Solids, Liquids, and gases dissolve to form solutions.

What is a solution?

A mixture of 2 or more substances that are uniformly mixed with each other.

What does it mean to dissolve?

When one substance mixes into another and completely disappears. For example, NaCl in H2O.

The Solution ProcessIn order for a solute to be dissolved in a solvent, the solute and solvent must be attracted to each other. The solute and the solvent molecules in a solution are then expanded (spread out), and able to mix with each other. In the example below, NaCl is broken apart because +/- water molecules are attracted to the +Na and the -Cl. The water breaks down the crystal, dissolving the salt.

Three types of interactions in the solution process:• solvent-solvent interaction• solute-solute interaction• solvent-solute interaction

Δ Hsoln = Δ H1 + Δ H2 + ΔH3

Energy Changes in SolutionTo determine the enthalpy

change, we divide the process into 3 steps.

1. Separation of solute particles.

2. Separation of solvent particles to make ‘holes’.

3. Formation of new interactions between solute and solvent.

Enthalpy Changes in Solution

The enthalpy change of the overall process depends on H for each of these steps.

Start

End

EndStart

Enthalpy changes during dissolution

The enthalpy of solution, ΔHsoln, can be either positive or negative.

ΔHsoln = ΔH1 + ΔH2 + ΔH3

ΔHsoln (MgSO4)= -91.2 kJ/mol --> exothermic

ΔHsoln (NH4NO3)= 26.4 kJ/mol --> endothermic

The term solubility refers to the maximum amount of material that will dissolve in a given amount of solvent at a given temperature to produce a stable solution.

Ack! What does this mean? It means how much solute (stuff) can dissolve in a solvent.

It is dependant on temperature, how much solvent you have, how much solute you have, and whether or not you mix the solution.

Nature of the solute and solventA solute is soluble when it dissolves completely in a solvent.

A solute is insoluble when it cannot dissolve in a solvent.

Some solutes are partially soluble, meaning only a bit dissolves while the rest remains solid.

When two liquids totally mix they are said to be miscible.

An example of this would be alcohol and water.

When to liquids do not mix they are said to be immiscible.

An example would be oil and water:

polar dissolves polar

Nonpolar dissolves nonpolarlike dissolves like

“like dissolves like”Two substances with similar intermolecular forces are likely to be soluble in each other.

• non-polar molecules are soluble in non-polar solvents

CCl4 in C6H6

• polar molecules are soluble in polar solvents

C2H5OH in H2O

• ionic compounds are more soluble in polar solvents

NaCl in H2O or NH3 (l)

SOLUBILITY

The solubility of a solute in a solvent at a particular temperature is the number of grams of the solute necessary to saturate 100 gm of the solvent at that temperature.

FACTORS AFFECTING SOLUBILITY

There are six main factors that control solubility of a solute.

(1) Temperature ** (2) Nature of solute or solvent (3) Pressure (4) Concentration ** (5) Time (6) Mixing **

Effect of Temperature on Solubility

• Solubility increases when temperature goes up (most of the time).

• More solute dissolves in the solvent at higher temperatures.

• Solubility decreases when temperature goes down (most of the time).

• Less solute dissolves in the solvent at lower temperatures.

In endothermic reactions solubility increases with the increase in temperature and vice versa.

For example: solubility of potassium nitrate increases with the increase in temperature.

In exothermic reactions solubility decrease with the increase in temperature.

For example: solubility of calcium oxide decreases with the increase in temperature.

Gases are more soluble in cold solvent than in hot solvent.

Temperature and Solutions

Solubility: the maximum solute can dissolve in a given amount of solvent

(in a given T).

T Solubility

T Crystal is formed,Solubility

gas in liquid: T ↑ Solubility ↓

Gases, however, decrease in solubility with an increase in temperature.

By looking at the plot of solubilities below, you can see that most solids increase in solubility with an increase in temperature.

The y-axis (dependant) is solubility: how much solute dissolves in the solvent.

The x-axis (independent) is temperature.

How much solute can dissolve is dependant on the temperature.

Solubility Curve

Learning Check

A. Why would a bottle of carbonated drink possibly burst (explode) when it is left out in the hot sun ?

B. Why would fish die in water that gets too warm?

Effect of Pressure on SolubilityHenry's Law states that "The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of that gas in contact with the liquid..." What this means for divers is that gas molecules will dissolve into the blood in proportion to the partial pressure of that gas in the lungs (as "warm-blooded" creatures, our core body temperature remains relatively constant).

