Electrochemistry half-cell voltages

These equations (18.5,18.6) connect electrochemistry to the world of thermodynamics. It only remains to devise some way of determining the voltage associated with cells and half-cells, and we should then be able to determine free energies and perhaps other thermodynamic properties of cell reactions. 

At the heart of electrochemistry is the electrochemical cell. We will consider the creation of an electrochemical cell from the joining of two half-cells. When an electrical conductor such as a metal strip is immersed in a suitable ionic solution, such as a solution of its own ions, a potential difference (voltage) is created between the conductor and the solution. This system constitutes a half-cell or electrode (Fig. 15.1). The metal strip in the solution is called an electrode and the ionic solution is called an electrolyte. We use the term electrode to mean both the solid electrical conductor in a half-cell (e.g., the metal strip) and the complete half-cell in many cases, for example, the standard hydrogen electrode, the calomel electrode. Each half-cell has its own characteristic potential difference or electrode potential. The electrode potential measures the ability of the half-cell to do work, or the driving force for the half-cell reaction. The reaction between the metal strip and the ionic solution can be represented as... 

In conventional electrochemistry, a half cell is always combined with another (reference half cell), and its electrochemical equilibrium is investigated through the cell voltage (EMF). In contrast, the half cell of the present... 

The first two reactions we discussed in this chapter were the anodic and cathodic reactions for steel in concrete. The terms anode and cathode come from electrochemistry which is the study of the chemistry of electrical cells. Figure 2.5 is a basic Daniell cell which is used at high school to illustrate how chemical reactions produce electricity. The cell is composed of two half cells , copper in copper sulphate and zinc in zinc sulphate. The total voltage of the cell is determined by the metals used and by the nature and composition of the solutions. What is happening is that in each half cell the metal is dissolving and ions are precipitating, that is. 

With an external DC power supply connected to the electrolytic cell, the applied voltage that gives no DC current flow in the external circuit corresponds to the equilibrium potential of the half-cell (or actually the cell). It is the same voltage as read by a voltmeter with very high input resistance and virtually no current flow (pH meter). In electrochemistry, potentiometry is to measure the potential of an electrode at zero current flow, which is when the cell is not externally polarized. To understand the equilibrium potential with zero external current, we must introduce the concept of electrode reaction... 

When a conductive particle is exposed to an electric field, it causes the particle to polarize. As a consequence an overpotential r varying according to a cosine law is induced at the surface of the particle (Eq. 1, Fig. l) Thus, a maximum potential difference will occur at opposite poles of the particle. In order to carry out electrochemistry at the surface of the particle a critical voltage difference corresponding to the sum of two half-cell reactions must be reached. Thus, for a given particle of radius r and an applied electric field E there will exist two polar regions defined by a critical angle 0 within which electrochemistry will occur. (Eq. 2, Fig. 2). This forms the theoretical basis of toposelective electrodeposition. 

Important manifestations of electrochemistry include battery technology and corrosion. Different types of batteries can be identified by the half-reactions involved. Batteries are classified as either primary cells, which cannot be recharged, or secondary batteries, which are rechargeable. The chemical origin of these differences is that the reaction in a primary cell results in products that cannot be reversed to regenerate the original reactants, whereas in secondary cells, the reaction can be driven in reverse by an applied voltage. Corrosion is another example of electrochemistry with practical implications, albeit not productive ones. In... 

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