Redox Rules
What is the oxidation number of . . .
Problems 11 - 25

Ten Examples

Problems 1-10 Return to Redox menu


Hydrides are compounds of a metal and hydrogen. What are the oxidation numbers of the elements present in each formula given in 11-13?

Problem #11: NaH

We know from other compounds like NaCl that Na is a +1. The H is, therefore, a -1.

Problem #12: CaH2

Ca = +2. Each H is -1.

Problem #13: SH6

As of July 2017, this hydride has been predicted but not yet been discovered. The S is a +6 and each of the six hydrides is a -1.

Peroxides have the polyatomic ion O22¯ present. Two examples:

Problem #14: H2O2

Each hydrogen is a +1, the peroxide is -2. That means that each oxygen in the peroxide is a -1. However, be careful. If you make reference to the -1 value, you must make it clear that you are referring to the oxygens in a peroxide. If you simply refer to an O without any context, the assumption is that you are referring to an oxide, the oxidation number of which is -2.

Problem #15: CaO2

Calcium is +2, the peroxide is -2, when discussed as the group. If the question were to ask the oxidation number of each oxygen in a peroxide, the answer would be -1.

Comment: PbO2 is an example of possible confusion with identifying peroxides. Is the Pb = +4? In which case, the O2 porton refers to two oxides, each of which is -2. Or, is the Pb = +2? If that is the case, then the O2 portion is a peroxide and has a total charge of -2, each O being -1 in the peroxide.

How do you tell which one is meant? You would do so by the context in which the formula is presented. if context is lacking, ask your teacher for help. If it's an online course and live help is hard (or impossible) to get, pick the Pb = +4 answer. If it's wrong, give the other one.


Problem #16: HIO3, the iodine has an oxidation number of:

The three oxygens total to -6 and the hydrogen adds a +1, leaving -5 to be offset by the iodine. The iodine has an oxidation number of +5.

Problem #17: S2O32¯, thiosulfate:

Three oxygens, each of which is -2, total to -6. Since -2 is left over, a total of -4 must be offset by the two sulfurs. Each sulfur has an oxidation umber of +2.

Problem #18: What is the oxidation number of all free elements? Another way to ask this is to say that the elements are in their uncombined state

Zero.

By the way, molecular elements (the seven diatomics plus P4 and S8) are considered to be uncombined.


Problem #19: Determine the oxidation number of P in P2O5.

The five oxygens total to -10. Each P is +5.

Problem #20: Determine the oxidation number of S in Al2S3.

Aluminum takes on a +3 oxidation number in all common aluminum-containing compounds. Examples include AlCl3 and Al2O3.

Sulfur can take on several different oxidation numbers. When it is alone and in the nonmetal position of a binary compound, it takes on a -2 oxidation number. An example of this is H2S.

In this compound, Al is a +3 and S is a -2.


Problem #21: Determine the oxidation number of Te in Sc2Te3.

The compound H2Te is known to exist, which means that Te has an oxidation number of -2 when acting as a nonmetal.

The presence of three Te means a total negative oxidation number of -6.

To offset this, each Sc must be a +3.


Problem #22: Determine the oxidation number of Si in SiF62¯.

The F is a -1. We know this from the existence of HF. Six of the yields a total of -6. In order to have -2 left over, the Si must be a +4

Problem #23: All monatomic ions have oxidation numbers equal to the ______.

(a) difference of protons and electrons
(b) number of protons
(c) number of electrons
(d) difference of protons and neutrons

The answer is (a). Examples include Na+, Ca2+ and F¯.


Problem #24: On rare occasions, oxidation numbers do not have to be whole numbers. Determine the oxidation number for S in S4O62–.

There are six O, which contribute a total of -12 in oxidation number. Since -2 is left over, that leaves -10 to be offset by the four sulfurs.

Each sulfur is, therefore, +10/4. Reduced would be +5/2

In truth, the four sulfurs would have a total of +10 oxidation and each would be a whole value. For example, if three S were each +2 and one S was a +4, this would total up to +10. Chemical substances do not have fractional oxidation numbers.


Problem #25: What are the oxidation numbers for each Mn in Mn3O4?

Four oxygens gives a total of -8 in oxidation number.

The three Mn atoms must each have an integer oxidation number. This is satisfied if two Mn are each a +3 and the third Mn is a +2.


Bonus Problem: Superoxide is not commonly mentioned in introductory chemistry classes. it is the polyatomic ion O2¯. KO2 is the most well-known of the superoxides. What are the oxidation numbers on each oxygen in the superoxide anion?

It appears that, within the superoxide, each oxygen must have an oxidation number of -1/2.

The ChemTeam thinks, but is not 100% sure, that the electronic structure of the superoxide anion is such that one O has an oxidation number of zero and the other O has an oxidation number of -1.


Ten Examples

Problems 1-10 Return to Redox menu