Zinc-copper electrochemical cell, electron

Electrons in this zinc-copper electrochemical cell flow from the zinc strip to the copper strip, causing an electric current that powers the light bulb. As the spontaneous reaction continues, much of the zinc strip is oxidized to zinc ions and the copper ions are reduced to copper metal, which is d osited on the copper strip. If an outside voltage source is applied to reverse the flow of electrons, the original conditions of the cell are restored. 

A basic electrochemical cell is depicted in Figure 9.3 and is made of a copper wire in one container with a solution of copper sulfate and a zinc rod in a different container with a zinc sulfate solution. There is a salt bridge containing a stationary saturated KC1 solution between the two containers. Electrons flow freely in the salt bridge in order to maintain electrical neutrality. A wire is connected to each rod and then to a measuring device such as a voltmeter to complete the cell. 

In these redox reactions, there is a simultaneous loss and gain of electrons. In the oxidation reaction part of the reaction (oxidation half-reaction), electrons are being lost, but in the reduction half-reaction, those very same electrons are being gained. Therefore, in redox reactions there is an exchange of electrons, as reactants become products. This electron exchange may be direct, as when copper metal plates out on a piece of zinc or it may be indirect, as in an electrochemical cell (battery). 

A simple electrochemical cell can be made from two test tubes connected with a third tube (the crossbar of the H ), as shown in Figure 12-1. The hollow apparatus is filled by simultaneously pouring different solutions into the two test tubes, an aqueous solution (aq) of zinc sulfate into the left tube and a copper sulfate solution into the one on the right. Then a strip of zinc metal is dipped into the ZnS04 solution, a piece of copper is inserted into the CUSO4 solution, and the two ends of the metal strips are connected by wires to an voltmeter. The lateral connecting tube allows ionic migration necessary for a closed electrical circuit. The voltmeter will show the electrical potential of 1.10 volts, which leads to the movement of electrons in the wire from the zinc electrode toward the copper electrode. 

The left half of this electrochemical cell, containing a zinc electrode and zinc sulfate solution, engages in an oxidation reaction. This reaction liberates electrons and turns zinc atoms into zinc ions (Zn, which is a zinc atom that has lost two electrons and therefore has a net positive charge of 2). The zinc atoms come from the electrode, which is gradually depleted the zinc ions that are produced in the reaction enter the solution. In the right half of the cell, a reduction reaction occurs electrons combine with copper ions to produce neutral atoms of copper. Copper ions leave the solution in the process and collect at the copper electrode. Over time, the zinc electrode and copper solution will run out of material, causing the reaction to cease unless the material is replenished. 

In each compartment of the electrochemical cell a half reaction occurs. The two half reactions result in an overall reaction that generates a flow of electrons or current. In one cell compartment, zinc is oxidized according to the reaction Zn Zn + 2e . The reduction of copper takes place in the other cell s compartment Cu +,, + 2e Cu,.. Notice that these reactions are the same ones that take place... 

The anodic reaction is an oxidation reaction producing electrons in the anode, while the cathodic reaction is a reduction reaction consuming electrodic electrons at the cathode interface. We shall consider, as an example, an electrochemical cell consisting of a metallic zinc electrode and a metallic copper electrode, in which the anodic reaction of zinc ion transfer (zinc dissolution) is coupled with the cathodic reaction of copper ion transfer (copper deposition) as shown in the following processes ... 

Continuing with the examination of the electrochemical cell, notice that electrons are lost by the zinc side of the cell and taken in by the copper side of the cell. For this reason, electricity flows from left to right in the diagram. 

An electrochemical cell consists of two parts, called half-cells, in which the separate oxidation and reduction reactions take place. Each half-cell contains an electrode, which is the object that conducts electrons to or from another substance, usually a solution of ions. In Figure 21-1, the beaker with the zinc electrode is where the oxidation part of the redox reaction takes place. The beaker with the copper electrode is where the reduction part of the reaction takes place. The reaction that takes place in each half-cell is the half-reaction, sometimes called half-cell reaction, that you studied in Chapter 20. The electrode where oxidation takes place is called the anode of the cell. The electrode where reduction takes place is called the cathode of the cell. Which beaker in Figure 21-1 contains the anode and which contains the cathode ... 

