Chapter 11

Properties of solutions

Solutions A solution is a homogenous mixture.

The solvent does the dissolving. The solute is dissolved by the solvent.

There are various types solutions. Gas-gas: air, natural gas (gas solution) Liquid-liquid: antifreeze, vodka in water (liquid

solution) Solid – solid: brass (solid solution)

We will focus on aqueous solutions.

Solutions Gas-liquid: carbonated water (liquid solution) Solid – liquid: seawater, sugar water (liquid

solution) Gas-solid: hydrogen in platinum (solid

solution) We will focus on aqueous solutions.

Ways of Measuring

Molarity = moles of solute Liters of solution

Mass % = Mass of solute x 100 Mass of solution

Mole fraction of component A

BA

AA nn

n

Molality = moles of solute Kilograms of solvent

Molality is abbreviated m (lowercase)

Normality - read but don’t focus on it. Note: molarity is based on volume so it

changes slightly with temperature, molality does not because it depends on mass

Ways of Measuring

Parts per million (ppm)- used for very dilute concentration, usually

water or soil ppm = 1mg of substance

1 L H2O ppm = 1 mg of substance

1 kg of soil

Ways of Measuring

Example

A solution of phosphoric acid was made by dissolving 10.0 g of H3PO4 in 100.0 mL of water. The resulting volume was 104.0 mL. Calculate the density, mole fraction, molarity, and molality of the solution. Assume water has a density of 1.00 g/mL.

Solution FormationSolution Formation

Heat of SolutionHeat of Solution The heat of solution

is the amount of heat energy absorbed or released when a specific amount of solute dissolves in a solvent.

Heat of SolutionHeat of Solution Heat of solution is positive if energy is

absorbed Heat of solution is negative if energy is

released Equation for heat of solution

321ln HHHHso

Steps in Solution FormationSteps in Solution FormationHH11 Step 1 - Step 1 - Expanding the solute

Separating the solute into individual components

Steps in Solution FormationSteps in Solution FormationHH22 Step 2Step 2 -- Expanding the solvent

Overcoming intermolecular forces of the solvent molecules

Steps in Solution FormationSteps in Solution FormationHH33 Step 3 - Step 3 - Interaction of solute and solvent to form the solution

Predicting Solution FormationPredicting Solution Formation

Factors affecting Solubility

Structure Effects

Molecular structure determines polarity Polarity similarities between solute and

solvent favors solubility SO….there is a connection between

structure and solubility

Structure Effects

Consider vitamin A and vitamin C… Vitamin A is fat-soluble because it is

nonpolar Vitamin C is polar

Copyright © Houghton Mifflin Company. All rights reserved. 11–18

Figure 11.4 The Molecular Structures of (a) Vitamin A and (b) Vitamin C

Pressure effects

Changing the pressure doesn’t affect the amount of solid or liquid that dissolves

They are incompressible. It does affect gases.

Pressure effects

Pressure affects the amount of gas that can dissolve in a liquid.

Solubility of gases increase with pressure

The gas is at equilibrium with the dissolved gas in this solution.

The equilibrium is dynamic.

If you increase the pressure the gas molecules dissolve faster.

The equilibrium is disturbed.

The system reaches a new equilibrium with more gas dissolved.

Henry’s Law

C=kPP = pressure of the gas above the liquid (in atm)C = concentration of the dissolved gas in (in mol/L) k = Henry’s law constant (specific to the gas) (mol/L atm)

NOTE: only when there is no chemical reaction between the solute and solvent

ExampleA bottle of soda at 25C contains CO2 gas at a pressure of 5.0 atm over the liquid. Assuming the partial pressure of CO2 in the atmosphere is 4.0 x 10-4, calculate the equilibrium concentrations before and after the bottle is opened. (k= 3.1 x 10-2 mol/Latm.)

Temperature Effects

Increased temperature increases the rate at which a solid dissolves.

We can’t predict whether it will increase the amount of solid that dissolves.

We must read it from a graph of experimental data.

Temperature effects

Solids dissolve more rapidly at higher temperatures

The amount of most solids increase with temperature BUT not always.

Determined experimentally

Solubility ChartSolubility Chart

Temperature effects Solubility of gases increases

as temperature decrease

Vapor Pressures of solutions

Vapor Pressure of Solutions

What does the term “vapor pressure” mean? The pressure of the vapor

over a liquid at equilibrium.

What?!! I don’t get it! Can you explain it more? When the rate of

condensation is equal to the rate of evaporation, the pressure above the liquid is called “VAPOR PRESSURE”

Vapor Pressure of Solutions What happens to the vapor

pressure of a solvent if we add a nonvolatile solute? The vapor pressure of the

solvent decreases. OK, I can accept that….but

can you tell me WHY? Because the dissolved

nonvolatile solute decreases the number of solvent molecules AND so there are not as many solvent molecules that can escape

Vapor Pressure of Solutions

Raoult observed this to be true….and related them together through

Psolution = solventP0solvent

What happens when both the solute and solvent are volatile- (liquid-liquid solutions)?

