Elements in Redox Reactions

Metallic elements taking part in redox reactions, such as zinc in the reaction above, commonly act as reducing agents they are oxidized to cations such as Zn2+. Other reducing agents include hydrogen gas, which can be oxidized to H+ ions ... 

The concept of oxidation number is used to simplify the electron bookkeeping in redox reactions. For a monatomic ion (e.g., Na+, S2 ), the oxidation number is, quite simply, the charge of the ion (+1, —2). In a molecule or polyatomic ion, the oxidation number of an element is a pseudo-charge obtained in a rather arbitrary way, assigning bonding electrons to the atom with the greater attraction for electrons. 

Another way to determine what is undergoing oxidation and what is undergoing reduction is by looking at the change in oxidation numbers of the reactant species. Oxidation occurs when there is an increase in oxidation number. In the example above, the Zn metal went from an oxidation state of 0 to +2. Reduction occurs when there is a decrease in oxidation number. Cu2+ went from an oxidation state of +2 to 0. In order to determine if a particular reaction is a redox reaction, determine the oxidation numbers of each element in the reaction. If at least one element changes oxidation number, it is a redox reaction. Refer to your textbook for rules in assigning oxidation numbers. 

The usefulness of determining the oxidation number in analytical chemistry is twofold. First, it will help determine if there was a change in oxidation number of a given element in a reaction. This always signals the occurrence of an oxidation-reduction reaction. Thus, it helps tell us whether a reaction is a redox reaction or some other reaction. Second, it will lead to the determination of the number of electrons involved, which will aid in balancing the equation. These latter points will be discussed in later sections. 

O Explain why, in redox reactions, the total increase in the oxidation numbers of the oxidized elements must equal the total decrease in the oxidation numbers of the reduced elements. 

How do you determine whether a given transformation reaction of an organic compound is a redox reaction Which elements in an organic molecule are primarily involved in redox reactions ... 

Table III shows the abundance of various elements in the earth s crust and the oxidation states they frequently occupy. The table indicates that of the 14 most abundant elements, only six participate in redox reactions in the surface layers of the earth. [PH3 seems to be extremely rare (42) and will not be discussed.] Because by definition free oxygen as 02 is absent in the anoxic zone, it is evident that oxides of Fe(III) are the most important oxidizers in anoxic environment and that S042 and higher oxides of manganese are of importance only locally. Reducing compounds of importance are organic matter and sulfides, the latter frequently from volcanic emanations. Hydrogen is commonly combined with other elements, as in H20, CH4, and NH3 but may locally occur free as H2. Since iron is the most widespread element that can serve as an oxidizer in the anoxic environment the distribution of the valence states of iron in various rocks is of interest (see Table IV). Sandstones frequently have a high Fe203/Fe0 ratio, but shales and clays may also be highly oxidized as shown in Tables IV and V. Since approximately 75% of the earth s surface is covered with sediments and since the sediments...

The alkaline earth elements in group 2A—Be, Mg, Ca, Sr, Ba, and Ra—are similar to the alkali metals in many respects. They differ, however, in that they have ns2 valence-shell electron configurations and can therefore lose two electrons in redox reactions. Alkaline earth metals are thus powerful reducing agents and form ions with a +2 charge. 

As a general rule, the properties of interhalogen compounds are intermediate between those of their parent elements. For example, IC1 is a red solid that melts near room temperature, and BrF is a brownish gas that condenses to a liquid near room temperature. All six possible diatomic interhalogen compounds are known, and all act as strong oxidizing agents in redox reactions. 

PROBLEM 6.25 Although we haven t talked about group 6A elements in this chapter, what are they likely to do in redox reactions—gain or lose electrons How many ... 

When determining which substances were oxidized and reduced in a redox reaction, first assign the oxidation numbers to all the elements in the reaction. The formation for magnesium chloride serves as an example ... 

Besides the strict electron transfer function, hemeproteins may display intrinsic catalytic activities, generally in redox reactions. The simplest reactions, which are those usually found in anaerobes, involve the reduction or oxidation of small molecules, such as nitrogen oxides (NO and N02 [14]), sulfur oxides (SO3"), elemental sulfur and sulfide, dioxygen and hydrogen peroxide. More complex reactions may also be performed by heme proteins, namely those involving the oxidation of organic molecules, although this type of reaction has not yet been found in anaerobes. 

Any reaction that includes a free element as reactant or product is a redox reaction. In combination reactions, elements combine to form a compound, or a compound and an element combine. Decomposition of compounds by absorption of heat or electricity can form elements or a compound and an element. In displacement reactions, one element displaces another from solution. Activity series rank elements in order of reactivity. The activity series of the metals ranks metals by their ability to displace H2 from water, steam, or acid, or to displace one another from solution. Combustion typically releases heat and light energy through reaction of a substance with O2. 

The problems associated with the M state of the elements of group III have been noted previously (cf. Sections 25.1.13 and 25.2.3). The TP ion has been identified in pulse radiolysis decomposition of aqueous Tl2S04, and in y-irradiated frozen aqueous solutions. Thallium(II) has also been invoked as an intermediate in the photochemical reduction of aqueous Tl" solutionsbut there is dispute as to its involvement in redox reactions. It is safe to conclude that there is no equilibrium aqueous phase chemistry of TP" ". 

In redox reactions the oxidation numbers of elements change. To test whether an equation represents a redox... 

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