inter molecular forces l.
Download
Skip this Video
Download Presentation
Inter- molecular Forces

Loading in 2 Seconds...

play fullscreen
1 / 70

Inter- molecular Forces - PowerPoint PPT Presentation


  • 107 Views
  • Uploaded on

Inter- molecular Forces. Have studied INTRA molecular forces—the forces holding atoms together to form molecules. Now turn to forces between molecules — INTER molecular forces. Forces between molecules, between ions, or between molecules and ions. Ion-Ion Forces for comparison of magnitude.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Inter- molecular Forces' - reia


Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
inter molecular forces
Inter-molecular Forces

Have studied INTRAmolecular forces—the forces holding atoms together to form molecules.

Now turn to forces between molecules —INTERmolecular forces.

Forces between molecules, between ions, or between molecules and ions.

ion ion forces for comparison of magnitude
Ion-Ion Forcesfor comparison of magnitude

Na+—Cl- in salt

These are the strongest forces.

Lead to solids with high melting temperatures.

NaCl, mp = 800 oC

MgO, mp = 2800 oC

covalent bonding forces for comparison of magnitude

C=C, 610 kJ/mol

C–C, 346 kJ/mol

C–H, 413 kJ/mol

CN, 887 kJ/mol

Covalent Bonding Forcesfor comparison of magnitude
attraction between ions and permanent dipoles
Attraction Between Ions and Permanent Dipoles

Water is highly polar and can interact with positive ions to give hydrated ions in water.

attraction between ions and permanent dipoles5
Attraction Between Ions and Permanent Dipoles

Water is highly polar and can interact with positive ions to give hydrated ions in water.

attraction between ions and permanent dipoles6
Attraction Between Ions and Permanent Dipoles

Many metal ions are hydrated. This is the reason metal salts dissolve in water.

attraction between ions and permanent dipoles7
Attraction Between Ions and Permanent Dipoles

Attraction between ions and dipole depends on ion charge and ion-dipole distance.

Measured by ∆H for Mn+ + H2O --> [M(H2O)x]n+

-1922 kJ/mol

-405 kJ/mol

-263 kJ/mol

dipole dipole forces
Dipole-Dipole Forces

Such forces bind molecules having permanent dipoles to one another.

dipole dipole forces9
Dipole-Dipole Forces

Influence of dipole-dipole forces is seen in the boiling points of simple molecules.

Compd Mol. Wt. Boil Point

N2 28 -196 oC

CO 28 -192 oC

Br2 160 59 oC

ICl 162 97 oC

hydrogen bonding
Hydrogen Bonding

A special form of dipole-dipole attraction, which enhances dipole-dipole attractions.

H-bonding is strongest when X and Y are N, O, or F

hydrogen bonding in h 2 o
Hydrogen Bonding in H2O

H-bonding is especially strong in water because

  • the O—H bond is very polar
  • there are 2 lone pairs on the O atom

Accounts for many of water’s unique properties.

hydrogen bonding in h 2 o12
Hydrogen Bonding in H2O

Ice has open lattice-like structure.

Ice density is < liquid.

And so solid floats on water.

Snow flake: www.snowcrystals.com

hydrogen bonding in h 2 o13
Hydrogen Bonding in H2O

Ice has open lattice-like structure.

Ice density is < liquid and so solid floats on water.

One of the VERY few substances where solid is LESS DENSE than the liquid.

hydrogen bonding in h 2 o14
Hydrogen Bonding in H2O

H bonds ---> abnormally high specific heat capacity of water (4.184 J/g•K)

This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason thunderstorms release huge energy.

hydrogen bonding15
Hydrogen Bonding

H bonds leads to abnormally high boiling point of water.

hydrogen bonding in biology
Hydrogen Bonding in Biology

H-bonding is especially strong in biological systems — such as DNA.

DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O.

Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases.

—adenine with thymine

—guanine with cytosine

slide18

Double helix of DNA

Portion of a DNA chain

forces involving induced dipoles
FORCES INVOLVING INDUCED DIPOLES

How can non-polar molecules such as O2 and I2 dissolve in water?

The water dipole INDUCES a dipole in the O2 electric cloud.

Dipole-induced dipole

forces involving induced dipoles22
FORCES INVOLVING INDUCED DIPOLES

Solubility increases with mass the gas

forces involving induced dipoles23
FORCES INVOLVING INDUCED DIPOLES
  • Process of inducing a dipole is polarization
  • Degree to which electron cloud of an atom or molecule can be distorted in its polarizability.
im forces induced dipoles

d

-

I-I

I-I

d

The alcohol temporarily creates or INDUCES a dipole in I2.

