Current-consuming reaction

Current flow in cells is attended by an overall chemical reaction, more particularly a current-producing (or current-consuming) reaction in which electrons do not appear explicitly. In the example reported above, decomposition of dissolved zinc chloride,... 

Devices for the production of chemical energy differ from the ones described, in that different electrode reactions now occur at the cathode and anode. As the result of overall current-producing and current-consuming reactions, energy-rich products... 

A sheet of steel of thickness 0.50 mm is tinplated on both sides and subjected to a corrosive environment. During service, the tinplate becomes scratched, so that steel is exposed over 0.5% of the area of the sheet. Under these conditions it is estimated that the current consumed at the tinned surface by the oxygen-reduction reaction is 2 X 10 A m -. Will the sheet rust through within 5 years in the scratched condition The density of steel is 7.87Mg m . Assume that the steel corrodes to give Fe " ions. The atomic weight of iron is 55.9. 

Since the rate of movement is controlled by the rate of the electrochemical reaction, when we oxidize or reduce the conductins polymer of the device at constant current, we will have a uniform movement with perfect control of the movement rate the movement is stopped by stopping the current flow the movement is reversed by reversing the direction of the current flow. By doubling the current density, we obtain a movement rate that is twice the previous one. Rates and mechanical energy are proportional to the current consumed per mass unit (Fig. 25). 

For coulometric analysis, the substance being examined must react in 100% current yields [i.e., other (secondary) reactions must be entirely absent]. In efforts to avoid side reactions, coulometry most often is performed potentiostatically (amperometrically) (i.e., the electrode potential is kept constant during the experiment), and the current consumed at the electrode is measured. The current is highest at the start of the... 

Oxides of Platinum Metals Anodes of platinum (and more rarely of other platinum metals) are used in the laboratory for studies of oxygen and chlorine evolution and in industry for the synthesis of peroxo compounds (such as persulfuric acid, H2S2O8) and organic additive dimerization products (such as sebacic acid see Section 15.6). The selectivity of the catalyst is important for all these reactions. It governs the fraction of the current consumed for chlorine evolution relative to that consumed in oxygen evolution as a possible parallel reaction it also governs the current yields and chemical yields in synthetic electrochemical reactions. 

In the present example, of the 79% carbon consumed by the above mechanism, about 12% represents excess carbon consumed due to current inefficiency, the second consumption mechanism. Present-day cells have current efficiencies in the 85-95% range. Current inefficiency reactions involve back-reactions of electrolytic CO2 with reduced metal species from the cathode. An example of this reoxidation mechanism, for dissolved aluminum, is given by the following equation ... 

In other cases, a push-me-pull-you situation arises the faster (as defined by the current density) reaction cannot produce current any faster than the slower reaction can consume it. Corrosion engineers use this principle in several ways including sacrificial anodes and corrosion inhibitors. Examples can be found throughout the text. 

Chloramines were analysed in additional experiments by selective spectrophotomet-rical procedure (HACH method 10171). When the chloride concentration is nearly zero, de facto no chlorine or chloramines (measured as the difference between total and active chlorine) are visible. Ammonium is formed linearly in the cathodic reaction (A). In the second case B, active chlorine is consumed by the ammonium with chloramine formation. After switching off the electrolytic current, this reaction continues until all the chlorine is reacted. 

Eremin41 reported that the amount of methane cracked by a high frequency discharge is proportional to the current consumed and to the amount of excess hydrocarbons. The reaction was found to be of the first order and the rate was directly proportional to the discharge energy and inversely proportional to the original amount... 

The 21 formed in the second reaction is determined either by visual chemical titration with a reagent such as sodium thiosulfate in the presence of a suitable endpoint indicator or by amperometric, coulometric, or photometric titration methods. The most sensitive KF methods for the measurement of iodine are coulometric. For both the volumetric-amperometric and coulometric methods the endpoint is detected by a pair of platinum electrodes called the indicator electrodes. An electrical potential (100-400 mV) is applied across the electrodes to balance the circuit and the endpoint is reached when the concentration of I2 ( 50pmoll ) depolarizes the cathode deflecting a galvanometer. The volumetric method measures the amount of standardized reagent necessary to depolarize the platinum electrodes. The coulometric method utilizes, in addition to the indicator electrodes, a second pair of platinum electrodes (generator electrodes) that electrolytically convert the 1 to I2. The current consumed in this process is used to calculate the amount of water using the equation that describes Faraday s laws of electrolysis. 

Surface disorder has a great influence on minority generation rate. This can be seen in a variation of saturation currents for reactions which consume minorities,... 

In many cases, decomposition reactions compete with the redox system, and the stability towards decomposition is a matter of kinetics. Measurements of the photoelectrochemical stabilization efficiency have been made, where S = [current consumed by redox species]/[total current] [25]. S can be measured directly with rotating ring-disk electrode (RRDE) experiments, where the semiconductor comprises the... 

Methanol crossover to the cathode is quickly oxidized to CO2 via Eq. (6.42). This consumes O2, dilutes the cathode flow, and creates a mixed potential parasitic current Qp) reaction at the cathode ... 

A decrease in current efficiency with increasing current density is observed in many cases. This can enhance the uniformity of thickness of the deposit due to a negative feedback effect. Thus, in areas on the surface where the total current density is higher, the fraction of the current consumed for metal deposition is lower. The dependence of the current efficiency on current density is governed by the kinetic parameters of the two reactions involved. Thus, in the commonly encountered situation where hydrogen evolution is activation controlled while metal deposition is partially controlled by mass transport (0.05 < j/ji) < 0.7), the current efficiency will decrease with increasing current density. 

Since any current resulting from tire anodic reaction must be consumed by tire catlrodic reaction, tire catlrodic current,7, must be equal to tire airodic current As a consequence, tire equilibrium potential of a metal (e.g. Fe) tlrat is immersed into air aqueous electrolyte will be adjusted by tire condition tlrat = j This is... 

Without coke backfill, the anode reactions proceed according to Eqs. (7-1) and (7-2) with the subsequent reactions (7-3) and (7-4) exclusively at the cable anode. As a result, the graphite is consumed in the course of time and the cable anode resistance becomes high at these points. The process is dependent on the local current density and therefore on the soil resistivity. The life of the cable anode is determined, not by its mechanical stability, but by its electrical effectiveness. 

Many of the fission products formed in a nuclear reactor are themselves strong neutron absorbers (i.e. poisons ) and so will stop the chain reaction before all the (and Pu which has also been formed) has been consumed. If this wastage is to be avoided the irradiated fuel elements must be removed periodically and the fission products separated from the remaining uranium and the plutonijjm. Such reprocessing is of course inherent in the operation of fast-breeder reactors, but whether or not it is used for thermal reactors depends on economic and political factors. Reprocessing is currently undertaken in the UK, France and Russia but is not considered to be economic in the USA. 

In oilfield situations we are generally faced with corrosion attacks in aqueous environments. Basically all attacks in aqueous solutions are electrochemical in nature. This means that besides the chemical reaction there will also be a flow of electrons, resulting in a flow of current. The current flows from a higher potential to a lower one. Hence, there are two reactions taking place simultaneously in the system. One reaction occurs as the electrons are discharged from the surface, called the anode. The released electrons are consumed in the other... 

It will be seen that the impressed current electrode discharges positive current, i.e. it acts as an anode in the cell. There are three generic types of anode used in cathodic protection, viz, consumable, non-consumable and semi-consumable. The consumable electrodes undergo an anodic reaction that involves their consumption. Thus an anode made of scrap iron produces positive current by the reaction ... 

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