Oxidation-reduction reactions redox numbers

Today, many reactions in aqueous solutions can be described as oxidation-reduction reactions (redox reactions). Oxidation is the process in which the oxidation number of atoms increases. Reduction is the process in which the oxidation number of atoms is decreased or made more negative. In another definition, oxidation is the loss of electrons by an atom, and reduction is the gain of electrons. Let us look at the following reaction ... 

First, balance the number of atoms then, balance all of the transferred electrons. However, these simple rules are often difficult to apply in oxidation-reduction reactions (redox reactions). 

The reactions that this sodium-chlorine case typifies are called oxidation-reduction reactions. The term oxidation refers to the loss of electrons, while the term reduction refers to the gain of electrons. A number of oxidation-reduction reactions (nicknamed redox reactions) are useful in titrimetric analysis, and many are encountered in other analysis methods. 

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. 

A reaction in which the oxidation number of an element is increased. Examples (1, 2, 3) 2 Mg(s) + 02(g) - 2 MgO(s) (2, 3) Mg(s) - Mg2+(s) + 2e". oxidation number The effective charge on an atom in a compound, calculated according to a set of rules (Toolbox K.l). An increase in oxidation number corresponds to oxidation, and a decrease, to reduction, oxidation-reduction reaction See redox reaction. oxidation state The actual condition of a species with a specified oxidation number. 

Oxidation-reduction reactions, often called redox reactions, occur because many elements can occur in more than one oxidation state, and can, therefore, influence chemical speciation. At normal temperatures and pressures, the number of elements involved in such reactions in relatively small. 

When a biochemical half-reaction involves the production or consumption of hydrogen ions, the electrode potential depends on the pH. When reactants are weak acids or bases, the pH dependence may be complicated, but this dependence can be calculated if the pKs of both the oxidized and reduced reactants are known. Standard apparent reduction potentials E ° have been determined for a number of oxidation-reduction reactions of biochemical interest at various pH values, but the E ° values for many more biochemical reactions can be calculated from ArG ° values of reactants from the measured apparent equilibrium constants K. Some biochemical redox reactions can be studied potentiometrically, but often reversibility cannot be obtained. Therefore a great deal of the information on reduction potentials in this chapter has come from measurements of apparent equilibrium constants. 

One of the main purposes for using oxidation numbers is to follow the movement of electrons during an oxidation-reduction reaction. Doing so helps to predict the products and determine the outcomes of such reactions. There are a few different ways to analyze redox reactions, but we will focus on only one the ion-electron method (also called the half-reaction method). The procedure requires that you know the reactants and products of the reaction, but, by going through the process, you will gain a better understanding of the mechanisms by which these reactions proceed. 

OXIDATION AND REDUCTION All the reactions mentioned in the previous sections were ion-combination reactions, where the oxidation number (valency) of the reacting species did not change. There are however a number of reactions in which the state of oxidation changes, accompanied by the interchange of electrons between the reactants. These are called oxidation-reduction reactions or, in short, redox reactions. 

The many reactions that involve the transfer of electrons from one species to another are called oxidation-reduction reactions, or simply, redox reactions. We use oxidation numbers to keep track of electron transfers. The systematic naming of compounds (Sections 4-11 and 4-12) also makes use of oxidation numbers. 

You can always recognize a redox reaction by analyzing oxidation numbers. First determine the oxidation number of each element wherever it appears in the reaction. If no elements change in oxidation numbers, the reaction is not an oxidation-reduction reaction. If changes do occur, the reaction is an oxidation-reduction reaction. Remember that oxidation and reduction must always occur together if some atoms increase in oxidation numbers, then others must decrease. 

Whereas redox reactions on metal centres usually only involve electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer, but hydrogen transfer as well — hence the frequently used denomination dehydrogenase . Note that most of these dehydrogenase reactions are reversible. Redox reactions in biosynthetic pathways usually use NADPH as their source of electrons. In addition to NAD and NADP+, which intervene in redox reactions involving oxygen functions, other cofactors like riboflavin (in the form of flavin mononucleotide, FMN, and flavin adenine dinucleotide, FAD) (Figure 5.3) participate in the conversion of [—CH2—CH2— to —CH=CH—], as well as in electron transfer chains. In addition, a number of other redox factors are found, e.g., lipoate in a-ketoacid dehydrogenases, and ubiquinone and its derivatives, in electron transfer chains. 

Identifying oxidation-reduction reactions and their components Reduction is the gain of an electron by a molecule, atom, or ion, thus decreasing its oxidation number. Oxidation is the loss of an electron, thus increasing the oxidation number of the molecule, atom, or ion. These two processes always occur together in oxidation-reduction reactions, also called redox reactions. Electrons lost by one substance are gained by the other. 

We know that in an oxidation-reduction reaction we must ultimately have equal numbers of electrons gained and lost, and we can use this principle to balance redox equations. For example, in this case, 2 Ag ions must be reduced for every Cu atom oxidized ... 

The normality (N) of a solution is the number of equivalents of solute per liter of solution. The equivalent is usually defined in terms of a chemical reaction. For acid-base reactions, an equivalent is the amount of substance that will react or form 1 mole of hydrogen (H ) or hydroxide (OH ) ions. For redox (oxidation-reduction) reactions, an equivalent is the amount of substance that will react or form 1 mole of electrons. 

SECTION 4.4 Oxidation is the loss of electrons by a substance, whereas reduction is the gain of electrons by a substance. Oxidation numbers keep track of electrons during chemical reactions and are assigned to atoms using specific rules. The oxidation of an element results in an increase in its oxidation number, whereas reduction is accompanied by a decrease in oxidation number. Oxidation is always accompanied by reduction, giving oxidation-reduction, or redox, reactions. 

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