Daniell redox reaction

The redox reaction takes place in a galvanic cell when an external circuit, such as a metal wire, connects the electrodes. The oxidation half-reaction occurs in one half-cell, and the reduction half-reaction occurs in the other half-cell. For the Daniell cell ... 

The zinc anode and copper cathode of a Daniell cell are both metals, and can act as electrical conductors. However, some redox reactions involve substances that cannot act as electrodes, such as gases or dissolved electrolytes. Galvanic cells that involve such redox reactions use inert electrodes. An inert electrode is an electrode made from a material that is neither a reactant nor a product of the cell reaction. Figure 11.6 shows a cell that contains one inert electrode. The chemical equation, net ionic equation, and half-reactions for this cell are given below. 

The Daniell cell illustrates the basic features of an electrochemical cell. Electrochemical cells always involve a redox reaction. Oxidation occurs at the cathode of the cell and reduction takes place at the anode. Electrons always flow from the anode to the cathode. Electrochemical cells come in many arrangements. To gain an appreciation for the variety of electrochemical cells, consider all the types of batteries available. 

The Daniell cell is an early example of a galvanic cell. It was invented by the British chemist John Daniell in 1836, when the growth of telegraphy created an urgent need for a reliable, steady source of electric current. Daniell knew that the redox reaction... 

An electrochemical cell is an experimental apparatus for generating electricity by using a redox reaction. The Daniel cell for the system Zn(s) + C oi) Zn(aq) + Cu(s) is shown in Figure 1.2. 

The Nemst equation enables us to calculate E as a function of reactant and product concentrations in a redox reaction. For example, for the Daniell cell in Figure 19.1... 

The two solutions in the Daniell cell are connected by a salt-bridge (e.g. gelatine containing aqueous KCl or KNO3), which allows the passage of ions between the half-cells without allowing the Cu(II) and Zn(II) solutions to mix too quickly. When the Daniell cell is assembled, redox reaction 7.8 occurs spontaneously. 

Similar analyses for other metals can be carried out. For example, Cu and Zn are adjacent rf-block metals, and it is interesting to investigate factors that contribute to the difference between E° values for the Cu +/Cu and Zn +/Zn redox couples, and thus reveal how a balance of thermodynamic factors governs the spontaneous reaction that occurs in the Daniell cell (reaction 7.8). Table 7.3 lists relevant thermodynamic data it is apparent that the crucial factor in making E°q i+ significantly more positive than ii°zn2+/zn is the greater enthalpy of atomization of Cu Na (aq)-I-e Na(s)... 

In many secondary school textbooks, the topic of electrochemical cells is introduced after redox reactions by demonstrating a galvanic cell, very often the Daniell cell (zinc-copper cell). But for students, this experiment is overwhelming because of this cell does not look like a battery. Moreover, the experiment is confusing for students because of the large number of new phenomena and the complexity of the explanations. We would propose to start with a more simple experiment that clarifies some basic characteristics of cells, especially the concepts of electrolyte, electrode and electrode reaction. 

A combination of any two dissimilar metallic conductors can be used to construct a galvanic cell. The cell potential defines the measure of the energy available in a cell. A high cell potential signifies a vigorous spontaneous redox reaction. The unit of potential is the volt, V. A Daniell cell, for example, has a potential of 1.1 V. 

When two such half-cells are combined in an electrical circuit, a redox reaction occurs if there is a potential difference between the half-cells. This is illustrated in Figure 8.1 by the Daniell cell, in which a Cu /Cu half-cell (equation 8.6) is combined with a Zn IZn half-cell (equation 8.7). 

When the and Zn ions are in their standard states (that is, both have an activity of 1 in the solution), we find that the emf of a Daniell cell is 1.10 V at 25°C (see Figure 13.4). This emf must be related directly to the redox reactions, but how Just as the overall cell reaction can be thought of as the sum of two half-cell reactions, the measured emf of the cell can be treated as the sum of the electric potentials at the Zn and Cu electrodes. Knowing one of these electrode potentials, we could obtain the other by subtraction (from 1.10 V). It is impossible to measure the potential of just a single electrode, but if we arbitrarily set the potential value of a particular electrode at zero, we can use it to determine the relative potentials of otha electrodes. The hydrogen electrode, shown in Figure 13.5, serves as the reference for this purpose. 

Thus, the overall redox reaction of the Daniell cell is... 

Electrochemical Cells and Redox Reactions Example of Daniell s Galvanic Cell... 

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 conductive connection between two electrolytes of different composition is designated by two vertical lines , The electrode pair is always given in such a way that the electrons flow from left to right in the equation. According to these rules, therefore, the composed redox reaction in the Daniell cell is given by the following cell diagram... 

ArmentroutPB (1999) Gas-Phase Organometallic Chemistry. 4 1-45 Astruc D, Daniel M-C, Ruiz J (2006) Metallodendritic Exo-Receptors for the Redox Recognition of Oxo-Anions and Halides. 20 121-148 Aubert C, Fensterbank L, Gandon V, MalacriaM (2006) Complex Polycyclic Molecules from Acyclic Precursors via Transition Metal-Catalyzed Cascade Reactions. 19 259-294... 

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) ... 

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