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Lecture17222

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Information about Lecture17222

Published on August 7, 2008

Author: uladzimir

Source: slideshare.net

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a supplemental resource for students
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The Properties of Mixtures: the Solution Process Lecture 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!

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. ∆ H solute > 0

Solute (aggregated) + heat  solute (separated)

To overcome intermolecular attractions, energy is needed.

So the process is endothermic.

∆ H solute > 0

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

Solvent (aggregated) + heat  solvent (separated)

To overcome intermolecular attractions, energy is needed.

So the process is endothermic.

∆ H solvent > 0

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

Solute (separated) + solvent (separated)  solution + heat

The particles attract each other, energy is released.

So the process is exothermic.

∆ H mix < 0

The three events in solution

Heat of solution (∆H soln ) is the total enthalpy change that occurs when a solution forms from solute and solvent. May be both exothermic and endothermic.

Thermochemical solution cycle: ∆ H soln = ∆H solute + ∆H solvent + ∆H mix 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: ∆ H soln = ∆H solute + (∆H solvent + ∆H mix ) ∆ H hydr = ∆H solvent + ∆H mix ∆ H soln = ∆H solute + ∆H hydr

Heat of hydration NaCl (g)  Na + (g) + Cl - (g) Na + (g) + 6H 2 O (l)  [Na(H 2 O) 6 ] + (aq) Cl - (g) + 6H 2 O (l)  [Cl(H 2 O) 6 ] - (aq) ------------------------------------------- NaCl (s) + 6H 2 O (l)  [Na(H 2 O) 6 ] + (aq) +[Cl(H 2 O) 6 ] - (aq) M + (g) [or X - (g) ] + H 2 O  M + (aq) [or X - (aq) ] ∆ H hydr of the ion < 0, always

NaCl (g)  Na + (g) + Cl - (g)

Na + (g) + 6H 2 O (l)  [Na(H 2 O) 6 ] + (aq)

Cl - (g) + 6H 2 O (l)  [Cl(H 2 O) 6 ] - (aq)

-------------------------------------------

NaCl (s) + 6H 2 O (l)  [Na(H 2 O) 6 ] + (aq) +[Cl(H 2 O) 6 ] - (aq)

M + (g) [or X - (g) ] + H 2 O  M + (aq) [or X - (aq) ]

∆ H hydr 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  H hydr is.

Coulomb’s law A 2+ ion attracts H 2 O molecules more strongly than a 1+ ion of similar size; A small 1+ ion attracts H 2 O molecules more strongly than a large 1+ ion.

A 2+ ion attracts H 2 O molecules more strongly than a 1+ ion of similar size;

A small 1+ ion attracts H 2 O 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 group increase across a period of ions (Na + —Mg 2+ —Al 3+ ) - 3rd period

decrease down a group of ions (Li + —Na + —K + —Rb + —Cs + —Fr + ) - 1A

(F - —Cl - —Br - —I - ) - 7A group

increase across a period of ions (Na + —Mg 2+ —Al 3+ ) - 3rd period

The heat of solution for ionic compounds in water:  H soln =  H lattice +  H hydration of the ions  H lattice is always positive  H hydration is always negative

Dissolving ionic compounds in water

Hot (CaCl 2 ) and cold (NH 4 NO 3 ) packs

The heat of solution  H soln is only one of two factors determining whether a solute dissolves in a solvent. The other factor is entropy S.

 

 

Entropy is 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

Ludwig Eduard Boltzmann (1844–1906), Austrian scientist

Freedom of particle motion and entropy S liquid > S solid ; ∆S melting > 0 S gas > S liquid ; ∆S vaporization > 0 S solid > S gas ; ∆S sublimation > 0

S liquid > S solid ; ∆S melting > 0

S gas > S liquid ; ∆S vaporization > 0

S solid > S gas ; ∆S sublimation > 0

Solid state: minimum entropy

A solution usually has higher entropy than the pure solute and pure solvent: S soln > (S solute + S solvent ) ∆ S soln > 0

Systems tend toward a state of lower enthalpy and higher entropy.

Entropy is higher when mixed

THE END

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