Exam 4 Flashcards
Oxidation Number Rules
- Oxidation number of an element in standard state = zero
Pure element like Ag, He, Cu, Cl2, O2, S8= 0 - Oxidation number of a monoatomic ion is equal to its ion charge
- The sum of oxidation numbers is zero for a compound, and equal to the charge for a polyatomic ion.
- The more electronegative element has a negative oxidation number. NH3 N = -3, H = +1. -3 +3x(+1)= 0
- Hydrogen is +1 except with metals it is -1.
6.Oxygen is usually -2, except with F = +2, and in peroxide O22- = -1 - Fluorine is always -1
- For other elements not defined here, use rule 3 to determine the oxidation number.
For Group 1A is +1
For group 11A is +2
Other halogen is usually -1 but it depends
Li+ = +1
Ba2+ = +2
Fe=0
N2= 0 because it’s a free species
Cl- = -1
Oxidation
Increase in oxidation number
Gain of oxygen atoms
Loss of hydrogen atoms
Loss of electrons
Reduction
Decrease in oxidation number
Loss of oxygen atoms
Gain of hydrogen atoms
Gain of electrons
Oxidation State
is the physical state of the element corresponding
to its oxidation number. So we can calculate the theoretical oxidation number
of an atom in a molecule but say that it is in that oxidation state.
Formal Charge
- a means of identifying feasible electron-dot structures for covalent molecules. To calculate formal charge, we divided the bonding electrons equally between the constituent atoms
- splits electrons in a bond.
liquid Ionics
Liquid at room temp, very high boiling point
Recyclable = a green solvent
Allows Some reactions that do not take place in conventional solvents
Solvents #
classified by dielectric constant
Dielectric constant = ability to distort an electric field
50-100 = polar protic
20-50 = polar aprotic
0 -20 = nonpolar
Polar protic: water, ammonia, HF
Good for ionics
Solvents
function by interacting with solid solutes so that the reactant molecules and/or ions are free to collide and react. In order for a solvent to dissolve
a solute, the solvent-solute bonding interaction must be strong enough to overcome the lattice energy, the attraction between the particles in the solid crystal.
dielectric constant
to the ability of the compound to distort
an electrostatic field
Polar protic solvents
undergoes autoionization
contain one of the three very polar bonds H O F, O O H, or N O H. Solvation occurs by the strong ion-dipole forces between the anion and the hydrogen of the solvent and between the cation and the fluorine/oxygen/nitrogen
of the solvent. Solubility depends on the ion-dipole attractions being stronger
than the lattice energy
lattice energy
the electrostatic cation-anion
attractions in the crystal lattice
autoionization
a small proportion of
the solvent molecules undergo hydrogen ion exchange reactions to give their
conjugate acid and conjugate base simultaneously
acid
a solute that, either by direct dissociation or by reaction with the
solvent, gives the cation characteristic of the solvent.
base
is a solute that, either
by direct dissociation or by reaction with the solvent, gives the anion characteristic of the solvent
dissociation
“separation” and is used
when a solvent such as water separates the ions that are present in ionic compounds
Lewis Theory
A Lewis acid is an electron-pair
acceptor, while a Lewis base is an electron-pair donor.
Lewis acid would
be referred to as an electrophile and the Lewis base as a nucleophile.
Hard Acids
class a metal ions, consist of most of the
metal ions in the periodic table. They are characterized by low electronegativities and often high charge densities.
Soft acids
class b metal ions, are the group of metal
ions that are in the lower-right part of the metallic elements in the periodic
table They have low charge densities and tend to have among the
highest electronegativities of the metallic elements. With low charge densities,
these cations will be easily polarized; hence, they tend toward covalent bond
formation
Group Trends
Groups of elements tend to have characteristic properties. Descending a group in the periodic table, there are often smooth trends in these properties. Best for Groups 1, 2, 17, and 18. 13-16 less defined
Alkali Metals
-there is a decrease in melting and boiling point down the group
-reactivity increases spectacularly down the group.
-Down the alkali metals, the large size weakens metallic bond
Halogens
melting and boiling points increases down the group
the dispersion forces increase in strength with the number of electrons, thus accounting for the trend.
Going down a column the intermolecular forces weaken
Fluorine = explosive
Chlorine = Violent, but needs light catalysis
Bromine = slow
Iodine = Needs heat, equilibrium formed
The (n) Group and (n + 10) Group Similarities
There are similarities in chemical formulas and structures of the highest
oxidation state of some members of the (n) Group elements and of the members of the corresponding (n 1 10) Group elements.
Diagonal Relationship
There are similarities in chemical properties between an element and that to the lower right of it. This relationship is found primarily for elements in the upper-left corner of the periodic table.
The diagonal relationship is most chemically significant for three pairs of elements: lithium
and magnesium, beryllium and aluminum, and boron and silicon
The (n) Group and (n+10) Group Similarities 2
Melting point and reduction potentials of Al are similar to Sc
Isostructural compounds Na3MF6
B2H6 and Ga2H6 are molecular, but AlH3 is a covalent polymer.
