CHAPTER 16 Solutions 16.1 Solution Formation Solutions can be either solids, liquids, or gases Solutions are homogeneous mixtures that are grouped according to physical state. (mixtures = no bonding) The nature of solvent and solute affects whether a substance will dissolve. Mar 26 11:47 AM Mar 26 11:47 AM Solvation = Dissolving Solvation is a surface phenomenon! Water dissolving NaCl. Water can only dissolve ions they touch Solute is dissolved Na + O Cl Na + Na + Cl Cl Na + Na + Cl Cl Na + Cl H H Mar 26 11:45 AM Mar 26 11:47 AM What determines how fast solutes dissolve? 1. amount of surface area 2. force of collisions with solvent 3. frequency of collisions with solvent 3 factors increase the rate of solvation: Agitation Temperature Particle Size These increase the collisions between solvent and solute Mar 26 11:47 AM Mar 26 11:47 AM 1
Agitation solvent is brought into contact with the solute more frequently with greater force. Shaking and stirring Temperature increases the force and frequency of solvent and solute Heat makes particles move faster Mar 26 11:48 AM Mar 26 11:49 AM Particle Size Powder has less surface, but very little volume Crystals have more surface, but a larege volume Powder has a greater surface area:volume ratio Solubility A saturated solution contains the maximum amount of solute for a given amount of solvent at a constant temperature. How much solute can a given amount of solvent hold... Mar 26 11:49 AM Mar 26 11:50 AM The solubility of a substance is the amount of substance that dissolves in a given quantity of a solvent at a given temperature to produce a saturated solution. Solubility changes if temperature changes. Apr 10 11:58 AM Mar 26 11:51 AM 2
Solubility Maximum amount of solute dissolved in 100 ml of solvent at a given temperature. Solubility Example 36.2 grams of NaCl in 100 grams of water @ 25 C. 34.2 g KCl 37.2 g KCl 40.1 g KCl 36.2 g NaCl 100 g H 2O at 25 C 20 C 30 C 40 C Mar 26 7:54 AM Mar 26 11:51 AM Important!!!!! 100 g = 100 ml Dynamic Equilibrium in a saturated solution. Solubility is generally expressed in grams of solute per 100 g of solvent. grams of solute at 20 C 100 g H2O Rate of Solvation = Rate of Desolvation Dissolving = Crystallization Particles move from solid to solvated state and back to solid again. A solution that contains less solute than a saturated solution is unsaturated Mar 26 11:52 AM Mar 26 11:53 AM Factors Affecting Solubility The solubility of liquids and solids increases with temperature The solubility of gases decreases with temperature 20 C Supersaturated Solutions 30 C 20 C 100 g NaCl Saturated 125 g NaCl Saturated 125 g NaCl Supersaturated Mar 26 11:53 AM Apr 9 7:37 AM 3
Removing solute from a supersaturated solution Crystallization in a supersaturated solution can be initiated if a very small crystal, called a seed crystal, of the solute is added Seed Crystal Video http://www.metacafe.com/watch/404010/rapid_crystallization_supersaturated_solution_demo/ Supersaturated Solution http://www.teachertube.com/viewvideo.php?video_id=86887 Mar 26 2:43 PM Apr 11 12:06 PM Liquids and Liquids Two liquids are said to be miscible if they dissolve in each other. (Polar & Polar or Nonpolar & Nonpolar) Liquids that are insoluble in one another are immiscible (NonPolar & Polar) The solubilities of gases are greater in cold water than in hot water Hotter water has higher vapor pressure, which allows gas to escape The components of air become less soluble as the temperature of the water rises. Mar 26 2:44 PM Mar 26 2:44 PM Thermal Pollution Industrial plants remove cold water and dump hot water back into the lake. This increases the temperature of the water, decreasing the dissolved O 2. Warm vs. Cold Soda can Henry s Law at a given temperature the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the liquid In other words, as the pressure of the gas above the liquid increases, the solubility of the gas increases S 1 S 2 P 1 P 2 Mar 26 2:44 PM Mar 26 2:59 PM 4
