Oxidation number change

Rusting and oxidation number. As iron rusts, its oxidation number changes from 0 in Fe(s) to +3 in Fe3+. 

Verify that reaction (23) is an oxidation-reduction reaction and that the oxidation number change of carbon is balanced by the oxidation number change of the sulfur. 

The oxidation number of an element in a monatomic ion is the same as its charge. For example, the oxidation number of magnesium is +2 when it is present as Mg2+ ions, and the oxidation number of chlorine is — 1 when it is present as Cl" ions. The oxidation number of the elemental form of an element is 0 so magnesium metal has oxidation number 0 and chlorine in the form of Cl2 molecules also has oxidation number 0. When magnesium combines with chlorine, the oxidation numbers change as follows ... 

Step 3 Disproportionation in water nitrogen s oxidation number changes from +4 to +5 and +2 ... 

Although these reactions look similar, the redox reaction includes a pure element. Nearly all reactions involving pure elements are redox, because the oxidation number changes from zero to either a positive or a negative value. 

Figure 4.17 Summary of oxidation number changes by reaction stage for labelled atoms in Scheme 4.9 for triclosan synthesis.

In every reaction in which the oxidation number of an element in one reactant (or more than one) goes up, an element in some reactant (or more than one) must go down in oxidation number. An increase in oxidation number is called an oxidation. A decrease in oxidation number is called a reduction. The term redox (the first letters of reduction and oxidation) is often used as a synonym for oxidation-reduction. The total change in oxidation number (change in each atom times number of atoms) must be the same in the oxidation as in the reduction, because the number of electrons transferred from one species must be the same as the number transferred to the other. The species that causes another to be reduced is called the reducing agent in the process, it is oxidized. The species that causes the oxidation is called the oxidizing agent in the process, it is reduced. 

There are two essentially different methods to balance redox reactions—the oxidation number change method and the ion-electron method. The first of these is perhaps easier, and the second is somewhat more useful, especially for electrochemical reactions (Chap. 14). 

Balance the following equation by the oxidation number change method ... 

You can also monitor changes in oxidation numbers in reactions that involve covalent molecules. For example, oxidation number changes occur in the reaction of hydrogen and oxygen to form water. 

Seeing how oxidation numbers change in oxidation-reduction reactions... 

Section 16.1 introduces the concept of oxidation number and how to calculate the oxidation number of an element from the formula of the compound or ion of which it is a part. Section 16.2 describes how to use the oxidation numbers to name compounds, formalizing and extending the rules given in Chapter 6. Section 16.3 shows how to predict possible oxidation numbers from the position of the element in the periodic table and how to use these oxidation numbers to write probable formulas for covalent compounds. Section 16.4 presents a systematic method for balancing equations in which oxidation numbers change. 

Step 1 Determine the oxidation number of each element in the reaction. Identify the element that is reduced, and balance the number of atoms of that element, if necessary. In this example, the nitrogen atom is reduced from an oxidation state of +5 to -3. After the element whose oxidation number changes has been balanced, no element will change oxidation number in steps 2 through 4. 

Balance the equation for the reduction of HNO3 to NH4NO3 by Mn by the oxidation number change method. Add other compounds as needed. 

This reaction can be scrutinised for oxidation number changes. Iron goes from +3 to 0 and so has been reduced, and the carbon is oxidised from zero oxidation number to +2 in carbon monoxide. 

Equations for redox reactions are sometimes difficult to balance. Use the steps in Skills Toolkit 2 below to balance redox equations for reactions in acidic aqueous solution. An important step is to identify the key ions or molecules that contain atoms whose oxidation numbers change. These atoms are the starting points of the unbalanced half-reactions. For the reaction of zinc and hydrochloric acid, the unbalanced oxidation and reduction half-reactions would be as follows ... 

To see how oxidation numbers change, start hy assigning numbers, shown in Table 20-2, to all elements in the balanced equation. Then review the changes as shown in the accompanying diagram. 

How does the oxidation number change when an element is oxidized When it is reduced (20.1)... 

Oxidation Numbers Change/Atom Equalizing Changes Gives... 

One important use of oxidation numbers is in balancing redox equations. There are essentially two methods to balance redox reactions the oxidation number change method and the ion-electron method. In the former method, the changes in oxidation number are used to balance the species in which the elements that are oxidized and reduced appear. The numbers of atoms of each of these elements is used to give equal numbers of electrons gained and lost. If necessary, first balance the number of atoms of the element oxidized and/or the number of atoms of the element reduced. Then, balance by inspection, as was done in Chapter 7. 

Complexes of Fe " " and Mn " " ions with three soil and peat HAs have also been studied by XANES and EXAFS spectroscopies (Davies et al., 1997, 2001). No second-shell peaks are observed in EXAFS spectra of Fe " "- and Mn -HA complexes. Further, no evidence in EXAFS spectra is observed for oxidation number changes in these redox-active metals as a function of varying metal loading. 

---