The Properties of Mixtures: the Solution Process Lecture 17
Jun 23, 2015
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