Physical Properties of SolutionsChapter 13

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1. Most chemical reaction occurs when ions and molecular Interaction within solution phase 2. In Ch5 and Ch11, we mentioned the gas, liquid and solid properties. 3. In this chapter, we will focus on the molecular interaction in solution phase.

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A solution is a homogenous mixture of 2 or more substances

The solute is(are) the substance(s) present in the smaller amount(s)

The solvent is the substance present in the larger amount

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A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature.

An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature.

A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature.

Sodium acetate crystals rapidly form when a seed crystal isadded to a supersaturated solution of sodium acetate.

Crystallization

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Three types of interactions in the solution process:• solvent-solvent interaction• solute-solute interaction• solvent-solute interaction

Molecular view of the formation of solution

Hsoln = H1 + H2 + H3

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“like dissolves like”

Two substances with similar intermolecular forces are likely to be soluble in each other.

• non-polar molecules are soluble in non-polar solvents

CCl4 in C6H6

• polar molecules are soluble in polar solvents

C2H5OH in H2O

• ionic compounds are more soluble in polar solvents

NaCl in H2O or NH3 (l)

Solvation(溶合 ): Hydration is one kind of Solvation

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Concentration UnitsThe concentration of a solution is the amount of solute present in a given quantity of solvent or solution.

Percent by Mass

% by mass = x 100%mass of solutemass of solute + mass of solvent

= x 100%mass of solutemass of solution

Mole Fraction (X)

XA = moles of A

sum of moles of all components

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Concentration Units Continued

M =moles of solute

liters of solution

Molarity (M)

Molality (m)

m =moles of solute

mass of solvent (kg)

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What is the molality of a 5.86 M ethanol (C2H5OH) solution whose density is 0.927 g/mL?

m =moles of solute

mass of solvent (kg)M =

moles of solute

liters of solution

Assume 1 L of solution:5.86 moles ethanol = 270 g ethanol927 g of solution (1000 mL x 0.927 g/mL)

mass of solvent = mass of solution – mass of solute

= 927 g – 270 g = 657 g = 0.657 kg

m =moles of solute

mass of solvent (kg)=

5.86 moles C2H5OH

0.657 kg solvent= 8.92 m

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Selection of the concentration unit

Mole fraction: used to calculate the partial pressure of gas or the vapor pressure of a solution.Molarity : Ease for preparing the solution

Molality : When consider the temperature effect on solution using Molality will be better e.g.: 1.0 M solution prepared at 25 oC will change to 0.97M when temperature upto 45oC

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Temperature and Solubility

Solid solubility and temperature

solubility increases with increasing temperature

solubility decreases with increasing temperature

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Fractional crystallization(分項結晶 )

Ex: 90 g KNO3 mixed with 10g NaCl, when cooled to 0oC, (90-12) g=78g KNO3 can be obtained from mixed solution

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Temperature and Solubility

O2 gas solubility and temperature

solubility usually decreases with

increasing temperature

Heat west will strongly effect the environment due to the low solubilityof O2 in water.

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Pressure and Solubility of Gases

The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law).

c = kP

c is the concentration (M) of the dissolved gas

P is the pressure of the gas over the solution

k is a constant for each gas (mol/L•atm) that depends only on temperature

low P

low c

high P

high c

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1. Henry’s Law can be explained as the impact frequency between gas molecular and liquid surface

2. The kinetic equilibrium: Vapor rate v.s Solubility 3. Most gas will follow Henry’s law, however, some gas is excluded. EX: Some reaction make the solubility much higher: 1. NH3 + H2ONH4

+ + OH-

2. CO2+H2O H2CO3 3. Hb+4O2 Hb(O2)4

CO2 gas from soda

Henry’s law

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Colligative Properties

Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.

依數性質

1. The solution concentration is < 0.2 M2. Vapor-Pressure Lowering3. Boiling-Point Elevation4. Freezing-Point Depression5. Osmotic Pressure

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Raoult’s law

If the solution contains only one solute:

X1 = 1 – X2

P 10 - P1 = P = X2 P 1

0

P 10 = vapor pressure of pure solvent

X1 = mole fraction of the solvent

X2 = mole fraction of the solute

P1 = X1 P 10

Vapor-Pressure Lowering (蒸氣壓下降 )

溶液中之溶劑分壓 = 純溶劑蒸氣壓 X 溶劑莫耳分率

蒸氣壓下降與溶質莫耳數成正比

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PA = XA P A0

PB = XB P B0

PT = PA + PB

PT = XA P A0 + XB P B

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Ideal Solution

Volatile solution(both A and B are volatile)

Hsoln

Ideal solution:=0

溶解熱

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Nonideal gas:Case 1: The intermolecular forces between A and B are weaker than these between A or B molecular.Case 2: The intermolecular forces between A and B are stronger than these between A or B molecular.

