Water and Aqueous Systems Chapter 15

Properties of Water High surface tension Low vapor pressure High boiling point **All due to high intermolecular forces between water molecules due to HYDROGEN BONDING!

Surface Tension Surface tension is the inward force, or pull, that tends to minimize the surface area of the liquid A surfactant can interfere with the hydrogen bonding of water molecules and reduce surface tension

How does a lake freeze and not kill all the fish?

Homogeneous aqueous systems

What are Solutions? Solution : a uniform (homogeneous) mixture that may contain solids, liquids or gases. Parts of a solution: Solute: What gets dissolved in a solution – usually present in smaller amounts Solvent: What does the dissolving in a solution – usually present in larger amounts – WATER is the “UNIVERSAL SOLVENT”

Dissolving Process “Solvation” http://www.northland.cc.mn.us/biology/Biology1111/animations/dissolve.html This animation shows how the intermolecular forces between the ions and the polar water molecules overcome the ionic forces holding Sodium and Chloride ions together

Heterogeneous aqueous systems

Suspensions Suspensions have particles that are much larger than those in solutions AND the particles will eventually settle out

Colloids Colloids have particles smaller than those in suspensions and larger than those in solutions The particles of a colloid do not settle out The Tyndall Effect states that visible light can be scattered by colloidal/suspended particles

Solutions Chapter 16

Some terms associated with solutions Soluble: a substance that dissolves in a solvent is said to be “soluble” in that solvent Insoluble: a substance that does not dissolve in a solvent is said to be “insoluble” in a solvent Miscible: two substances are able to be mixed (think “mixable”) Immiscible: two substances are not able to be mixed (think “not mixable”)

More solution terms Aqueous : a solution with water as the solvent (something dissolved in water) Solubility : refers to the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature and pressure

Types of Solutions Saturated solution: contains the maximum amount of dissolved solute for a given amount of solvent at the specified temperature and pressure Unsaturated solution: contains less dissolved solute for a given temperature and pressure than a saturated solution (could still dissolve more) Supersaturated solution: contains more dissolved solute than a saturated solution (made by increasing temperature and/or pressure to dissolve and then cooled slowly)

Solution Concentration (cont’d) A “Concentrated” solution contains a large amount of solute relative to its solubility A “dilute” solution contains a small amount of solute relative to its solubility

Factors Affecting Solubility Solvation = process of surrounding solute particles with solvent particles to form a solution Ionic Compounds – charged ends of the water molecule attract the positive and negative ions of the ionic compound, breaking them apart and surrounding them Molecular compounds – each O-H bond in the molecule becomes a site for hydrogen bonding with the attractive forces between polar water molecules and other polar molecules “LIKES DISSOLVE LIKES” – Polar solvents dissolve polar solutes & nonpolar solvents dissolve nonpolar solutes

Factors Affecting Solubility Agitation  stirring or shaking  increases rate of solution Surface area  larger surface area (smaller particles)  increases rate of solution Temperature  higher temperature (more kinetic energy and collisions)  increases rate of solution

Henry’s Law States that at a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid S1 S2 = P1 P2 Solubility units will be mass/volume g/L or mg/ml

Henry’s Law Problem If 0.85 g of a gas at 4.0 atm of pressure dissolves in 1.0 L of water at 25°C, how much will dissolve in 1.0 L of water at 1.0 atm of pressure and the same temperature?

Units of Concentration

substance being dissolved Molarity Concentration of a solution. substance being dissolved total combined volume

Molarity 2M HCl What does this mean?

Molarity Calculations 6.02  1023 (particles/mol) molar mass (g/mol) MASS IN GRAMS MOLES NUMBER OF PARTICLES LITERS SOLUTION Molarity (mol/L)

Molarity Calculations How many grams of NaCl are required to make 0.500L of 0.25M NaCl? 0.500 L 0.25 mol 1 L 58.44 g 1 mol = 7.3 g NaCl

Molarity Calculations Find the molarity of a 250 mL solution containing 10.0 g of NaF. 10.0 g 1 mol 41.99 g = 0.238 mol NaF 0.238 mol 0.25 L M = = 0.95M NaF

The Dilution Equation M1V1 = M2V2 M1 = initial molarity (“stock solution”) V1 = initial volume (Liters) M2 = final (desired) molarity V2 = final volume (Liters) This equation is used when you have a “stock solution” of higher molarity than you need and you need to dilute it to a lower molarity by adding additional solvent.

