Presentation on theme: "Aqueous Solutions Solution: Homogeneous mixture; solid liquid, or gas Soluble: Capable of being dissolved Solute: Substance that is dissolved, present."— Presentation transcript:
Aqueous Solutions Solution: Homogeneous mixture; solid liquid, or gas Soluble: Capable of being dissolved Solute: Substance that is dissolved, present in lesser amounts Solvent: Dissolving medium; present in greater amounts
Types of Mixtures Solutions: particles < 1nm; cannot be seen; no scattering of light (salt water) Suspensions: particles > 100 nm; settle to the bottom (sand & water) Colloids: particles 1-100 nm; suspended throughout the medium; exhibit Tyndall effect Tyndall Effect: Scattering of light so that the beam of light can be seen like headlights in fog; seen in colloids
Hydration Hydrates: ionic compounds with water molecules in their structure specific ratios of water to compound heating can drive off the water and leave the anhydrous salt CuSO 4 5H 2 O (copper (II) sulfate pentahydrate
Hydrates Effloresce: If a hydrate has a vapor pressure greater than water vapor, than it will lose its water of hydration Hygroscopic: Hydrated salts that have a low vapor pressure remove water from moist air to form higher hydrates Used as drying agents or desiccants Silica gel Deliquescent: remove sufficient water from the air to dissolve completely and form solutions
Types of Solutes Electrolytes: Conduct an electric current when dissolved All Ionic Compounds Polar Molecules when dissolved in water (ionized) Weak Electrolyte: fraction of the solute exist as ions Strong Electrolyte: almost all of the solute exist as separate ions Nonelectrolytes: Do not conduct an electric current when dissolved Covalent compounds Polar Molecules in a pure state
Dissociation Sodium Chloride: NaCl (s) Na +1 (aq) + Cl -1 (aq) Magnesium nitrate: Mg(NO 3 ) 2(s) Mg +2 (aq) + 2NO 3 -1 (aq) Dissociation is the “breaking apart” of ions as an ionic compound dissolves in water.
Factors Affecting Rate of Dissolution Surface Area: Greater surface area (smaller particle size) = faster dissolution Agitation: stirring = faster dissolution Heating: generally means faster dissolution
Like Dissolves Like The polarity of water molecules plays an important role in the formation of solutions of ionic compounds and other polar compounds in water. The positive ends of the water molecules attract negative ions (dipoles) and the negative ends of water molecules attract positive ions (dipoles), pulling them from the surface of the crystal.
Miscible/Immiscible Nonpolar molecules do not attract ions of crystalline solids strongly enough to overcome forces holding them together and therefore cannot dissolve polar or ionic compounds. Miscible : Two liquids mix; soluble in one another (Ex: Ethanol & Water) Immiscible: Do not mix; insoluble in one another (Ex: Oil & Water)
Relative Concentration Concentration: a measure of the amount of solute that is dissolved in a given quantity of solvent Unsaturated: Less than the theoretical amount of solute is dissolved; there is room to dissolve more Saturated: The maximum amount of solute that can be dissolved at a certain temperature has been reached Supersaturated: More than the theoretical amount of solute is dissolved in solvent at a given temperature
Solubility/Recrystallization Solubility: A measure of how much solute can dissolve in a given amount of solvent at a specific temperature Dilute Solution: The amount of solute dissolved is small in relation to the amount of solvent present. Concentrated Solution: The amount of solute dissolved is large in relation to the amount of solute present. At the same time substances are breaking apart, there are also particles attracted to each other and remaining together – RECRYSTALLIZATION Solution Equilibrium: Rate of dissolution = Rate of recrystallization
Saturation Curve °C 0 0 20 40 60 80 140 100 120 160 180 g / 100 mL 102040608010030507090 SupersaturatedSaturated Unsaturated Changing the temperature of a solvent can have an effect on solubility!!
°C 0 0 20 40 60 80 140 100 120 160 180 g / 100 mL 102040608010030507090 A B Which solute has the greatest solubility at 20°C? Which solute has the greatest solubility at 50°C?
Gas Solubility Henry’s Law: S 1 /P 1 = S 2 / P 2 The solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the surface of the liquid. Open a can of pop!! Carbon dioxide is forced into solution of flavored water at 5-10 atm. Effervescence: The FIZZING that happens when you open the pop; the release of gas from a solution
Concentrations of Solutions Molarity = M = mol solute L solution Molality = m = mol solute kg solvent Percent by mass [% m/v] = mass solute x 100 mL solution Percent by volume [% v/v] = vol solute x 100 vol solution
Dilutions You have the following stock solutions available: 2.0 M NaCl; 4.0 M KNO 3 ; 0.5M MgSO 4 Calculate the stock volumes you must dilute to make the following solutions using M 1 V 1 = M 2 V 2 500 mL of a 0.50 M sodium chloride solution 2.0 L of a.20 M magnesium sulfate solution
Examples/Molarity What is the molarity of a solution that contains 212.5 g of sodium nitrate (NaNO 3 ) in 3.0 liters of solution? What mass of sucrose, C 12 H 22 O 11 is needed to make 300 mL of a 0.50 M solution?
Examples/Molality Calculate the molality of a solution made by dissolving 45.0 g of dextrose, C 6 H 12 O 6 in 500.0 g of water. What is the mass of water required to prepare a 1.00 molal solution containing 10.0 g of NaOH? Calculate the molality of a solution prepared by dissolving 6.3 moles of KCN in 633 g of water
Percent Problems % m/v: Calculate the number of grams of solute required to make the following solutions: 2.5 L of saline solution– 90% NaCl 50 mL of 4% magnesium chloride %v/v: 10 mL acetic acid is diluted with water to a total solution volume of 200 mL. What is the % v/v of acetic acid? 25mL of ethanol and 75 mL of water are mixed. What is the % v/v of the solution?
Colligative Properties Depend only on the number of particles dissolved in a given mass of solvent Vapor Pressure Lowering Boiling Point Elevation Freezing Point Depression
Vapor Pressure Lowering A nonvolatile solute has a lower vapor pressure than a pure solvent As solutes are added the equilibrium is disrupted and solvent particles form shells around the solute particles. This reduces the number of free solvent particles able to escape the liquid. Equilibrium is eventually re- established at a lower vapor pressure Decrease in VP is proportional to the number of particles the solute makes in solution Solute that disassociate in large #s have a greater effect of VP
Boiling Point Elevation The difference in temperature between the boiling point of a solution & that of a pure solvent Boiling points of solutions are higher than that of pure solvents The presence of solutes increase BP Additional KE is required for particles to overcome the attractive forces that keep them in liquid Boiling Point elevation (ΔT b ): directly proportional to the molal concentration ΔT b α m ΔT b = K b x m
Freezing Point Depression The Difference in temperature between the freezing point of a solution and that of the pure solvent More KE must be withdrawn from a solution than from a pure solution to solidify Freezing Point Depression (Δ T f ): directly proportional to the molal concentration Δ T f = K f x m