3.1- Flashcards
Molecular compounds boil at ____ temperatures than ionic compounds
lower
Lattice energy
Energy required to completely separate one mole of a solid ionic compound into gaseous ions
… measure of the stability of any ionic solid
Born-Haber cycle
Relates lattice energies of ionic compounds to ionization energies, electron affinities, and other atomic and molecular properties
… defines various steps that precede formation of an ionic solid
Bond enthalpy
Energy required to break a covalent bond
Born-Harbor cycle for LiF
Li(s) + 1/2(F_2) –> LiF (s)
(1) Convert solid lithium to lithium vapor (+H) (sublimation)
(2) Dissociate 1/2 mole of F_2 gas into separate gaseous F atoms (+H)
(3) Ionize 1 mole of gaseous Li atoms (+H)
(4) Add 1 mole of electrons to 1 mole of gaseous F atoms (-H)
(5) Combine 1 mole of gaseous Li+ and 1 mole of F- to form 1 mole of solid LiF (-H)
Why is lattice energy always positive?
Separation of ions in a solid into ions in the gas phase is an endothermic process, so the larger the lattice energy, the more stable the solid and more tightly held the ions
Why…
does MgCl_2 exist and not MgCl?
does NaCl exist and not NaCl_2?
Must balance between ionization energies and the stability, or lattice energy, from the formation of the solid
4 types of crystals
Ionic, covalent, molecular, metallic
Two important characteristics of ionic solids
(1) Composed of charged species
(2) Anions and cations are generally quite different in size
When do ionic solids conduct electricity?
Not in the solid state, because strong cohesive forces fix the ions in place.
In the molten state or dissolved in water, the ions are free to move and conduct electricity.
Why is diamond hard?
Each carbon is sp^3 hybridized, resulting in a very strong tetrahedral structure with a high melting point.
What holds covalent crystals together?
An extensive 3-D network of covalent bonds
Why is graphite a good conductor of electricity?
Carbon is arranged in six-membered rings, sp^2 hybridized, with the unhybridized 2p obital used in pi bonding. This delocalized molecular orbital allows electrons to move around. (covalent bonds = hard, layers of carbon rings can slide = lubricant, slippery)
Describe molecular crystals
Lattice points are occupied by molecules, and the attractive force that holds them together are van der Waals forces and/or hydrogen bonding
Soft, low melting point, poor conductor
Describe metallic crystals
Every lattice points is occupied by an atom of the same metal
Bonding is delocalized over the entire crystal, delocalized electrons make metals good conductors
