Chapter 13 – Properties of Solutions Many chemical reactions occur when the reactants are in the aqueous phase. Therefore, we need a way to quantify the amount of reactants in an aqueous solution. We quantify products and reactants in an aqueous solution by measuring the concentration.

Properties of Solutions Factors that affect the solubility of a compound. The process of a solute dissolving in a solvent is called solvation.

Solutions Factors that affect the solubility of a compound. Agitation – When a solute is agitated, or stirred, more water molecules can come in contact with the solute, thus speeding up solvation.

Solutions Factors that affect the solubility of a compound. Heat – An increase in the temperature causes the water molecules to move faster. So there will be more contacts between the water molecules and the solute, increasing the process of solvation.

Solutions Factors that affect the solubility of a compound. Increasing the surface area of the solute – breaking up or crushing the solute increase the surface area of the solute. As a result, more water molecules can contact the solute making is solvate faster.

Solutions Heats of Solution (ΔH soln ). Solvation is a three part process – 1.Breaking up of the solute. 2.Breaking up the solvent. 3.Formation of new intermolecular (or ion-dipole) forces between te solute and the solvent.

Solutions Heats of Solution (ΔH soln ). Solvation is a three part process –

Solutions Heats of Solution (ΔH soln ). Solvation is a three part process – 1.If the sum of the three parts is positive (+ΔH soln ), then the process is endothermic.

Solutions Heats of Solution (ΔH soln ). Solvation is a three part process – 1.If the sum of the three parts is negative (-ΔH soln ), then the process is exothermic.

Solutions Calculating Solution Concentration Expressing Concentrations Molarity (M) Molality (m) % by volume or mass Mole fraction

Solutions Calculating Solution Concentration Molarity (M): Molarity is defined as the number of moles of solute per liter of solution. Molarity (M) = moles of solute 1.0 L of H 2 O

Solutions What is the molarity of a solution that was made by dissolving 5.0 grams of NaCl in 500. mL of water.

Solutions How many moles of calcium chloride would be contained in 30.0 mL of a 1.5 M calcium chloride solution?

Solutions Calculating Solution Concentration Molality (m): Molality (m) is defined as the number of moles of solute per kilogram of solvent. Molality (m) = moles of solute kg of solvent

Solutions Calculating Solution Concentration Calculate the molality of solution that was made by dissolving 3.0 grams of ammonium chloride in mL of water.

Solutions Calculating Solution Concentration How many grams of water must 60.0 grams of sodium sulfate be dissolved in to make a 0.80 m sodium sulfate solution?

Solutions Calculating Solution Concentration % by volume: It is what the name says it is, the percentage of the whole solution that is the solute. % by volume = volume of solute volume of solution

Solutions Calculating Solution Concentration % by mass: The percentage of the mass of the solution that is the solute. % by mass = mass of solute mass of solution

Solutions Calculating Solution Concentration Vinegar is made by adding 3.0 grams of concentrated acetic acid to 97.0 grams of water. What is the % by mass of acetic acid in vinegar?

Solutions Calculating Solution Concentration Most red wines are 12.0 % ethyl alcohol by volume. How many milliters of ethyl alcohol are contained in a 1.0 L bottle of red wine?

Solutions Calculating Solution Concentration Mole Fractions – Is defined as the number of moles of solute per moles of solvent plus the number of moles of solute. Mole fraction = moles solute moles solvent + moles solute

Solutions Calculating Solution Concentration Calculate the mole fraction of a solution that is made by dissolving 15.0 grams of sucrose, C 12 H 22 O 11, in mL of water.

Solutions Colligative Properties A physical property of a substance that varies depending on the number of solute particles dissolved in the solution.

Solutions Colligative Properties Solute particles affect; The vapor pressure of a solution The boiling point of a solution The freezing point of a solution The rate of diffusion of water particles (osmosis)

Solutions Colligative Properties Vapor pressure – Defined as the pressure of the vapor of the solvent when dynamic equilibrium is reached between the rate of evaporation and the rate of condensation.

Solutions Colligative Properties Vapor pressure of water at 25°C is 3.17 kPa. If we let pure water reach dynamic equilibrium with its vapor, it will exert a pressure of 3.17 kPa on its container.

Solutions Colligative Properties

Solutions Colligative Properties Vapor Pressure If solute particles are added to pure water, they will interfere with the evaporation of the water molecules. The interference of the water particles by the solute particles causes the rate of evaporation to slow down. Solute particles cause the vapor pressure to decrease.

Solutions Colligative Properties Calculating Vapor Pressure P A = X A (P° A ) The vapor pressure of a solution is equal to the product of the mole fraction of the solvent and vapor pressue of the pure solvent (at a specified temperature).

Solutions Colligative Properties Calculating Vapor Pressure P A = X A (P° A ) Calculate the vapor pressure of a 1.50 M MgCl 2 solution.

