much less KCl is dissolved at 20 o C than at 60 o C in 100g H 2 O? l Read the line value: 34g at 20 o C l Subtract it from the given value of chart at 60 o C: 44g 44g – 34g = 10 g 20

Example 2: lHlHow much more KNO3 is required to saturate the solution if 20g are dissolved at 40 o C? lRlRead the line value: 44g lSlSubtract the given value: 44g – 20g = 14 g more can be added! 21

Your turn! 1. 1 How much NaCl will dissolve in 100g H 2 O at 35 o C? 2. 2 How much NH 4 Cl will dissolve at 50 o C? 3. 3 What is the solubility of NH 3 at 25 o C? 4. 4 What is the solubility of potassium chlorate in 200 grams of water at 35 o C? 5. 5 If 85 grams of potassium iodide are mixed with 100 grams of water at 0ºC, how much more must be added to saturate the solution? g

6.What minimum temperature needed to dissolve 80 grams of sodium nitrate in 100 grams of water? 7. What minimum temperature needed to dissolve 42 grams of potassium chloride in 100 grams of water? 8. What minimum temperature needed to dissolve 20 grams of KClO 3 in 100 grams of water? 9.If 95 grams of potassium nitrate are mixed with 100 grams of water at 45ºC, how much will not dissolve? 10.If 85 grams of potassium iodide are mixed with 100 grams of water at 0 º C, how much more must be added to saturate the solution? 23

11. If 55 grams of potassium chlorate are mixed with 100 grams of water at 55ºC, how much will not dissolve? 12. If 125 grams of potassium iodide are mixed with 100 grams of water at 10ºC, how much more must be added to saturate the solution? 13. What is the solubility of potassium chlorate in 50 grams of water at 35ºC? Solubility Activity W/S 24

Measuring Concentration is...M&m l a measure of the amount of solute dissolved in a given quantity of solvent l A concentrated solution has a large amount of solute l A dilute solution has a small amount of solute – These are qualitative descriptions l But, there are ways to express solution concentration quantitatively (NUMBERS!) 25

Concentrated vs. Dilute Lots of solute, in a small amount of solvent. Small amount of solute in a large amount of solvent. Notice how dark the solutions appears. Notice how light the solution appears. CONCENTRATEDDILUTE 26

Molarity: a unit of concentration l Molarity = moles of solute liters of solution Abbreviated with a capital M, such as 6.0 M This is the most widely used concentration unit used in chemistry. 27

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Dilution Adding water to a solution will reduce the number of moles of solute per unit volume but the overall number of moles remains the same! Think of taking an aspirin with a small glass of water vs. a large glass of water You still have one aspirin in your body, regardless of the amount of water you drank, but a larger amount of water makes it more diluted. 29

Dilution l The number of moles of solute in solution doesn’t change if you add more solvent! l The # moles before = the # moles after l Formula for dilution: M 1 x V 1 = M 2 x V 2 l M 1 and V 1 are the starting concentration and volume; M 2 and V 2 are the final concentration and volume. l Stock solutions are pre-made solutions to known Molarity. Sample 16.4, p

Percent solutions can be expressed by a) volume or b) mass l Percent means parts per 100, so l Percent by volume: = Volume of solute x 100% Volume of solution l indicated %(v/v) l Sample Problem 16.5, page

Percent solutions l Percent by mass: = Mass of solute(g) x 100% Volume of solution (mL) l Indicated %(m/v) l More commonly used l 4.8 g of NaCl are dissolved in 82 mL of solution. What is the percent of the solution? l How many grams of salt are there in 52 mL of a 6.3 % solution? 32

Percent solutions l Another way to do mass percentage is as mass/mass: l Percent by mass: = Mass of solute(g) x 100% Mass of solution (g) l Indicated %(m/m) 33

Section 16.3 Colligative Properties of Solutions l OBJECTIVES: – Explain & Identify: vapor pressure, freezing point, and boiling point and how they may differ in solution from those same properties of the pure solvent. 34

Colligative Properties -These depend only on the number of dissolved particles -Not on what kind of particle -Three important colligative properties of solutions are: 1)Vapor pressure lowering 2)Boiling point elevation 3)Freezing point lowered 35

- Page 488 Glucose will only have one particle in solution for each one particle it starts with. NaCl will have two particles in solution for each one particle it starts with. CaCl 2 will have three particles in solution for each one particle it starts with. Colligative Properties Some particles in solution will IONIZE (or split), while others may not. 36

Vapor Pressure is LOWERED 1) Surface area is reduced, thus less evaporation, which is a surface property 2) The bonds between molecules keep molecules from escaping. So, in a solution, some of the solvent is busy keeping the solute dissolved. l This lowers the vapor pressure l Electrolytes form ions when they are dissolved, making more pieces. NaCl  Na + + Cl - (this = 2 pieces) l More pieces = a bigger effect 37

Boiling Point is ELEVATED l The vapor pressure determines the boiling point. (Boiling is defined as when the vapor pressure of liquid = vapor pressure of the atmosphere). l Lower vapor pressure means you need a higher temperature to get it to equal atmospheric pressure l Salt water boils above 100ºC l The number of dissolved particles determines how much, as well as the solvent itself. 38

Freezing Point is LOWERED l Solids form when molecules make an orderly pattern called “crystals” l The solute molecules break up the orderly pattern. – Makes the freezing point lower. – Salt water freezes below 0ºC – Home-made ice cream with rock salt? l How much lower depends on the amount of solute dissolved. 39

- Page 494 The addition of a solute would allow a LONGER temperature range, since freezing point is lowered and boiling point is elevated. 40

Section 16.4 Calculations Involving Colligative Properties l OBJECTIVES: – Describe & calculate how freezing point depression and boiling point elevation are related to molality. 41

Molality (abbreviated m) l a new unit for concentration l m = Moles of solute kilogram of solvent 42

Freezing Point Depression l The size of the change in freezing point is also determined by molality.  T f = K f x m x n  T f is the change in freezing point l K f is a constant determined by the solvent (Table 16.2, page 494). l n is the number of pieces it falls into when it dissolves. 43

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Boiling Point Elevation l The size of the change in boiling point is determined by the molality.  T b = K b x m x n  T b is the change in the boiling point l K b is a constant determined by the solvent (Table 16.3, page 495). l n is the number of pieces it falls into when it dissolves. l Sample Problem 16.9, page

Mole fraction l This is another way to express concentration l It is the ratio of moles of solute to total number of moles of solute plus solvent (Fig , p.522) n a n a + n b X = Sample 16.7, page