Sc3+ and Al3+ both hydrolyze significantly in water.
Reaction with OH-: next
Reaction with S2-: Sc3+ and Al3+ precipitate the hydroxide, but Ga3+ gives the sulfide.
TiO2 and SnO2 are isostructural.
V and P , metal and a nonmetal
Many oxo species
VO43- and PO43- are tetrahedral ions and strong bases.
Both elements form polyatomic ions with multiple P or V
Cr compounds are good oxidizers, but S compounds are not
Both are strong acids with Sulfuric the stronger of the two
Mg and Zn -soluble of sulfates and chlorides
carbonates are insoluble
Mg and Zn species at various pH values, zn complex at high pH
Mn and Cl species at various pH values,- Both strong acids
Beryllium and Aluminum
Both form a strong protective oxide layer.
Both oxides are amphoteric forming [M(OH)4]n- with excess hydroxide.
Both form carbides with C4-. Other Group II form acetylides with C22-
Differences: Hydrated Be2+ has four water molecules (probably because it is so small.) Al3+ has six water molecules.
Charge density explains these similarities as well.
Boron and Silicon
Both form an acidic solid oxide. Aluminum oxide is amphoteric, and carbon dioxide is acidic, but gaseous.
Boric acid, H3BO3, and silicic acid, H4SiO4, are very weak acids. Al(OH)3 is amphoteric.
Borates and Silicates which share oxygen atoms are common
Boron and silicon form flammable hydrides
The “Knight’s Move” Relationship
Found in lower members of group 11 to 15
Element in a group (n) and Period (m) have similarities to an element two groups right (n+2) and one period down (n+1)
“L” shift knight in chess
Chlorides form M(OH)Cl precipitates
Alloys with copper Sn gives bronze and Zn gives brass
Sn(IV) Po(IV)
Chlorides form SnCl4 and PoCl4 molecular
Excess chlorine forms complexes [MCl6]2-
Stable nitrates of the two are (4+) like Sn(NO3)4
(4+) oxides SnO2 and PoO2
Ag(I)Tl (I) and K(I)
Insoluble halides except for fluorides
Similarities in melting point of chlorides and nitrates
l(I) is a soft acid that binds to S atoms
Treatment for thallium poisoning:
1) flood system with K+
2) Precipitate Tl(I) with [Fe3+Fe2+(CN)6]+
Similar size and charge = similar charge densities = similar chemistry. Due to poor shielding by d and f electrons.
Shrinks (n+2) group and (m+1) period atoms to about the same as n group and m period atom
Why do lower main group atoms have two oxidation states?
Inert pair effect. Relativistic effects of the 6s orbital increase the ionization energy of the electron making then harder to remove.
Combo Elements
a subset of isoelectronic behavior in which the
sum of the valence electrons of a pair of atoms of one element matches the sum
of the valence electrons of two horizontal neighboring elements:
A combo element can be defi ned as the combination of an (n 2 x) group element with an (n 1 x) group element to form compounds that parallel those of
the (n) group element.
Strontium for calcium
Strontium is similar to calcium
Strontium-90 in nuclear fallout may substitute for Ca in bones and teeth. The radioactivity destroys tissues in marrow killing immune cells.
Strontium especially affects children with growing bones. Until the ban on atmospheric testing of nuclear bombs in 1964, Strotium-90 levels steadily increased in children.
Self Ionization
also called autoionization, when a species transfers a +1 or ion to for a cation + anion.
Leveling solvent
solvent that converts a very strong base to (-) form of solvent and very strong acid to (+) form of solvent.
amphiprotic
a substance that can be both an acid and a base.
polyprotic
A substance that can donate or accept more than one H+
the trends in melting point for Group 2 elements
Mg is unusual but Ca> Sr– BaSr from a trend that melting point Ba decreases down as metal Bond weakens with bigger atoms
the trends in melting point for Group 17
The trend is increasing from F2-> CL2 -> Br2 -> I2
diatomics -only dispersion increases with # of e-
Clathrates
a substance in which molecules or atoms are trapped within the crystalline framework of other molecules.
IMF for Halogens
IMF Weaken as you go down the column. As you go down the melting point and boiling point increase
Group 15
Transition from non metals (N,P) to metalloid (As) to metals (Sb, Bi).
Reactivity not clear cut
All form oxyanions, harder for Sb and Bi
Metals form (3+) cations
Isoelectronic Series in Covalent Compounds
Isoelectronic species have the same number of valence electrons and total electrons
Species with the same number of valence electron, but different total electrons are valence isoelectronic
Mn and Cl
(ClO4-) colorless (MnO4- )purpleboth strong oxidizers
Cl2O7 colorless Mn2O7 reddish-brown highly explosive liquids
ClO2 yellow gas MnO2 black solid Unusually oxidation state
