T16 Phase Changes And IMFs Flashcards
What are the 3 IMFs?
Hydrogen bonding, dipole-dipole, and (London) dispersion forces. All are weaker than ionic bonds. All polar molecules have a separation of charge called a dipole–one side has a partial positive charge and the other has a partial negative charge. Dipole-dipole attractions are the electrostatic forces between the dipole moments of two polar molecules, when the partially positive end of one polar molecule is attracted to the partially negative end of another polar molecule. London dispersion forces are constantly present in all atoms or molecules, but they are weak, and are only significant when the molecules are very close. Bc electrons are in constant motion, any atom or molecule can develop a temporary, instantaneous dipole if its electrons are distributed asymmetrically; the presence of this dipole can distort the electrons of a neighboring atom/molecule and produce an induced dipole. This results in a relatively weak electrostatic interaction between the species. The larger and heavier an atom or molecule is, the stronger the dispersion forces they exhibit are. Molecular geometry (or shape of a molecule) also affects the magnitudes of dispersion forces between them. More spread-out shape means more surface area/points of contact and therefore stronger London dispersion forces; more compact shape means weaker dispersion forces.
Van der Waals interactions
All of the forces (or IMFs) between neutral atoms and molecules, including dipole-dipole, dipole-induced-dipole, and London dispersion forces. Does not include forces due to covalent or ionic bonding, or the attraction between ions and molecules. (Dipole-dipole: electrostatic attraction of partial negative end of one dipolar molecule for the partial positive end of another; All molecular substances have van der Waals interactions, but they are not predominant if there are stronger forces present. Nonpolar molecules (like methane, CH4) lack other IMFs so van der Waals predominates.
What do boiling point and melting point indicate?
Higher boiling and melting point means stronger IMFs. In molecules for which London dispersion forces are predominant, the larger and heavier the atom/molecule is, the stronger the dispersion forces, and therefore the higher the boiling and melting points are. Consider that F2 and Cl2 are gases at room temp, while Br2 is a liquid, and I2 is a solid–F2 and Cl2 exhibit the attractive forces, Br2 has stronger attractive forces, and I2 has the strongest. Larger molecules hold electrons farther away from the nucleus–meaning they are more easily polarizable.
Which molecules can form hydrogen bonds?
Only molecules in which a hydrogen atom is directly bonded to a highly electronegative atom (oxygen, nitrogen, fluorine) can form H bonds. Look out for polyatomic ions within a compound, like hydroxide (OH-), which can form H bonds. Nonpolar molecules, and molecules whose atoms have similar electronegativities (like hydrocarbons - C and H have similar electronegativities), cannot form H bonds.
What does vapor pressure tell you about IMFs?
Vapor pressure is an indication of how easy it is for molecules to “escape” from IMFs to become a gas, and is measured by letting a liquid evaporate in a closed container. Think of molecules flying away, exerting pressure on container walls with their increased movement and increased collisions. The higher the vapor pressure, the easier it is for molecules to escape. Lower vapor pressure indicates stronger IMFs. High vapor pressure indicates weak IMFs.
What can be used in place of pressure of gas in stoichiometry calculations?
pressure of a gas is proportional to the mole number of the gas (at constant P,T,V) thus gas pressure can be used in place of mole in stoichiometry calculations.
What is pressure?
average Force/Area
Heat vs temperature
Temperature is a measure of kinetic energy of the particles and Heat is a type of transfer of energy from one body to another. Consider a graph of the heat vs temperature of water as heat is being added to the water; the energy steadily increases, but sometimes the temperature doesn’t change (temp doesn’t change during phase changes until the phase change is complete)