We modify Raoult’s law to Ptotal = Psolute+ Psolvent= soluteP0

solute + solventP0solvent

Ideal solutions The application of Raoult’s law works for an ideal

solution. So what is an IDEAL solution?!!!

A solution where the solute and solvent interactions are similar.

Nonideal solutions What makes a solution nonideal?

If hydrogen bonding occurs When ∆Hsoln is large and negative

Both cause a negative deviation from Raoult’s law (lower vapor pressure)

Can you explain this? Both of these cause interactions between the solute

and solvent. So less molecules will escape and result in a lower vapor pressure.

Nonideal solutions What if ∆Hsoln is positive, what kind of deviation would

occur? Cause a positive deviation from Raoult’s law (higher

vapor pressure)

Colligative Properties, Osmotic Pressure and

Electrolytes

Colligative Properties

What are the colligative properties? Boiling point elevation Freezing point depression Osmotic pressure

What solute characteristic do these have in common? Only dependent on the NUMBER of solute

particles in an ideal solution

Boiling point Elevation If atmospheric pressure is 1 atm, what is the vapor

pressure when a liquid boils? The vapor pressure of the liquid must be..1 atm! In order for a liquid to boil, vapor and atmospheric

pressure must be equal. How would a nonvolatile solute affect vapor

pressure? It lowers it.

How does this affect the boiling point? It raises it. We need more energy (heat) to raise the

vapor pressure.

Boiling point Elevation The equation is: T = Kbmsolute

T is the change in the boiling point Kb is a constant determined by the solvent. msolute is the molality of the solute

I know you can use this equation to calculate the elevated boiling point….

We can also calculate the molar mass of the solute using this equation…YIPEE!

Boiling point Elevation A 2.00 gram sample of a biomolecule was

dissolved in 15.0 g of CCl4. The boiling point of this solution was determined to be 77.85 oC. Calculate the molar mass of this biomolecule? The boiling point of pure CCl4 is 76.50 oC and Kb for CCl4 is 5.03 oC ·kg/mol

Freezing point Depression

Why will a nonvolatile solute dissolved in a liquid lower its freezing point? Use water as the liquid in your explanation. The solute causes the vapor pressure of

water to be lower than ice. In order for the vapor pressure of ice = the vapor pressure of liquid, the temperature must decrease.

Freezing point Depression

The equation is: T = -Kfmsolute

T is the change in the freezing point Kf is a constant determined by the solvent

msolute is the molality of the solute

Hey! We can also use this to calculate molar mass of the solute! COOL!

Freezing point Depression

When 0.455 g of thyroxine is dissolved in 10.0 g benzene, the freezing point of the solution is depressed by 0.300 oC. What is the molar mass of thyroxine?

Kf for benzene = 5.12 OC · kg/mol

Osmotic Pressure What is osmosis?

Flow of solvent through a semipermeable membrane Semipermeable membrane! What so special about that?

It’s special because it allows solvent molecules to pass through but blocks solute molecules

Osmotic Pressure What does this have to do with pressure?

When the system has reached equilibrium, liquid levels are different. So the pressure on the solution is greater than that of the pure solvent. We call this OSMOTIC PRESSURE

Osmotic Pressureπ=MRT

∏ is the osmotic pressure M is the molarity of the solution T is the temperature in Kelvin

What do you think we can do with osmotic pressure? Yep! We can calculate the molar mass of

a solute! Wow! Colligative proerties are useful!

Electrolytes in solution Colligative properties depend on the number

of particles. So….. Would electrolytes have a different affect on

colligative properties than non electrolytes? You betcha they would! When electrolytes dissolve they dissociate. This

creates more particles. 1 mole of NaCl makes 2 moles of ions. 1mole Al(NO3)3 makes 4 moles ions.

Electrolytes in solution How many moles of particles would

sucrose make? Why it would only have 1 mole of particles

since its nonelectrolyte!

Electrolytes have a bigger impact on melting and freezing points per mole because they make more pieces.

Relationship is expressed using the van’t Hoff factor i i = Moles of particles in solution Moles of solute dissolved The expected value can be determined from the

formula of the compound. We can change our formulas to = iKm and ∏= i MRT

Electrolytes in solution

The actual value is usually less because At any given instant some of the ions in

solution will be paired up. Ion pairing increases with concentration. i decreases with increasing concentration.

Expected vs Observed van’t Hoff Factors