Half-reactions, description

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. 

The description of the battery states that one electrode is Pb in contact with PbSOq, and the other electrode is Pb02 in contact with PbSOq. This information identifies the two half-reactions ... 

Description of cell Cell formula Half-reaction Net cell reaction... 

The rusting process is complex, and the equations may be written in various ways. A simplified description of the half-reactions and the overall cell reaction is given here. 

Next, we want to consider how to describe a galvanic cell fully, given just its half-reactions. This description will include the cell reaction, the cell potential, and the physical setup of the cell. Let s consider a galvanic cell based on the following half-reactions ... 

Figure 21-6 describes this potential calculation. The graph shows the zinc half-cell with the lower reduction potential (the oxidation half-reaction) and the copper half-cell with the higher reduction potential (the reduction halfreaction). You can see that the space between the two (Fceii) difference between the potentials of the individual half-cells. The Example Problem that follows gives a step-by-step description of calculating cell potentials. 

You are given the half-cell descriptions for a voltaic cell and standard reduction potentials in Table 21-1. In any voltaic cell, the half-reaction with the lower reduction potential will proceed as an oxidation. With this information, you can write the overall cell reaction and calculate the standard cell potential. 

Iodine is a weak oxidizing agent used primarily for the determination of strong reductants. The most accurate description of the half-reaction for iodine in these applications is... 

Describe the reaction that takes place between an uncharged metal, like zinc, and a cation, like the copper(II) ion, in a water solution. Your description should include the nature of the particles in the system before the reaction takes place, the nature of the reaction itself, and the nature of the particles in the system after the reaction. Your description should also include the equations for the half-reactions and the net ionic equation for the overall reaction. 

Given a description of a voltaic cell, including the nature of the half-reactions, identify which electrode is the cathode and which is the anode. 

These values and descriptions apply to the diatomic species X2, where X represents a halogen atom. The half-reaction is... 

In most chemistry education curricula, the topic of electrochemical cells is introduced after redox reactions, which can be considered as descriptions by means of half reactions. This order may be reversed by a phenomenological introduction of the particulate half reactions, which subsequently can serve as means for describing direct redox reactions. 

Analyze We are given the equation for a spontaneous reaction that takes place in a voltaic cell and a description of how the cell is constructed. We are asked to write the half-reactions occurring at the anode and at the cathode, as well as the directions of electron and ion movements and the signs assigned to the electrodes. 

As the concentration of free electrons in a metal is much greater than that of eiectroactive compounds in an electrolytic solution, then one can assume that this concentration is constant, and it no longer features in the equation for the apparent rate iaw of the eiementary redox half-reactions at a metal electrolyte interface. This would be different if we wanted to write kinetic iaws for a redox half-reaction at an interface between a semiconductor and an eiectroiyte. However, here the description will be confined to metaiiic eiectrodes. 

This simple description of a reaction affords a facile procedure for the computer to generate products from substrates, or substrates from products, by adding the appropriate Azrc-list to the ztc-values of each reacting strand of carbons -in the case of constructions, for the two half-reactions out from each end of the designated C-C bond. 

A complete description of a galvanic cell usually includes four items y The cell potential (always positive for a galvanic cell where ceii = >° (cathode) — (anode)) and the balanced cell reaction. y The direction of electron flow, obtained by inspecting the half-reactions and using the direction that gives a positive iceii-> Designation of the anode and cathode. 

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