12.1-12.3 Solutions: Type
and Energy
Solutions: What are they made up of?
All solutions are mixtures, but not all mixtures are
solutions. What type of mixture is a solution?
(Homogeneous or heterogeneous) and why?
Solutions are homogeneous mixtures because they are
made up of two or more substances that dissolve each
other causing one phase (or layer)
Solutions are made up of solutes (what is being dissolved,
typically the minority component), and solvents (what is
doing the dissolving, typically the majority component).
Types of Solutions
Can solutions be made of solids, and/or gases?
YES! Examples would be air (a mixture of gases) and brass (a mixture of
solids).
• In aqueous solutions, water is the solvent, and a solid, liquid or gas is the
solute.
• Remember, as a simple check to see if one substance will dissolve another,
“likes dissolve likes”. This means, most of the time, polar dissolves polar,
and non-polar dissolves non-polar, but the two won’t form a solution.
• Solubility – the amount of the substance that will dissolve in a given amount
of solvent.
– Example: the solubility of NaCl in water at 25oC is 36 g NaCl per 100 g H2O.
Entropy
• Gases mix, but not to lower potential energy. Gases mix to increase
entropy.
• Entropy – a measure of energy randomization or energy dispersal in a
system.
• The pervasive, or universal, tendency for energy to spread out, or disperse,
is the reason that two ideal gases mix (such as neon and argon).
Here is another example of energy dispersal. What would happen if
continually heated a substance with metal tongs and the tongs remained in the
burner flame for too long?
The heat, thermal energy, would disperse up the tongs, and you would burn
your hand. Remember heat travels from hot to cold.
The Effect of Intermolecular Forces
• A solution always forms if the solvent-solute interaction are
greater than or comparable to the solvent-solvent and the solutesolute interactions.
• Remember, if two liquids dissolve in each other, such as hexane
and pentane, they are said to be miscible.
• Here’s the tricky part, even if the solvent-solute interactions are
weak compared to the solvent-solvent and the solute-solute
interactions, a solution MAY or MAY NOT form. This all depends
on intermolecular forces.
– Example – Water and hexane do not mix because the energy disparity,
or inequality, is too large, and water can not form hydrogen bonds to
hexane. However, water can mix with N2, CO2 and O2 by inducing a
dipole in these molecules. Here, the energy disparity is small.
Let’s Try a Practice Problem!
Determine whether each compound is soluble in hexane.
a.) water (H2O)
b.) propane (CH3CH2CH3)
c.) ammonia (NH3)
d.) hydrogen chloride (HCl)
a.) No
b.) Yes
c.) No
d.) No
If a molecule has both a polar and a nonpolar end, the longer the nonpolar end
becomes, the more fat soluble it becomes and the less water soluble.
Energetics of Solution Formation
•
In determining whether a solution, when formed, will emit or absorb heat, three steps must
be considered:
– Step 1: the separating of solute particles. This is always endothermic.
– Step 2: the separating of solvent particles from each other. This is also always
endothermic.
– Step 3: the mixing of solute and solvent particles, joining together by IMFs. This is
always exothermic.
•
Enthalpy of solution formation can be determined using Hess’s Law. Here, the enthalpy
change is the sum of the changes in enthalpy in the three steps above.
– If the sum of exo. is approximately equal to endo., the heat of solution is about zero.
Example: NaCl(aq). Here there is an increase of entropy that result in a solution, but the
energy change of the solution remains nearly constant.
– If the sum of the endo. < exo., the solution emits heats (is exothermic), when formed.
Here, energy decreases, and entropy increases. Example: CaCl2(aq)
– In the sum of endo.>exo., the solution absorbs hear (is endothermic), when formed. As
long as the heat of solution is not too large, entropy will increase and a solution will be
formed. Example: NaHCO3(aq)
ΔHsoln = ΔHsolute + ΔHsolvent + Hmix
Aqueous Solutions and Heat of
Hydration
• In ionic aqueous solutions, ΔHsolvent and ΔHmix, can be
combined into one term called the heat of hydration,
ΔHhydration.
• ΔHhydration :the enthalpy change that occurs when 1 mol
of the gaseous solute ions is dissolved in water. The
heat of hydration is always largely negative due to the
much stronger ion-dipole attraction between solvent
and solute, than the hydrogen bonds between water
molecules.
ΔHsoln = ΔHsolute + ΔHhydration
Let’s Try a Practice Problem!
The enthalpy of solution for cesium fluoride is -36.8
kJ/mol. What can you conclude about the relative
magnitudes of ΔHsolute and the ΔHhydration?
The ΔHhydration must have been greater than the
ΔHsolute.
12.1-12.3 pgs. 588-589 #’s 29, 31, 35 & 37
Read 12.4-12.5 pgs. 555-565