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 H 2 O) Induced Dipole-Induced Dipole or London Dispersion Forces (as in H 2 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:

Solvation Or Hydration

The overall Δ  three steps. ΔH sln = ΔH 1 + ΔH 2 + ΔH 3 But only ΔH 3 is exothermic! 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 ΔH sln 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 -ΔH 3 is similar to ΔH 1 + ΔH 2, and they will mix. If the solute and solvent have different types of intermolecular forces, then -ΔH 3 is much less than ΔH 1 + ΔH 2, 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, CHCl 3, 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, CH 3 Cl, 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, CH 2 CH 3 OCH 3, 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

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

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

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

This is Henry’s Law: s = k H P Where s is the solubility in M, P is the partial pressure of the gas, and k H 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)