Intermolecular Interactions

Forces between molecules

III.  Intermolecular Interactions:

A.  Dependent on the  3D structure of the molecule.

1.  Molecules are not always flat, 2D objects which we depict on paper. 

2.  Molecules have a 3 dimensional shape which affects properties like

a. 

b.

c. polarity

1) polarity = unequal charge distribution in molecule = partial charges ( d⁺  ) (   d^- )

a)  based on presence of polar covalent bonds

b)  presence of non-bonding pairs of electrons

c)  3D shape of molecule:  presence of asymmetry

2)  examples of types of polar molecules:

 

3) examples of types of non-polar molecules:

 

4) determining polarity in this class:

a) non-polar molecules:

 

b) polar molecules

 

3.  Shape/polarity affects how molecules interact with each other

a.  intermolecular interactions or cohesive forces

b.  how molecules "see" each other:  attractive forces between molecules

4.  3D shape is determined by how many sets of electrons radiated out from a central atom.

a.  remember, electrons carry a negative charge therefore, covalent bonds and non-bonding pairs of electrons will try to get as far away from each other as possible.

b.  2 covalent bonds present:    angle =             shape =

c.  3 covalent bonds present:    angle =             shape =

d.  4 covalent bonds present:    angle =             shape =

e.  what happens to the shape of molecules when there are non-bonding pairs on the central atom?

1)  2 covalent bonds, 1 non-bonding pair:  angle =             shape =

2)  3 covalent bonds, 1 non-bonding pair:  angle =             shape =

3)  2 covalent bonds, 2 non-bonding pairs: angle =             shape =

5.  types of 3D shapes "observed" with simple molecules

Molecular Structure	Bond Angle	Example	symmetry/polarity
linear	180o		symmetrical  non-polar
trigonal planar	120o		symmetrical   non-polar
angular	<120o		asymetrical        polar
tetrahedral	~109o		symmetrical     non-polar
trigonal pyramidal	<107o		asymmetrical     polar
angular	<104.5o		asymmetrical      polar

B.  Types in intermolecular interactions

1.  Hydrogen bonds

a.  when do they occur?

 

b.  how strong is the interaction?

 

c.  how is this phenomenon depicted (cartooned?)

 

d.  why does it happen?

 

2.  Dipole interactions or polar interactions

a.  when do they occur?

 

b.  how strong is the interaction?

 

c.  how is this phenomenon depicted?

 

d  why does it happen?

 

3.  Hydrophobic interactions (non-polar interactions)

a.  when do they occur?

 

b.  how strong is the interaction?

 

c.  why does it happen?

1)

2)

3)

4)

example of size dependent bp:

CH₄ = -161^oC    C₂H₆= -88^oC    C₃H₈= -42.1^oC

C₄H₁₀ = -0.5^oC    C₅H₁₂= 35-36^oC    C₆H₁₄= 69^oC

C₇H₁₆= 98^oC         C₈H₁₈= 125-127^oC

II.  Intermolecular Interactions and the States of Matter

A.  Three states of matter:

B.  States of matter defined on the molecular level

1.  Based on the Kinetic-Molecular Theory

a.

b.

c.

d.

2. Table describing states of matter:

	DISTANCE between particles	INTERMOLECULAR INTERACTIONS	ORGANIZATION	MOVEMENT
GAS
LIQUID
SOLID

C.  Phase changes represent breaking intermolecular interactions.

1.  solid to liquid

a.

b.

2.  liquid to gas

a.  evaporation

1)

2)

 

b.  boiling

1)

2) dependent on two things:

a).

b).

3) definition:

3.  The stronger the intermolecular interactions between molecules, the harder it is to break them apart (the more energy required to break them apart), the higher the melting point and boiling point of that substance will be.

4.  gas to liquid =

5.  liquid to solid =

6.  solid to gas =

D.  Phase change demos

1.  Collapsing can: Why does the can collapse?

2.  "Volcano":  What causes the gas formation?

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