Factors Affecting Solution FormationPt. 2

By Shawn P. Shields, Ph.D.

This work is licensed by Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Recall: Solution Formation

The formation of a solution is a physical process (as opposed to chemical).

Solutions form based on two factors:

The tendency toward mixing (increased entropy)

The “match” between the intermolecular forces for the solute and the solvent.

Enthalpy of Solution (Hsoln)

The energetics of solution formation can basically be divided into three processes:

First, solvent molecules need to be “pushed apart,” which requires energy to overcome IM attractions.

Solute molecules need to be “pushed apart” as well, which also requires energy (to overcome interparticle attractions).

Enthalpy of Solution (Hsoln)Finally, solution formation (if energetically possible).

If the solute-solvent interactions are at least as favorable as solute-solute or solvent-solvent interactions, energy will be released (exothermic, negative Hsoln)

If the solute-solvent interactions are less favorable than the solute-solute or solvent-solvent interactions, energy will be absorbed (endothermic, positive Hsoln)

Enthalpy of Solution (Hsoln)The enthalpy of solution formation can also be expressed as

Hsoln = H1 + H2 + H3 H1 is the energy required to push apart solvent molecules.H2 is the energy required to separate solute particles H3 is the amount of energy released upon solution formation.

Enthalpy and Solution Formation

http://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0m/s17-01-factors-affecting-solution-for.html

Effect of Entropy on Solution Formation

Why would a solution form spontaneously if the overall process is endothermic?

Entropy- the tendency toward disorder (or mixing).

Entropy increases in a spontaneous process.

Effect of Entropy on Solution Formation

Entropy is increased when a solute is dissolved in a solvent and distributed throughout the volume.

When Hsoln is positive (endothermic), entropy is the driving force for solution formation.

H

O HH

O

HH

OHH

O

H

Na+

HO

HCl-

HO

H

H

OH

HO

H

H

OH

NaCl Dissolves into Ions in Solution

Energy is required to push apart H2O molecules and overcome the lattice energy of NaCl.Energy is released due to ion-dipole attractions and increased entropy.

+

Summary of the Energetics of Solution Formation

Substances with similar intermolecular forces tend to be soluble in one another.

“Like dissolves like”Nonpolar solvents dissolve nonpolar solutes

Polar solvents are more likely to dissolve ionic and polar solutes.

Pressure Effects on Solubility

Henry’s Law (applies to gases dissolved in a solvent)

The solubility of a gas in any solvent increases with increasing partial pressure of the gas above the solvent.

gas (solute)

Pressure Effects on Solubility

Henry’s Law: Sg = kPg

Sg = the solubility of the gas (M)

Pg = the partial pressure of the gas above the solventk = the Henry’s law constant (specific for each solvent-gas pair at a given T) (look this up)

gas (solute)

Temperature Effects on Solution Formation

The general trend is that solubility of most solids in water increases with increasing temperature.

The solubility of gases in water decreases with increasing temperature.

Solubility vs Temperature

http://2012books.lardbucket.org/books/principles-of-general-chemistry-v1.0m/s17-04-effects-of-temperature-and-pre.html

Next up…

Colligative Properties