Half-reactions The two parts

Half-reactions The two parts of an oxidation-reduction reaction one representing oxidation, the other reduction. 

Half-life (of a reaction) the time required for a reactant to reach half of its original concentration. (15.4) Half-reactions the two parts of an oxidation-reduction reaction, one representing oxidation, the other reduction. (4.11 11.1)... 

This shows that the voltage of a given cell may be thought of as being made up of two parts, one part characteristic of one of the half-reactions and one part characteristic of the other halfreaction. Chemists call these two parts half-cell potentials, a term that emphasizes the relation between voltage and potential energy. The halfcell potentials are symbolized °. 

The key to balancing complicated redox equations is to balance electrons as well as atoms. Because electrons do not appear in chemical formulas or balanced net reactions, however, the number of electrons transferred in a redox reaction often is not obvious. To balance complicated redox reactions, therefore, we need a procedure that shows the electrons involved in the oxidation and the reduction. One such procedure separates redox reactions into two parts, an oxidation and a reduction. Each part is a half-reaction that describes half of the overall redox process. 

The basic idea of this method is to split a complicated equation into two parts called half-reactions. These simpler parts are then balanced separately, and recombined to produce a balanced overall equation. The splitting is done so that one of the half-reactions deals only with the oxidation portion of the redox process, whereas the other deals only with the reduction portion. What ties the two halves together is the fact that the total electrons lost by the oxidation process MUST equal the total gained by the reduction process (step 6). 

Equations such as these represent half-reactions. A half-reaction is one of the two parts of a redox reaction— the oxidation half alone or the reduction half alone. Table 20-3 shows a variety of reduction half-reactions that can accept electrons from iron, oxidizing it from Fe to Fe ". ... 

The two parts of an electrochemical cell are the half-cells, and the reactions at the electrodes are the half-cell reactions. A salt bridge allows ions to flow between the half-cells. 

The possible situations in which external overlap can occur are analyzed in Fig. 6, which shows that there are two kinds of external overlap. In primary overlap the previous construction was at the same site as the one under consideration. Hence only the a-carbon of the synthon is now part of the reactive strand for the new half-reaction, the rest of the strand lying across the previously constructed bond into the product /-list of the previous partner synthon, as in 10. Thus primary overlap can involve only half-reactions of s =2 or 3. With secondary overlap the prior construction has occurred at the /8-carbon of the present reactive strand and allows external overlap to involve only the first (a ) carbon of the prior partner synthon and so only present half-reactions of s =3. 

Oxidation-reduction equations can be balanced by inspection or by the half-reaction method. This method involves splitting a reaction into two parts (the oxidation half-reaction and the reduction halfreaction). 

When students are happy writing redox equations, they could be asked to balance equations of electron transfers. For example, the iron ions in iron(II) chloride solution are oxidised to iron(III) ions by passing chlorine through iron(II) chloride solution. We can look at the two parts of the reaction and write half equations to show what is happening. 

To relate this value to the standard reduction pertential in part (a), we first need to recognize that we can write the equilibrium reaction as the sum of two half-reactions. The reduction step is the reaction from part (a), and the oxidation step is the... 

A transfer of electrons causes changes in the oxidation states of one or more elements. Any chemical process in which elements undergo changes in oxidation number is an oxidation-reduction reaction. This name is often shortened to redox reaction. An example of a redox reaction can be seen in Figure 1.4, in which copper is being oxidized and NO from nitric acid is being reduced. The part of the reaction involving oxidation or reduction alone can be written as a half-reaction. The overall equation for a redox reaction is the sum of two half-reactions. Because the number of electrons involved is the same for oxidation and reduction, they cancel each other out and do not appear in the overall chemical equation. 

The rate at which the corrosion of the 2iac proceeds depends on the rates of the two half reactions (eqs. 8 and 12). Equation 8, a necessary part of the desired battery reaction, fortunately represents a reaction that proceeds rather rapidly, whereas the reaction represented by equation 12 is slow. le, the generation of hydrogen on pure 2iac is a sluggish reaction and thus limits the overall corrosion reaction rate. 

In any redox reaction, some species are oxidized and others are reduced. To reveal the number of electrons transferred, we must separate the oxidation and reduction parts. This is most easily accomplished by dividing the redox reaction into two half-reactions, one describing the oxidation and the other describing the reduction. In half-reactions, electrons appear as reactants or products. 

The underlying problem in testing the validity of the additivity principle in corrosion, mineral extraction, and electroless plating is that the electrode metal itself forms part of one of the half-reactions involved, e.g., zinc in equation (5) and copper in equations (8) and (12). A much better test system is provided by the interaction of two couples at an inert metal electrode that does not form a chemical part of either couple. A good example is the heterogeneous catalysis by platinum or a similar inert metal of the reaction... 

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