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A Review of Molecular Geometry a Precursor to Understanding Intermolecular Forces PowerPoint PPT Presentation


A Review of Molecular Geometry a Precursor to Understanding Intermolecular Forces. http://www2.gasou.edu/chemdept/general/molecule/quiz/frame4b.htm. http://www2.gasou.edu/chemdept/general/molecule/quiz/frame4a.htm. Intermolecular forces. Molecular attractions.

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A Review of Molecular Geometry a Precursor to Understanding Intermolecular Forces

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A Review of Molecular Geometry a Precursor to Understanding Intermolecular Forces

http://www2.gasou.edu/chemdept/general/molecule/quiz/frame4b.htm

http://www2.gasou.edu/chemdept/general/molecule/quiz/frame4a.htm


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Intermolecular forces

Molecular attractions

Attractions between molecules are classified as

  • Dipole-dipole interactions

  • London forces also known as

  • van der Waals forces

  • Hydrogen bonding


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Relative magnitudes of forces

The relative size of these interactions is important so the relative effects are understood.

Relative strengths for the different interactions are listed here.

Hydrogen bonding >

Dipole-dipoleinteractions >

London forces

  • Covalentbonds >

400 kcal >

12-16 kcal >

2-0.5 kcal >

less than 1 kcal

Clearly normal covalent bonds are almost 40 times the strength of hydrogen bonds. Covalent bonds are almost 200 times the strength of dipole-dipole forces, and more than 400 times the size of London forces.


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Dipole – Dipole Forces

What is required of a molecule or molecules to allow for Dipole-Dipole forces?


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  • Dipole-dipole forces - only polar covalent molecules have the ability to form dipole-dipole attractions between molecules. Polar covalent molecules act as little magnets, they have positive ends and negative ends which attract each other.


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London forces

London forces exist in nonpolar molecules.

These forces result from temporary charge imbalances. The temporary charges exist because the electrons in a molecule or ion move randomly in the structure. The nucleus of one atom attracts electrons form the neighboring atom. At the same time, the electrons in one particle repel the electrons in the neighbor and create a short lived charge imbalance.


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These temporary charges in one molecule or atom attract opposite charges in nearby molecules or atoms. A local slight positivecharged+ in one molecule will be attracted to a temporary slight d-negative charge in a neighboring molecule.


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  • London forces - all molecules have the capability to form London forces. These are solely dependent on the surface area and the polarizability of the surface of the molecule. These are the only types of forces that non-polar covalent molecules can form. They result from the movement of the electrons in the molecule which generates temporary positive and negative regions in the molecule.


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Melting Points and Boiling Points of Similar Substances with Increasing Formula Weights

Substance

FW (g/mol)

mp (°C)

bp (°C)

F2

38

-220

-188

Cl2

71

-100.98

-34.6

Br2

160

-7.2

58.78

I2

254

113.5

184.35

As the size of the halogens increases, the melting and boiling points increase. The energy required to move and separate the molecules from one another increases as the size of the molecules increases. If it takes more energy to separate the molecules, the attractions between molecules must be greater. The types of intermolecular forces responsible for the increase in melting points and boiling points of these non-polar covalent compounds are called London forces or dispersion forces.


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Hydrogen bonding

Hydrogen bonding is a unique type of intermolecular molecular attraction. There are two requirements.

The first is a covalent bond between a H atom and either F, O, or N. (These are the three most electronegative elements.)

The second is an interaction of the H atom in this kind of polar bond with a lone pair of electrons on a nearby atom of F,O, or N.


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The normal boiling point for water is 100 degrees Celsius. (The graph below has some artistic problems locating 100.) The observed bp is high compared to the expected value. The predicted bp from the trend of boiling points for H2Te, H2Se, H2S and H2O is very low. If the trend continued the predicted boiling point would be below -62 oC. The "anomalous" boiling point for water is the result of hydrogen bonding between water molecules.


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Methane, CH4

methyl ether, CH3OCH3

Hydrogen peroxide, H2O2

methyl alcohol, CH3OH

Which of the following molecules display hydrogen bonding?


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  • Answer: The hydrogen peroxide and methyl alcohol have hydrogen bonding between molecules.

  • The methane lacks highly electronegative atoms bonded to the hydrogen atoms.

