Solutions nt/Solutions_30SE.ppt.

Solutions

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Definitions

A solution is a homogeneous mixture of a solute dissolved in a solvent.

The solvent is generally in excess an aqueous solution has water as solvent.

Example

The solution NaCl(aq) is sodium chloride NaCl(s) dissolved in water H2O(l)

The solute is NaCl(s) and the solvent is H2O(l).

Air is an example of a solution with one “solvent” (nitrogen) and many “solutes” (oxygen, helium, argon, carbon dioxide, etc.)

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DefinitionsDefinitionsSolutions can be classified as Solutions can be classified as

saturatedsaturated or or ununsaturatedsaturated..A A saturatedsaturated solution contains the solution contains the

maximum quantity of solute that maximum quantity of solute that dissolves at that temperature.dissolves at that temperature.

A saturated solution represents equilibrium: rate of dissolving equals to rate of crystallization

An An unsaturatedunsaturated solution contains solution contains less than the maximum amount less than the maximum amount of solute that can dissolve at a of solute that can dissolve at a particular temperatureparticular temperature

A SUPERSATURATED A SUPERSATURATED SOLUTIONS SOLUTIONS contain more solute contain more solute than is possible to be dissolvedthan is possible to be dissolved

http://www.authorstream.com/Presentation/Margot-


Supersaturated solutions Supersaturated solutions are are unstable. The supersaturation is unstable. The supersaturation is only temporary, and usually only temporary, and usually accomplished in one of two ways:accomplished in one of two ways:

1.1. Warm the solvent so that it will Warm the solvent so that it will dissolve more, then cool the dissolve more, then cool the solution solution

2. Evaporate some of the solvent carefully so that the solute does not solidify and come out of solution.

Electrolyte and Non-electrolyte

Electrolyte: a substance that conducts electricity when dissolved in water. – Acids, bases and soluble ionic solutions

are electrolytes. Non-electrolyte: a substance that

does not conduct electricity when dissolved in water. – Molecular compounds and insoluble

ionic compounds are non-electrolytes.

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Electrolytes Some solutes can dissociate into ions.

Electric charge can be carried.

Types of solutes

Na+

Cl-

Strong Electrolyte -100% dissociation,all ions in solution

high conductivity


Types of solutes

CH3COOH

CH3COO-

H+

Weak Electrolyte -partial dissociation,molecules and ions in solution

slight conductivity


Types of solutes

sugar

Non-electrolyte -No dissociation,all molecules in solution

no conductivity



Types of Electrolytes

Weak electrolyte partially dissociates.

– Fair conductor of electricity. Non-electrolyte does not dissociate.

– Poor conductor of electricity.

• Strong electrolyte dissociates completely.– Good electrical conduction.


Representation of Electrolytes using Chemical Equations

MgCl2(s)+H2O → Mg2+(aq) + 2 Cl- (aq)

A strong electrolyte:

A weak electrolyte:

CH3COOH(aq) ← CH3COO -(aq) +H+(aq)→

CH3OH(aq)

A non-electrolyte:





Strong ElectrolytesStrong acids: HNO3, H2SO4, HCl, HClO4

Strong bases: MOH (M = Na, K, Cs, Rb etc)

Salts: All salts dissolving in water are completely ionized.

Stoichiometry & concentration relationship

NaCl (s) +H2O Na+ (aq) + Cl– (aq)

Ca(OH)2 (s) +H2O Ca2+(aq) + 2 OH– (aq)

AlCl3 (s) +H2O Al3+ (aq) + 3 Cl– (aq)

(NH4)2SO4 (s) +H2O 2 NH4 + (aq) + SO42– (aq)

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3 Stages of Solution Process Separation of Solute

– must overcome IMF(Intermolecular forces) or ion-ion attractions in solute

– requires energy, ENDOTHERMIC ( + H)

Separation of Solvent– must overcome IMF of solvent particles– requires energy, ENDOTHERMIC (+ H)

Interaction of Solute & Solvent– attractive bonds form between solute particles and solvent

particles– “Solvation” or “Hydration” (where water = solvent)– releases energy, EXOTHERMIC (- H)


Dissolution at the molecular level?

Consider the dissolution of NaOH in H2O


Factors Affecting Solubility

1. 1. Nature of Solute / SolventNature of Solute / Solvent- Like dissolves like (IMF)

2. 2. Temperature -Temperature -i) Solids/Liquids- Solubility increases with Temperature

Increase K.E. increases motion and collision between solute / solvent.

ii) Gas - Solubility decreases with Temperature

Increase K.E. result in gas escaping to atmosphere.

