Solutions

–Review of Solutions–Intermolecular Forces and Solutions(or Like Dissolves Like)

–Factors in Solubility–Types of Solutions–Factors in Rate of Dissolution

What are Solutions?• Homogeneous mixtures• Substances are soluble or miscible

• They can be in all phases–Solid Solutions–Liquid Solutions–Gas Solutions

Why do Solutions Form?

Can we predict when a solution will form?

What will happen if…?

Solutions• You know that oil and water don’t mix together to form a solution.

• The burning question is WHY???

• They are both liquids, why shouldn’t they mix together easily to form a solution?

• And why does NaCl dissolve in water but NOT in oil?

Rehash of Intermolecular Forces

• Dipole-Dipole (as in HCl)• Hydrogen Bonding (as in HF or H2O)

• Induced Dipole-Induced Dipole or London Dispersion Forces (as in H2 or He)

Intermolecular Forces

• Ion-Dipole– Soluble salt

dissolves into ions– Ions are attracted

to partial charges

Intermolecular Forces

Intermolecular Forces and Solutions• You’ve learned to predict which type of force predominates based on the polarity of the molecule.

• You learned that the intermolecular forces are responsible for the boiling point (and melting point) of a substance.

• But that’s not all that intermolecular forces do!

Intermolecular Forces and Solutions

• Intermolecular forces drive the solution process.

• If we understand how intermolecular forces work, we can predict whether two substances will mix or not, AND explain WHY!

The Solution Process

• What happens when we mix two substances together?

• We can visualize the mixing as occurring in three steps:

SolvationOr Hydration

• The overall Δ H is the sum ofthree steps.

ΔHsln = ΔH1 + ΔH2 + ΔH3

• But only ΔH3 is exothermic!

The Solution Process

The Solution Process• So the solution process can be

exothermic or endothermic.• Exothermic processes are

favored by nature, but endothermic processes also occur naturally.

• How can we tell whether they will occur naturally?

• The magnitude of ΔHsln is crucial!

The Solution Process

• If the energy released forming the solute-solvent intermolecular forces is small compared to the energy required to break the solute-solute and solvent-solvent forces, then the solution process is highly endothermic.

• The two substances won’t mix!

The Solution Process• If the energy released forming

the solute-solvent intermolecular forces is comparable to the energy required to break the solute-solute and solvent-solvent forces, then the solution process is exothermic or slightly endothermic.

• The two substances will mix!

The Solution Process

• What this also means is the strength and thus the “type” of intermolecular forces must be similar between the solute and solvent.

The Solution Process

• If the solute and solvent have similar kinds of intermolecular forces, then -ΔH3 is similar to ΔH1 + ΔH2, and they will mix.

• If the solute and solvent have different types of intermolecular forces, then -ΔH3 is much less than ΔH1 + ΔH2, and they will NOT mix.

The Solution Process

• This is summed up as the rule “Like dissolves like.”

• What types of forces are similar?

Like Dissolves Like

• Ionic compounds are held together by ion-ion attractions. Ions have full charges.

• Polar molecules are held together by dipole-dipole or hydrogen bonding. The molecules have partial charges.

• Nonpolar molecules are held together by London forces (induced dipoles), the weakest intermolecular force.

Like Dissolves Like

• So polar molecules tend to mix with other polar molecules.

• Ionic compounds tend to mix with polar compounds.

• Nonpolar molecules (or atoms) tend to mix with other nonpolar substances.

Like Dissolves Like

• It’s actually even more complicated.

• Here’s an example:• Chloroform, CHCl3, is a polar

molecule, with a dipole moment of 1.04 D.

• But it is not very water soluble. (1 mL dissolves in 200 mL water)

Like Dissolves Like

• Methyl chloride, CH3Cl, is polar with a dipole moment of 1.9 D.

• It is considered to be slightly water soluble (100x more than chloroform).

• Ethyl methyl ether, CH2CH3OCH3, is polar with a dipole moment of 1.12 D.

