Electrolytic and Galvanic Cells

So far the spontaneous functioning of an electrochemical cell has been described, which corresponds to the transformation of energy obtained in a chemical reaction into electron movement, that is electrical energy. This type of cell is a galvanic cell. 

Let us consider the charges on the electrodes in the two cases. At the anode in a galvanic cell, since the oxidation is spontaneous, there is an excess of electrons at the electrode. On the other hand, in an electrolytic cell where oxidation is forced to occur, there is a shortage of electrons and a positive charge. The two situations are  

Some electrochemical cells can function as galvanic or electrolytic cells. A well-known example is the lead-acid car battery. Under discharge 

PURPOSE OP EXPERIMENT Construct several galvanic cells, and measure their voltages construct an electrolytic cell, and determine the copper oxidized and hydrogen liberated during electrolysis of a dilute H2SO4 solution. 

Oxidation-reduction reactions are the basis of the branch of chemistry called electrochemistry. Such a reaction may occur spontaneously and produce electrical energy, as in a galvanic cell. If the reaction does not occur spontaneously, the addition of electrical energy may initiate a chemical change, a process called electrolysis. 

All electrochemical cells involve two half-reactions an oxidation half-reaction in which electrons are released, and a reduction half-reaction in which electrons are taken up. The net voltage of the cell, the only quantity that can be measured experimentally, is the algebraic sum of the potentials for the two half-reactions. Each potential is a measure of the relative ability of a given half-reaction to occur. However, since the potential for a half-reaction cannot be measured directly, numerical values for half-reaction potentials are arbitrary and must be based on a reference potential. The hydrogen half-reaction serves as the reference for all electrochemical potentials. 

As soon as you have made up the cell, use the following procedure to determine which electrode is releasing electrons to the external circuit and which is taking up electrons. Pour approximately 2 mL of a starch-potassium iodide, KI, solution into a small beaker. Add 2 drops of 3 M hydrochloric acid, HCl, solution and stir. Thoroughly wet a piece of filter paper in the beaker, and place it on a watch glass. With the very clean electrode wires from the cell about O.5-1.0 cm apart, touch the wires to the filter 

Immediately determine the voltage of your galvanic cell by connecting the wires to the proper terminals of the voltmeter as indicated by your starch-potassium iodide test. Continue to read the voltmeter for several minutes to make sure that you observe the maximum voltage of your cell. Record the maximum voltage in TABLE 29.lA. Remove the zinc electrode, clean it, and return it to the reagent shelf. 

In an electrochemical cell, the anode is where oxidation takes place the cathode is where reduction takes place. As the reaction proceeds in a galvanic cell, the electrons released at the anode travel through the external circuit (Fig. 5.5). They re-enter the cell at the cathode, where they bring about reduction. This flow of current in the external circuit, from anode to cathode, corresponds to the cathode having a higher potential than the anode and arises from the tendency of 

The simplest type of galvanic cell has a single electrolyte common to both electrodes (as in Fig. 5.3). In some cases it is necessary to immerse the electrodes in different electrolytes, as in the DanieU cell (Fig. 5.7), in which the redox couple at one electrode is Cu VCu and at the other is Zn VZn. In an electrolyte concentration cell, which would be constructed Hke the cell in Fig. 5.4, the electrode compartments are of identical composition except for the concentrations of the electrolytes. In an electrode concentration cell the electrodes themselves have different concentrations, either because they are gas electrodes operating at different pressures or because they are amalgams (solutions in mercury) with different concentrations. 

In a gas electrode (Fig. 5.8), a gas is in equilibrium with a solution of its ions in the presence of an inert metal. The inert metal, which is often platinum, acts as a source or sink of electrons but takes no other part in the reaction except perhaps to act as a catalyst. One important example is the hydrogen electrode, in which hydrogen is bubbled through an aqueous solution of hydrogen ions and the redox couple is HVHj. 