The main processes occurring in electrochemical cells are simultaneous oxidation and reduction reactions, or redox reactions. At one electrode, the anode, a reduced species is oxidized here meaning to release electrons, while at the other electrode, the cathode, an oxidized species absorbs electrons and is reduced. It is common to think of an electrochemical cell as consisting of two half-cells. (one containing the anode and the second containing the cathode) and to describe the processes in terms of halfcell reactions. For example, one common cell consists of a copper cathode in a copper sulfate solution, and a zinc anode in a zinc sulfate solution. The overall reaction is... 

An example of an experiment is given by Barral et al. (1992). In this student experiment, a sandpapered zinc bar is put in a beaker with a diluted solution of sulphuric acid, and a clean copper bar is placed in another beaker with the same solution of sulphuric acid. Both experiments are well known to students. Subsequently, both bars are cleaned again, connected with a metal wire and placed in a third beaker of the solution. Within a minute, bubbles can be observed at each bar. The teacher should not explain that this is due to zinc losing electrons to the hydrogen ions via copper, nor point out that an electric current is flowing, that is, an electrochemical cell has been created. Instead, students are asked to write down their observations and explain the production of the bubbles by themselves. 

Sacrificial Anodes Incontrastto the impressed current technique, the use of sacrificial anodes does not depend on the creation of driven electrochemical cell. Rather, a galvanic cell is formed between the structure and the sacrificial anode in which electrons pass spontaneously from the latter to the former (Fig. 9). Thus, the source of the electrons (the sacrificial anode) must have a more negative electrode potential than the structure. It was for this reason that Humphrey Davy chose zinc or iron to protect copper, and it also explains why magnesium, aluminum and zinc alloys are used to protect steel today. 

One common example of a redox reaction in electrochemistry involves the transfer of electrons from zinc (Zn) metal to copper (Cu) ions in an electrochemical cell. 

FIGURE 10.4 (a) Components of an electrochemical cell. Oxidation (loss of electrons) occurs in the anode compartment and electrons flow out of the cell through the external circuit. Electrons re-enter the cell at the cathode, and reduction (gain of electrons) takes place in the cathode compartment. A porous salt bridge (or other connection) allows ions to flow between the two compartments to maintain charge balance, (b) A simple electrochemical cell. When the electrodes are connected by a conducting circuit, electrons flow from the zinc electrode, where zinc is oxidized, to the copper electrode, where copper is reduced. The overall reaction in this cell... 

It will be helpful at this point to explain in detail how to make an electrochemical cell and what processes are occurring there. We will start with a classical copper-zinc cell. First, we find a copper block Cu(s) and immerse it into a solution of copper sulfate, Cu (aq) S04 (aq), Fig. 9.1. Next, we connect the copper block with a source of electrons. Fig. 9.2. When two electrons, 2e , are supplied to the copper block... 

Redox Reactions. For many electrochemists the paramount concern of their discipline is the reduction and oxidation (redox) reaction that occurs in electrochemical cells, batteries, and many other devices and applications. Reduction takes place when an element or radical (an ionic group) gains electrons, such as when a double positive copper ion in solution gains two electrons to form metallic copper. Oxidation takes place when an element or radical loses electrons, such as when a zinc electrode loses two electrons to form a doubly positive zinc ion in solution. In electrochemical research and applications the sites of oxidation and reduction are spatially separated. The electrons produced by chemical processes can be forced to flow through a wire, and this... 

Exercise 6.6. An electrochemical cell has electrodes made of zinc and copper and operates under standard conditions. Which electrode is the anode and which the cathode Which way will electrons flow in the external (wire) portion of the circuit What is the maximum electric potential difference that this cell can generate Will Zn(s) spontaneously reduce Cu (aq), or will Cu(s) spontaneously reduce Zn (aq) ... 

