Oxygen oxidation number

In Chap. 6 we placed Roman numerals at the ends of names of metals to distinguish the charges on monatomic cations. It is really the oxidation number that is in parentheses. This nomenclature system is called the Stock system. For monatomic ions, the oxidation number is equal to the charge. For other cations, again the oxidation number is used in the name. For example, Hg2 + is named mercury(I) ion. Its charge is 24- the oxidation number of each atom is 4-1. Oxidation numbers are also used for other cations, such as dioxovanadium(V) ion, V02". The prefix 0x0- stands for oxygen. Oxidation numbers can be used with nonmetal-nonmetal compounds, as in sulfur(VI) oxide for SO3, but the older system using prefixes (Table 6-2) is still used more often. 

Combined oxygen, oxidation number -2 We know from part (b) that the oxidation number of the phosphorus atoms in H3PO4 is +5. 

As we have just seen the reaction of alkanes with oxygen to give carbon dioxide and water IS called combustion A more fundamental classification of reaction types places it m the oxidation—reduction category To understand why let s review some principles of oxidation-reduction beginning with the oxidation number (also known as oxidation state)... 

The carbon m methane has the lowest oxidation number (—4) of any of the com pounds m Table 2 4 Methane contains carbon m its most reduced form Carbon dioxide and carbonic acid have the highest oxidation numbers (+4) for carbon corresponding to Its most oxidized state When methane or any alkane undergoes combustion to form carbon dioxide carbon is oxidized and oxygen is reduced A useful generalization from Table 2 4 is the following... 

A neutral carbon atom has four valence electrons Five electrons are assigned to the CH2OH carbon therefore it has an oxidation number of -1 Seven electrons are assigned to the CH3 carbon therefore it has an oxidation number of-3 As expected this method gives an oxidation number of -2 for oxygen and +1 for each hydrogen... 

Combustion of alkanes is an example of oxidation-reduction Although It IS possible to calculate oxidation numbers of carbon m organic mole cules It IS more convenient to regard oxidation of an organic substance as an increase m its oxygen content or a decrease m its hydrogen content... 

A peroxide oi peioxo compound contains at least one pair of oxygen atoms, bound by a single covalent bond, in which each oxygen atom has an oxidation number of —. The peroxide group can be attached to a metal, M, through one (1) or two (2) oxygen atoms, or it can bridge two metals (3) ... 

The oxidation number of an element in a monatomic ion is equal to the charge of that ion. In the ionic compound NaCl, sodium has an oxidation number of +1, chlorine an oxidation number of — 1. The oxidation numbers of aluminum and oxygen in A1203 (Al3+, O2- ions) are +3 and —2, respectively. 

Oxygen is ordinarily assigned an oxidation number of —2 in its compounds. (An exception arises in compounds containing the peroxide ion, 022-, where the oxidation number of oxygen is —1.)... 

The transition metals, unlike those in Groups 1 and 2, typically show several different oxidation numbers in their compounds. This tends to make their redox chemistry more complex (and more colorful). Only in the lower oxidation states (+1, +2, +3) are the transition metals present as cations (e.g., Ag+, Zn2+, Fe3+). In higher oxidation states (+4 to +7) a transition metal is covalently bonded to a nonmetal atom, most often oxygen. 

Strategy The structure can be obtained by removing an oxygen atom from H O, (Figure 21.8). Relative acid strengths can be predicted on the basis of the electronegativity and oxidation number of the central nonmetal atom, following the rules cited above. 

Is this an oxidation-reduction reaction Historically, it surely is, for the term oxidation originally referred specifically to reactions with oxygen. Yet our electron-transfer view of oxidation-reduction reactions provides no help in deciding so. Where in reaction (76) is there any evidence of electrons being gained or lost In such a doubtful case, our oxidation number scheme provides an answer. Applying the same assumptions used in treating the HSOf-HSOi"... 

In reaction (76) the oxidation number of sulfur changes from +4 to 4-6. According to this, two electrons are released by each sulfur atom oxidized. Show that these electrons are gained by oxygen if we assume oxygen has oxidation number equal to zero in O2. 

For the present, we will limit ourselves to molecules containing hydrogen and/or oxygen along with the element to which we wish to assign an oxidation number. The rules we will utilize are as follows ... 

Sulfur reacts with molecular oxygen to form compounds in which sulfur is assigned positive oxidation numbers, +4 and +6. The reactions are those used in the manufacture of sulfuric acid (see Chapter 13) ... 

The oxidation number of all the halogens is - 1 unless the halogen is in combination with oxygen or another halogen higher in the group. The oxidation number of fluorine is —1 in all its compounds. 

The oxidation number of oxygen is —2 in most of its compounds. Exceptions are its compounds with fluorine (in which case, the previous statement takes precedence) and its occurrence as peroxides (022-), superoxides (02 ), and ozonides (03 ). 

SOLUTION The process is oxidation if the oxidation number of sulfur increases, reduction if it decreases. We need to assign the oxidation numbers of sulfur in S02 and S042, then compare them. To assign an oxidation number to sulfur we represent that number by x and solve for x, by using the rules in Toolbox K.l. The oxidation number of oxygen is —2 in both compounds. 

K.8 Some compounds of hydrogen and oxygen are exceptions to the common observation that H has oxidation number -1-1 and O has oxidation number —2. Assuming that each metal has the oxidation number of its most common ion, find the oxidation numbers of FI and O in each of the following compounds (a) K02 (b) LiAIH4 (c) Na202 (d) NaH (e) K03. 

Maximum oxidation numbers are often found in the oxoanion with the most oxygen atoms. For minimum oxidation numbers, consult Fig. C.6. (b) Describe anv patterns you see in the data. 

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