+

d

-

d

O

-

O

R

H

R

H

d

+

d

+

IM FORCES — INDUCED DIPOLES

Consider I2 dissolving in ethanol, CH3CH2OH.

forces involving induced dipoles25
FORCES INVOLVING INDUCED DIPOLES

The induced forces between I2 molecules are very weak, so solid I2 sublimes (goes from a solid to gaseous molecules).

Formation of a dipole in two nonpolar I2 molecules.

Induced dipole-induced dipole

forces involving induced dipoles26
FORCES INVOLVING INDUCED DIPOLES

The magnitude of the induced dipole depends on the tendency to be distorted.

Higher molec. weight ---> larger induced dipoles.

Molecule Boiling Point (oC)

CH4 (methane) - 161.5

C2H6 (ethane) - 88.6

C3H8 (propane) - 42.1

C4H10 (butane) - 0.5

boiling points of hydrocarbons

C4H10

C3H8

C2H6

CH4

Boiling Points of Hydrocarbons

Note linear relation between bp and molar mass.

measuring equilibrium vapor pressure
Measuring Equilibrium Vapor Pressure

Liquid in flask evaporates and exerts pressure on manometer.

Active Fig. 13.17

liquids equilibrium vapor pressure

ether

alcohol

water

O

O

O

C

H

H

C

H

C

H

H

2

5

H

5

2

5

2

dipole-

extensive

dipole

H-bonds

H-bonds

increasing strength of IM interactions

Liquids Equilibrium Vapor Pressure

FIGURE 13.18:VP as a function of T.

1. The curves show all conditions of P and T where LIQ and VAP are in EQUILIBRIUM

2. The VP rises with T.

3. When VP = external P, the liquid boils.

This means that BP’s of liquids change with altitude.

4. If external P = 760 mm Hg, T of boiling is the NORMAL BOILING POINT

5. VP of a given molecule at a given T depends on IM forces. Here the VP’s are in the order

liquids
Liquids

HEAT OF VAPORIZATION is the heat req’d (at constant P) to vaporize the liquid.

LIQ + heat ---> VAP

Compd. ∆Hvap (kJ/mol) IM Force

H2O 40.7 (100 oC) H-bonds

SO2 26.8 (-47 oC) dipole

Xe 12.6 (-107 oC) induced dipole

transitions between phases section 13 10
TRANSITIONS BETWEEN PHASESSection 13.10

Lines connect all conditions of T and P where EQUILIBRIUM exists between the phases on either side of the line.

(At equilibrium particles move from liquid to gas as fast as they move from gas to liquid, for example.)

phase diagram for water
Phase Diagram for Water

Solid phase

Liquid phase

Gas phase

phase equilibria water
Phase Equilibria — Water

Solid-liquid

Gas-Liquid

Gas-Solid

triple point water
Triple Point — Water

At the TRIPLE POINTall three phases are in equilibrium.

phases diagrams important points for water
Phases Diagrams—Important Points for Water

T(˚C) P(mmHg)

Normal boil point 100 760

Normal freeze point 0 760

Triple point 0.0098 4.58

critical t and p
Critical T and P

As P and T increase, you finally reach the CRITICAL T and P

Above critical T no liquid exists no matter how high the pressure.

At P < 4.58 mmHg and T < 0.0098 ˚C solid H2O can go directly to vapor. This process is called SUBLIMATION

This is how a frost-free refrigerator works.

critical t and p40
Critical T and P

COMPD Tc(oC) Pc(atm)

H2O 374 218

CO2 31 73

CH4 -82 46

Freon-12 112 41(CCl2F2)

Notice that Tc and Pc depend on intermolecular forces.

solid liquid equilibria
Solid-Liquid Equilibria

In any system, if you increase P the DENSITYwill go up.

Therefore — as P goes up, equilibrium favors phase with the larger density (or SMALLERvolume/gram).

Liquid H2OSolid H2O

Density 1 g/cm3 0.917 g/cm3

cm3/gram 1 1.09

solid liquid equilibria42

P

Liquid

H

O

2

Solid

Normal

H

O

2

freezing

point

760

mmHg

0 °C

T

Solid-Liquid Equilibria

Raising the pressure at constant T causes water to melt.

The NEGATIVE SLOPE of the S/L line is unique to H2O. Almost everything else has positive slope.

The behavior of water under pressure is an example of

LE CHATELIER’S PRINCIPLE

At Solid/Liquid equilibrium, raising P squeezes the solid.

