Electrical potentials, Voltaic cells

A voltaic cell consists essentially of three parts two electrodes, from which the positive and negative electricity leave the cell, and an electrolyte in which the electrodes are contained. Its form is therefore that of an electrolytic cell, and the difference between the two lies only in the condition that in the former we produce an electric current through the agency of the material changes, whereas in the latter we induce these material changes by a current supplied from an external source the same arrangement may therefore serve as either. The direction in which the current flows through the cell will depend on the potential difference between its terminals. 

In addition, this review has been prepared to promote the term voltaic cell in honor of Alessandro Volta, the inventor of the pile, i.e., an electrochemical generator of electricity. Up to now this name has been used in only a few papers. This term is a logical analogue to the term galvanic cell, particularly in discussions of Volta potential and Gal-vani potential concepts. 

The surface potential of a liquid solvent s, %, is defined as the difference in electrical potentials across the interface between this solvent and the gas phase, with the assumption that the outer potential of the solvent is zero. The potential arises from a preferred orientation of the solvent dipoles in the free surface zone. At the surface of the solution, the electric field responsible for the surface potential may arise from a preferred orientation of the solvent and solute dipoles, and from the ionic double layer. The potential as the difference in electrical potential across the interface between the phase and gas, is not measurable. However, the relative changes caused by the change in the solution s composition can be determined using the proper voltaic cells (see Sections XII-XV). 

A voltaic cell converts chemical energy into electrical energy. It consists of two parts called half-cells. When two different metals, one in each half-cell, are used in the voltaic cell, a potential difference is produced. In this experiment, you will measure the potential difference of various combinations of metals used in voltaic cells and compare these values to the values found in the standard reduction potentials table. 

The electrochemical cell with zinc and copper electrodes had an overall potential difference that was positive (+1.10 volts), so the spontaneous chemical reactions produced an electric current. Such a cell is called a voltaic cell. In contrast, electrolytic cells use an externally generated electrical current to produce a chemical reaction that would not otherwise take place. 

Electrodes in a voltaic cell, however, are connected to circuits— paths by which electrons flow. Voltaic cells are sources of electricity, so they can be used to drive electrolytic reactions or perform other activities that require electricity. The term voltaic honors the Italian scientist Alessandro Volta (1745-1827), a pioneer of electrochemistry. A simple voltaic cell can form a battery, invented by Volta in 1800. The unit of electric potential, the volt, also honors Volta. 

VOLTAIC CELL. Two conductive metals of different potentials, in contact with an electrolyte, which generate an electric current. The original voltaic cell was composed of silver and zinc, with brine-moistened paper as electrolyte Semisolid pastes are now used electrodes may be lead, nickel, zinc, of cadmium. 

A battery (or galvanic or voltaic cell) is a device that uses oxidation and reduction reactions to produce an electric current. In an electrolytic cell, an external source of electric current is used to drive a chemical reaction. This process is called electrolysis. When the electric potential applied to an electrochemical cell is just sufficient to balance the potential produced by reactions in the cell, we have an electrochemical cell at equilibrium. This state also occurs if there is no connections between the terminals of the cell (open-circuit condition). Our discussion in this chapter will be limited to electrochemical cells at equilibrium. 

A voltaic cell (also known as a galvanic cell) is a device that allows for the transfer of electrons (in a redox reaction) to be completed in a separate pathway from the reaction mixtures. In a voltaic cell, the two half-reactions are physically separated from each other by placing them into two separate reaction vessels. The electrons are transferred from one vessel to the other by a connecting wire (see Figure 18.1). In voltaic cells, the reactions in each vessel must be spontaneous. In figure 18.1, in the reaction on the left, a zinc strip is placed in a zinc sulfate solution, where zinc from the strip replaces zinc in solution (Zn —> Zn2+ + 2 c ). In the reaction vessel on the left, the zinc strip will lose mass over time. Electrons create an electric potential difference across the wire, which is also known as a voltage. The voltage across the wire will allow electrons to be forced from the zinc strip, across the wire, to the copper strip. However, an electric current cannot be established until the circuit is completed. 

There are several terms you should be familiar with for voltaic cells. First, the voltage that is impressed across the circuit (that is, the difference in electrical potential between the zinc strip and the copper strip) is known as the cell voltage, which is also occasionally called the cell potential or the electromotive force, EMF. The copper electrode, because it becomes negatively charged and attracts cations, is known as the cathode. The zinc electrode becomes positively charged and is known as the anode. You are expected to know which part of the reaction takes place at the cathode and which part takes place at the anode. These can sometimes be difficult to remember, so a simple mnemonic device can help you distinguish between the two. Oxidation occurs at the Anode (note how each term starts with a vowel), and deduction occurs at the Cathode (note how each term starts with a consonant). 

