Aqueous overall redox reaction

The electrical current needed to start an automobile engine is provided by a lead storage battery. This battery contains aqueous sulfuric acid in contact with two electrodes. One electrode is metallic lead, and the other is solid Pb02. Each electrode becomes coated with solid PbSOq as the battery operates. Determine the balanced half-reactions, the overall redox reaction, and the anode and cathode in this galvanic cell. 

Any complete redox reaction in an aqueous medium is a combination of two half-reactions, because electrons can be neither stored in nor removed from water free electrons are too unstable to persist for long as isolated species in water. When two half-reactions are combined into one reaction, the AG°s are added. The AG° value for the overall redox reaction may be used to calculate the equilibrium constant, as discussed in Section 1.6.3. 

Balancing the chemical equation for a redox reaction by inspection can be a real challenge, especially for one taking place in aqueous solution, when water may participate and we must include HzO and either H+ or OH. In such cases, it is easier to simplify the equation by separating it into its reduction and oxidation half-reactions, balance the half-reactions separately, and then add them together to obtain the balanced equation for the overall reaction. When adding the equations for half-reactions, we match the number of electrons released by oxidation with the number used in reduction, because electrons are neither created nor destroyed in chemical reactions. The procedure is outlined in Toolbox 12.1 and illustrated in Examples 12.1 and 12.2. 

The first step In balancing a redox reaction is to divide the unbalanced equation into half-reactions. Identify the participants in each half-reaction by noting that each half-reaction must be balanced. That Is, each element In each half-reaction must be conserved. Consequently, any element that appears as a reactant In a half-reaction must also appear among the products. Hydrogen and oxygen frequently appear in both half-reactions, but other elements usually appear In just one of the half-reactions. Water, hydronium ions, and hydroxide ions often play roles In the overall stoichiometry of redox reactions occurring in aqueous solution. Chemists frequently omit these species in preliminary descriptions of such redox reactions. 

Cr(VI).Other remediation processes for Cr(VI) contaminated soils include H2S injection, aqueous Fe(II) injection, and the use of reduced Fe solids. Aqueous-phase Cr(VI)-Fe(II) redox reactions may be significant if Fe2+ concentrations are in equilibrium with relatively soluble, ferric hydroxide-like phases (Tokunaga et al., 2003). The overall interactions involving microbial activity, organic carbon degradation, Fe2+, and mineral surfaces control the net rates of Cr(VI) reactions in soils. 

An alternative to the oxidation-number method for balancing redox reactions is the half-reaction method. The key to this method is to realize that the overall reaction can be broken into two parts, or half-reactions. One half-reaction describes the oxidation part of the process, and the other half-reaction describes the reduction part. Each half is balanced separately, and the two halves are then added to obtain the final equation. Let s look at the reaction of aqueous potassium dichromate (K2Cr2C>7) with aqueous NaCl to see how the method works. The reaction occurs in acidic solution according to the unbalanced net ionic equation... 

Any surface reaction that involves chemical species in aqueous solution must also involve a precursory step in which these species move toward a reactive site in the interfacial region. For example, the aqueous metal, ligand, proton, or hydroxide species that appear in the overall adsorption-desorption reaction in Eq. 4.3 cannot react with the surface moiety, SR, until they leave the bulk aqueous solution phase to come into contact with SR. The same can be said for the aqueous selenite and proton species in the surface redox reaction in Eq. 4.50, as another example. The kinetics of surface reactions such as these cannot be described wholly in terms of chemically based rate laws, like those in Eq. 4.17 or 4.52, unless the transport steps that precede them are innocuous by virtue of their rapidity. If, on the contrary, the time scale for the transport step is either comparable to or much longer than that for chemical reaction, the kinetics of adsorption will reflect transport control, not reaction control (cf. Section 3.1). Rate laws must then be formulated whose parameters represent physical, not chemical, processes. 

Another example of an internal redox reaction induced by an external activator is given by [PtIICl2(Me2SO)2] which, upon addition of HC1, converts to [PtIVCl4(Me2S)2].152 An overall internal redox between a Pd11 center and ethylene in aqueous solution is a basic reaction in the Wacker process, i.e., metal-catalyzed conversion of ethylene to acetaldehyde. 

It is sometimes convenient to ignore hydrolysis of ions in aqueous solutions. Leaving out the hydrolysis step may give the felse impression that ions exist in solution. The overall net ionic equation for this redox reaction will be the same, however, with or without consideration of hydrolysis of the S ions. 

When an aqueous solution of iron(III) ions is reacted with sulfur dioxide gas (this forms an acid solution), aqueous iron(II) ions and the sulfate ion (SO ) are produced. Write a balanced overall redox equation for the reaction. 

In Chapter 7, we learned how to balance chemical equations by inspection. Some redox reactions can be balanced in this way. However, redox reactions occurring in aqueous solutions are usually difficult to balance by inspection and require a special procedure called the half-reaction method of balancing. In this procedure, the overall equation is broken down into two half-reactions one for oxidation and one for reduction. The half-reactions are balanced individually and then added together. For example, consider tiie redox reaction ... 

SECM [8,15,67,68]. ET can be probed using a feedback mode of the SECM. A tip UME is placed in the upper liquid phase (e.g., organic solvent) containing one form of the redox species (e.g., the reduced form, R). When the tip is held at a positive potential, R reacts at the tip surface to produce the oxidized form of the species, O. When the tip approaches the ITIES, the mediator can be regenerated at the interface via the bimolecular redox reaction between O in the organic phase and a reduced form of aqueous redox species (Figure 5.16A). In addition to the ET step, the overall interfacial process includes the transfer of a common ion between two liquid phases and the mass transfer in the bottom phase. If these steps are rapid, the current-distance curves are described by Equation 5.35. Otherwise, a more complicated theory may be required [15,68]. 

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