Intermolecular Forces Forces between molecules

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

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

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

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

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

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

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

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

Is Carbon Dioxide Polar?

Is carbon tetrafluoride polar?

Is Water Polar?

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

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

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

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

What elements can do this? Which elements have the strongest electronegativity? Which elements have the strongest electronegativity? Anything with an N-H bond, O-H bond, or F-H bond will have hydrogen bonding. 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) Often called “induced dipoles” Often called “induced dipoles” A momentary change in where the electrons are in one molecule, “induces” a dipole in another molecule. A momentary change in where the electrons are in one molecule, “induces” a dipole in another molecule.

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

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

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

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

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

Practice Problem Explain why ammonia (-33.4 º C)has a higher boiling point than phosphine, PH 3 (-87.8 º C). Justify your answer. Explain why ammonia (-33.4 º C)has a higher boiling point than phosphine, PH 3 (-87.8 º C). Justify your answer. Figure out ammonia’s formula Figure out ammonia’s formula Draw Lewis structures for both compounds Draw Lewis structures for both compounds Figure out the molecular geometry for both compounds Figure out the molecular geometry for both compounds Figure out what IMFs each compound has. Figure out what IMFs each compound has. Compare the two compounds. Compare the two compounds.

Practice Problems Explain why ammonia (-33.4 º C)has a higher boiling point than phosphine, PH 3 (-87.8 º C). Justify your answer. Explain why ammonia (-33.4 º C)has a higher boiling point than phosphine, PH 3 (-87.8 º C). Justify your answer.

Practice Problem 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. 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.