Example A gas has a solubility in water of 10.5 g/l at 15 C and 455 kpa of pressure. What is the solubility of the gas in water at 15 C and 615 kpa of pressure? 10.5 g/l 455 kpa 615 kpa 16.2 Concentrations of Solutions The concentration of a solution is a measure of the amount of solute that is dissolved in a given quantity of solvent. The unit for concentration is Molarity Concentration= amount of solute dissolved in a solvent. Dilute solutions have a small amount of solute. Concentrated solutions have a large amount of solute. Molarity (M) Moles of solute Liters of solution v n M n = moles v = volume M = molarity Mar 26 2:47 PM Mar 26 2:46 PM Key Concept! Is it possible to have 1 ml of 5.0M solution? 200 ml 2.0 M HCl 300 ml 2.0 M HCl If you poured 5.0M HCl into 3 different size containers, would the molarity in each container change? Are the substances different because they have different amounts in them? Does each flask have 2 moles of HCl dissolved? Mar 26 2:46 PM Apr 11 12:48 PM 5
Which flask has more moles of HCl dissolved? 200 ml 300 ml 2.0 M HCl 2.0 M HCl Example #1 What is the molarity of NaCl in sea water if it contains 4 moles of NaCl per 500 ml? 4 moles 2.0 moles 2.0 moles 0.5 L 1.0 L 0.2 L 1.0 L 0.3 L Apr 11 12:48 PM Mar 28 10:55 AM Example #2 What is the molarity of NaCl in sea water if it contains 20.5 g of NaCl per 750 ml? Example #3 How many moles of NaCl are dissolved in 100.0 ml of a 5.0M solution? 20.5 g NaCl.350 moles 0.75 L 1 mole 58.5 g 5.0 moles n = M x V = x 0.1 L = 1.0 L Mar 28 10:54 AM Mar 28 10:54 AM Example #4 How many grams of NaCl are contained in 450.0 ml of a 2.0M solution? Example #5 What volume (in ml) of 12.0M HCl is needed to have 5.00 moles of HCl? 2.0 moles n = M x V = x 0.45 L = 1.0 L 0.9 moles 58.5 g 1 mole n V = = M 5.0 moles 12.0 moles 1.0 L Mar 28 10:53 AM Mar 28 10:52 AM 6
Making Dilutions Adding water to an existing solution. Increasing the volume to lower Molarity 0.5 L 8.0 M HCl 2.0 M HCl The number of moles of solute does not change when a solution is diluted How much water must be added to make 2.0 M HCl? Mar 26 2:59 PM Apr 11 12:48 PM Dilution Formula Example #6 Concentrated H 2SO 4 is 18.0M, what volume is needed to make 4.50 L of 1.00M solution? How much water must be added to it? M 1 x V 1 = M 2 x V 2 Volume can be in ml or L as long as they are the same on both sides. The number of moles are the same on both sides. Stock solution is M 1 or the solution of greater concentration. M1: 18.0 M V1: M2: 1.0 M V2: 4.50 L The number of moles in each solution are the same!!! The difference between the 2 solutions is the amount of water you add. Mar 26 2:49 PM Mar 28 10:51 AM Mixing Solutions Calculate the final concentration if 5.0 L of 2.0M NaCl and 3.0 L of 4.0M NaCl are mixed? Step 1: Find total volume (Same units) Example #7 Calculate the final concentration if 4.00 L of 5.0M NaCl and 6.00 L of 2.0M NaCl are mixed? Step 2: Find total moles Step 3: Calculate molarity Mar 26 2:48 PM Mar 26 2:48 PM 7
Example #8 Calculate the final concentration if you added 5.0 L of water to the mixture in example #7. Percent Solutions Determining the percentage of solute in any given solution. Not the same as molarity because every solute has a different molar mass Important when comparing different solutions!! comparing 1.0 M HCI vs. 1.0 M H 2SO 4 Mar 26 2:49 PM Mar 26 2:50 PM How do I compare 1.0-M HCl with 2.0-M HCl? How do I compare 1.0-M HCl with 1.0-M H 2SO 4? Percent Solution g % V mass of solute ml of solution g = mass v = volume (ml) % = % solution Apr 15 7:47 AM Mar 26 2:46 PM Percent Solutions % m/v is always expressed in g/ml. If you are given molarity, then you can always determine the mass per 1.0 L. % m/v of 2.0 M NaCl Molar Mass NaCl = 58.5 % m/v of 2.0 M MgCl2 Molar Mass MgCl2 = 95 100 g of NaCl is dissolved in 2.00 L of solution. When there is a Liquid and a Solid. %(mass/vol.) = mass of solute (g) volume of solution (ml) Must be in g/ml! Mar 26 2:51 PM 8