Dilution Equation (cont’d) What volume of 3.00M KI stock solution would you use to make 0.300 L of a 1.25M KI solution?

Other Concentration Units

Percent by Mass mass of solute Percent by mass = ___________________ x 100% mass of solution In order to maintain a sodium chloride (NaCl) concentration similar to ocean water, an aquarium must contain 3.6 g NaCl per 100.0g of water. What is the percent by mass of NaCl in the solution?

Percent by Volume volume of solute Percent by volume = _______________ x 100% volume of solution What is the percent by volume of ethanol in a solution that contains 35 mL of ethanol dissolved in 155 mL of water?

Molality moles of solute Molality (m) = ________________ kg of solvent In the lab, a student adds 4.5 g of sodium chloride (NaCl) to 100.0 g of water. Calculate the molality of the solution.

Mole Fraction A mole fraction equals the number of moles of solute or solvent in a solution divided by the total number of moles of solute and solvent XA = nA nA + nB XA = mole fraction of substance A nA = moles of A nB = moles of B

Mole Fraction Example What is the mole fraction of NaOH in an aqueous solution that contains 22.8% NaOH by mass?

Let’s stop here for today… Enough Already! Let’s stop here for today…

Colligative Properties Colligative properties depend on the number of solute particles in a solution, not on the identity of the solute particles Vapor Pressure Lowering Boiling point elevation Freezing point depression Osmotic pressure

Vapor Pressure Lowering Vapor pressure is the pressure exerted in a closed container by liquid particles that have escaped the liquid’s surface and entered the gaseous state. With fewer solvent particles (more solute particles) at the surface, fewer particles enter the gaseous state, and the vapor pressure is lowered

Raoult’s Law *out of order* Psoln = XsolventP0solvent Where: Psoln = observed vapor pressure of the solution Xsolvent = mole fraction of the solvent P0solvent = vapor pressure of the pure solvent

Raoult’s Law Problem *some changes were made to slide Calculate the expected vapor pressure at 25°C for a solution prepared by dissolving 158.0 g of sucrose (molar mass = 342.3 g/mol) in 641.6 g of water. At 25°C, the vapor pressure of pure water is 23.76 torr.

Boiling Point Elevation A liquid boils when its vapor pressure equals the atmospheric pressure. More solute particles in solution cause a lower vapor pressure and thus it takes a higher temperature to reach boiling.

Calculating BP Elevation ΔTb = Kbm Where: ΔTb = bp elevation Kb = molal boiling point elevation constant (will be given to you) m = molality (either given to you, or may need to calculate this)

Freezing Point Depression The freezing point of a solution is always lower than that of a pure solvent. At a solvent’s freezing point, the particles no longer have enough kinetic energy to overcome intermolecular forces and the particles form into an ordered solid. In a solution, the solute particles interfere with the intermolecular forces between solvent particles and the temperature needs to be even lower in order to freeze and become solid

Calculating FP Depression ΔTf = Kfm Where: ΔTf = fp depression Kf = molal freezing point depression constant (will be given to you) m = molality (may need to calculate this!)

Sample calculation What are the boiling point and freezing point of a 0.40m solution of sucrose in ethanol? For Ethanol, Kb = 1.22 °C/m, Kf = 1.99 °C/m, boiling point = 78°C, freezing point = -114°C

Osmotic pressure Osmosis is the diffusion of a solvent through a semi-permeable membrane (allows some particles through and not others) During osmosis, water molecules can move in both directions (from dilute to concentrated) across the membrane, but the solute particles cannot Osmotic pressure is the amount of additional pressure caused by the water molecules that moved in to the concentrated solution

Determining Molar Mass from Osmotic Pressure Π = MRT Where: Π = osmotic pressure in atmospheres M = molarity of solution R = universal gas law constant (0.08206 L*atm/mol*K) T = temperature in Kelvin

Determining Molar Mass from Osmotic Pressure To determine the molar mass of a certain protein, 1.00 g is dissolved in enough water to make 1.00 L of solution. The osmotic pressure of this solution was found to be 0.00147 atm at 298 K. Calculate the molar mass of the protein.