The term redox electrode is normally reserved for an electrode in which the couple consists of the same element in two nonzero oxidation states (Fig. 5.9). An example is an electrode in which the couple is Fe +/Fe +. In general, the reaction is 

For the electrode corresponding to the Fe VFe couple the reduction halfreaction and reaction quotient are 

---

The correct statement is (d). Electrons are produced at the anode and move toward the cathode, regardless of the electrode material. The electrons do not move through the salt bridge ions do. Electrons do not leave the cell they provide current within the circuitry. Reduction occurs at the cathode in both galvanic and electrolytic cells—in all types of electrochemical cells, in fact. 

In the operation of both galvanic and electrolytic cells, there is a reaction occurring on the surface of each electrode. For example, the following reaction takes... 

Figure 16.2 shows a comparison of a galvanic and electrolytic cell for the Sn/Cu system. On the left-hand side of Figure 16.2, the galvanic cell is shown for this system. Note that this reaction produces 0.48 Y But what if we wanted the reverse reaction to occur, the nonsponta-neous reaction This can be accomplished by applying a voltage in excess of 0.48 V from an external electrical source. This is shown on the right-hand side of Figure 16.2. In this electrolytic cell, electricity is being used to produce the nonspontaneous redox reaction.

How are galvanic and electrolytic cells built, and how do they function What equations are used to describe these types of cells How can you solve quantitative problems related to electrolysis ... 

Redox reactions, which involve a transfer of electrons, can occur in acidic and basic conditions. Electrochemistry explains the creation of galvanic and electrolytic cells. You find out about both topics in this part. 

Thus far, we ve been concerned only with galvanic cells—electrochemical cells in which a spontaneous redox reaction produces an electric current. A second important kind of electrochemical cell is the electrolytic cell, in which an electric current is used to drive a nonspontaneous reaction. Thus, the processes occurring in galvanic and electrolytic cells are the reverse of each other A galvanic cell converts... 

This holds good for galvanic and electrolytic cells. In the case of electrolysis of water at 25 °C the following values have to be considered ... 

Why is the electrolyte necessary in both galvanic and electrolytic cells ... 

Garnett, P.J., Treagust, D.F. Conceptual difficulties experienced by senior high school students of electrochemistry Electrochemical (Galvanic) and electrolytic cells. Journal Research of Science Teaching 29 (1992), 1079... 

The symbol for the fuel cell and electrolysis cell is derived from the battery symbol the longer and shorter lines represent, respectively, the cathode and anode, and the dashed line represents the electrolyte. An arrow drawn in the direction of positive current flow points toward an electrolyte with negative charge carriers, as in the manner of the transistor symbol. Galvanic and electrolytic cells are distinguished by the location of the positive terminal a positive terminal at the cathode indicates a galvanic cell, while a positive terminal at the anode indicates an electrolytic cell. The outer box represents the system enclosure, which may or may not be open. The values of potential and overpotential are consistent with Table 2. 

What are the differences between galvanic and electrolytic cells Recall that both are electrochemical cells. 

In both galvanic and electrolytic cells, oxidation always occurs at the anode and reduction always occurs at the cathode. However, the polarities of the electrodes are different in the two kinds of cells. In a galvanic cell the cathode is + and the anode is -. In an electrolytic cell the cathode is - and the anode is . 

Every galvanic and electrolytic cell has two electrodes. The one which introduces electrons to the cell is the cathode the one which removes them from the cell is the anode. In this chapter we will consider some of the basic concepts of cells the meaning of cell and electrode potentials will be examined in detail. We shall define electrolytic cells and galvanic cells discuss the difference between surfaces and bulk of matter consider the location of the site of the electrode reaction, and the forces and laws which control the flow of current and make one electrode the source and the other the sink for electrons. 

The overpotential for galvanic and electrolytic cells can be defined as deviation of the cell potential from its equilibrium value, excluding the potential drop due to the cell internal resistance I , ... 

---