When both solvent and solute are Liquids! % by vol. = vol. Solute X 100% vol. Solution Example #9 What is the %(v/v) of ethanol when 75 ml is diluted to a volume of 250 ml with water? Answer in: %(volume/volume) Example #10 What is the percent (mass/volume) of NaCl in a 200.0 ml solution containing 14 grams of NaCl? Example #11 How many grams of glucose would you need to prepare 2.0 L of 2% (m/v) glucose solution? Example #12 What is the %(v/v) of ethanol when 50 ml is diluted with 150 ml of water? 16.3 Colligative Properties of Solutions Colligative properties depend on the number of particles dissolved in a given mass of solvent 1. Freezing Point 2. Vapor Pressure 3. Boiling Point 9
3 Colligative Properties Factors that change when Particles dissolve For the same substance, we compare Molarity! For different substances, we compare the number particles dissolved for 1 mole of a substance. Dissolved Particles The more dissolved particles, the greater change in the colligative properties. Glucose: 1 molecule (1 particle) NaCl: 2 ions (2 particles) CaCl 2: 3 ions (3 particles) C6H12O6 NaCl CaCl2 Which will have the most dissolved particles in 1 L of solution? 3.0 M NaCl 1.8 M AlCl 3 3.0 moles x (6.02 x 10 23 ) 1.8 moles x (6.02 x 10 23 ) x 2 particles (Na + & Cl ) x 4 particles (Al 3+, Cl, Cl, Cl ) 1 mole 1 mole 1 mole 6.02 x 10 23 molecules 6.02 x 10 23 form units 6.02 x 1023 form units x 1 dissolved particle x 2 dissolved particles x 3 dissolved particles Mar 28 10:29 AM Apr 16 8:16 AM 3 Colligative Properties 1. vapor pressure lowering 2. boiling point elevation 3. freezing point depression Water Vapor Pressure 4.24 kpa Boiling Point = 100ºC Vapor pressure of the water must be 101.3 kpa to boil. 30º Celsius Salt Water Vapor Pressure 3.69 kpa Boiling Point = 102 ºC Vapor pressure of the water must be 101.3 kpa to boil. Mar 26 2:56 PM 10
Vapor Pressure Substances with high vapor pressure evaporate quickly. Substances with low vapor pressure evaporate slowly. Vapor Pressure Mercury 23 C = 0.000219 kpa Water 20 C = 2.3 kpa Water 90 C = 69.9 kpa Acetone 22.7 C = 26.7 kpa * Notice that hot water has a higher vapor pressure than cold water. Mar 26 3:05 PM Mar 26 3:05 PM Freezing Point The more dissolved particles, the colder it needs to be for a solvent to freeze. Antifreeze in cars. Why is CaCl 2 better to melt ice than NaCl? Boiling Point Vapor pressure of liquid equals vapor pressure of the air. The higher percentage of solute, the higher the boiling point. As water evaporates, the concentration of solute rises. Mar 28 10:27 AM Mar 26 2:54 PM Boiling Point Salt water has lower vapor pressure than fresh water. Salt water will boil at 102 C at a location with an atmospheric pressure of 1 atm. Pasta will cook faster at 102 C than 100 C. High Altitude Directions At high altitudes, the atmospheric pressure is less than 101.3 kpa. (Denver = 84.0 kpa) In Denver, water boils at 94 C so water reaches a boil faster but food cooks slower than at sea level. Mar 26 2:54 PM Mar 26 2:53 PM 11
This is the solubility of salt in water at 25 C. 36.2 g NaCl 100 g H2O at 25 C What happens to the solubility if temperature increases or decreases? How many grams of NaCl can be dissolved in 300 grams of water? Solubility Curves A visual representation of how much of a substance can be dissolved in 100 grams of water, based on temperature In general, the solubility of a substance will increase with temperature Apr 17 12:52 PM Apr 12 9:07 PM 12