INTERMOLECULAR FORCES

A Quick Introduction Intermolecular forces exist everywhere Intermolecular forces exist everywhere Short-range attractive forces operating between the particles that make up the units of a liquid or solid substance Short-range attractive forces operating between the particles that make up the units of a liquid or solid substance Intermolecular forces also cause gases to liquefy or solidify at low temperatures and high pressures Intermolecular forces also cause gases to liquefy or solidify at low temperatures and high pressures

Did you know… Intermolecular forces are generally weaker than ionic or covalent bonds Intermolecular forces are generally weaker than ionic or covalent bonds Many properties such as boiling point reflect the strengths of the intermolecular forces Many properties such as boiling point reflect the strengths of the intermolecular forces The stronger the attractive force, higher the temp. when liquid boils The stronger the attractive force, higher the temp. when liquid boils Melting points of solids increase as strengths of IF increase Melting points of solids increase as strengths of IF increase

An Attraction Force: Ion-dipole Forces Exist between an ion and the particle charge on the end of a polar molecule Exist between an ion and the particle charge on the end of a polar molecule Polar molecules = dipoles (has both poles + and -) Polar molecules = dipoles (has both poles + and -) + ion are attractive to – end of dipole…and vice versa + ion are attractive to – end of dipole…and vice versa Magnitude of attraction increases as the magnitude or ion charge of the dipole moment increases Magnitude of attraction increases as the magnitude or ion charge of the dipole moment increases

Types of Intermolecular Forces Dipole-dipole forces Dipole-dipole forces Ion – Dipole forces Ion – Dipole forces London Dispersion forces London Dispersion forces Hydrogen-bonding forces Hydrogen-bonding forces

Dipole-dipole forces Exist between neutral polar molecules Exist between neutral polar molecules Weaker than ion-dipole forces Weaker than ion-dipole forces Molecules with equal mass or size, strengths of intermolecular attraction increases with increasing polarity Molecules with equal mass or size, strengths of intermolecular attraction increases with increasing polarity

A Few Queries… What about non-polar molecules? What about non-polar molecules? So how does a non-polar gas liquefy? So how does a non-polar gas liquefy? Because you need some sort of attraction between particles Because you need some sort of attraction between particles

…well… 1930 – Fritz London proposed origin of this type of attraction 1930 – Fritz London proposed origin of this type of attraction Recognized that motion of electrons in atom/molecule can create an instantaneous dipole moment Recognized that motion of electrons in atom/molecule can create an instantaneous dipole moment

Example Helium is spherically symmetrical Helium is spherically symmetrical Molecules -> non-polar; and possess no permanent dipole moment Molecules -> non-polar; and possess no permanent dipole moment However, instantaneous distribution of electrons is diff. from avg. distribution However, instantaneous distribution of electrons is diff. from avg. distribution If we freeze the electrons in helium atom, both electrons could be on same side of nucleus; giving it instantaneous dipole moment If we freeze the electrons in helium atom, both electrons could be on same side of nucleus; giving it instantaneous dipole moment

Example

London Dispersion Forces Electrons repel one another, motion of electrons on one atom influence the neighboring atoms. Electrons repel one another, motion of electrons on one atom influence the neighboring atoms. So temporary dipole moment on one atom can induces a similar dipole on a adjacent atom So temporary dipole moment on one atom can induces a similar dipole on a adjacent atom This is significant only when molecules are close together. This is significant only when molecules are close together.

Polarizability Ease with which the charge distribution can be distorted by an external electric field Ease with which the charge distribution can be distorted by an external electric field Think of it as “squashiness” of an electron cloud Think of it as “squashiness” of an electron cloud The greater the polarizability of molecule, the more easily its electron cloud can be distorted to a momentary dipole The greater the polarizability of molecule, the more easily its electron cloud can be distorted to a momentary dipole

So… More polarizable molecules have stronger London dispersion forces More polarizable molecules have stronger London dispersion forces larger molecules have greater polarizability- more electrons farther from the nuclei larger molecules have greater polarizability- more electrons farther from the nuclei So London dispersion forces increase with increasing size and molecular weight So London dispersion forces increase with increasing size and molecular weight

Other factors that determine strength: Shape Shape Ex. N-pentane and neopentane Ex. N-pentane and neopentane N-pentane attraction is higher because more molecules come in contact- because it is more cylindrically shaped N-pentane attraction is higher because more molecules come in contact- because it is more cylindrically shaped

Summary of: Comparing Strengths of Intermolecular Forces Molecules that have comparable eights and shapes have dispersion forces that are approx. equal Molecules that have comparable eights and shapes have dispersion forces that are approx. equal When molecules differ greatly in weights, dispersion forces are accordingly different When molecules differ greatly in weights, dispersion forces are accordingly different

Hydrogen Bonding Hydrogen bonding is a special type of intermolecular attraction that exists between the hydrogen atom in a polar bond and an unshared electron pair on a nearby small electronegative ion or atom Hydrogen bonding is a special type of intermolecular attraction that exists between the hydrogen atom in a polar bond and an unshared electron pair on a nearby small electronegative ion or atom Tend to be the strongest type of intermolecular forces Tend to be the strongest type of intermolecular forces

More About Hydrogen Bonding Can be considered unique dipole-dipole attractions Can be considered unique dipole-dipole attractions F, O, and N are so electronegative, that a bond between them and hydrogen is quite polar, with hydrogen on positive end F, O, and N are so electronegative, that a bond between them and hydrogen is quite polar, with hydrogen on positive end

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