Electrochemistry a Chem1 Supplement Text Stephen K. Lower Simon Fraser University Contents 1 Chemistry and electricity 2 Electroneutrality .............................. 3 Potential dierences at interfaces ..................... 4 2 Electrochemical cells 5 Transport of charge within the cell .................... 7 Cell description conventions ........................ 8 Electrodes and electrode reactions .................... 8 3 Standard half-cell potentials 10 Reference electrodes ............................ 12 Prediction of cell potentials ........................ 13 Cell potentials and the electromotive series ............... 14 Cell potentials and free energy ...................... 15 The fall of the electron ........................... 17 Latimer diagrams .............................. 20 4 The Nernst equation 21 Concentration cells ............................. 23 Analytical applications of the Nernst equation .............. 23 Determination of solubility products ................ 23 Potentiometric titrations ....................... 24 Measurement of pH .......................... 24 Membrane potentials ............................ 26 5 Batteries and fuel cells 29 The fuel cell ................................. 29 1 CHEMISTRY AND ELECTRICITY 2 6 Electrochemical Corrosion 31 Control of corrosion ............................ 34 7 Electrolytic cells 34 Electrolysis involving water ........................ 35 Faraday’s laws of electrolysis ....................... 36 Industrial electrolytic processes ...................... 37 The chloralkali industry. ....................... 37 Electrolytic rening of aluminum .................. 38 1 Chemistry and electricity The connection between chemistry and electricity is a very old one, going back to Allesandro Volta’s discovery, in 1793, that electricity could be produced by placing two dissimilar metals on opposite sides of a moistened paper. In 1800, Nicholson and Carlisle, using Volta’s primitive battery as a source, showed that an electric current could decompose water into oxygen and hydrogen. This was surely one of the most signicant experiments in the history of chemistry, for it implied that the atoms of hydrogen and oxygen were associated with positive and negative electric charges, which must be the source of the bonding forces between them. By 1812, the Swedish chemist Berzelius could propose that all atoms are electried, hydrogen and the metals being positive, the nonmetals negative. In electrolysis, the applied voltage was thought to overpower the attraction between these opposite charges, pulling the electried atoms apart in the form of ions (named by Berzelius from the Greek for “travellers”). It would be almost exactly a hundred years later before the shared electron pair theory of G.N. Lewis could oer a signicant improvement over this view of chemical bonding. 1 CHEMISTRY AND ELECTRICITY 3 dissoluti on of Zn as Zn2+ causes - electric charges to build up in the - - tw o phases which inhibi ts further e e AAZn dissoluti on AAA AA AAA Zn2+(aq) AA AAA AA Zn2+(aq) AAA AA Zn in metal Figure 1: Oxidation of metallic zinc in contact with water Meanwhile, the use of electricity as a means of bringing about chemical change continued to play a central role in the development of chemistry. Humphrey Davey prepared the rst elemental sodium by electrolysis of a sodium hydrox- ide melt. It was left to Davey’s former assistant, Michael Faraday, to show that there is a quantitative relation between the amount of electric charge and the quantity of electrolysis product. James Clerk Maxwell immediately saw this as evidence for the “molecule of electricity”, but the world would not be receptive to the concept of the electron until the end of the century. Electroneutrality Nature seems to very strongly discourage any process that would lead to an excess of positive or negative charge in matter. Suppose, for example, that we immerse a piece of zinc metal in pure water. A small number of zinc atoms go into solution as Zn2+ ions, leaving their electrons behind in the metal: 2+ Zn(s) Zn +2e (1) → As this process goes on, the electrons which remain in the zinc cause a negative charge to build up which makes it increasingly dicult for additional positive ions to leave the metallic phase. A similar buildup of positive charge in the liquid phase adds to this inhibition. Very soon, therefore, the process comes to a halt, resulting in a solution in which the concentration of Zn2+ is so low 10 (around 10 M ) that the water can still be said to be almost “pure”. There would be no build-up of charge if the electrons could be removed from the metal as the positive ions go into solution. One way to arrange this is to drain o the excess electrons through an external circuit that forms part of a complete electrochemical cell; this we will describe later. Another way to remove electrons is to bring a good electron acceptor (that is, an oxidizing agent) into contact with the electrode. A 1 CHEMISTRY AND ELECTRICITY 4 suitable electron acceptor would be hydrogen ions; this is why acids attack many metals. For