Unit #2 Flashcards
(94 cards)
what is the average trend in melting and boiling point
down a group, they decrease
from left to right, they increase until the metalloids then decrease significantly
what is the trend in physical properties
they are very difficult to predict across a period (mp, hardness and conductivity) but usually increase until metaloids then tend to decrease
why are metalloids “hard solids”
it is due to the strong covalent bonds holding atoms together in their extensive covalent network
what is the 2 important classes of reactions?
reduction-oxidation (redox) and acid-base reactions
what are redox reactions
the tendency of a species to lose or gain electrons, there is a transfer of electrons (reflected in a change of oxidation state)
- RA : loses e-s, is oxidized by OA, oxi number increases
- OA : gains e-s, is reduced by RA, oxi number decreases
steps:
- write rxn
- determine 1/2 rxns (which is being oxidized and reduced)
identify whether these are good oxidizing or reducing: Mg, O2, MnO4-
Mg - RA (goes to 2+)
O2 - OA (goes to 2-)
MnO4 - OA (@ 7+ so it wants to go down)
what are acid-base reactiosn
metal oxides and nonmetal oxides differ in their reactivity w/ water
- basic oxide: Li2O + H2O -> 2LiOH (metal)
- acidic oxide: SO2 + H2O -> H2SO3 (non-metal)
aka anhydrides (w/o water)
periodic trend: basic oxides -> acidic oxides
metalloids are usually amphoteric
what are the results of these reactions?
- MgO + H2O
- SO3 + H2O
- MgO + H2O -> Mg(OH)2
- SO3 + H2O -> H2SO4
why do bonds form?
electrostatic attractions: bonding lowers the potential energy between positive and negative particles -> lower energy = more stability
quantum mechanics perspective: atoms form bonds in order to make their outer shell more stable
what properties play a role in bonding
properties such as the IE, EA, Zeff, and size all play a role in their bonding (how they come together and bond).
metallic behaviour (Larger size, low IE) increases down a ground and decrease left to right across a period
what are the 3 types of bonding
ionic bonding, covalent bonding, and metallic bonding
what is ionic bonding
metals and nonmetals bonding with a complete electron transfer (forms cations and anions) arranged in a extended structure.
formula = lowest whole number ration
strength (related to coulombs law, E ∝ q1q2/d) is related to the magnitude of charges and the size of ions (lattice energy is released when the gaseous ions combine to form the ionic solid)
what physical properties does the ionic bonding model account for
- hard and rigid - the positive and negative ions are strongly attracted to eat other and are difficult to separate (req lots of energy)
- brittle - when stress is applied to the ionic lattice, the layers shift slighty -> ions of like charges are force closer together, this increases electrostatic repulsion which results in the structure breaking down
- poor conductivity in solids but good in liquid - solid ionic compounds do not conduct because the ions are locked to a rigid lattice. the dissociation of ions in the liquid phase allow for the ions to move out of the lattic structure and conduct electricity
what is covalent bonding
nonmetal and nonmetal sharing electrongs between atoms (form a molecule). each atom holds onto its own electrons tightly (high IE) and attracts other electrons as well (highly negative EA)
- typically exist as liquids or gases w/ low mp and bp. to explain you need to consider the distinction between the strong covalent bonds within molecules and the weak forces between molecules
what is metallic bonding?
metal and metal, delocalized “sea of electrons” in an extended structure.
- typically malleable and ductile w/ moderately high mp and bp, as well as excellent conductors of heat and electricity
what physical properties does the metallic bonding account for
- malleability and ductility - can be deformed because of the electron sea prevents repulsions among the cations
- moderately high mp: cations can move without disrupting their attraction to surrounding electrons
- thermal/electrical conductivity: mobile, delocalized electrons in the metallic structure
what is bond energy
the energy required to overcome the attraction and is defines as the standard enthalpy change for breaking the bond in 1 mol of gaseous molecules
- breaking a bond is always endothermic (req energy)
- forming a bond is always exothermic (release energy)
what is bond strength related to
the strength of the bond depends on the magnitude of the mutual attraction between bonded nuclei and shared electrons
- be energy increases with increasing the bond orders and decreasing the bond length
- the more bonds there are, the stronger the bond is
rank the relative bond length and strengths and explain why: Si-F, Si-C, Si-O
Si-F>Si-O>Si-C (strength) - Si-F<Si-O<Si-C (length)
Atomic radius decreases across a period so flourine is smaller than O which is smaller than C. Therefore, bond length decreases across a period going C->O->F and bond strength increases
as bond order increases, bond length
as bond order increases, bond energy
decreases
increases
the net energy change results:
from the difference in bond energies:
ΔHrxn » Sum of BE (bonds broken) – Sum of BE (bonds formed)
exo: energy released during the formation of bonds is greater than the energy required to break bonds
endo: energy released during bond formation is smaller than the energy required to break bonds
In other works: the heat released or absorbed during a chemical change is due to differences between reactant bond energies and product bond energies
what is electronegativity?
the relative ability of an atom bonded within a molecule to attract shared electrons to itself. the larger the number, the stronger the attraction of the electrons by that atom. It is a relative scale in which atoms are ranked on their ability to attract electrons
- homogeneous bonds -> non-polar
- heterogenous bonds -> polar (the more EN atom takes a greater share of the bonding e- = partial negative charge while the other is slightly positive)
what are the electronegativity trends?
REMEMBER F>O>Cl,N>Br>I, C, S>H
- across a period electronegativity increases - if the valence shell of an atom is less than half full, it requires less energy to lose an electron than to gain one
- down a group electronegativity decreases - there is an increased distance between the valence electrons and nucleus (larger AR)
increase in ΔEN results in:
larger partial charges and higher partial ionic character. decreasing ΔEN the bond becomes more covalent and the character of the substance