Oxidation numbers. See

Considering the changes in oxidation number during a reaction can dramatically simplify the concept of oxidation and reduction oxidation is an increase in oxidation number and reduction is a decrease in oxidation number (see Chapter 7). Be aware, though, oxidation numbers rarely correlate with the charge on an ion. For example, consider the sulphate anion S04 (IV). 

Since so much can be predicted or at least rationalized on the basis of the periodicity of a few simple atomic properties, especially Pauling electronegativity, size, and charge (or oxidation number) see Oxidation Number) of the atoms or ions involved, we begin by looking at these trends in some detail (beyond that done in General Chemistry courses) and... 

The oxidation number (see Sections IR-4.6.1 and IR-9.1.2.8) of an element is indicated by a Roman numeral placed in parentheses immediately following the name (modified by the ending ate if necessary) of the element to which it refers. The oxidation number may be positive, negative or zero (represented by the numeral 0). An oxidation number is always non-negative unless the minus sign is explicitly used (the positive sign is never used). Non-integral oxidation numbers are not used for nomenclature purposes. 

Most r/-block elements have more than one common oxidation number and one or more less common oxidation numbers (see text Figure 16.6). 

We first assign oxidation numbers to elements that exhibit a single common oxidation number (see Table 4-10). We recall that oxidation numbers are represented per atom and that the sum of the oxidation numbers in a compound is zero, and the sum of the oxidation numbers in an ion equals the charge on the ion. 

Even the reactions in the lead accumulator can be interpreted with the help of oxidation numbers (see Fig. 8.5). If, in the model of the accumulator, two lead plates are dipped into 20% sulfuric acid solution and electrolyzed by the voltage of about 5 V, one of the lead plates is covered with a dark layer of lead oxide (see E8.10). If the transformer is taken away and one joins both plates with a voltmeter, the voltage of 2 V will be measured (see E8.10). The following equilibrium exists if a pure lead plate and a lead oxide plate are separately dipped into 20% sulfuric acid solution (see Fig. 8.5) ... 

Note This rule applies to the transition metals and metals on the right side of the periodic table, which often have more than one oxidation number. See Table 7.8. It does not apply to group 1 and group 2 cations, as they have only one oxidation number. Examples ... 

You w3l nodoe that die compounds of iron are named ironfil) sulphate and irondll) sulphate to show which of its valencies iron is using in the compound. This is always done with tbe compounds of elements of variable valency. For valency and oxidation number, see Chapter 8, p. 72. 

Rule 3 The sum of all oxidation numbers in a neutral compound is zero. The sum of all oxidation numbers in a polyatomic (manynatom) ion is equal to the charge on the ion. This rule often allows chemists to calculate the oxidation number of an atom that may have multiple oxidation states, if the other atoms in the ion have known oxidation numbers. (See Chapter 6 for examples of atoms with multiple oxidation states.)... 

The reader should check Reactions (6.9g) and (6.9h) for consistency between electron losses or gains and changes in oxidation numbers -see step 5 in Exercise 6.2.]... 

Any combination with something higher on the list that necessitates its having a different oxidation number (see rule 2 for assigning oxidation numbers). Examples H2O5 and KO2—the oxidation number of O for H2O2 is 1 and for KOt is —i. 

During the course of reaction (12.1), Zn lost two electrons. By definition, it is said to have been oxidized. Zn + represents the oxidized form Ox of the couple Zn +/Zn, whereas Zn is its reduced form. Red. Likewise, Cu + captures two electrons during the course of reaction (12.1). It has been reduced. Cu and Cu + are, respectively, the reduced and oxidized forms of the couple Cu +/Cu. Cu + and Zn + are said to be in a state of oxidation upper than Cu and Zn. We shall see in the following section that the oxidation states are labeled by oxidation numbers (see Sect. 12.2). 

In some cases, telling what s been oxidized and reduced is easy in other cases, it isn t. Start by going through the example reaction and assigning oxidation numbers (see the earlier section Oxidation numbers for details) ... 

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