the very active metals such as sodium, H2O is a suciently good electron acceptor. The degree of charge unbalance that is allowed produces dierences in elec- tric potential of no more than a few volts, and corresponds to concentration un- balances of oppositely charged particles that are not even detectable by ordinary chemical means. There is nothing mysterious about this prohibition, known as the electroneutrality principle; it is a simple consequence of the thermodynamic work required to separate opposite charges, or to bring like charges into closer contact. The additional work raises the free energy G of the process, making it less spontaneous. The only way we can get the reaction in Eq 1 to continue is to couple it with some other process that restores electroneutrality to the two phases. A simple way to accomplish this would be immerse the zinc in a solution of copper sulfate instead of pure water. As you will recall if you have seen this commonly-performed experiment carried out, the zinc metal quickly becomes covered with a black coating of nely-divided metallic copper. The reaction is a simple oxidation-reduction process, a transfer of two electrons from the zinc to the copper: 2+ 2+ Zn(s) Zn +2e Cu +2e Cu(s) → → The dissolution of the zinc is no longer inhibited by a buildup of negative charge in the metal, because the excess electrons are removed from the zinc by copper ions that come into contact with it. At the same time, the solution remains electrically neutral, since for each Zn2+ introduced to the solution, one Cu2+ is removed. The net reaction Zn(s) +Cu2+ Zn2+ +Cu(s) → quickly goes to completion. Potential dierences at interfaces Electrochemistry is the study of reactions in which charged particles (ions or electrons) cross the interface between two phases of matter, typically a metallic phase (the electrode) and a conductive solution, or electrolyte. A process of this kind is known generally as an electrode process. Electrode processes (reactions) take place at the surface of the electrode, and produce a slight unbalance in the electric charges of the electrode and the solution. The result is an interfacial potential dierence which, as we saw above, can materially aect the rate and direction of the reaction. Much of the impor- tance of electrochemistry lies in the ways that these potential dierences can be related to the thermodynamics and kinetics of electrode reactions. In particular, 2 ELECTROCHEMICAL CELLS 5 Ð + direction of electron flow in external circuit AAA AAACu direction of conventional current flow Zn porous fritted AAA gl ass barrier AAA AAA A AAA Cu2+ AAAZn2+ A AAA AAA A AAA NO Ð 2+ Ð 2+ Ð AAA A 3 AAA Zn ê Zn ,NO3 êê Cu , NO3 ê Cu A Figure 2: A simple electrochemical cell manipulation of the interfacial potential dierence aords an important way of exerting external control on an electrode reaction. The interfacial potential dierences which develop in electrode-solution sys- tems are limited to only a few volts at most. This may not seem like very much, but it is important to understand that what is important is the distance over which this potential dierence exists. In the case sof an electrode immersed in a solution, this distance corresponds to the thin layer of water molecules and ions that attach themselves to the electrode surface– normally only a few atomic diamters.a only a few atomic diameters. In this way a very small voltage can produce a very large potential gradient For example, a potential dierence of 8 one volt across a thickness of only 10 cm amounts to a potential gradient of 100 million volts per centimetre: a very signicant value indeed! Actually, interfacial potentials exist between any two phases in contact, even in the absence of chemical reactions. In many forms of matter, they are the result of adsorption or ordered alignment of molecules caused by non-uniform forces in the interfacial region. Thus colloidal particles in aqueous suspenctions selectively adsorb a given kind of ion, positive for some colloids, and negative for others. The resulting net electric charge prevents the particles from coming together and coalescing, which they would otherwise tend to do under the inuence of ordinary van der Waals attractions. 2 Electrochemical cells The electron-transfer reactions that occur at the surface of a metal immersed in a solution take place near the surface of the electrode, so there is no way that the electrons passing between the solution and the electrode can be channeled through an instrument to measure their voltage or to control the rate of the reaction. However, if we have two such metal-solution interfaces, we can easily measure a potential diference between them. Such an arrangement is called a 2 ELECTROCHEMICAL CELLS 6 galvanic cell. A typical cell might consists of two pieces of metal, one zinc and the other copper, each immersed each in a solution containing a dissolved salt of the corresponding metal (see Fig. 2). The two solutions are connected by a tube containing a porous barrier that prevents them from rapidly mixing but allows ions to diuse through.