Electrolytic and Voltaic Cells

GOAL 1 Distinguish among electrolytic cells, voltaic cells, and galvanic cells. 

2 Describe and identify the parts of an electrolytic or voltaic (galvanic) cell and explain how it operates. 

An electrolytic cell is made up of a container holding an ionic solution called an electrolyte and two electrodes (Fig. 19.1 [a]). When the electrodes are connected to an outside source of electricity, they become charged, one positively and one negatively. Ions in the electrolyte move to the oppositely charged electrode, where chemical reactions occur. The movement of ions is an electric current. The whole process is called electrolysis. 

There is another kind of cell in which electrolysis occurs. A voltaic cell, also called a galvanic cell, does not have to be connected to an outside source of electricity to make current flow (Fig. 19.1 [b]). Instead, chemical changes at the electrodes cause electricity to flow in an outside circuit. In fact, a voltaic cell may be the outside source that causes electricity to flow in an electrolytic cell. All common batteries are voltaic cells. 

Sodium ion migrates Reduced to to cathode sodium metal 

Will a reaction occur when copper metal is placed in an iron(II) sulfate solution  

SOLUTION No, copper lies below iron in the series, loses electrons less easily than iron, and therefore will not displace iron(II) ions from solution. In fact, the reverse is true. When an iron nail is dipped into a copper(II) sulfate solution, it becomes coated with free copper. The equations are 

From Table 17.2, we may abstract the following pair in their relative position to each other  

According to Step 2 in Problem-Solving Strategy Using the Activity Series, we can predict that free iron will react with copper(II) ions in solution to form free copper metal and iron(II) ions in solution  

Indicate whether these reactions will occur  

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The cathode is defined as the electrode at which reduction occurs, i.e., where electrons are consumed, regardless of whether the electrochemical cell is an electrolytic or voltaic cell. In both electrolytic and voltaic cells, the electrons flow through the wire from the anode, where electrons are produced, to the cathode, where electrons are consumed. In an electrolytic cell, the dc source forces the electrons to travel nonspontaneously through the wire. Thus, the electrons flow from the positive electrode (the anode) to the negative electrode (the cathode). However, in a voltaic cell, the electrons flow spontaneously, away from the negative electrode (the anode) and toward the positive electrode (the cathode). 

Ions flow in the electrolytes of both electrolytic and voltaic cells. Electrons only flow in the external circuit of all types of electrochemical cells. 

Electrochemistry is concerned with the interconversion of electrical and chemical energy. Electrical effects occur as a result of the movement of electrical charge, either as mobile ions in an aqueous solution or melted liquid or as delocalized electrons in a conductor. Electrolytic and voltaic cells are the two types of electrochemical cells. 

The difference between electrolytic and voltaic cells is illustrated in Figure 19.1. Both kinds of cells have many applications. With the exception of barium, electrolytic cells are used to industrially produce all of the Group lA/1 and 2AJ2 metals. Sodium... 

There is a fundamental difference between the electrolytic and voltaic cells in Figure 19.1. What is that difference ... 

A comparison of electrolytic and voltaic cells can be seen in Figure 3.1. 

There is another way in which electrons can be rearranged in a chemical reaction, and that is through a wire. Electrochemistry is redox chemistry wherein the site for oxidation is separated from the site for reduction. Electrochemical setups basically come in two flavors electrolytic and voltaic (also known as galvanic) cells. Voltaic cells are cells that produce electricity, so a battery would be classed as a voltaic cell. The principles that drive voltaic cells are the same that drive all other chemical reactions, except the electrons are exchanged though a wire rather than direct contact. The reactions are redox reactions (which is why they produce an electron current) the reactions obey the laws of thermodynamics and move toward equilibrium (which is why batteries run down) and the reactions have defined rates (which is why some batteries have to be warmed to room temperature before they produce optimum output). 

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. 

Recent progress and main problems of the study of electrochemical equilibrium properties are reviewed for interfaces between two immiscible liquid electrolyte solutions. The discussed properties are mainly described in terms of the Galvani, Volta, zero charge, and surface (dipolar) potentials at the liquid-liquid interfaces and free liquid surfaces. Different galvanic and voltaic cells with liquid-liquid, mainly water-nitrobenzene interfaces, are described. These interfaces may be polarizable or reversible with respect to one or several ions simultaneously. 

