Reduction half cell

The thermodynamically stable oxidation state of a metal in a given environment is a function of the prevailing oxidation potential. The value of the potential is given by the Nemst equation, which is described in Chapter 5 for the generic reduction half-cell ... 

A We write down the oxidation half-equation with the method of Chapter 5, and obtain the reduction half-equation from Table 21-1, along with the reduction half-cell potential. 

A cell is a complete electroanalytical system consisting of an electrode at which reduction occurs, as well as an electrode at which oxidation occurs, and including the connections between the two. A half-cell is half of a cell in the sense that it is one of the two electrodes (and associated chemistry) in the system, termed either the reduction half-cell or the oxidation half-cell. The anode is the electrode at which oxidation takes place. The cathode is the electrode at which reduction takes place. An electrolytic cell is one in which the current that flows is not spontaneous, but rather due to the presence of an external power source. A galvanic cell is a cell in which the current that flows is spontaneous. 

When using a multiplier to equalize electron loss and gain in reduction half-cell potentials, do not use the multiplier on the voltage of the half-cell. 

Equations 7.2 and 7.3 are examples of electrochemical half-cell reactions. Since free electrons are not found in nature, half-cell reactions always occur in pairs such that the electrons generated by one are consumed by the other. The half-cell reaction that releases electrons is referred to as an oxidation reaction. The half-cell reaction that consumes electrons is referred to as a reduction reaction. For the redox reaction shown in Eq. 7.1, the oxidation and reduction half-cell reactions are given by Eqs. 7.2 and 7.3,... 

A laboratory device used to connect the oxidation and reduction half-cells, which usually comes in two types glass tube or filter paper. 

A review of photo-assisted electrolysis studies performed with p-type semiconductor photocathode/dark Pt anode systems suggests that a complementary phenomena arising from the presence of OH ions produced during the reduction half-cell reaction,... 

The corresponding oxidation half-cell potential is the negative of this value, i.e., 7iV/ri = +0.76 V Some standard reduction half-cell potentials are given in Table 15-1. 

E°cen = (reduction half-cell)-ii °(oxidation half-cell)... 

Table 15-1. Standard Reduction Half-Cell Potentials...

Hydrogen gas is bubbled over a platinum surface that is coated with platinum black, an electrolytically deposited coating of colloidal platinum, which is an excellent catalyst for the above equilibrium. The hydrogen electrode has been selected as the standard against which the potentials of other electrodes are measured. Equations of the type of reaction (I) are called half-cell reactions, because they include electrons. Reaction I is a reduction half-cell reaction. 

Given the values for two half reactions, you can easily predict the potential difference of the corresponding cell simply add the reduction potential of the reduction half-cell to the negative of the reduction potential (that is, to the oxidation potential) of the oxidation reaction. 

Potential difference, Ecell, between oxidation and reduction half-cells under nonstandard conditions. 

Similarly, the reduction half-cell of the model has been constructed. In the latter system (Figure 6B) the electron acceptor dimethyl-4,4 -bipyridinium (methylvlologen, MV2+) and Ru(bipy)5 ( or the water soluble Zn-porphyrins (3) and (4) were dissolwd in the aqueous phase of the water-S-oil micro-emulsion with the electron donor, thlophenol, being concentrated at the water-oil interface. Illumination of this system results in the production of the viologen radical cation. This photosensitized electron transfer process results in the separation of the reduced photoproduct, M, from the oxidized product, diphenyldisul-flde, which is in the toluene phase. 

Note that reduction of iodine has the higher reduction potential. This half-reaction will proceed in the forward direction as a reduction. The iron half-reaction will proceed in the reverse direction as an oxidation. Rewrite the half-reactions in the correct direction. l2(s) -> 21 (aq) (reduction half-cell reaction)... 

A carbon (graphite) rod in the center of the dry cell serves as the cathode, but the reduction half-cell reaction takes place in the paste. An electrode made of a material that does not participate in the redox reaction is called an inactive electrode. The carbon rod in this type of dry cell is an inactive cathode. (Contrast this with the zinc case, which is an active anode because the zinc is oxidized.) The reduction half-cell reaction for this dry cell follows. 

The overall reaction is written as the sum of a reduction half-cell reaction and an oxidation half-cell reaction. 

It is shown subsequently that simple electrode-kinetics theory leads to the following equations for the oxidation and reduction half-cell reactions, respectively ... 

The reduction half-cell. In this case, the cathode compartment consists of a copper bar (the cathode) immersed in a Cu " electrolyte [such as a solution of copper(II) sulfate, CUSO4]. Copper metal is the product in the reduction halfreaction, and the bar conducts electrons into its half-cell. 

The components of the anode compartment (oxidation half-cell) are written to the left of the components of the cathode compartment (reduction half-cell). 

We can generalize this result for any voltaic cell the standard cell potential is the difference between the standard electrode potential of the cathode (reduction) half-cell and the standard electrode potential of the anode (oxidation) half-cell ... 

The lUPAC recipe for half-cell manipulations s first, half-cells are always written and tabulated as reductions when combined with the SHE and second, calculate the cell voltage as the right (reduction) half-cell voltage minus the left (oxidation) voltage, whether using the SHE or not. This is the opposite of the way we wrote equations (18.7) and (18.8) for the zinc-hydrogen cell, so we rewrite them (according to the first rule) as... 

---