Intermolecular forces may be attractive or repulsive. Johannes D van der Waals, Dutch, was the first to postulate intermolecular forces in developing a theory to account for properties of real gases.

Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together.

Intermolecular Forces They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.

Intermolecular Forces These intermolecular forces as a group are referred to as Van der Waals forces.

Van der Waals Forces Van der Waals forces are also known as London forces. They are weak interactions caused by momentary changes in electron density in a molecule. They are the only attractive forces present in non polar compounds.

Weakest intermolecular force; present in all organic molecules The surface area of a molecule determines the strength of the Van der Waals interactions between molecules. The larger the surface area, the larger the attractive force between two molecules, and the stronger the intermolecular forces. Van der Waals forces are also affected by polarizability. Polarizability is a measure of how the electron cloud around an atom responds to changes in its electronic environment.

Dipole-Dipole Interactions Molecules that have permanent dipoles are attracted to each other. The positive end of one is attracted to the negative end of the other & vice-versa. These forces are only imp. when the molecules are close to each other.

Dipole-Dipole Interactions The more polar the molecule, the higher is its boiling point. Between two polar compounds

Hydrogen Bonding The dipole-dipole interactions experienced when H is bonded to N, O, or F are usually strong. We call these interactions hydrogen bonds.

Hydrogen Bonding Hydrogen bonding arises in part from the high electro negativity of nitrogen, oxygen, and fluorine. Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed.

Summarizing Intermolecular Forces

Boiling point – defined as the temperature at which the vapor pressure of the substance equals the pressure of the atmosphere above it.

Boiling Point The boiling point of a compound is the temperature at which liquid molecules are converted into gas. In boiling, energy is needed to overcome the attractive forces in the more ordered liquid state. The stronger the intermolecular forces, the higher the boiling point.

Consider the example below Consider the example below. Note that the relative strength of the intermolecular forces increases from pentane to butanal to 1-butanol. The boiling points of these compounds increase in the same order. For two compounds with similar functional groups: The larger the surface area, the higher the boiling point. The more polarizable the atoms, the higher the boiling point.

Data and results: N- butyl alcohol -C4H9OH -Has a boiling point of 118 C. (Literature boiling point)

Data and results: Tert- Butyl alcohol ; C4H10O - Has a boiling point of 82 C

Data and results: N-amyl Alcohol C5H11OH - Has a boiling point of 136-138 C

CONCLUSION Non volatile impurities usually decrease the vapor pressure of the liquid, causing an increase in the boiling point. Volatile impurities usually decrease the boiling point due to the increase in the vapor pressure

Compounds having strong bonds need higher boiling point Compounds having strong bonds need higher boiling point. Weak bonds on the other hand exhibit lower boiling points. More Carbon atoms mean stronger bonds, increase in boiling point Less carbon atoms mean weaker bonds, decrease in boiling point

If the compounds have the same number of carbon atoms, depend on the structure: *Straight chained- Strong bonds; needed high boiling point *Branched chain- weaker bonds; low boiling point

Reference: http://www.slideshare.net/junita55/boiling-presentation http://www.suu.edu/faculty/howard/courseinfo.html http://www.google.com.ph/imgres?q=boiling+point+gif http://www.slideshare.net/gadams61/intermolecular-forces-3015692 http://www.slideshare.net/kwarne/intermolecular-forces-properties http://www.slideshare.net/gjr2323/lecture5-123101

Reference: http://www.slideshare.net/opalair1020/intermolecular-chemical-bonding http://www.digitalofficepro.com/ppt/nature-environment-powerpoint-templates.html http://www.slideshare.net/kwarne/intermolecular-forces-properties http://www.slideshare.net/elmochem/chapter-11-lecture-intermolecular-forces-liquids-solids?src=related_normal&rel=5787060 http://www.slideshare.net/search/slideshow?searchfrom=header&q=Molecular+forces

Group Members: Morana, Kevin Caunan, Marisol De Leon, Lourdez Delos Reyes, Pam Principe, Geene