The Properties of Mixtures: the Solution

ProcessLecture 17

Similia similibus solvuntur

Macroscopic rule “like dissolves like”

is based on microscopic interactions.

How do enthalpy and entropy change in solute-solvent

interaction?

Three events in the process of solution:Solute particles separate from each other (some energy must be absorbed);Some solvent particles separate to make room for the solute particles;Solute and solvent particles mix together (some energy must be released).

There must be change in enthalpy!

Solution: separating particles

Solute particles separate from each other:Solute (aggregated) + heat solute (separated)To overcome intermolecular attractions, energy is needed.So the process is endothermic.∆Hsolute > 0

Solvent particles separate from each other:Solvent (aggregated) + heat solvent (separated)To overcome intermolecular attractions, energy is needed.So the process is endothermic.∆Hsolvent > 0

Solute and solvent particles mix:Solute (separated) + solvent (separated) solution + heatThe particles attract each other, energy is released.So the process is exothermic.∆Hmix < 0

The three events in solution

Heat of solution (∆Hsoln)

is the total enthalpy change that occurs when a solution

forms from solute and solvent.May be both exothermic and

endothermic.

Thermochemical solution

cycle:∆Hsoln = ∆Hsolute + ∆Hsolvent + ∆Hmix

Resembles Hess’s law and Born-Haber cycle.

Enthalpy components of the heat of solution

Solution implies solvation.

Solvation is a process of surrounding a solute particle

with solvent particles.Hydration is a process of

surrounding a solute particle with water molecules.

Heat of hydration:

∆Hsoln = ∆Hsolute + (∆Hsolvent + ∆Hmix)

∆Hhydr = ∆Hsolvent + ∆Hmix

∆Hsoln = ∆Hsolute + ∆Hhydr

Heat of hydration

NaCl(g) Na+(g) + Cl-

(g)

Na+(g) + 6H2O(l) [Na(H2O)6]+

(aq)

Cl-(g) + 6H2O(l) [Cl(H2O)6]-

(aq)

-------------------------------------------NaCl(s) + 6H2O(l) [Na(H2O)6]+

(aq)+[Cl(H2O)6]-

(aq)

M+(g)[or X-

(g)] + H2O M+(aq)[or X-

(aq)]

∆Hhydr of the ion < 0, always

Charge density of an ion

is the ratio of the ion’s charge to its volume.

In general, the higher the charge density is,

the more negative Hhydr is.

Coulomb’s law

A 2+ ion attracts H2O molecules more strongly than a 1+ ion of similar size;A small 1+ ion attracts H2O molecules more strongly than a large 1+ ion.

Charge densities and heats of hydration

decrease down a group of ions (Li+—Na+—K+—Rb+—Cs+—Fr+) - 1A(F-—Cl-—Br-—I-) - 7A groupincrease across a period of ions (Na+—Mg2+—Al3+) - 3rd period

The heat of solution

for ionic compounds in water:Hsoln = Hlattice + Hhydration of the ions

Hlattice is always positive

Hhydration is always negative

Dissolving ionic compounds in

water

Hot (CaCl2) and cold (NH4NO3) packs

The heat of solution Hsoln

is only one of two factors determining whether a solute

dissolves in a solvent.The other factor is entropy S.

Entropyis directly related to the

number of ways that a system can distribute its energy.

It is closely related to the freedom of motion of the

particles and the number of ways they can be arranged.

Ludwig Eduard Boltzmann (1844–1906), Austrian scientist

Freedom of particle motion and entropy

Sliquid > Ssolid ; ∆Smelting > 0

Sgas > Sliquid ; ∆Svaporization > 0

Ssolid > Sgas ; ∆Ssublimation > 0

Solid state: minimum entropy

A solutionusually has higher entropy than

the pure solute and pure solvent:

Ssoln > (Ssolute + Ssolvent)

∆Ssoln > 0

Systems tendtoward a state of lower enthalpy

and higher entropy.

Entropy is higher when mixed

THE END