Henry’s Law is used in breathalizers to determine how much alcohol a person has drunk.

Pressure and Solutions

P Solubility (gas in liquid)Henry’s law

Pressure and Solubility of Gases

12.5

The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law).

c = kPc is the concentration (M) of the dissolved gas

P is the pressure of the gas over the solution

k is a constant (mol/L•atm) that depends onlyon temperature

low P

low c

high P

high c

Effect of Concentration: Degrees of Saturation• When referring to solutions, there are three degrees of saturation — unsaturated, saturated, and supersaturated.

• If a solution is unsaturated, the solvent is capable of dissolving more solute. • When the solution is saturated, the solvent has dissolved the maximum amount of solute

that it can at the given temperature. • At this point we say that the solution is in a state of dynamic equilibrium—the

processes of dissolving and precipitation are happening at the same rate.

• A supersaturated solution is one in which the solvent contains more solute than it can theoretically hold at a given temperature.

• Supersaturated solutions are often formed by heating a solution and dissolving more solute, then cooling the solution down slowly. These solutions are unstable and crystallize readily.

Seeding

A surface on which to being crystallizing.

Supersaturated solution

When a solution reaches saturation, some of the solid precipitates out and forms a solid on the bottom of the beaker.

Sometimes a reaction happens when a solution is made, but one of the products is insoluble in the solvent and precipitates out.

A precipitate is a solid that forms from a solution, typically because the solid is either insoluble in the solvent, or is no longer soluble.

Concentration TermsSolutions are often referred to as being concentrated or dilute. These two terms are very general. While concentrated indicates that there is a lot of solute dissolved in the solvent (perhaps the solution is near to being saturated) and dilute indicates that a small amount of solute is dissolved in the solvent, we often need to be exact with quantities in chemistry.

Effect of Time: Diffusion & Dissolving

As time goes on, due to the random (Brownian) movement of the molecules, the solute will completely dissolve in the solvent.

Effect of Mixing

You have done several experiments in which you were instructed to add a solute to a solvent and stir. What was the reason behind the stirring?

When you stir a solute in a solution, you increase the movement of the molecules. When the molecules of the solute move more and faster, they come in higher contact with the molecules of the solution. The solution is able to attack the solute and pull it apart, dissolving the solution.

Mixing causes the solute to dissolve in the solvent faster than if not mixed.

ExampleCalculate the molarity of a solution prepared by dissolving 20.0 g of solid NaOH in enough water to make 100 mL of solution.

ExplanationConvert grams to moles:

Then convert mL to liters:

Then divide:

Molarity (M)The molarity of a solution is a measure of the number of moles of solute per liter of solution. This is the most common concentration unit used in chemistry. Let’s now run through how you calculate the molarity of a solution.

Molality (m)

The molality of a solution is a measure of the number of moles of solute per kilogram of solvent. The molality is dependent on the mass of the solvent in the solution. Try an example:

A solution is prepared by mixing 80.0 g of sodium hydroxide (NaOH) with 500.0 g of water. Calculate the molality of this solution.ExplanationConvert grams of solute to moles:

Convert grams of solvent to kg:

Divide:

.

What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is 0.927 g/mL?

m =moles of solute

mass of solvent (kg)M =

moles of solute

liters of solution

Assume 1 L of solution:5.86 moles ethanol = 270 g ethanol927 g of solution (1000 mL x 0.927 g/mL)

mass of solvent = mass of solution – mass of solute= 927 g – 270 g = 657 g = 0.657 kg

m =moles of solute

mass of solvent (kg)=

5.86 moles C2H5OH

0.657 kg solvent= 8.92 m

Changing Molarity to Molality

If we know the density of the solution, we can calculate the molality from the molarity, and vice versa.

Mass Percent (Weight Percent)The mass percent of a solution is another way of expressing its concentration.

Mass percent is found by dividing the mass of the solute by the mass of the solution and multiplying by 100.

A solution of NaOH that is 28% NaOH by mass contains 28 g of NaOH for each 100 g of solution. Here’s the equation:

Now try a problem involving the equation:A solution is prepared by mixing 5.00 g ethanol (C2H5OH) with 100.0 g water. Calculate the mass percent of ethanol in this solution.