Electrochemical cells are of two types voltaic and electrolytic. A voltaic cell uses a spontaneous chemical reaction to generate an electric current. It does this by physically separating the reaction into its oxidation and reduction halfreactions. These half-reactions take place in half-cells. The half-cell in which reduction occurs is called the cathode the... 

Figure 19.1 Electrolytic and voltaic (galvanic) cells, (a) An outside source, such as a battery or a generator, causes a flow of electricity (electric current) through an electrolytic cell and an external circuit. The cell consists of two identical electrodes immersed in an ionic solution (the electrolyte). The wires and any device to which they are connected make up the external circuit. The flow of electricity through the electrolyte consists of the movement of ions to oppositely charged electrodes. The current... 

An electrochemical cell is a device in which the electrons transferred in an oxidation-reduction reaction are made to pass through an electrical circuit. (See also electrolytic cell and voltaic cell.) An electrode is a surface on which an oxidation or a reduction half-reaction occurs. 

The principles discussed in this chapter have a host of practical applications. Whenever you start your car, turn on your cell phone, or use a remote control for your television or other devices, you are making use of a voltaic cell. Many of our most important elements, including hydrogen and chlorine, are made in electrolytic cells. These applications, among others, are discussed in Section 18.6. ... 

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. 

Similar considerations apply of course to the opposing electromotive forces of polarisation during electrolysis, when the process is executed reversibly, since an electrolytic cell is, as we early remarked, to be considered as a voltaic cell working in the reverse direction. In this way Helmholtz (ibid.) was able to explain the fluctuations of potential in the electrolysis of water as due to the variations of concentration due to diffusion of the dissolved gases. It must not be forgotten, however, that peculiar phenomena—so-called supertension effects—depending on the nature of the electrodes, make their appearance here, and com-... 

The basic principle of every measurement of the Volta potential and generally of the investigations of voltaic cells too, in contrast to galvanic cells, may thus be presented for systems containing metal/solution (Fig. 2) and liquid/liquid interfaces (Fig. 3), respectively. This interface is created at the contact of aqueous and organic solutions (w and s, respectively) of electrolyte MX in the partition equilibrium. Of course, electrolyte MX, shown in Fig. 2 and other figures of this chapter, may be different in organic (s) and aqueous (w) phases. 

Voltaic cells have been used by Malov et al. to investigate several silver salts, including superionic electrolytes synthesized on the basis of Agl, such as AgRbb, AgSI, and Agf,W04l4. 

The non situ experiment pioneered by Sass uses a preparation of an electrode in an ultrahigh vacuum through cryogenic coadsorption of known quantities of electrolyte species (i.e., solvent, ions, and neutral molecules) on a metal surface. " Such experiments serve as a simulation, or better, as a synthetic model of electrodes. The use of surface spectroscopic techniques makes it possible to determine the coverage and structure of a synthesized electrolyte. The interfacial potential (i.e., the electrode work function) is measured using the voltaic cell technique. Of course, there are reasonable objections to the UHV technique, such as too little water, too low a temperature, too small interfacial potentials, and lack of control of ionic activities. ... 

There are two types of cells electrolytic (which requires a battery or external power source) and voltaic (which requires no battery or external power source). The reaction in the diagram is voltaic and therefore spontaneous. In a voltaic cell, the anode is the negative terminal, and oxidation occurs at the anode. Remember the OIL portion of OIL RIG (Oxidation Is Losing electrons) and AN OX (ANode is where Oxidation occurs). 

Long after the above work on electrochemical element migration described above was published, Hamilton (1998) pointed out that Govett (1976) and B0lviken L0gn (1975) had described an electrolytic cell (which would not work as described) rather than a voltaic cell that should work. Hamilton was, of course, correct. I have passed over to him some unpublished data on Thalanga and I look forward to his interpretation of the dispersion patterns there. 

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. 

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