Negative plates equilibrium potentials

Kinetic factors may induce a variation of electrode potential with current the difference between this potential and the thermodynamic equilibrium potential is known as the overvoltage and the electrode is said to be polarized. In a plating bath this change of potential can be attributed to the reduced concentration of depositing ions in the double layer which reduces the rate of transfer to the electrode, but the dissolution rate from the metal increases. Since the balance of these rates determines the electrode potential, a negative shift in the value occurs the concentration polarization Olconc)- Anodic effects are similar but in the opposite direction. 

The electrochemical reactions that proceed during formation of negative plates can be represented by the following equations. represents the equilibrium potential... 

The paste of cured negative plates is composed of 3BS, PbO and residual Pb as well as additives including expanders, carbon, etc. The lead compounds are reduced to Pb through electrochemical reactions. We can judge about the sequence of these reactions by the respective equilibrium potentials (E ) of their reduction at 298.15K [1]. All equilibrium potentials are referred to a standard hydrogen electrode. 

The above sequence of reactions indicates that formation of negative plates will proceed in two stages. The difference of 300mV between the equilibrium potentials of the PbO Pb and 3BS Pb... 

Both Pb02 and Pb are thermodynamically imstable in sulphuric acid. The equilibrium potential of the Pb02/PbS04 couple is more anodic to the O2/ H2O couple by 0.46 V, and the equilibrium potential of Pb/PbS04 is more cathodic to the H2/H+ couple by 0.36 V as shown in Figure 7.1. Hence, even under open-circuit conditions, O2 evolution at the positive plate and H2 evolution at the negative plate can occur according to the reactions ... 

The zinc electrode is negatively charged because the equilibrium of Cu atoms and Cu2+ions lies more on the side of the Cu atoms of the more noble metal copper (see Fig. 8.3). So, automatically, the copper plate becomes the positive pole in the electric cell an electrical voltage can be measured between both metals. One can state that every metal has a specific electrical potential, but when present by itself, is not possible to measure. With the voltmeter, it is possible to measure the potential difference between two metals dipped in salt or in acidic solutions. 

Consider the following situation. The oxidized form of a reactant consists of metal ions in solution, and the electrode material (metal) is the reduced form. Such electrochemical system is called class 1 electrode. An example may be a copper plate immersed into a solution of copper sulfate. If some external source of current is used to create a potential on such an electrode and the potential is more negative that the equilibrium one, then copper ions will be reduced on the cathode. Ions near the electrode will be reduced first. This will lead to the decrease in concentration of the discharging particles and new portions of the oxidized form will diffuse from the bulk of the electrode. 

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