  • The methyl ether has an oxygen but the carbons are bonded to the "O" . The hydrogen atoms are not bonded to very electronegative "O" atom.


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When can hydrogen bonding exist?

Possible combinations where hydrogen bond can exist. The first entry shows the covalent bond to the O or N atom. These atoms form two and three covalent bonds. The single covalent bond between O,N,F is shown and the dashed line shows the hydrogen bond. NOTICE the H atom is attracted to a lone pair on the nearby N,O,F atom.

A covalent bond between -O-H ---- :O-

A covalent bond between -N-H----- :O-

A covalent bond between F-H ------ :O-

A covalent bond between -O-H ---- :N-

A covalent bond between -N-H---- :N-

A covalent bond between F-H ----- :N-

A covalent bond between -O-H ----- :F-

A covalent bond between -N-H ---- :F-

A covalent bond between F-H ------ :F-


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Hydrogen bonding in an ice crystal

Summary on hydrogen bonding

Hydrogen bonding is responsible for the expansion of water when it freezes. The water molecules in the solid have tetrahedral spatial arrangement for the two lone pairs and two single bonds radiating out from the oxygen. The lone pairs on the "O" atoms are attracted to nearby water molecules through hydrogen bonds. A cage like structure results. The cage has an hexagon shaped opening.


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Melting Points and Boiling Points of Substances with Similar Formula Weights

Substance

FW (g/mol)

mp (°C)

bp (°C)

F2

38

-220

-188

NO

30

-164

-152

CH3OH

32

-94

65

Ca

40

893

1484

NaF

42

993

1695

All the substances in this table have similar formula weights thus they have similar London forces. If the only attractions between substances have to do with size, then they should have similar melting points and boiling points. They do not. Let us look more closely at the nature of the substance to see if we can relate the structure of the material with its properties.


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Fluorine and Nitrogen Monoxide

Fluorine and nitrogen monoxide are similar in size and thus have similar London forces.  Fluorine is a non-polar covalent molecule while nitrogen monoxide is a polar covalent molecule - it has a positive and a negative end, like a magnet. Since nitrogen monoxide has the higher melting point and boiling point, it must have the stronger intermolecular forces. Given the same size, polar covalent molecules must have stronger forces of attraction than non-polar covalent molecules. These forces of attractions are called dipole-dipole forces.


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Nitrogen Monoxide and Methanol

Nitrogen monoxide and methanol are similar in size and thus have similar London forces. Nitrogen monoxide and methanol are polar covalent molecules and thus have dipole-dipole forces. Since methanol has the higher melting point and boiling point, it must have the stronger intermolecular forces. The difference in these molecules is the presence of a certain extremely polar bond present in methanol that is not present in nitrogen monoxide. This is the oxygen - hydrogen bond.

Oxygen is more electronegative than hydrogen and pulls the electron density in the oxygen - hydrogen bond towards it. This leaves very little electron density around the hydrogen since hydrogen has no core electrons. The part of hydrogen directed away from the oxygen - hydrogen bond has very little electron density shielding the nucleus. Thus that part of the hydrogen nucleus which is exposed can interact with the non-bonding electrons on another methanol molecule. This interaction of a non-bonding pair with a hydrogen attached to an electronegative element such as oxygen is called a hydrogen bond.


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  • Calcium and Sodium Fluoride

  • A large jump in melting points and boiling points is observed when we turn from covalent compounds to metals and ionic compounds. Both metals and ionic compounds involve the interaction of particles with full charges.

    • Metals. Metal ions interact with the sea of electrons that surround them. This attraction must be very strong as the melting point and boiling point of calcium is much higher than the covalent compounds which share a similar formula weight.

    • Ionic Compounds. Substances which bear full charges, anions and cations, are attracted very strongly as evidenced by the melting point and boiling point of sodium fluoride.

  • The types of interactions responsible for the extremely high melting and boiling points of metals and ionic compound are called electrostatic forces and are the strongest of all the intermolecular forces.


Http www2 gasou edu chemdept general molecule quiz frame4b htm l.jpg

http://www2.gasou.edu/chemdept/general/molecule/quiz/frame4b.htm

http://www2.gasou.edu/chemdept/general/molecule/quiz/frame4a.htm


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