Intermolecular ForcesSummary: http://itl.chem.ufl.edu/2045/lectures/lec_g.html

http://wps.prenhall.com/esm_brown_chemistry_9/2/660/169060.cw/index.htmlhttp://www.ausetute.com.au/intermof.htmlVideo: http://www.youtube.com/watch?v=DVCYlST6mYQ&feature=related

http://faculty.sdmiramar.edu/fgarces/zCourse/All_Year/Ch100_OL/aMy_FileLec/04OL_LecNotes_Ch100/05_CompoundBonding/505_IMF_Water/505_StMatterIMF.htm

Sites

• Why ice floats on water:• http://www.sumanasinc.com/webcontent/animations/content/

propertiesofwater/water.html

• http://itl.chem.ufl.edu/2041_f97/lectures/lec_g.html

• http://web.jjc.edu/staff/pmills/CHM100/Intermolecular%20Forces.pdf

Types of Intermolecular Forces

1. Hydrogen bondinghttp://www.sumanasinc.com/webcontent/animations/content/propertiesofwater/water.html

2. Dipole-Dipole forces

3. Van der Waals forces

1. Hydrogen Bonding• Occurs when H is bonded directly to a small highly electronegative

element such as F, O or N.

• Ice structure showing the intermolecular hydrogen bonding in red, holding the molecules of water into a 'diamond' like ring structure. The blue bonds represent the covalent bonds within the water molecules (intramolecular)

• The normal boiling point for water is 100 degrees Celsius.

• The "anomalous" boiling point for water is the result of hydrogen bonding between water molecules

2. Dipole - Dipole attractions2. Dipole - Dipole attractions

• Occurs between polar molecules that are attracted to each other by electrostatic forces.

• The positive end of a dipole is attracted to the negative end of a nearby molecule.

Mr = 58 BP -0.5°C Mr = 58 BP=56.2°C

London Dispersion ForcesDispersion forces, also known as London

forces, are the temporary attractive force that results when electrons in two adjacent atoms take positions that make the atoms form temporary dipoles.

When two non polar molecules are close to each other, the electron cloud of one molecule repels the electron cloud of the other molecule making the density around each nucleus greater in one region of each cloud. This makes each molecule form a temporary dipole.

IMPORTANT:

The larger the molecule, the greater it’s dispersion forces.

F2 (g) < Cl2 (g) < Br2 (l) < I2 (s)

Larger and heavier atoms and molecules exhibit stronger dispersion forces than smaller and lighter ones.

London dispersion forces between the larger Br2 molecules are sufficient to cause them to form a liquid at 25oC, whereas London dispersion forces between the smaller Cl2 molecules are not.

Dispersion Forces• The more electrons that are present in the molecule,

the stronger the dispersion forces will be. • Dispersion forces are the only type of intermolecular

force that operates between non-polar molecules• Dispersion forces exist between non-polar

molecules such as

– hydrogen (H2)

– chlorine (Cl2)

– carbon dioxide (CO2)

– methane (CH4)

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Animation

• http://ezchemistry4idiots.blogspot.com/2011/01/animations-on-intermolecular-forces.html

Examples:

• CH3CH3 184 CH3Cl 249

• CH3CH2CH3 231 CH3CH2Cl 285

• CH3CH2CH2CH3 272 CH3CH2CH2Cl 319

• The polar molecule has a slightly higher boiling point than the non-polar one, but not by very much.

• The increase in intermolecular attractions due to the additional dipole-dipole interactions is almost negligible compared with the effect of the dispersion forces

Relative Strength of Bonds

• The relative strengths for the different interactions are listed assuming similar size molecules.

• (CH4) -161ºC, ammonia (NH3) -33ºC, water (H2O) 100ºC and hydrogen fluoride (HF) 19ºC,

Ionic > Covalent > Hydrogen bond > Dipole-dipole > London

NaCl 801°C (H2O) 100ºC (HF) 19ºC (CH4) -161ºC

Relative Strength of Bonds

• The relative strengths for the different interactions are listed assuming similar size molecules.

• (CH4) -161ºC, ammonia (NH3) -33ºC, water (H2O) 100ºC and hydrogen fluoride (HF) 19ºC,

Ionic > Covalent > Hydrogen bond > Dipole-dipole > London

NaCl 801°C (H2O) 100ºC (HF) 19ºC (CH4) -161ºC