CHEM1312H - States of Matter and IMFs

state of the function depends on…

-the balance between kinetic energy and IMFs

-the stronger the forces between the molecules, the closer they are to each other

intramolecular forces

-forces within a molecule (holds molecules together)

-through sharing or transfer of electrons to form bonds (covalent, ionic)

intermolecular forces

-forces between molecules

-electrostatic interaction between molecules

IMFs

-determine whether substances are solids, liquids, or gases

-this can be related to the structure of the particles which in turn determines the strength of IMFs

-strong IMFs tend to result in solids or liquids

-weak IMFs result in gases

types of IMFs

-van der waal forces

• london dispersion forces

• dipole-dipole interactions

• dipole-induced dipole forces

-hydrogen bonding

-ion-dipole forces

london dispersion forces

a temporary attractive force that results when the electrons in two adjacent atoms or molecules occupy positions that make the atoms or molecules form temporary dipoles

how LDFs work

-the motion of electrons in one atom causes an instantaneous dipole that can influence the motions of electrons in a neighboring atom

-this causes the adjacent atom to have a temporary dipole moment and eventually the atoms are attracted to each other

LDFs are present

-in all atoms and molecules

-they are the main interaction between nonpolar atoms and molecules

• ex: noble gases, halogens, hexane, benzene, etc

polarizability

the ease with which the electron cloud of an atom, or molecule, can be distorted by an electric field or another atom

LDFs and polarizability

the strength of dispersion forces increases with increasing atomic or molecular size

LDFs and size of particles

dispersion forces increase with molar mass as molecules or atoms with higher molar mass generally have more electrons dispersed over a greater volume

LDFs and surface area of particles

linear molecules have a larger surface area of interaction which enhances intermolecular contact and increases dispersion forces

dipole

-two equal but opposite charges (a partial positive and a partial negative charge)

-caused by the difference in electronegativity between polar covalent bonds

dipole moment

-the measurement of the separation of positive and negative in a molecule

• vector quantity that has both direction and magnitude

• the larger the ΔEN the larger the dipole moment

• helps us quantify the polarity of a molecule

Dipole moments and polarity

-in some molecules the bonds can be polar, but the molecule itself can be polar or nonpolar

• if the molecular geometry results in dipole moments adding up (vectors sum up) to a net dipole, then the molecule is polar

• if the molecular geometry results in dipole moments of individual bonds cancelling each other, then the molecule is non-polar

6 symmetrical geometries that result in an overall non-polar molecular polarity

-linear

-trigonal planar

-tetrahedral

-square planar

-trigonal bipyramidal

-octahedral

6 symmetrical geometries thing to remember

the symmetry in these molecules can be broken if one of the atoms around the central molecule is substituted by a different atom resulting in a polar molecule

polarity of large organic molecules

-using the molecular geometry, polarity around parts or one group of a larger molecule can also be found

-for the type of molecule, we look at if one part of the molecule is polar, the entire molecule is polar

dipole-dipole forces

-a result of electrostatic interaction between polar molecules

-electrostatic attraction exists between the partially positive end of one molecule and then negative end of a neighboring molecule

-polar molecules have higher melting and boiling points than non-polar molecules given that both molecules have comparable sizes (molar mass)

when comparing substances of comparable molecular size

dipole forces can produce significant differences

when comparing substances of widely different molecular size

dispersion forces are usually more significant than dipole forces

comparing substances (size) reasoning

as molecules get larger, their electron clouds become more polarizable and have greater surface areas, causing dispersion forces to increase much more rapidly compared to dipole-dipole forces

hydrogen bonding

-occurs between two molecules in which one has a hydrogen covalently bonded to a highly electronegative atom such as N, O, or F (FON) and the other molecules (polar) with a lone pair on N, O, F

-displays strong dipole-dipole forces (a super dipole-dipole force)

why are hydrogen bonds strong?

-polarity of the bonds (due to large ΔEN between H and FON

-small size of hydrogen, oxygen, nitrogen, and fluorine atoms allow the close approach of dipoles

-hydrogen bonding are stronger in comparison to dipole-dipole and dispersion forces for this course

types of hydrogen bonding

-within a single molecule (intramolecular hydrogen bonding)

-between identical molecules (intermolecular hydrogen bonding)

-between two different molecules (intermolecular hydrogen bonding)

hydrogen bonding and physical properties

review in notes

ion dipole forces

-occurs when an ionic compound mixes with a polar compound and it’s the strongest IMF

-this results in electrical interactions between ions and the partial charges on a polar molecule

debye forces

-type of IMF that exists between polar and nonpolar molecules

-a permanent dipole on a polar compound can induce a temporary dipole on a neighboring non-polar molecule

summary of LDFs

-dispersion forces are present in all molecules and atoms and increase with increasing molar mass

-they are the main forces in non-polar molecules

summary of dipole-dipole forces

-present in polar molecules

-when comparing substances of comparable molecular sizes, dipole forces can produce significant differences

-when comparing substances of widely different molecular sizes, dispersion forces are usually more significant than dipole forces

summary of hydrogen bonding

present in molecules containing hydrogen bonded to F, O, or N

summary of ion-dipole forces

present in mixtures of ionic compounds and polar compounds