Oxidation-reduction reactions spontaneous change

The thermodynamic criterion for spontaneity (feasibility) of a chemical and electrochemical reaction is that the change in free energy, AG have a negative value. Free-energy change in an oxidation-reduction reaction can be calculated from knowledge of the cell voltage ... 

The energy made available by this spontaneous electron flow (the free-energy change for the oxidation-reduction reaction) is proportional to AE ... 

Electrochemistry is best defined as the study of the interchange of chemical and electrical energy. It is primarily concerned with two processes that involve oxidation-reduction reactions the generation of an electric current from a spontaneous chemical reaction and the opposite process, the use of a current to produce chemical change. 

Oxidation-reduction reactions are the basis of the branch of chemistry called electrochemistry. Such a reaction may occur spontaneously and produce electrical energy, as in a galvanic cell. If the reaction does not occur spontaneously, the addition of electrical energy may initiate a chemical change, a process called electrolysis. 

We determine whether a given chemical reaction is an oxidation-reduction reaction by keeping track of the oxidation numbers oxidation states) of the elements involved in the reaction. — (Section 4.4) This procedure identifies whether the oxidation state changes for any elements involved in the reaction. For example, consider the reaction that occurs spontaneously when zinc metal is added to a strong acid (T FIGURE 20.1) ... 

Some oxidation-reduction reactions do not occur spontaneously but can be driven by electrical energy. If electrical energy is required to produce a redox reaction and bring about a chemical change in an electrochemical cell, it is an electrolytic cell. Most commercial uses of redox reactions make use of electrolytic cells. 

Because reaction (19.3) is a spontaneous reaction, the displacement of Zn " (aq) by Cu(s)—the reverse of reaction (19.3)—does not occur spontaneously. This is the observation made in Figure 19-1. In Section 19-3, we will discuss how to predict the direction of spontaneous change for oxidation-reduction 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... 

In a redox reaction, electrons move spontaneously toward atoms or molecules having more positive reduction potentials. In other words, a compound having a more negative reduction potential can transfer electrons to (i.e., reduce) a compound with a more positive reduction potential. In this type of reaction, the change in electric potential A is the sum of the reduction and oxidation potentials for the two half-reactions. The AE for a redox reaction is related to the change in free energy AG by the following expression ... 

In a spontaneous reaction between two half-ceUs, the half-ceU with the more positive potential in Table 2.2 undergoes reduction, while the one with the more negative potential undergoes oxidation. The charge transfer changes the standard electrode potential due to the change of the composition of the electroactive species in the electrolyte. When a final ratio of the activities of the reactive species, as defined in Eq. (2.38), is equal to the equilibrium constant of the reaction, the system will be in equilibrium. 

The criteria for spontaneous change developed in Chapter 13 apply to reactions of all types—precipitation, acid-base, and oxidation-reduction (redox). We can devise an additional useful criterion for redox reactions, however. 

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