Electrochemical Cells

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 15:28

When an electrical current flows through matterAnything that occupies space and has mass; contrasted with energy., permanent chemical changes often occur. In some cases electrical energyA system's capacity to do work. supplied from an outside source can cause a chemical reactionA process in which one or more substances, the reactant or reactants, change into one or more different substances, the products; chemical change involves rearrangement, combination, or separation of atoms. Also called chemical change. to take place. Such a process is called electrolysis, and the system to which electricity is supplied is called an electrolytic cellAn electrochemical cell used to drive a reaction that would otherwise be non-spontaneous (reactant-favored).. A typical example of electrolysis is the laboratory preparation of H2(g) and O2(g) by passing electrical current through-water. Electrolysis is quite important in chemical industry, being involved in manufacture of aluminum, chlorine, copper, and numerous other substances.

It is also possible to produce a flow of electricity as a result of a spontaneousCapable of proceeding without an outside source of energy; refers to a reaction in which the products are thermodynamically favored (product-favored reaction). chemical reaction. A chemical system which can cause a current to flow in this way is called a galvanic cell or a voltaic cellAn electrochemical cell in which a spontaneous reaction occurs. Such a cell can be used to generate electricity. Also called Galvanic cell.. An example of a galvanic cell with which you are almost certainly familiar is a flashlight batteryA series of voltaic cells connected together, used to produce electricity. Also, a single voltaic cell for commercial use.. Since an electrical current is a flow of electrons or other charged particles, it should come as no surprise that both electrolytic and galvanic cells involve redox reactions.

In an electrolytic cell electric energy supplied from an outside source causes a nonspontanious reaction to occur. A galvanic (or voltaic) cell, on the other hand. harnesses a spontaneous reaction to produce electric current. In either kind of cell the electrodeIn an electrochemical cell, a surface on which oxidation or reduction occurs; an electrode conducts electric current into or out of a cell. at which oxidationThat part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a reactant must occur. occurs is called the anodeThe electrode in an electrochemical cell where oxidation occurs. The positively charged electrode in a vacuum tube. and the electrode at which reductionThat part of a chemical reaction in which a reactant gains electrons; simultaneous oxidation of a reactant must occur. occurs is the cathodeThe electrode in an electrochemical cell where reduction occurs; the negatively charged electrode in a vacuum tube..

Electrolytic cells have numerous commercial applications. Chlorine, sodium hydroxide, hydrogen, aluminum, magnesium, sodium, calcium, and high-purity copper are some of the more important chemicals produced by electrolysis. Electro-plating of metals such as chromium, silver, nickel, zinc, and tin is also quite important. In any electrolysis reaction the amount of substance consumed or produced can be related to the electric charge which passes through the cell by means of the Faraday constantThe electric charge carried by one mole of electrons, 9.648 670 x 104 C mol-1; abbreviated F. F, which equals 9.649 × 104 C mol–1.

A galvanic cell may be represented by an abbreviated notation such as

Zn│Zn2+ (1 M)║ Ag2+(1 M)│Ag

When a cell is written this way, it is always assumed that the left-hand electrode is an anode and an oxidation half-equation occurs there. The right-hand electrode must then be taken as the cathode and a reduction half-equation is assumed to occur there. The cell reaction is the sum of these two half-equations. If it is spontaneous, our assumptions about anode on the left and cathode on the right were correct. Electrons will be forced into an external circuit on the left and the cell emf is taken to be positive. If the cell reaction written according to the above convention turns out to be nonspontaneous, then its reverse will be spontaneous. Our assumptions about which electrode is the anode and which the cathode must also be reversed, and the cell emf is given a negative sign.

Because cell emf values indicate whether a process is spontaneous, they are quite useful. They are additive and are conventionally reported as standard electrode potentials. These refer to the emf of a cell with a hydrogen-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 on the left and the electrode whose potential is reported on the right. The standard electrode potential is directly related to the standard free energyGibbs energy: a thermodynamic function corresponding to the tendency for spontaneous change in a system; represented by the symbol G. change for a reaction, thus allowing direct determination of ΔG°.

Many galvanic cells are of commercial importance. These include dry cells, mercury cells, rechargeable Ni-Cd batteries, and lead storage cells. Fuel cells, in which a continuous supply of both oxidizing and reducing agentA chemical species that donates electrons in order to reduce another species. In the process the reducing agent is itself oxidized. is supplied, may eventually become important because of their high efficiencies.