Lecture 1 Chapter 12 Sections 1-2

• Solutions

• Solubility

Chapt 12: Nature of Solutions

• Solution – a homogeneous mixture of two or more substances, in which one is called the solvent and the other is the solute– Solvent – main medium, what is there in the largest

quantity– Solute – dissolved in the solvent

X

? ?

Without bubbles! X

Solution Concentration

• Concentration units we have already mentioned:– Mole Fraction – (see Section 5.5)

– ppm and ppb – (see Section 5.5)• Really the same thing as mole fraction (remember pph = percent)

– Molarity – common measure for solutions (see Section 3.6)

• A new concentration unit:– Molality

total

AA n

nX =

solution

solute

VnM =

Mole per mole unitless

Moles per liter

Molality (m)

• Useful for applications where the temperature of a solution changes.

• Defined as the number of moles of solute divided by the mass of the solvent in kg

• Units are mol/kg

solvent

solutem m

nc =

Example: Aqueous ammonia

Concentrated aqueous ammonia is 14.8 M and has a density of 0.898 g/mL. What is the molality of ammonia in this solution?

Assume 17.0 g/mol as the molecular weight of NH3

Solubility

• Solubilities vary tremendously• Chemists often use qualitative descriptions, for example:

– Miscible: when two liquids mix in all proportions (Acetone and Water)

– Immiscible: when two liquids DO NOT mix at all (Oil and water)

– Insoluble: when a solid does not dissolve in a solvent (NaCl in gasoline)

– Saturated: a solution that has dissolved the maximum possible solute.

Solubility

• A more quantitative measure of solubility is the highest concentration solution that can be created. – A saturated solution of material X in water is 2.5 M– A saturated solution of material Y in water is 0.3 M– So, X is more soluble than Y

Supersaturation

• A solution holding the maximum amount of solute is saturated

• A solution that could hold more solute is unsaturated

• It is possible in some cases to add more solute to a solution that is already saturated. This solution is then called supersaturated– This is possible because solubility of solids in liquids

increases with heat. (Gasses become less soluble with heat.)– So, we make a saturated solution at high temp then carefully

cool it– If everything is just right no excess solute will crystallize out

of solution.

Supersaturation – deaux

• Crystallization is a complicated process• Some kind of nucleation point is needed.

– Imperfection in glass wall– Piece of dust that is correct shape– Already existing crystal

• Sodium acetate works well for this because it forms a complicated crystal shape.– So, really the only thing that makes a good nucleation point

is another crystal of sodium acetate.

Supersaturated Sodium Acetate

Five pieces must come together in just the right pattern…

Na+ CH3COO- 3 H2O

NaCH3COO 3H2O(s)

Sodium acetate trihydrate

Supersaturated Sodium Acetate

Note that in going from a “liquid” state to a solid state, heat is released.

The supersaturated solution was not the most stable state. Energy is released as the system becomes more stable.

Like Dissolves Like

• What does this mean?– Vinegar (acetic acid) dissolves in water, oil does not

• We are talking about polarity here…– Compounds with similar polarity will be soluble in

each other.– In other words, substances that dissolve in each other

usually have similar types of intermolecular interactions.

Which is the most soluble in hexane?

1. Hexanol2. NaCl3. Octane 4. Water

Example

Observe the following trend of alcohol solubilities in water:

n-Propanol CH3CH2CH2OH Completely Misciblen-Butanol CH3CH2CH2CH2OH 1.1 Mn-Pentanol CH3CH2CH2CH2CH2OH 0.30 Mn-Hexanol CH3CH2CH2CH2CH2CH2OH 0.056 M

Solubility of Solids

Remember there are 4 basic solids?1. Network solids (diamond, graphite) cannot dissolve

without breaking covalent bonds.2. Molecular solids – “like dissolve like”3. Metals – do not dissolve in water (some will react,

but not dissolve)4. Ionic Solids – remember the solubility rules?

Which of the following vitamins are fat-soluble (dissolve fatty hydrocarbons) and which are water soluble?

Example

Vitamins are organic molecules that are required for proper function but are not synthesized by the human body. Thus, vitamins must be present in the foods people eat. Vitamins fall into two categories: fat-soluble, which dissolve in fatty hydrocarbon-like tissues and water-soluble.

Alloys

• Alloy: a mixture of substances with metallic properties. (a solid solution)

• Some are true solutions homogeneous (brass)

• Others are heterogeneous (solder)

Solubility of Salts(Remember Chapt 4)

Remember: You are done with the homework when you understand it!

Today• Read syllabus• Start CAPA 1 (due Monday)

Friday• Finish reading Chapt 12• Plan to attend seminar