Intermolecular Forces between molecules Intermolecular Forces IMFs l

Intermolecular Forces between molecules

Intermolecular Forces (IMFs) l Different molecules have different forces that act between them. l These forces attracting the separate molecules together control many physical properties l l l Boiling Point Melting Point Solubility Viscosity Surface Tension

Three Main IMFs l Dipole-dipole forces l Hydrogen bonding l London dispersion forces (LDFs)

Dipole-Dipole Forces l Electronegativity – the ability of an atom in a compound to attract electrons to itself. l Fluorine has the highest electronegativity l The electrons in a compound spend more time around the most electronegative atoms than the other atoms.

Electronegativity l If the electrons are spending more time around the one particular atom, how will that atom be different from the other atoms? It will have a more negative charge than the other atoms in the compound. l Does not have a full negative charge but a partial negative charge (δ - , lower case delta) l

Hydrofluoric Acid (HF) The red end (fluorine) has a partial negative charge.

Polar Molecules l One end of the molecule is positive while the other end of the molecule is negative. l This difference in charge is called a “dipole”

What effect does this have? l How does this change the way two molecules interact? l The positive end of the molecule is attracted to the negative end of a different molecule.

How to spot dipole-dipole forces? l Look for molecular shapes that have uneven placements of atoms. Bent l Trigonal pyramidal l Anything that has more than one type of atom around the outside l

Is Carbon Dioxide Polar?

Is carbon tetrafluoride polar?

Is Water Polar?

Why is Polarity Important l Things that are polar or have charges dissolve in things that are polar. l Things that are nonpolar dissolve in things that are nonpolar. “Like dissolves Like” l Opposing types do not dissolve in each other. l

A Bio Reminder l Hydrophilic – “water loving” – polar l Hydrophobic – “water fearing” - nonpolar

Dissolution Process l How are strong ionic bonds broken in water? l Dissolving_Na. Cl-Electrolyte. exe l The polar nature of water creates attractions between the water and ionic compound.

Hydrogen Bonding l Special case of dipole -dipole forces. l The difference in electronegativity between some atoms and hydrogen is so strong that it creates a very strong dipole

What elements can do this? l Which elements have the strongest electronegativity? l Anything with an N-H bond, O-H bond, or F-H bond will have hydrogen bonding.

Hydrogen Bonding is Very Important It is the reason why ice floats.

Hydrogen Bonding is Very Important

DNA Base Pairs

London Dispersion Forces (LDFs) l l Often called “induced dipoles” A momentary change in where the electrons are in one molecule, “induces” a dipole in another molecule.

LDFs l The more electrons you have in a molecule, the more likely you are to have momentary imbalances in charges. l The more electrons in an atom, the stronger the London Dispersion Forces. l Any molecule can have London Dispersion Forces.

LDFs l l This explains why the boiling point goes up as you move down a column. Hydrogen telluride has more electrons than hydrogen sulfide Hydrogen telluride has stronger LDFs Hydrogen telluride has a higher boiling point

Crude Oil l Crude oil is refined based on differences in LDFs. l Longer carbon chains have higher boiling points l Have larger number of electrons

IMF Comparison l LDFs are the weakest l Dipole-dipole are in the middle l Hydrogen bonding is the strongest.

Practice Problems l List all of the intermolecular forces acting on two phosphorus trichloride molecules l Figure out the formula l Draw a Lewis structure l Figure out the molecular geometry l Check to see what IMFs it has.

Practice Problem l Explain why ammonia (-33. 4ºC)has a higher boiling point than phosphine, PH 3 (-87. 8ºC). Justify your answer. l Figure out ammonia’s formula l Draw Lewis structures for both compounds l Figure out the molecular geometry for both compounds l Figure out what IMFs each compound has. l Compare the two compounds.

Practice Problems l Explain why ammonia (-33. 4ºC)has a higher boiling point than phosphine, PH 3 (-87. 8ºC). Justify your answer.

Practice Problem l Hexane (C 6 H 14) is a liquid at room temperature. Its Lewis structure has each carbon connected to another in a long chain. Will sodium chloride dissolve in hexane? Justify your answer.