3. 3. Pressure Factor Pressure Factor --i) Solids/Liquids - Very little effect

Solids and Liquids are already close together, extra pressure will not increase solubility.

ii) gas - Solubility increases with Pressure.

Increase pressure squeezes gas solute into solvent.

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Solubility curve

Saturated

Unsaturated

Supersaturated


Solubility curve

Any solution can be made saturated, unsaturated, or supersaturated by changing the temperature.


Solubilities of Solids vs Temperature

Solubilities of several ionic solid as a function of temperature. MOST salts have greater solubility in hot water.

A few salts have negative heat of solution, (exothermic process) and they become less soluble with increasing

temperature.


The rate of solution

The rate of solution is a measure of how fast a substance dissolves. Some of the factors determining the rate of solution are:

size of the particles -- When a solute dissolves, the action takes place only at the surface of each particle. When the total surface area of the solute particles is increased, the solute dissolves more rapidly. Breaking a solute into smaller pieces increases its surface area and hence its rate of solution.

(Sample problem: a cube with sides 1.0 cm long is cut in half, producing two pieces with dimensions of 1.0 cm x 1.0 cm x 0.50 cm. How much greater than the surface area of the original cube is the combined surface areas of the two pieces?

2.0 cm2


The rate of dissolution stirring -- With liquid and solid solutes, stirring

brings fresh portions of the solvent in contact with the solute, thereby increasing the rate of solution.

amount of solute already dissolved -- When there is little solute already in solution, dissolving takes place relatively rapidly. As the solution approaches the point where no solute can be dissolved, dissolving takes place more slowly.

temperature -- For solid, liquid and gaseous solutes, changing the temperature not only changes the amount of solute that will dissolve but also changes the rate at which the solute will dissolve.


Temperature & the Solubility of GasesThe solubility of gases decreases at higher temperatures WHY???


The effect of partial pressure on solubility of gases Henry’s Law

At pressure of few atmosphere or less, solubility of gas solute follows Henry Law which states that the amount of solute gas dissolved in solution is directly proportional to the amount of pressure above the solution.

c = k Pc = k P

c = solubility of the gas (M)k = Henry’s Law ConstantP = partial pressure of gas

Henry’s Law Constants (25°C), k

N2 8.42 •10-7 M/mmHg

O2 1.66 •10-6 M/mmHg

CO2 4.48•10-5 M/mmHg

How does Henry’s Law apply?? & Soft Drinks

Henry’s Law & Soft Drinks Soft drinks contain “carbonated

water” – water with dissolved carbon dioxide gas.

The drinks are bottled with a CO2 pressure greater than 1 atm.

When the bottle is opened, the pressure of CO2 decreases and the solubility of CO2 decreases, according to Henry’s Law.

Therefore, bubbles of CO2 escape from solution.

Colligative PropertiesColligative PropertiesOn adding a solute to a solvent, the properties of On adding a solute to a solvent, the properties of

the solvent are modified.the solvent are modified. Vapor pressure Vapor pressure decreasesdecreases Melting point Melting point decreasesdecreases Boiling point Boiling point increasesincreases Osmosis is possible (osmotic pressure)Osmosis is possible (osmotic pressure)

These changes are called These changes are called COLLIGATIVE COLLIGATIVE PROPERTIESPROPERTIES. .

They depend only on theThey depend only on the NUMBERNUMBER of solute of solute particles relative to solvent particles, not on the particles relative to solvent particles, not on the KINDKIND of solute particles. of solute particles.


Vapor Pressure Lowering for a Solution The diagram below shows how a phase diagram is affected by

dissolving a solute in a solvent. Notice the changes in the freezing & boiling points.


Vapor Pressure Lowering

The presence of a non-volatile solute means that fewer solvent particles are at the solution’s surface, so less solvent evaporates!


Raoult’s LawDescribes vapor pressure lowering mathematically

The lowering of the vapour pressure when a non-volatile solute is dissolved in a volatile solvent (A) can be described by Raoult’s Law:

PA = AP°A

PA = vapour pressure of solvent A above solution

XA = mole fraction of the solvent A in solution.

P°A = vapour pressure of pure solvent A .

only the solvent (A) contributes to the vapour pressure of the solution


Mixtures of Volatile LiquidsBoth liquids evaporate & contribute to the vapor pressure


Raoult’s Law: Mixing Two Volatile Liquids

Since BOTH liquids are volatile and contribute to the vapour, the total vapor pressure can be represented using Dalton’s Law:

PT = PA + PB

The vapor pressure from each component follows Raoult’s Law:

PT = AP°A + BP°B

Also, A + B = 1 (since there are 2 components) Ideal solutions obtained if solute-solute, solute-solvent, and solvent-solvent interactions are similar, i.e. ΔHsoln = 0.


