Oxidation-reduction reaction redox changes

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. 

In a complexation reaction, a Lewis base donates a pair of electrons to a Lewis acid. In an oxidation-reduction reaction, also known as a redox reaction, electrons are not shared, but are transferred from one reactant to another. As a result of this electron transfer, some of the elements involved in the reaction undergo a change in oxidation state. Those species experiencing an increase in their oxidation state are oxidized, while those experiencing a decrease in their oxidation state are reduced, for example, in the following redox reaction between fe + and oxalic acid, H2C2O4, iron is reduced since its oxidation state changes from -1-3 to +2. 

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. 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/ donors. One of the most important is that based on NAD/NADP. NAD+ can accept what is essentially two electrons and a proton (a hydride ion) from a substrate such as ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidized product, acetaldehyde and the reduced cofactor NADH plus a proton (Figure 5.2). Whereas redox reactions on metal centres usually involve only electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer but... 

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. 

This is a very unsatisfactory definition because many oxidation-reduction or redox reactions do not involve changes in hydrogen or oxygen content, as the following example illustrates ... 

Reactions in which there is a change in the charges of some or all of the reactants are called oxidation-reduction (redox) reactions. Because there are changes in charge, equations can be considered with the inclusion of electrons showing the movement of electrons from one participant in the reaction to another. The reaction of metallic copper and sulfur is an example of an oxidation-reduction reaction. 

Redox titration — A - titration method in which electrons are transferred between the - titrant and the - analyte. Usually, the - end point of oxidation/reduction reactions is measured by chemical or potentiometric methods. The chemical method involves an - indicator that usually has a change in color at the end point, while the other method is a - potentiometric titration [i]. 

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. 

Redox reactions are more conveniently described in terms of relative electrical potentials instead of the equivalent changes in Gibbs free energy. The electrons in Equation 6.8 come from or go to some other redox couple, and whether or not the reaction proceeds in the forward direction depends on the relative electrical potentials of these two couples. Therefore, a specific electrical potential is assigned to a couple accepting or donating electrons, a value known as its oxidation-reduction or redox potential. This redox potential is compared with that of another couple to predict the direction for spontaneous electron flow when the two couples interact—electrons spontaneously move toward higher redox potentials. The redox potential of species /, ), is defined as... 

Actual electron transfer does occur in oxidation/reduction, or "redox", reactions. In this type of reaction, there is a change in the oxidation state of the adsorbate. A simple example is the chemisorption of an alkali atom, in which it becomes a 1+ ion, transferring its outer electron to empty electron orbitals of the substrate. It is the large electric dipole moment created by this charge transfer process that lowers the work function of surfaces on which alkali atoms are adsorbed (e.g., "cesiation") by up to several eV. This type of bonding is generally strong, and it can also be either molecular or dissociative. 

The chemistry of oxidation-reduction reactions is not limited to atoms of an element changing to ions or the reverse. Some redox reactions involve changes in molecular substances or polyatomic ions in which atoms are covalently bonded to other atoms. For example, the following equation represents the redox reaction used to manufacture ammonia. 

In every reaction in w hich the oxidation number of an element in one reactant (or more than one) increases, the oxidation number of another reactant (or more than one) must decrease. An increase in oxidation number is called oxidation a decrease is reduction. The term redox is often used as a synonym for oxidation-reduction. The total change in oxidation number must be the same in the oxidation as in the reduction, because the... 

A fundamental understanding of oxidation-reduction reactions is vital to the inorganic chemist in contexts ranging from energy transduction - chemical to electrical and the converse, in technical matters in corrosion processes and metallurgy, redox processes in environmental chemistry and metalloenzymes and metallo-proteins involved in electron transfer. Electron-transfer reactions of transition metal complexes are accompanied by a change in the oxidation state of the metal... 

A way to obtain the redox potential is to calculate the fi ee energy change AG° of an oxidation-reduction reaction... 

Oxidation-Reduction Reactions — Chemical reactions in which reactants undergo PAIRED CHANGES in their oxidation states. (They may be caiied Redox and Eiectron Transfer Reactions.)... 

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