Download Rules for Assigning Oxidation Numbers (115)

Rules for Assigning Oxidation Numbers
1.
Elements in molecules consisting of just that element are assigned an oxidation number of “zero”.
Ex. Na, Fe, Cl2, N2, P4, S8
2.
Monatomic ions have an oxidation number equal to the ion charge.
Ex. Cl-, S-2, Al+3, Fe+2, MnCl2, PbBr2, TiCl4
3.
In compounds: Group IA metals assigned an oxidation number = +1
Group IIA metals assigned an oxidation number = +2
Group IIIA (Al) assigned an oxidation number = +3
4.
In its compounds fluorine is assigned an oxidation number = -1.
5.
In its compounds hydrogen is assigned an oxidation number = +1.
Exceptions: Metal hydrides like NaH and CaH2 (H = -1. Rule 3 takes priority.)
6.
In its compounds oxygen is assigned an oxidation number = -2.
Exceptions: Peroxides:
H2O2, Na2O2, BaO 2 (O = -1; Rules 5 and 3 take priority)
Superoxides: KO2, RbO2, CsO 2 (O = -1/2; Rule 3 takes priority.)
Others:
OF2 (O = +2; Rule 4 takes priority.)
7.
Apply the oxidation number rules in the order given.
8.
The sum of oxidation numbers is equal to the overall charge on the species.
Balancing Redox Reactions by the Half-Reaction Method
A.
To Balance Acidic Half-Reactions:
1.
2.
3.
4.
B.
To Balance Basic Half-Reactions:
1.
2.
3.
4.
5.
6.
C.
Balance atoms being oxidized or reduced.(Usually atoms other than “H” or “O”)
Add H2O to balance the oxygen.
Add protons, H+, to balance the hydrogen.
Add electrons, e-, to balance the charge.
Balance atoms being oxidized or reduced. (Usually atoms other than “H” or “O”)
Add H2O to balance the oxygen.
Add protons, H+, to balance the hydrogen.
Add electrons, e-, to balance the charge.
Add OH- to neutralize the H+.
NOTE: OH- is added to both sides of eqn.
Cancel out extra H2O.
To Balance a Complete Redox Reaction: (involving both oxidation and reduction half reactions)
1.
2.
3.
4.
5.
Split the redox reaction into two half-reactions; one half-reaction for the
oxidation and one for the reduction.
Separately balance the half-reactions for acidic or basic solution.
Add the balanced half-reactions together so that the number of electrons cancel.
(ie. so that # e- lost during oxidation = # e- gained during reduction)
Cancel out species that appear on both sides of the equation in the same form..
Reduce coefficients to smallest set of whole # coefficients.
Balancing Redox Reactions by the Oxidation Number Method
A.
To Balance a Complete Redox Reaction: (involving both oxidation and reduction)
1.
Assign oxidation numbers and determine species oxidized and reduced.
2.
Balance atoms oxidized and reduced.
3.
Determine the total number of electrons lost by the reactant oxidized (reducing agent)
and the total number of electrons gained by the reactant reduced (oxidizing agent).
4.
Adjust coefficients of reactants so that # e- lost are equal to the # e- gained.
5.
Balance the rest of the atoms by inspection. However.....
A.
If the redox eqn. is in the form of a net ionic eqn., balance as follows:
1.
Acidic- add H2O to balance “O” and then add H+ to balance H.
2.
Basic - add OHG to balance charge and then add H2O to balance
H and “O”.