FIGURE 14-2 An electrochemical cell. In this cell, electrons flow from the zinc anode to the copper cathode. Zinc atoms are oxidized toZn ions and ions are reduced to copper atoms. 

An electrochemical cell, such as the Daniell cell, operates by the oxidation reaction producing electrons in the zinc anode, which are then pulled round the external circuit (wires, bulbs, voltmeter, etc.) by the reduction reaction at the copper cathode. As long as the overall reaction is not at equilibrium, the oxidation reaction pushes electrons into the external circuit, and the reduction reaction pulls them out. The cell is described as doing work since it produces a force that moves electrons around the external circuit. This work can light a bulb, drive an electric motor, etc. 

Reconsider the voltaic cell shown in Figure 2.1. There are two electrodes, Zn and Cu. These two metals each have different tendencies for accepting electrons. This tendency for the haif-reaction of either copper or zinc to occur as a reduction half-reaction in an electrochemical cell can be quantified as a reduction potential. There are two half-cells in Figure 2.1 a strip of zinc placed in a solution of ZnSO and a strip of copper placed in a solution of CuSO. The difference in potential between an electrode and its solution is known as electrode potential. When these two half-cells are coimected and the reaction begins, a difference in potential is observed between the electrodes. This potential difference, or voltage, is proportional to the energy required to move a certain electric charge between the electrodes. A voltmeter connected across the Zn Cu voltaic cell measures a potential difference of about 1.10 V when the solution concentrations of Zn + and Cu2+ ions are each 1 M. 

An electrochemical cell consists of two electrodes that are electrically interconnected by an electrolyte. Each electrode consists of an electric conductor in which the electric current is transmitted by electrons. As examples of electrode material, metallic copper, Cu, and zinc, Zn, can be mentioned. The electrolyte connecting the electrodes contains mobile electrically charged ions that serve as carriers of electric current. The positively charged ions are named cations and the negatively charged ions are named anions. As examples of electrolytes, water, aqueous salt solutions, and melts of ionically bound substances can be mentioned. 

Electrochemical Reactions. Consider a simple galvanic cell, composed of two metal electrodes, zinc and copper, immersed in two different aqueous solutions of unit activity—in this case, 1.0 M ZnS04 and 1.0 M CUSO4, respectively, connected by an electrical circuit, and separated by a semipermeable membrane (see Figure 3.8). The membrane allows passage of ions, but not bulk flow of the aqueous solutions from one side of the cell to the other. Electrons are liberated at the anode by the oxidation (increase in the oxidation number) of the zinc electrode ... 

The electrochemical processes occurring in this cell are the oxidation of zinc and the production of zinc sulfate and electrons at the anode, the absorption of electrons an the reduction and deposition of copper at the cathode, the flow of electrons through an external electrical circuit (resulting in electrical work), and a balancing flow of sulfate ions through the salt bridge. 

The Daniell cell is a typical example of what is known as a galvanic vokaic, or electrochemical ceil The fact that the electrons flow from the zinc to the copper electrode indicates that the tendency for Zn + 2e" to occur is greater than... 

In this extimple of in electrochemiccopper sulfate, respectively, connected by a salt bridge of potassium nitrate. Electrons are given up by the zinc anode and transferred to the copper cathode, generating an electrical current. 

Metallic copper precipitates spontaneously, therefore, the reaction according to the equilibrium condition in eqn. (5.7) necessarily involves a decrease of free energy (AG < 0). The reaction is qualitatively explained in the following way Zinc atoms at the surface of the electrode show a certain tendency to dissolve into ions Zn++(aq) and leave 2 free electrons in the electrode metal. The corresponding tendency to dissolve is less for copper atoms. Copper ions Cu++ in the solution, therefore, take up the liberated electrons and are reduced to Cu(s) that is precipitated on the electrode. In the following, we shall see how this phenomenon is decisive to the build-up of an electrochemical potential in galvanic cells. 

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