ExplanationPlugging the values we were given into the mass percent equation, we get:

Convert % mass to Molarity

• What is the Molarity of a 95% acetic acid solution? (density = 1.049 g/mL)

If you assume 1 L, that amount of solution = 1049 g

95% of the solution is acetic acid

1049 g solution x 0.95 = 997 g solute

997 g X 1 mol/60.05 g = 16.6 mol solute

Since we assumed 1 L, that’s 16.6 mol / 1 L or 16.6 M

Percent concentration:

Weight / volume (W / V)% =Weight solute

Volume of solution (mL)× 100

Weight / Weight (W / W)% =Weight solute

Weight of solution× 100

Volume / volume (V / V)% =Volume solute (mL)

Volume of solution (mL)× 100

Solubility

The maximum amount of solute that can dissolve in a specific amount of solvent usually 100 g.

g of solute 100 g water

Learning Check

At 40C, the solubility of KBr is 80 g/100 g H2O. Indicate if the following solutions are

(S) saturated or (U) unsaturated

A. ___60 g KBr in 100 g of water at 40C

B. ___200 g KBr in 200 g of water at 40C

C. ___25 KBr in 50 g of water at 40C

Parts per Million (ppm):

Concentration

ppm =g soluteg solvent × 106

Parts per billion (ppb):

ppb =g soluteg solvent × 109

Dilution is the process of taking a more concentrated solution and adding water to make it less concentrated. The more concentrated solution before the dilution is performed is known as the stock solution. You can relate the concentration of the stock solution to the concentration of the diluted solution using the equation below:

C1V1 = C2V2

where C is concentration (molarity) and V is the volume, in liters, of the solution. Try the following example using this equation.

What volume of 6.0 M sulfuric acid (H2SO4) must be used to prepare 2.0 L of a 0.10 M H2SO4 solution?ExplanationJust plug the numbers into the formula! Be careful to read closely.

C1V1 = C2V2

(6.0 M) (V1) = (0.10 M) (2.0 L)V1 = 0.033 L or 33 mL should be measured out and then diluted by adding enough water to make 2.00 L total volume.

This should be a review!!!

Dilution is the procedure for preparing a less concentrated solution from a more concentrated solution.

Dilution

Add Solvent

Moles of solutebefore dilution (i)

Moles of soluteafter dilution (f)

=

MiVi MfVf=

How would you prepare 60.0 mL of 0.2 MHNO3 from a stock solution of 4.00 M HNO3?

MiVi = MfVf

Mi = 4.00 Mf = 0.200 Vf = 0.06 L Vi = ? L

Vi =MfVf

Mi

= 0.200 x 0.064.00

= 0.003 L = 3 mL

3 mL of acid + 57 mL of water = 60 mL of solution

 Partial Pressure in terms of mole fraction:XAPtotal = PA (XA = mole fraction of A)            Example:  If there are 3 moles of gas A, 4 moles of gas B and 5 moles of gas C in a mixture of gases and the pressure of A is found to be 2.5 atm, what is the total pressure of the sample of gases?  

What is the mole Fraction of Gas A in mixture I?

What is the mole fraction of Gas B in mixture II?

Fractional crystallization is the separation of a mixture of substances into pure components on the basis of their differing solubilities.

Suppose you have 90 g KNO3 contaminated with 10 g NaCl.

Fractional crystallization:

1. Dissolve sample in 100 mL of water at 600C

2. Cool solution to 00C

3. All NaCl will stay in solution (s = 34.2g/100g)

4. 78 g of PURE KNO3 will precipitate (s = 12 g/100g). 90 g – 12 g = 78 g

Fractional Distillation Apparatus

Gravimetric Analysis1. Dissolve unknown substance in water

2. React unknown with known substance to form a precipitate

3. Filter and dry precipitate

4. Weigh precipitate

5. Use chemical formula and mass of precipitate to determine amount of unknown ion

TitrationsIn a titration a solution of accurately known concentration is added gradually added to another solution of unknown concentration until the chemical reaction between the two solutions is complete.

Equivalence point – the point at which the reaction is complete

Indicator – substance that changes color at (or near) the equivalence point

Slowly add baseto unknown acidUNTIL

the indicatorchanges color

What volume of a 1.420 M NaOH solution isRequired to titrate 25.00 mL of a 4.50 M H2SO4 solution?

