dra10 dissolution solution solubility

8/10/2019 DRA10 Dissolution Solution Solubility

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Solutions are homogeneous mixtures of two

or more pure substances. In a solution, the solute is dispersed

uniformly throughout the solvent .


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Molecular Solutions Molecular compounds with similar

chemical structures and olarities tend to

be miscible. Homolo ous alcohol series have olar and

non-polar ends.


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Ionic Solutions

Solubility affected by: -

affects the solubility. Also called,

Lattice energy (energy holding the ions. to the charge on ions; larger charge means

. Inversely proportional to the size of the ion;

.


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Solubility increases with increasing ion size, due

soluble), Ca(OH) 2, Sr(OH) 2, Ba(OH) 2(mostsoluble lattice ener chan es dominant .

Energy of hydration increases with for smallerions than bigger ones; thus ion size. MgSO 4(mostsoluble),... BaSO 4 (least soluble.) Hydrationenergy dominant.


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How does a solid

dissolve into a liquid?

What drives the

dissolution process?

What are the energetics


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1. Solvent molecules attracted to surface ions.

2. Each ion is surrounded by solvent molecules.3. Enthalpy ( H) changes with each interactionro en or orme .

Ionic solid dissolving in water


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1. Solvent molecules attracted to surface ions.

2. Each ion is surrounded by solvent molecules.3. Enthalpy ( H) changes with each interactionro en or orme .


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The ions are solvated

(surrounded bysolvent).If the solvent is water,

the ions arey ra e .The intermolecular

-dipole.


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To determine the enthalpy

,process into 3 steps.

.particles.

2. Separation of solventparticles to make holes.

3. Formation of new

solute and solvent.


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Enthalpy Changes in Solution

The enthalpychange of theoverall process

depends on

H for .

Start

n

EndStart


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Enthalpy changes during dissolution

Hsoln = H1 + H2 + H3

The enthalpy ofsolution, Hsoln , can

e e er pos ve ornegative.

Hsoln (MgSO 4)= -91.2 kJ/mol --> exothermic Hsoln (NH 4NO 3)= 26.4 kJ/mol --> endothermic


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Dispersal of energy inthe s stem.

Number of microstates

(arrangements) in thesys em.

. , is the favored state


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Entro chan es durin dissolution

change in entropy.1. Separation of solute

particles.

2. Separation of solvent .3. Formation of new

interactions between soluteand solvent.


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SAMPLE EXERCISE 1 Assessing Entropy ChangeIn the process illustrated below, water vapor reacts with excess solid

.reaction is


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Entropy Usuall FavorsMaking SolutionsBecause There Are More

Possible Arrangements of


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G = H - T SSolubility increases with T if G becomes morenega ve w en ncreasesWhen S is positive, -T S is negative and G

will increase when T increases.

When S is negative, -T S is positive and G.will decrease when T increases.

CompareKCl(s) K+ (aq) + Cl - (aq) S positiveCe SO s 2 Ce 3+ a + 3 SO 2- a S ne ative


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Ni(s) + HCl(aq) NiCl 2(aq) + H 2(g) NiCl2(s)dry

original solute by evaporating the solvent.

If you cant, the substance didnt dissolve, it reacted.


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Saturated solution

Solvent holds as muchsolute as is possible ata empera ure.

Undissolved solidremains in flask.Dissolved solute is indynamic equilibrium

w so so u eparticles.


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Unsaturated SolutionLess than themaximum amount ofsolute for thattem erature isdissolved in thesolvent.

o so rema ns nflask.


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upersa ura eSolvent holds more solute than is normally

.These solutions are unstable; crystallization canoften be stimulated by adding a seed crystal orscratching the side of the flask.


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,

How much solute can be dissolved in a solution?


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ac ors ec ng o u ySolute-Solvent Interactions o ar qu s en o sso ve n po ar so ven s. Miscible liquids: mix in any proportions. Immiscible li uids: do not mix. Intermolecular forces are important: water and ethanol are

miscible because the broken hydrogen bonds in both pure- . The number of carbon atoms in a chain affect solubility: the

more C atoms the less soluble in water.


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Factors Affecting SolubilitySolute-Solvent Interactions


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Factors Affecting SolubilitySolute-Solvent Interactions

The number of -OH groups within a moleculeincreases solubility in water.


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Solute-Solvent Interactions

Generalization: like dissolves like. The more polar bonds in the molecule, the better itdissolves in a polar solvent.

The less polar the molecule the less it dissolves in apolar solvent and the better is dissolves in a non-polar solvent.

e wor so s o no sso ve ecause e s rongintermolecular forces in the solid are not re- .


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Factors Affecting SolubilityPressure Effects Solubility of a gas in a liquid is a function of thepressure o e gas. The higher the pressure, the more molecules of gas

of a gas molecule striking the surface and enteringthe solution.

Therefore, the higher the pressure, the greater the solubility.

The lower the pressure, the fewer molecules of gas are close to the solvent and the lower the

.


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Factors Affecting Solubility

Experience tells us that sugar dissolves better in.

As temperature increases, solubility of solids.

Sometimes, solubility decreases as temperature. . .


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Chemists use the axiomlike dissolves like:Polar substances tend todissolve in polar solvents.

to dissolve in nonpolarsolvents.


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intermolecularattractions betweensolute and solvent,

the more likely theso u e w sso ve.Example: ethanol in water

Ethanol = CH 3CH 2OHIntermolecular forces = H-bonds; dipole-dipole; dispersion

Ions in water also have ion-dipole forces.


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hydrogen bonding) isvery soluble in water.Cyclohexane (which

only has dispersionforces) is not water-soluble.


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Vitamin A is soluble in nonpolar compounds

. Vitamin C is soluble in water.


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Whichv am n swater-solublean w c sfat-soluble?


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In general, thesolubility of gases inwater increases withincreasing mass.

y Larger molecules

ave s rongerdispersion forces.


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QuickTime and aTIFF (LZW) decompressor

are needed to see this picture.


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liquids and solidsdoes not changeIncreasing

appreciably with

pressure.

above

solution But, the solubility of a

gas in a liquid isforcesmore gasto dissolve.

its pressure.


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g g

whereg

the gas;

constant for that gas inthat solvent

P g is the partialressure of the as

above the liquid.


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Tem erature De endenceof Gas Solubility G = H - T S S is negative -T S is positive G becomes more positive

w en ncreases Therefore, solubility


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,

solubility of solidsolutes in liquidsolvents increases

with increasingempera ure.


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The opposite is true of.

temperature drivesgases out of solution.

Carbonated soft drinks

stored in therefrigerator.

arm a es ave essO2 dissolved in themthan cool lakes.

dra10 dissolution solution solubility

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