It responds by going to phase with greater density, i.e., the liquid phase.

co 2 phases
CO2 Phases

Separate phases

Increasing pressure

More pressure

Supercritcal CO2

Figure 13.41

dissolving an ionic solid
Dissolving An Ionic Solid

Active Figure 14.9

energetics of the solution process47
Energetics of the Solution Process

If the enthalpy of formation of the solution is more negative that that of the solvent and solute, the enthalpy of solution is negative.

The solution process is exothermic!

colligative properties
Colligative Properties

On adding a solute to a solvent, the props. of the solvent are modified.

  • Vapor pressure decreases
  • Melting point decreases
  • Boiling point increases
  • Osmosis is possible (osmotic pressure)

These changes are called COLLIGATIVE PROPERTIES.

They depend only on the NUMBER of solute particles relative to solvent particles, not on the KIND of solute particles.

concentration units
Concentration Units

MOLE FRACTION, X

For a mixture of A, B, and C

MOLALITY, m

WEIGHT % = grams solute per 100 g solution

calculating concentrations
Calculating Concentrations

Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g of H2O. Calculate mol fraction, molality, and weight % of glycol.

understanding colligative properties
Understanding Colligative Properties

To understand colligative properties, study the LIQUID-VAPOR EQUILIBRIUM for a solution.

understanding colligative properties52
Understanding Colligative Properties

VP of H2O over a solution depends on the number of H2O molecules per solute molecule.

Psolventproportional to Xsolvent

Psolvent = Xsolvent • Posolvent

VP of solvent over solution = (Mol frac solvent)•(VP pure solvent)

RAOULT’S LAW

raoult s law
Raoult’s Law

An ideal solution is one that obeys Raoult’s law.

PA = XA • PoA

Because mole fraction of solvent, XA, is always less than 1, then PA is always less than PoA.

The vapor pressure of solvent over a solution is always LOWERED!

raoult s law54
Raoult’s Law

Assume the solution containing 62.1 g of glycol in 250. g of water is ideal. What is the vapor pressure of water over the solution at 30 oC? (The VP of pure H2O is 31.8 mm Hg; see App. E.)

changes in freezing and boiling points of solvent
Changes in Freezing and Boiling Points of Solvent
  • For a 2-component system where A is the solvent and B is the solute
  • ∆PA = VP lowering = XBPoA
  • VP lowering is proportional to mol frac solute!
  • For very dilute solutions, ∆PA = K•molalityB where K is a proportionality constant.
  • This helps explain changes in melting and boiling points.
elevation of boiling point
Elevation of Boiling Point

Elevation in BP = ∆TBP = KBP•m

(where KBP is characteristic of solvent)

change in boiling point
Change in Boiling Point

Dissolve 62.1 g of glycol (1.00 mol) in 250. g of water. What is the BP of the solution?

KBP = +0.512 oC/molal for water (see Table 14.3).

change in freezing point
Change in Freezing Point

Ethylene glycol/water

solution

The freezing point of a solution is LOWERthan that of the pure solvent.

FP depression = ∆TFP = KFP•m

Pure water

lowering the freezing point
Lowering the Freezing Point

Water with and without antifreeze

When a solution freezes, the solid phase is pure water. The solution becomes more concentrated.

freezing point depression
Freezing Point Depression

Calculate the FP of a 4.00 molal glycol/water solution.

KFP = -1.86 oC/molal (Table 14.4)

freezing point depression63
Freezing Point Depression

How much NaCl must be dissolved in 4.00 kg of water to lower FP to -10.00 oC?.

freezing point depression64
Freezing Point Depression

How much NaCl must be dissolved in 4.00 kg of water to lower FP to -10.00 oC?.

boiling point elevation and freezing point depression
Boiling Point Elevation and Freezing Point Depression

∆T = K•m•i

A generally useful equation

i = van’t Hoff factor = number of particles produced per formula unit.

Compound Theoretical Value of i

glycol 1

NaCl 2

CaCl2 3

osmosis
Osmosis

The semipermeable membrane allows only the movement of solvent molecules.

Solvent molecules move from pure solvent to solution in an attempt to make both have the same concentration of solute.

Driving force is entropy

osmosic pressure

Osmotic pressure

Osmosic Pressure, ∏

Equilibrium is reached when pressure — the OSMOTIC PRESSURE, ∏ — produced by extra solution counterbalances pressure of solvent molecules moving thru the membrane.

∏ = cRT

(c is conc. in mol/L)

reverse osmosis water desalination
Reverse OsmosisWater Desalination

Water desalination plant in Tampa

osmosis calculating a molar mass
Osmosis Calculating a Molar Mass

Dissolve 35.0 g of hemoglobin in enough water to make 1.00 L of solution. ∏ measured to be 10.0 mm Hg at 25 ˚C. Calculate molar mass of hemoglobin.

ad