WRITE THE CHEMICAL EQUATION!

volume acid moles acid moles base volume base

H2SO4 + 2NaOH 2H2O + Na2SO4

4.50 mol H2SO4

1000 mL solnx

2 mol NaOH1 mol H2SO4

x1000 ml soln

1.420 mol NaOHx25.00 mL = 158 mL

M

acid

rx

coef.

M

base

Electrolytes

+ -electrolyte

Electrolyte: conduct an electric current.

strong electrolytes: molecules dissociate completely to ions (NaCl).

weak electrolytes: molecules dissociate partially to ions (CH3COOH).

nonelectrolytes: molecules do not dissociate to ions (DI water).

Ionization

NaCl → Na+ + Cl-

Na+

Cl-

bulb

An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity.

A nonelectrolyte is a substance that, when dissolved, results in a solution that does not conduct electricity.

nonelectrolyte weak electrolyte strong electrolyte

osmotic pressure

Osmotic Pressure

Semipermeable membrane

Higher concentration → Higher osmotic pressure

Osmotic Pressure (p)Osmosis is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one.A semipermeable membrane allows the passage of solvent molecules but blocks the passage of solute molecules.Osmotic pressure (p) is the pressure required to stop osmosis.

dilute moreconcentrated

Osmolarity (osmol) = M × i

M: molarityi: number of particles

Osmolarity ↑ → Osmotic pressure ↑

HighP

LowP

Osmotic Pressure (p)

p = MRT

M is the molarity of the solutionR is the gas constantT is the temperature (in K)

HemolysisCrenation

Isotonic solution Hypotonic solution Hypertonic solution

A cell in an:

isotonicsolution

hypotonicsolution

hypertonicsolution

Dialysis

Dilute solution

Chemistry In Action: Desalination

Freezing Point Depression• The freezing point of a substance is defined as the temperature at which the vapor pressure of the solid and the liquid states of that substance are equal. • If the vapor pressure of the liquid is lowered, the freezing point decreases.• Why is a solution’s freezing point depressed below that of a pure solvent?

• The answer lies in the fact that molecules cluster in order to freeze. They must be attracted to one another and have a spot in which to cluster; if they act as a solvent, solute molecules get in the way and prevent them from clustering tightly together. The more ions in solution, the greater the effect on the freezing point. We can calculate the effect of these solute particles by using the following formula:

Tf = Kf msolute iwhereTf = the change in freezing pointKf = molar freezing point depression constant for the substance (for water = 1.86ºC/m)m = molality of the solutioni = number of ions in solution (this is equal to 1 for covalent compounds and is equal to the number of ions in solution for ionic compounds)

Freezing-Point Depression

ΔTf = T f – Tf0

T f > Tf0 ΔTf > 0

T f is the freezing point of the pure solvent

0

T f is the freezing point of the solution

ΔTf = Kf m

m is the molality of the solution

Kf is the molal freezing-point depression constant (0C/m)

Change in Freezing Point Common Applications

of Freezing Point Depression

Propylene glycol

Ethylene glycol – deadly to small animals

At what temperature will a 5.4 molal solution of NaCl freeze?

Solution

∆TFP = Kf • m • i

∆TFP = (1.86 oC/molal) • 5.4 m • 2

∆TFP = 20.1 oC

FP = 0 – 20.1 = -20.1 oC

Freezing Point Depression

Boiling Point Elevation• The boiling point of a substance is the temperature at which the vapor pressure equals atmospheric pressure. • Because vapor pressure is lowered by the addition of a nonvolatile solute, the boiling point is increased. Why?

• Since the solute particles get in the way of the solvent particles trying to escape the substance as they move around faster, it will take more energy for the vapor pressure to reach atmospheric pressure, and thus the boiling point increases.

• We can calculate the change in boiling point in a way that’s similar to how we calculate the change in freezing point:

Tb = KbmsoluteiwhereKb = molar boiling point elevation constant (for water = 0.51˚C/m)

Boiling-Point Elevation

ΔTb = Tb – T b0

Tb > T b0 ΔTb > 0

T b is the boiling point of the pure solvent

0

T b is the boiling point of the solution

ΔTb = Kb m

m is the molality of the solution

Kb is the molal boiling-point elevation constant (0C/m)

Change in Boiling Point Common Applications of

Boiling Point Elevation

Now try a problem that deals with freezing point depression and boiling point elevation.

