Properties of Soln.

7/28/2019 Properties of Soln.

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Chapter 11- Properties of

Solutions

Sections 13.1 - 13.3

Dissolving Solubility

Read pg 529 543

pg 564 #1, 2, 3, 4, 6, 12, 13, 17, 19, 22, 25, 27, 29, 31,

84


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Solutions

Solutions are homogeneous mixtures of two

or more pure substances.

In a solution, the solute is dispersed uniformlythroughout the solvent.


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How Does a Solution Form?

As a solution forms, the solvent pulls solute

particles apart and surrounds, orsolvates,

them.


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QuickTime and a

Sorenson Video decompressorare needed to see this picture.


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How Does a Solution Form

If an ionic salt is

soluble in water, it is

because the ion-dipole interactions

are strong enough

to overcome the

lattice energy of thesalt crystal.


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From weakest to strongest, rank the

following solutions in terms of solvent

solute interactions: NaCl in water, butane

(C4H10) in benzene (C6H6), water in ethanol.

1. NaCl in water < C4H10 in C6H6 < water in ethanol2. Water in ethanol < NaCl in water < C4H10 in C6H6

3. C4H10 in C6H6 < water in ethanol < NaCl in water


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Correct Answer:

Butane in benzene will have

only weak dispersion forceinteractions. Water in

ethanol will exhibit much

stronger hydrogen-bondinginteractions. However, NaCl

in water will show iondipole

interactions because NaCl

will dissolve into ions.

1. NaCl in water < C4H10 in C6H6 < water in ethanol

2. Water in ethanol < NaCl in water < C4H

10in C

6H

6

3. C4H10 in C6H6 < water in ethanol < NaCl in water


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Energy Changes in Solution

Simply put, three

processes affect the

energetics of the

process:Separation of solute

particles

Separation of solventparticles

New interactions

between solute and

solvent


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Energy Changes in Solution

The enthalpy

change of the

overallprocess

depends on

Hfor each ofthese steps.


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Why Do Endothermic

Processes Occur?

Things do not tend

to occur

spontaneously (i.e.,

without outside

intervention) unless

the energy of the

system is lowered.


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Why Do Endothermic

Processes Occur?

Yet we know that

in some

processes, like

the dissolution of

NH4

NO3

in water,

heat is absorbed,

not released.


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Enthalpy Is Only Part of the Picture

The reason is that

increasing the

disorder orrandomness

(known as

entropy) of asystem tends to

lower the energy

of the system.


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Enthalpy Is Only Part of the Picture

So even though

enthalpy may

increase, theoverall energy of

the system can

still decrease ifthe system

becomes more

disordered.


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Water vapor reacts with excess solid

sodium sulfate to form the hydrated formof the salt. The chemical reaction is

Does the entropy increase or decrease?

SAMPLE EXERCISE 13.1 Assessing Entropy Change


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Student, Beware!

Just because a substance disappears when it

comes in contact with a solvent, it doesntmean the substance dissolved.


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Student, Beware!

Dissolution is a physical changeyou can

get back the original solute by evaporatingthe solvent.

If you cant, the substance didnt dissolve,

it reacted.


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Types of Solutions

Saturated

Solvent holds as

much solute as is

possible at thattemperature.

Dissolved solute is in

dynamic equilibriumwith solid solute

particles.


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Types of Solutions

Unsaturated

Less than the

maximum amount

of solute for that

temperature is

dissolved in thesolvent.


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Types of Solutions

Supersaturated

Solvent holds more solute than is normally

possible at that temperature.

These solutions are unstable; crystallization

can usually be stimulated by adding a seed

crystal or scratching the side of the flask.


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Factors Affecting Solubility

Chemists use the

axiom like dissolves

like:Polar substances

tend to dissolve in

polar solvents.

Nonpolar

substances tend to

dissolve in nonpolar

solvents.


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The more similar

the intermolecular

attractions, themore likely one

substance is to be

soluble in

another.


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Glucose (which has

hydrogen bonding)

is very soluble in

water, while

cyclohexane (which

only has dispersion

forces) is not.


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Vitamin A is soluble in nonpolar

compounds (like fats).

Vitamin C is soluble in water.


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SAMPLE EXERCISE 13.2 Predicting Solubility Patterns

Predict whether each of the following

substances is more likely to dissolve in carbontetrachloride (CCl4) or in water: C7H16, Na2SO4,

HCl, and I2.SolutionC7H16 is a hydrocarbon, so it is molecular

and nonpolar. Na2SO4, a compound containing ametal and nonmetals, is ionic; HCl, a diatomic

molecule containing two nonmetals that differ in

electronegativity, is polar; and I2, a diatomic

molecule with atoms of equal electronegativity, is

nonpolar. We would therefore predict that C7H16

and I2 would be more soluble in the nonpolar CCl4

than in polar H2O, whereas water would be thebetter solvent for Na SO and HCl.


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Answer :C5H12 < C5H11 Cl < C5H11 OH

Properties of Soln.

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