Deviations from ideality occur if, , there are strong solute-solvent

interactions as may be in H-bonding between solute and solvent.

Such solutions are called nonideal solutions.

― Deviations from Raoult’s law

ΔHsoln << 0 negative deviation⇒ΔHsoln >> 0 positive deviation⇒Benzene - Toluene mixture:

– The vapor pressure from each component follows Raoult's Law.

Recall that with only two components, Bz + Tol = 1

Benzene: when Bz = 1, PBz = P°Bz = 384 torr &

when Bz = 0 , PBz = 0

Toluene: when Tol = 1, PTol = P°Tol = 133 torr & when Tol = 0, PBz = 0

384 torr

133 torr

X Benzene

X Toluene

0 1

1 0

P (Total)

P (Benzene)

P (Toluene)

133 torr

384 torr



Normal Boiling ProcessNormal Boiling Point: BP of Substance @ 1atm

When solute is added , BP > Normal BP

Boiling point is elevated when solute inhibits solvent from escaping.


Boiling Point Elevation

ΔTb = (Tb -Tb°) = i ·m ·kb

Where, ΔTb = BP. Elevation

Tb = BP of solvent in solution

Tb° = BP of pure solvent

m = molality , kb = BP Constant

Some Boiling Point Elevation and Freezing Point Depression Constants

Normal bp (°C) Kb Normal fp (°C) Kf

Solvent pure solvent (°C/m) pure solvent (°C/m)

Water Water 100.00100.00 +0.5121 +0.5121 0.0 0.0 1.861.86BenzeneBenzene 80.10 80.10 +2.53+2.53 5.50 5.50 4.904.90CamphorCamphor 207 207 +5.611 +5.611 179.75 179.75 39.739.7Chloroform Chloroform 61.70 61.70 +3.63+3.63 - 63.5 - 63.5 4.70 4.70 (CH(CH33Cl)Cl)

Some Boiling Point Elevation and Freezing Point Depression Constants

Normal bp (°C) Kb Normal fp (°C) Kf

Solvent pure solvent (°C/m) pure solvent (°C/m)

Water Water 100.00100.00 +0.5121 +0.5121 0.0 0.0 1.861.86BenzeneBenzene 80.10 80.10 +2.53+2.53 5.50 5.50 4.904.90CamphorCamphor 207 207 +5.611 +5.611 179.75 179.75 39.739.7Chloroform Chloroform 61.70 61.70 +3.63+3.63 - 63.5 - 63.5 4.70 4.70 (CH(CH33Cl)Cl)

Boiling Point Elevation and Boiling Point Elevation and Freezing Point DepressionFreezing Point Depression

∆∆T = i K m T = i K m i = van’t Hoff factor = number of particles per i = van’t Hoff factor = number of particles per

molecule/formula unit. molecule/formula unit. For covalent compounds, i = 1. For covalent compounds, i = 1. For ionic compounds, i = the number of ionsFor ionic compounds, i = the number of ions

CompoundCompound Theoretical Value of iTheoretical Value of iglycolglycol 1 1NaClNaCl 22CaClCaCl22 33CaCa33(PO(PO44))22 55

When solution freezes the solid form is almost always pure.

Solute particles does not fit into the crystal lattice of the solvent because of the differences in size. The solute essentially remains in solution and blocks other solvent from fitting into the crystal lattice during the freezing process.


Freezing Point Depression Normal Freezing Point: FP of Substance @ 1atm When solute is added, FP < Normal FP FP is depressed when solute inhibits solvent from crystallizing.


Freezing Point Depression

Phase Diagram and the lowering of the freezing point.

Tf = i ·m ·kf

Where, Tf = FP depression

i = van’t Hoff Factorm = molality , kf = FP

Constant

Generally freezing point depression is used to determine the molar mass of an unknown substance.

Derive an equation to find molar mass from the equation above.


Osmotic pressure

Osmosis is the spontaneous movement of water across a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration

Osmotic Pressure - The Pressure that must be applied to stop osmosis

= i CRT

where P = osmotic pressure i = van’t Hoff factor C = molarity R = ideal gas constant T = Kelvin temperature


Osmosis and Blood Cells

(a) A cell placed in an isotonic solution. The net movement of water in and out of the cell is zero because the concentration of solutes inside and outside the cell is the same.

(b) In a hypertonic solution, the concentration of solutes outside the cell is greater than that inside. There is a net flow of water out of the cell, causing the cell to dehydrate, shrink, and perhaps die.

(c) In a hypotonic solution, the concentration of solutes outside of the cell is less than that inside. There is a net flow of water into the cell, causing the cell to swell and perhaps to burst.

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