Daniell element

Primary cells are non rechargeable cells, in which the electrochemical reaction is irreversible. They contain only a fixed amount of the reacting compounds and are discharged only once. If the educts are consumed by discharging, the cell cannot, or should not, be used again. A well-known example of a primary cell is the Daniell element, consisting of zinc and copper. 

From the chemical viewpoint, the galvanic cell is a current source in which a local separation of oxidation and reduction process exists. This is explained below by the example of the Daniell element (Fig. 3). Here the galvanic cell contains copper as the positive electrode, zinc as the nega-... 

For the Daniell element in Fig. 3, a potential difference A is obtained by calculation from the values in Fig. 5 according to Eq. (11) under equilibrum conditions the potential difference corresponds to the terminal voltage of the cell. 

Comparison of the Daniell element, the nickel/cadmium accumulator, and, the lithium/manganese dioxide primary cell, as examples, shows the influence of the electrode materials on different cell parameters (Table 1). 

For the Daniell element the electron-donating reaction is the oxidation of zinc. The active mass m which is necessary to deliver a capacity of 1 Ah, is calculated as follows ... 

During the determination of standard electrode potentials an electrochemical equilibrium must always exist at the phase boundaries, e.g. that of the elec-trode/electrolyte. From a macroscopic viewpoint no external current flows and no reaction takes place. From a microscopic viewpoint or a molecular scale, a continuous exchange of charges occurs at the phase boundaries. In this context Fig. 6 demonstrates this fact at the anode of the Daniell element. 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

Using the reaction free energy AG, the cell voltage Aelectrons exchanged during an electrode reaction must be determined from the cell reaction. For the Daniell element (see example), two moles of electrons are released or received, respectively ... 

It was mentioned earlier that the equilibrium cell voltage A%, is equal to the difference between the equilibrium potentials of its half-cells e.g., for the Daniell element,... 

If the concentrations of the copper and zinc ions within a Daniell element are known, the cell voltage As0 results as follows ... 

From experiments it is possible to obtain the temperature coefficient for the Daniell element,... 

Let us now consider a galvanic cell with the redox couples of equation 8.164. This cell may be composed of a Cu electrode immersed in a one-molal solution of CUSO4 and a Zn electrode immersed in a one-molal solution of ZnS04 ( Dan-iell cell or Daniell element ). Equation 8.170 shows that the galvanic potential is positive the AG of the reaction is negative and the reaction proceeds toward the right. If we short-circuit the cell to annul the potential, we observe dissolution of the Zn electrode and deposition of metallic Cu at the opposite electrode. The flow of electrons is from left to right thus, the Zn electrode is the anode (metallic Zn is oxidized to Zn cf eq. 8.167), and the Cu electrode is the cathode (Cu ions are reduced to metallic Cu eq. 8.168) ... 

Lord Kelvin once assumed that there was a complete transformation of thermal into electrical energy in the chemical action of a galvanic element. Measurements made by Joule and himself with a Daniell element gave results in harmony with theory. The agreement was afterwards shown to be illusory. Success in explaining facts is not necessarily proof of the validity of an hypothesis, for, as Leibnitz puts it, le vrai peut etre tir6 du faux, in other words, it is possible to infer the truth from false premises. 

For the Daniell element under standard conditions (T=298K),... 

A typical feature of a redox reaction is an exchange of electrons between at least two reaction partners. It is characterized by the fact that oxidation and reduction always occur at the same time. For the Daniell element, the copper ions are the oxidizing agent and the zinc ions the reducing agent. Both together form the corresponding redox pair ... 

A metal ion potential exists if a metal and its ions are present in balanced phases, for example, zinc and zinc ions at the anode of the Daniell element... 

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