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Application: Fractional distillation ( 分餾結晶 or 蒸餾 )

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Boiling-Point Elevation

Tb = Tb – T b0

Tb > T b0 Tb > 0

T b is the boiling point of the pure solvent

0

T b is the boiling point of the solution

Tb = Kb m

m is the molality of the solution

Kb is the molal boiling-point elevation constant (0C/m) for a given solvent

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Freezing-Point Depression

Tf = T f – Tf0

T f > Tf0 Tf > 0

T f is the freezing point of the pure solvent

0

T f is the freezing point of the solution

Tf = Kf m

m is the molality of the solution

Kf is the molal freezing-point depression constant (0C/m) for a given solvent

EX: 灑鹽讓雪融化EG 乙二醇抗凍劑

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How to explain the freezing-point depression:

A transition from the disordered state to the order state. For This to happen, more energy needs to be removed from theSystem. Because solution has greater disorder than the solvent,More energy need to removed. 亂度

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What is the freezing point of a solution containing 478 g of ethylene glycol (antifreeze) in 3202 g of water? The molar mass of ethylene glycol is 62.01 g.

Tf = Kf m

m =moles of solute

mass of solvent (kg)= 2.41 m=

3.202 kg solvent

478 g x 1 mol62.01 g

Kf water = 1.86 oC/m

Tf = Kf m = 1.86 oC/m x 2.41 m = 4.48 oC

Tf = T f – Tf0

Tf = T f – Tf0 = 0.00 oC – 4.48 oC = -4.48 oC

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Osmotic Pressure ()

Osmosis is the selective passage of solvent molecules through a porous membrane from a dilute solution to a more concentrated one.

A semipermeable membrane allows the passage of solvent molecules but blocks the passage of solute molecules.

Osmotic pressure () is the pressure required to stop osmosis.

dilutemore

concentrated

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HighP

LowP

Osmotic Pressure ()

= MRT

M is the molarity of the solution

R is the gas constant

T is the temperature (in K)

solvent solution

time

Ex: 1.紅血球放在低滲溶液中 , 使血球爆破。2.用大量糖保存食物。 (高滲溶液 )

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A cell in an:

isotonicsolution

hypotonicsolution

hypertonicsolution

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海水淡化科技

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Colligative Properties

Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.

Vapor-Pressure Lowering P1 = X1 P 10

Boiling-Point Elevation Tb = Kb m

Freezing-Point Depression Tf = Kf m

Osmotic Pressure () = MRT

一般可由實驗方式量測溶液的待測濃度 (m or M)

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Free ions v.s bounded ion pair

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Colligative Properties of Electrolyte Solutions

0.1 m NaCl solution 0.1 m Na+ ions & 0.1 m Cl- ions

Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles.

0.1 m NaCl solution 0.2 m ions in solution

van’t Hoff factor (i) = actual number of particles in soln after dissociation

number of formula units initially dissolved in soln

nonelectrolytesNaCl

CaCl2

i should be

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考慮電解質的依數性質

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Boiling-Point Elevation Tb = i Kb m

Freezing-Point Depression Tf = i Kf m

Osmotic Pressure () = iMRT

Colligative Properties of Electrolyte Solutions

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Colloids ( 膠體 )Between homogenous solution and Nonhomogenous solution. A colloid is a dispersion of particles of one substance throughout a dispersing medium made of another Substance.

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How to identify the solution and colloid?

One way to distinguish a solution from A colloid is be the Tyndall effect.

When a beam of light passes through A colloid, it is scattered by the dispersedPhase.

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Hydrophilic and Hydrophobic Colloids 1.Hydrophilic: water-loving2.Hydrophobic: water-fearing

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E.g: 河流道出海口形成淤泥的現象

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A long hydrocarbon tail that is nonpolar.

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HW CH1313.17 Calculate the molalities of these aqueous solutions: (a) 1.22 M sugar (C12H22O11) solution (density of solution = 1.12 g/mL), (b) 0.87 M NaOH solution (density of solution = 1.04 g/mL), (c) 5.24 M NaHCO3 solution (density of solution = 1.19 g/mL).

13.49 The vapor pressure of benzene is 100.0 mmHg at 26.1°C. Calculate the vapor pressure of a solution containing 24.6 g of camphor (C10H16O) dissolved in 98.5 g of benzene. (Camphor is a low-volatility solid.)

13.75 The osmotic pressure of 0.010 M solutions of CaCl2 and urea at 25°C are 0.605 atm and 0.245 atm, respectively. Calculate the van't Hoff factor for the CaCl2 solution.