Galvanic Cells

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 15:33


In an electrolytic cell electrical energyA system's capacity to do work. is consumed and an otherwise spontaneous redox reaction is reversed. A galvanic cellAn electrochemical cell in which a spontaneous reaction occurs. Such a cell can be used to generate electricity. Also called voltaic cell., on the other hand, produces electrical energy as a result of a spontaneous redox process. The electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. transfer characteristic of such a process is made to occur in two separate half-cells. Electrons released during an oxidationThat part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a reactant must occur. half-equation must flow through a wire or other external circuit before they can be accepted in a reductionThat part of a chemical reaction in which a reactant gains electrons; simultaneous oxidation of a reactant must occur. half-equation. Consequently an electrical current is made to flow.

A typical galvanic cell, the Daniell cell, was used to power telegraphs 100 years ago. This cell is based on the spontaneous redox reaction


Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)      (1)


(You can verify that this reaction is spontaneous by dipping a piece of zinc metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. in a copper sulfate solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture.. In a short time the surface of the zinc will become plated with red-brown copper metal.) The half-equations


                Zn(s) → Zn2+(aq) + 2e        (1a)

  Cu2+(aq) + 2e → Cu(s)                     (1b)


indicate that for each mole of zinc which is oxidized and goes into solution as zinc ions, 2 mol electrons are transferred to copper ions, converting them to copper atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume..

To produce electrical current we must prevent the Zn(s) from contacting the Cu2+(aq) ions and transferring the electrons directly. This is done in the Daniell cell by pouring a concentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). copper sulfate solution into the bottom of a glassA solid material that does not have the long-range order of a crystal lattice; an amorphous solid. A glass melts over a range of temperatures instead of having the definite melting temperature characteristic of crystalline solids. jar and then carefully pouring a layer of less concentrated zinc sulfate solution above it. Because it contains less soluteThe substance added to a solvent to make a solution. per unitA particular measure of a physical quantity that is used to express the magnitude of the physical quantity; for example, the meter is the unit of the physical quantity, length. volume, the zinc sulfate solution is less dense. It floats on the copper sulfate and does not mix with it. Therefore a copper electrodeIn an electrochemical cell, a surface on which oxidation or reduction occurs; an electrode conducts electric current into or out of a cell. placed in the bottom of the jar contacts only Cu2+(aq) ions, and a zinc electrode suspended in the zinc sulfate solution contacts only Zn2+(aq) ions.

Figure 1 A simple galvanic cell.
In the laboratory it is more convenient to set up a cell based on the Zn + Cu2+ reaction, as shown in Fig. 1. Two electrodes or half-cells are separated by a salt bridgeA connection that permits ions to pass but that restricts the flow of solution between the anode half cell and the cathode half cell in an electrochemical cell.. This contains an electrolyteA substance that dissolves to produce a solution containing ions, which cause the solution to conduct electricity., KCl, so that current can flow from one half-cell to the other, but the contents of the two half-cells cannot mix. The left-hand electrode in Fig. 1 is a Zn rod dipping in a solution of ZnSO4. Thus both components of the Zn2+/Zn redox couple are present, and the metal electrode can conduct electrons produced by Eq. (1a) to the wire in the external circuit. Since oxidation of Zn to Zn2+ occurs at the left-hand electrode, this electrode is the anodeThe electrode in an electrochemical cell where oxidation occurs. The positively charged electrode in a vacuum tube..

The right-hand electrode is a strip of Cu dipping in a solution of CuSO4. Here both components of the Cu2+/Cu redox couple are present, and Eq. (1b) can occur. Electrons supplied by the external circuit are conducted through Cu to the electrode surface, where they combine with Cu2+ ions to produce more Cu. Since reduction occurs at this right-hand electrode, this electrode is the cathodeThe electrode in an electrochemical cell where reduction occurs; the negatively charged electrode in a vacuum tube.. The net effect of the two half-cells is that electrons are forced into the external circuit at the anode and withdrawn from it at the cathode. This will cause current to flow, or, if current is prevented from flowing by a device such as the voltmeter in Fig. 1, it will cause an electrical potential difference (voltage) to build up.

The components of the redox couples at the electrodes in a galvanic cell need not always be a solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. and a species in solution. This is evident from Fig. 2. In this case the spontaneous redox reaction


2Fe2+(aq) + Cl2(g) → 2Fe3+(aq) + 2Cl(aq)      (2)


is involved. The oxidation half-equation at the anode is


Fe3+(aq) → Fe2+(aq) + e      (2a)


Thus at the right-hand electrode in Fig. 2 both components of the redox couple are in aqueousDescribing a solution in which the solvent is water. solution. Reaction (2a) occurs at the surface of the platinum wire, which conducts the released electrons to the external circuit.

The left-hand electrode in Fig. 2 is a gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. electrode. It consists of a platinum strip dipping in a solution which contains chloride ions. The electrode is surrounded by a glass tube through which chlorine gas can be pumped. At this electrode the reaction is a reduction:


Cl2(g) + 2e→ 2Cl(aq)      (2b)


Therefore the left-hand electrode is the cathode. Since electrons are forced into the external circuit at the anode and withdrawn at the cathode, electrons flow from right to left in this cell.


Figure 2 A example of a galvanic cell.