intermolecular forces

pt 1

intermolecular force

interaction within a single molecule (covalent bond)

nature of imf

imfs are coulombic and generally weaker

imf types

dipole-dipole interactions, LDF, hydrogen bonding, ion-dipole interaction

dipole-dipole interactions

• occur between any two polar molecules

• can be attractive/repulsive

• molecules orient themselves to maximize attraction

• strength of interactions related to dipole magnitude

dipole-induced dipole interactions

• dipole of water approaches nonpolar oxygen molecule

• electrons in oxygen are repelled by the - of water

• oxygen forced to have induced dipole

london dispersion forces

• all molecules exhibit ldfs, including polar molecules

• ldfs are the primary type of interaction between nonpolar molecules

• strength of the ldfs depend on how easily the electrons can disperse

• the larger the electron cloud, the more polarizable it is, greater strength

ldfs and boiling point

• the larger the molecule, the more polarizable the electron cloud, resulting in stronger ldfs, higher boiling point

• ldfs can become quite strong as molecules become large

hydrogen bonding

• interaction between molecules, not within

• electronegative oxygen atoms draws electrons to itself and away from the hydrogen atoms

• the small hydrogen atom partially loses its electron to the oxygen atom, leaving a bare proton

• the oxygen atom from a different h2o molecule interacts very strongly with the hydrogen atom, hydrogen bond

• strong type of imf

• only takes place between an H atom covalently bonded to a highly electronegative atom (FON)

ion-dipole interactions

• when ionic compounds dissolve in aqueous solution, the dipole of the water interacts with charged ions and cause them to separate

• interactions between the ions and water are called ion-dipole interactions

• stronger than hydrogen bonding

properties of substances/relationship

• properties increase, imf increase

• properties decrease, imf decrease

which properties increase as imf increases

• melting/boiling point

• surface tension

• viscosity

which properties decrease as imf decreases

• vapor pressure

• volatility

when two molecules have a similar number of electrons (similar mass), how do intermolecular forces rank in strength?

hydrogen bonding > dipole-dipole > ldf

if two molecules have the same type of intermolecular forces, but different sizes, which one has stronger imfs and why?

the larger molecules experience stronger imf

interaction key

• polar + polar = dipole dipole

• polar + nonpolar = dipole induced dipole

• nonpolar + nonpolar = ldfs (all have this)

• ion + polar = ion dipole

properties of solids

• strong interactions between particles

• definite shape and volume

• regular, crystalline structure

• fixed arrangement of particles

• vibrational degree of freedom

types of solids

1. ionic

2. molecular

3. metallic

4. covalent network

ionic solid

ex. NaCl

• formed by cation and anion, each type surrounded by each other in 3d lattice held together by lattice energy

• formula represents ratio between ions, not discrete particles

• high melting/boiling point due to strong coulombic attraction

why are ionic solids so brittle?

because there is mechanical force, there is increased ionic repulsions, which therefore causes a fracture

explain the conductivity of ionic solids

poor conductors of electricity in the solid state, good conductors when liquid and aqueous, ions must be free to flow for it to conduct

molecular solids

• formed by distinct, individual neutral molecules, turn form molecular lattice structures

• formed exclusively of nonmetal atoms, and chemical formula represents the actual # of atoms in each molecule

• relatively low melting/boiling point due to weak IMF

explain the conductivity of molecular solids

poor conductors of electricity in all states, as electrons are held tightly in covalent bonds

metallic solids

• formed by metallic elements

• show metallic bonding where valence electrons are free to flow from atom-atom (sea)

• malleable, ductile

• melting points vary

• great conductors of heat/electricity

covalent network

ex. diamond

• formed by distinct atoms all bonded together covalently in a 3d network

• formed by carbon, metalloids

• very high melting point and hardness

• poor conductors of electricity as electrons are held tightly in a covalent bond

graphite

• more common allotrope of carbon

• carbon in graphite is sp² hybridized, form very large sheets of carbon atoms in trigonal planar

• weak IMF hold sheets together, graphite is soft bc layers can slide past each other

• excellent conductor of electricity, delocalized electrons can flow across the sheets

identifying solids with formula

• metal + nonmetal = ionic

• nonmetal + nonmetal = molecular/network

• only metals = metallic

• Si/C networks = usually covalent networks

phases of matter

• states of matter are dictated by the kinetic energy of particles and a substance’s heats of fusion/vaporization, pressure, temperature

• particles retain their chemical identity in all three states

• volume, density, and interparticle distances are different

solid water

• below 0C, water molecules are in fixed positions as ice

• molecules are moving but not past each other, vibrational degrees of freedom

• molecules in a solid are not necessarily closer to each other than they are in a liquid

liquid water

• above water’s melting point, water molecules are moving too fast for their mutual attraction to maintain them locked in place

• molecules are able to slide past one another, translational degree of freedom

• the molecules at the surface might evaporate air pressure affects vaporization

gaseous water

• above 100C, attraction between water molecules is not sufficient to hold molecules together

• molecules in the gas phase move randomly in straight lines between collisions, all degrees of freedom (vibrational, translational, rotational)

• space between molecules is larger than liquids and solids

dipole dipole interactions exists in what kind of molecule?

any 2 polar molecules