• It is water soluble.

Like Dissolves Like

• What’s going on?• Why aren’t the polar molecules

methyl chloride and chloroform water soluble?

• Because “like dissolves like” can be stated more specifically:

Like Dissolves Like

• Substances which dissolve in each other usually have similar types of intermolecular forces.

• This can explain why chloroform and methyl chloride aren’t very water soluble.

Like Dissolves Like

• Chloroform, methyl chloride, and ethyl methyl ether have dipole-dipole intermolecular forces.

• Water has H-bonding.• So they don’t seem very “like”

water.• Why is ethyl methyl ether

water soluble?

Like Dissolves Like

• Ethyl methyl ether can H-bond to water as it contains an O.

• Chloroform and methyl chloride can’t.

• So ethyl methyl ether is more “like” water and is water soluble.

Like Dissolves Like

• It also gets more complicated for larger molecules with polar and nonpolar regions.

• If the molecule has lots of polar regions (particularly H-bonding), it is more likely to be water soluble.

Like Dissolves Like

Like Dissolves Like

• Some molecules like soap or ethanol are soluble in both polar and nonpolar solvents.

• This is because they have both polar and nonpolar regions and so can dissolve in both types of solvents.

Like Dissolves Like

Like Dissolves Like

Like Dissolves Like

• Now you can answer the questions:

• Why doesn’t water (a polar molecule) mix with oil (a nonpolar molecule)?

• Why does NaCl (an ionic compound) dissolve in water but NOT in oil?

Other Solubility Factors• Now you can understand and

predict whether 2 substances will mix.

• But are there any other factors in solubility?– Temperature– Pressure (for gas solubility)

Temperature & Solubility• For most substances, the

solubility increases with increasing temperature.

• This is not true for ALL compounds.

• For gases, the reverse is true: the solubility decreases with increasing T.

• This is why sodas go “flat” when open at room temperature.

Temperature & Solubility

Temperature & Solubility

Pressure & Solubility of Gases• As gases are compressible, we

can affect the solubility by changing the pressure.

• The higher the vapor pressure of a gas, the more it dissolves.

Pressure & Solubility of Gases

Pressure & Solubility of Gases• This is Henry’s Law:

s = kHP

Where s is the solubility in M, P is the partial pressure of the gas, and kH is Henry’s Constant.

• Soda and sparkling wine manufacturers rely on Henry’s Law!

• Deep sea divers HATE this law! Why?

Solution Types• Unsaturated

• Saturated

• Supersaturated

Solution Types• Unsaturated

–Solutions where LESS solute is dissolved than is possible at that temperature.

–So the solute is not at its solubility limit (usually in M or g/mL or g/L)

–Solution is CLEAR!

Solution Types• Saturated

–Solutions where as much solute is dissolved as is possible at that temperature.

–Solute is at the solubility limit

–Solution may be clear or there may be solute at the bottom.

Solution Types• Supersaturated

–Solutions where MORE solute is dissolved as is possible at that temperature.

–Solute is over the solubility limit!

–Solution is clear.–Unstable sln; may crystallize

or “crash” easily.

Solution Types• How do you tell them

apart? – If there is undissolved solute,

it IS saturated.– If there is no undissolved

solute, then it could be any of the 3.

Solution Types• How do you tell them

apart? –Add a small amount of the

solute– If it “crashes”, then it was

supersaturated.– If the solute doesn’t dissolve,

then it was saturated.– If the solute dissolves, then

it was unsaturated.

Solution Types

Factors in Rate of Dissolution• Now you know the factors

which determine whether substances will mix.

• But what factors help determine how FAST they will mix?

• You actually know all of these already!

• How do YOU get sugar to dissolve in water?

Factors in Rate of Dissolution• Stirring

• Temperature

• Surface Area

Solution Concentration Units• Mass %• Volume %• Mass/Volume %• Molarity (you know)• Mole Fraction (you know for

gas)• Molality (abbrev. m)