ELECTROCHEMISTRY II. ELECTROCHEMISTRY Introduction In 1812 Humphry Davy wrote: “If a piece of zinc and a piece of copper be brought in contact with each other, they will form a weak electrical combination, of which the zinc will be positive, and the copper negative; this may be learnt by the use of a delicate condensing electrometer”. One can consider what happens when electrodes (zinc and copper in Davy’s experiment) are immersed in electrolyte solutions and connected via an external metallic conductor. Such an arrangement is a typical electrochemical cell. Electromotive force (EMF) of the cell One can consider a cell built from a zinc electrode immersed into a solution of ZnSO 4 and a copper electrode immersed in CuSO4 (Daniel’s cell). The two solutions are separated by a porous barrier, which allows electrical contact but prevents excessive mixing of the solutions by interdiffusion. This cell can be represented by a scheme: Zn|Zn2+|Cu2+|Cu where vertical line denotes phase boundaries. According to definition given by IUPAC the electromotive force (emf), E, is defined as follows: the emf is equal in sign and in magnitude to the electrical potential of the metallic conducting lead on the right when that of the similar lead on the left is taken as zero, the cell being open. Thus it can be written that: E Eright Eleft 1 where Eright and Eleft would be the potentials of the right and left leads relative to some common standard. The meaning of left and right refers to the cells as written. Measurement of emf – the potentiometer The definition of emf states that the potential difference is measured while the cell is open, i.e., while no current is being drawn from the external leads. In practice E is being measured under conditions in which the current drawn from the cell is so small as to be negligible. The Lodz University of Technology, Faculty of Chemistry, Institute of Applied Radiation Chemistry, Laboratory of Chemistry ELECTROCHEMISTRY method, devised by POGGENDORF used a circuit known as potentiometer (a basic one is shown in Figure 1). Figure 1. Potentiometer scheme The slide wire is calibrated with a scale so that any setting of the contact corresponds to a certain voltage. With a double through switch in the standard cell position S, slide wire is set to the voltage reading of standard cell, and rheostat is being adjusted until no current flows through the galvanometer G. At this point the potential difference between A and B, the IR along the section AB of the slide wire, just balances the emf of the standard cell. Then switch of the unknown cell is being set to the X position and slide wire is being readjusted until no current flows through galvanometer. From the new setting the emf of the cell can be read directly from the scale of the slide wire. The most widely used standard is the Weston cell written as: Cd(Hg)|CdSO4 · 8/3 H2O|CdSO4 (sat.sol.)|Hg2SO4|Hg The cell reaction is: 8 8 Cds Hg SO s H Ol CdSO H Os 2Hgl 2 4 3 2 4 3 2 The accuracy of the compensation method for measuring an emf is limited only by the accuracy of the standard E and of the various resistances in the circuit. The precision of the method is determined mainly by the sensitivity of the galvanometer used to detect the balance between unknown and standard emf. Reversible cells An electrode immersed in a solution is said to constitute a half cell. The typical cell is combination of two half cells. One should be primarily interested in so called reversible cells, which can be recognized by the following criterion: the cell is connected with a potentiometer arrangement for emf measurement by compensation method. The emf of the cell is measured: Lodz University of Technology, Faculty of Chemistry, Institute of Applied Radiation Chemistry, Laboratory of Chemistry ELECTROCHEMISTRY a) with a small current flowing through the cell in one direction b) then with an imperceptible flow of current c) and finally with a small flow in opposite direction If the cell is reversible, its emf changes only slightly during this sequence, and there is no discontinuity in the value of the emf at the point of balance (b). Reversibility implies that any chemical reaction occurring in the cell can proceed in either direction, depending on the flow of current, and the null point of the driving force