lecture17222
DESCRIPTION
a supplemental resource for studentsTRANSCRIPT
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The Properties of Mixtures: the Solution
ProcessLecture 17
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Similia similibus solvuntur
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Macroscopic rule “like dissolves like”
is based on microscopic interactions.
How do enthalpy and entropy change in solute-solvent
interaction?
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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!
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Solution: separating particles
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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
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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
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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
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The three events in solution
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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.
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Thermochemical solution
cycle:∆Hsoln = ∆Hsolute + ∆Hsolvent + ∆Hmix
Resembles Hess’s law and Born-Haber cycle.
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Enthalpy components of the heat of solution
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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.
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Heat of hydration:
∆Hsoln = ∆Hsolute + (∆Hsolvent + ∆Hmix)
∆Hhydr = ∆Hsolvent + ∆Hmix
∆Hsoln = ∆Hsolute + ∆Hhydr
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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
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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.
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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.
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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
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The heat of solution
for ionic compounds in water:Hsoln = Hlattice + Hhydration of the ions
Hlattice is always positive
Hhydration is always negative
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Dissolving ionic compounds in
water
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Hot (CaCl2) and cold (NH4NO3) packs
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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.
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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.
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Ludwig Eduard Boltzmann (1844–1906), Austrian scientist
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Freedom of particle motion and entropy
Sliquid > Ssolid ; ∆Smelting > 0
Sgas > Sliquid ; ∆Svaporization > 0
Ssolid > Sgas ; ∆Ssublimation > 0
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Solid state: minimum entropy
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A solutionusually has higher entropy than
the pure solute and pure solvent:
Ssoln > (Ssolute + Ssolvent)
∆Ssoln > 0
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Systems tendtoward a state of lower enthalpy
and higher entropy.
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Entropy is higher when mixed
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THE END