Calculate the freezing point and boiling point of a solution of 100 g of ethylene glycol (C2H6O2) in 900 g of water.Explanation:

Calculate molality:

Freezing point depression = Tf = (m)(Kf)(i)

Tf = (1.79)(1.86)(1) = 3.33ºC

Freezing point = 0ºC - 3.33ºC = -3.33ºC

Boiling point elevation = Tb = (m)(Kb)(i)

Tb = (1.79)(0.51)(1) = 0.91ºC

Boiling point = 100ºC + 0.91ºC = 100.91ºC

What is the freezing point of a solution containing 478 g of ethylene glycol (antifreeze) in 3202 g of water? The molar mass of ethylene glycol is 62.01 g.

ΔTf = Kf mi

m =moles of solute

mass of solvent (kg)= 2.41 m=

3.202 kg solvent

478 g x 1 mol

62.01 g

Kf water = 1.86 0C/m

ΔTf = Kf mi = 1.86 0C/m x 2.41 m x 1 = 4.48 0C

ΔTf = T f – Tf0

Tf = T f – ΔTf0 = 0.00 0C – 4.48 0C = -4.48 0C

Colligative Properties of Nonelectrolyte Solutions

Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.

Vapor-Pressure Lowering (Raoult’s Law)

Raoult’s law

If the solution contains only one solute:

X1 = 1 – X2

P 10 - P1 = DP = X2 P 1

0

P 10 = vapor pressure of pure solvent

X1 = mole fraction of the solvent

X2 = mole fraction of the solute

P1 = X1 P 10

SAMPLE EXERCISE Calculation of Vapor-Pressure Lowering

Glycerin (C3H8O3) is a nonvolatile nonelectrolyte with a density of 1.26 g/mL at 25°C. Calculate the vapor pressure at 25°C of a solution made by adding 50.0 mL of glycerin to 500.0 mL of water. The vapor pressure of pure water at 25°C is 23.8 torr.

SolutionAnalyze: Our goal is to calculate the vapor pressure of a solution, given the volumes of solute and solvent and the density of the solute.Plan: We can use Raoult’s law to calculate the vapor pressure of a solution. The mole fraction of the solvent in the solution, XA, is the ratio of the number of moles of solvent (H2O) to total solution (moles C3H8O3 + moles H2O).Solve: To calculate the mole fraction of water in the solution, we must determine the number of moles of C3H8O3 and H2O:

The vapor pressure of the solution has been lowered by 0.6 torr relative to that of pure water.

We now use Raoult’s law to calculate the vapor pressure of water for the solution:

Colligative Properties of Nonelectrolyte Solutions

Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.

Vapor-Pressure Lowering P1 = X1 P 10

Boiling-Point Elevation DTb = Kb m

Freezing-Point Depression DTf = Kf m

Osmotic Pressure (p) p = MRT

Colligative Properties of Electrolyte Solutions

0.1 m NaCl solution 0.1 m Na+ ions & 0.1 m Cl- ions

Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.

0.1 m NaCl solution 0.2 m ions in solution

van’t Hoff factor (i) = actual number of particles in soln after dissociationnumber of formula units initially dissolved in soln

nonelectrolytesNaClCaCl2

i should be

123

Which would you use for the streets of New Glarus to lower the freezing point of ice and why? Would the temperature make any difference in your decision?

a) sand, SiO2

b) Rock salt, NaCl

c) Ice Melt, CaCl2

Change in Freezing Point

Boiling-Point Elevation ΔTb = i Kb m

Freezing-Point Depression ΔTf = i Kf m

Osmotic Pressure (p) p = iMRT

Colligative Properties of Electrolyte Solutions

Tyndall Effect

The Tyndall Effect is caused by reflection of light by very small particles in suspension in a transparent medium. It is often seen from the dust in the air when sunlight comes in through a window, or comes down through holes in clouds. It is seen when headlight beams are visible on foggy nights, and in most X-File episodes when Moulder and Sculley check out some dark place with flashlights.

Tyndall effect is easily seen using a laser pointer aimed at the the mist from this ultrasonic humidifier's mist

In liquids the tyndall effect can be easily seen by using a laser pointer. If you dilute milk to where it is almost clear, or if you have any type of sol, such as colloidal silver, then the beam of the laser can be easily seen as it travels through the liquid.

Tyndall effect is seen here using a laser pointer. The glass on the left contains 5 ppm of HVAC colloidal silver and the one on the right is from the tap after the bubbles have settled out.