Water and Aqueous Systems Chapter 15. Properties of Water High surface tension Low vapor pressure...
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Transcript of Water and Aqueous Systems Chapter 15. Properties of Water High surface tension Low vapor pressure...
Water and Aqueous SystemsChapter 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
Molarity• Concentration of a solution.
solution of liters
solute of moles(M)Molarity
total combined volume
substance being dissolved
Molarity
2M HCl
L
molM
L 1
HCl mol 2HCl 2M
What does this mean?
Molarity Calculationsmolar mass
(g/mol)
6.02 1023
(particles/mol)
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
= 7.3 g NaCl
58.44 g
1 mol
L 1
mol0.25 0.25M
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
L
molM
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 solutePercent 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 solutePercent 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 soluteMolality (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?
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 atmospheresM = molarity of solutionR = 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.