Solutions Colligative Properties Boiling Point Temperature The formal definition of ‘boiling point’ is the temperature at which the vapor pressure of a liquid becomes equal to the atmospheric pressure around the liquid.

Solutions Colligative Properties Boiling Point Temperature If the vapor pressure is lowered by the addition of solute particles, then it must take more energy for the water molecules to evaporate. As a result, the aqueous solution will boil at a higher temperature.

Solutions Colligative Properties Boiling Point Elevation The change in the boiling temperature (ΔT b ) is equal to the boiling point constant for water (k b ) times the molality of the solution (m) times the Van Hoff Factor (i) *. ΔT b = k b. m. i * i = the # of particles the Solid produces when it dissolves.

Solutions Colligative Properties Calculate the boiling point of a solution that is made by dissolving 40.0 grams of NaCl in mL of water. ΔT b = k b. m. I (k b = °C/m)

Solutions Colligative Properties Freezing Point Depression Solute particles can interfere with the arrangement of water molecules when they freeze. As a result, a colder temperature must be reached before water molecules can arrange themselves to form the solid (ice).

Solutions Colligative Properties Freezing Point Depression

Solutions Colligative Properties Freezing Point Depression The change in the freezing point of water (ΔT f ) is equal equal to the freezing point constant of water(k f ) times the molality of the solution (m) times the Van Hoff Factor (i). ΔT f = k f. m. i (k f = 1.86 °C/m)

Solutions Colligative Properties Freezing Point Depression Calculate the freezing point of an aqueous solution consisting of 10.0 grams of CaCl 2 dissolved in 1.0 L of water. ΔT f = k f. m. i (k f = 1.86 °C/m)

Solutions Colligative Properties Osmotic Pressure Osmosis is the movement of water molecules through a semi-permeable membrane. Osmotic pressure is a measure of the pressure that the water molecules expert on the semi-permeable membrane. Solute particles can cause water molecules to migrate to area where the water molecules are less concentrated.

Solutions Colligative Properties Osmotic Pressure Now you know why salt and snails don’t mix.

Solutions Colligative Properties Calculating Osmotic Pressure Π = MRT Osmotic pressure (Π) is equal to the product of the molar mass (M), the gas law constant (R), and the temperature.

Solutions Colligative Properties Calculating Osmotic Pressure Π = MRT What is the osmotic pressure at 20.0°C of a M solution of sucrose (C 12 H 22 O 11 )?

Solutions Colloids Solutions that have small particles suspended in solvent-like medium. The particles are so small (5 – 1000 nm)that they won’t settle out of the solution under the influence of gravity.

Solutions Colloids Some colloids consist of particles so small that they are microscopic. But the particles can scatter and reflect light. This is called the Tyndall Effect.

Solutions Colloids There are many forms of colloids; Gases dispersed in liquids Gases dispersed in solids Liquids dispersed in liquids Liquids dispersed in gases Liquids dispersed in solids Solids dispersed in gases Solids dispersed in liquids Solids dispersed in solids

Solutions Colloids Biochemistry o The blood is a colloid consisting of many suspended particles. o Proteins stay suspended from interaction between the polar water molecules and the polar ends on the surface of a protein.

Solutions Colloids Biochemistry o Hydrophobic colloids, ‘water fearing’, can stay suspended in the blood by cations coating the surface of the suspended particle.

Solutions Solubility of Gases and Liquids Solubility of Gases in Water – Generally, as the temperature of water increases, the solubility of a gas decreases. Fast moving water molecules are no longer in contact with dissolved gases long enough to keep it dissolved.

Solutions Solubility of Gases and Liquids Solubility of Gases in water Consequences -

Solutions Solubility of Gases and Liquids Solubility of Gases in Water Henry’s Law – The solubility of a gas increases as the pressure of the gas increases. S 1 = S 2 P 1 P 2

Solutions Solubility of Gases and Liquids Solubility of Gases in Water Henry’s Law Problem – At standard pressure, the solubiligy of oxygen gas is g/L. If you wanted to increase the solubility of O 2 to g/L, to which pressure would you have to pressurize the O 2 ? S 1 = S 2 P 1 P 2

Solutions Solubility of Gases and Liquids Solubility of Solids in Water - Generally, the solubility of a solid increases as the temperature of the water increases. Some solids become less soluble as the temperature of the water increases.

Solutions Solubility of Gases and Liquids Solubility of Solids in Water - The solubility graph represents the maximum amount of solute that can dissolve in 1 gram of water as a function of the temperature. Saturation occurs when a solution can no longer hold any more solute.

Solutions Solubility of Gases and Liquids Solubility of Solids in Water - An unsaturated solution does not have the maximum amount of solute dissolved. A supersaturated solution has more than the maximum allowable dissolved solute.

Solutions Solubility of Gases and Liquids Solubility of Solids in Water - What mass of KNO3 would precipitate from a saturated solution that was cooled from 23°C to 10°C?