To summarize voltaic cells, let s review the components that create the cell. First, you need two half-cells, each of which contains an electrode immersed in an electrolytic solution (typically containing the cation of the metal in the electrode). A spontaneous reaction must occur between the electrode and the solution. A wire connects the two electrodes and will allow the external flow of electrons from the anode to the cathode. In Figure 18.1, a voltmeter is shown as part of the circuit between the two electrodes. This is not a necessary part of the circuit—it is simply there to measure the voltage across the circuit. The salt bridge completes the electric circuit and allows the flow of cations and anions between the two half-reactions. Sometimes a porous disc is used in place of a salt bridge. The driving force for the current is the difference in potential energies between the two half-cells. 

In voltaic cells, it is possible to carry out the oxidation and reduction halfreactions in different places when suitable provision is made for transporting the electrons over a wire from one half-reaction to the other and to transport ions from each half-reaction to the other in order to preserve electrical neutrality. The chemical reaction produces an electric current in the process. Voltaic cells, also called galvanic cells, are introduced in Section 17.1. The tendency for oxidizing agents and reducing agents to react with each other is measured by their standard cell potentials, presented in Section 17.2. In Section 17.3, the Nernst equation is introduced to allow calculation of potentials of cells that are not in their standard states. 

Learning a few electrical variables and their nnits will enable us to do electrochemical calculations, both for voltaic cells and for electrolysis cells. These are presented in Table 17.1. In this section, potential, also called voltage, is the important unit. Potential is the tendency for an electrochemical half-reaction or reaction to proceed. In this section, we will be using the standard half-cell potential, symbolized e°. Standard half-cell potentials can be combined into standard cell potentials, also symbolized e°. The snperscript ° denotes the standard state of the system, which means that the following conditions exist in the cell ... 

Recall from Chapter 16 that an object s potential energy is due to its position or composition. In electrochemistry, electrical potential energy is a measure of the amount of current that can be generated from a voltaic cell to do work. Electric charge can flow between two points only when a difference in electrical potential energy exists between the two points. In an electrochemical cell, these two points are the two electrodes. The potential difference of a voltaic cell is an indication of the energy that is available to move electrons from the anode to the cathode. 

The basic components of a voltaic cell are a wire, two electrodes and two partially-separated solutions. When the electrodes are placed in their respective solutions and the wire is used to connect them, a spontaneous flow of electrons occurs in the wire from one electrode to the other. The impetus for current flow comes from the difference between the oxidation potentials of the electrodes and the solutions, or between the electrodes themselves or between the two solutions in which the electrodes are immersed. A chemical redox reaction occurs between these separated species such that the oxidation half of the reaction occurs in one solution and the reduction half occurs in the other. The partial separation of the solution can be accomplished by a membrane or a salt bridge, which allows an electrolytic connection but does not allow a general mixing of the two solutions. Within the cell, electrical current moves in the form of free electrons in the wire and as ions in the electrolyte. 

Fig. 3-1. An example of a voltaic cell spontaneously generating current in a wire. The impetus for electron movement in the wire comes from the difference in oxidation potential between Zn,s) and Cu. The reducing agent, Zn(s), gives up electrons at the anode to become Zn ". The oxidising agent, Cu acquires electrons at the cathode and plates-out on the copper electrode as Cu(s). The semi-permeable membrane allows ions to move between the two solutions preventing charge imbalances and completing the electrical circuit (from Hamilton, 1998).

A voltaic cell is an electrochemical cell that converts chemical energy into electrical energy. Electrolysis is the opposite of a battery. It converts electrical energy into chemical potential energy. 

There are two kinds of electrochemical cells, voltaic (galvanic) and electrolytic. In voltaic cells, a chemical reaction spontaneously occurs to produce electrical energy. The lead storage battery and the ordinary flashlight battery are common examples of voltaic cells. In electrolytic cells, on the other hand, electrical energy is used to force a nonspontaneous chemical reaction to occur, that is, to go in the reverse direction it would in a voltaic cell. An example is the electrolysis of water. In both types of these cells, the electrode at which oxidation occurs is the anode, and that at which reduction occurs is the cathode. Voltaic cells wOl be of importance in our discussions in the next two chapters, dealing with potentiometry. Electrolytic cells are important in electrochemical methods such as voltammetry, in which electroactive substances like metal ions are reduced at an electrode to produce a measurable current by applying an appropriate potential to get the nonspontaneous reaction to occur (Cha]pter 15). The current that results from the forced electrolysis is proportional to the concentration of the electroactive substance. 

However, we can make this process happen by supplying from an external source an electric potential greater than eii- In effect, we have converted the voltaic cell into an electrolytic cell and changed the nature of the electrodes—anode is now cathode, and cathode is now anode (Figure 21.23B) ... 

---