of the reaction is just balanced by the compensating emf of the potentiometer. One of the sources of irreversibility in cells is the liquid junction, like in the Daniel’s cell presented before. Another one can be salt bridge which consists of a connecting tube filled with a concentrated solution of a salt, usually KCl. Than cell scheme can be written as: Zn|Zn2+||Cu2+|Cu A better way to avoid irreversible effects is to avoid liquid junctions altogether, by using single electrolyte, like in the Weston cell (CdSO4 solution saturated with sparingly soluble Hg2SO4. Types of half cells One of the simplest half cells consists of metal electrode in contact with solution containing ions of the metal e.g. silver and silver-nitrate solution. Gas electrodes can be constructed by placing a strip of nonreactive metal (platinum or gold) in contact with both the solution and the gas stream. The hydrogen electrode consists of a platinum strip exposed to a current of hydrogen and partly immersed in an acid solution. Overall reaction is: 1 H H e 2 2 2 In nonmetal-nongas electrodes, the inert metal passes into a liquid or solid phase e.g. - bromine-bromide half cell: Pt|Br2|Br . In an oxidation-reduction electrode an inert metal dips into a solution containing ions in two different oxidation states, e.g. ferric and ferrous ions in the half cal Pt|Fe2+|Fe3+. When electrons are supplied to the electrode the reaction is Fe3+ + e Fe2+. Since it is a function of Lodz University of Technology, Faculty of Chemistry, Institute of Applied Radiation Chemistry, Laboratory of Chemistry ELECTROCHEMISTRY electrodes either to accept electrons from, or to donate electrons to ions in the solution they are all in the sense oxidation-reduction electrodes. Metal, insoluble salt electrodes consist of a metal in contact with one of its slightly soluble salts; in the half cell, this salt is in turn in contact with a solution containing common anion. - An example is the silver, silver chloride half cell Ag|AgCl|Cl (c1) and overall electrode reaction is AgCls e Ag s Cl . Metal, insoluble oxides electrodes are similar to the metal, insoluble salt one e.g. antimony, — antimony trioxide electrode with a scheme Sb|Sb2O3|OH and an overall reaction 1 3 Sbs 3OH Sb O H Ol 3e . An antimony rod is covered with a thin layer of 2 2 3 2 2 oxide and dips into a solution containing OH- ions. Classification of cells When two suitable cells are connected an electrochemical cell is given. The connection is made by bringing the solutions in the half cells into contact so that ions can pass between them. If these two solutions are the same, there is no liquid junction, and one can have a cell without transference. If the solutions are different, the transport of ions across the junction will cause irreversible changes in the two electrolytes, and one can have a cell with transference. Cells in which the driving force is the change in concentration are called concentration cells. The change in concentration can occur either in the electrolyte or in electrodes. The variety of electrochemical cells is given in Figure 2. Electrochemical cells Chemical cells Concentrationl cells Without With Electrolyte Electrode transference transference Concentration Concentration cells cells Without With transference transference Lodz University of Technology, Faculty of Chemistry, Institute of Applied Radiation Chemistry, Laboratory of Chemistry ELECTROCHEMISTRY Figure 2. Electrochemical cells Emf and standard emf of the cell For generalized cell reaction: aA bB cC dD free-energy change in terms of the activities of the reactants is: c d 0 aC aD G G RT ln a b 3 a AaB Since G=-|z|FE, division by -|z|F gives c d 0 RT aC aD E E ln a b 4 z F a AaB called Nernst equation. E0 is called a standard emf of the cell. Determination of this value is one of the most important procedures in electrochemistry. As an example lets consider cell consisting of a hydrogen electrode and a silver-silver chloride electrode immersed in a solution of hydrochloric acid: Pt(H2)|HCl(m)|AgCl|Ag. The overall reaction is: 1 AgCl H H Cl Ag 2 2 The emf of the cell is RT a Ag a a E E 0 ln Cl H 5 F 1 a a 2 AgCl H 2 Setting the activities of the solid phases equal to the unity, and choosing hydrogen pressure so that aH2=1 (for ideal gas P=1atm) following reaction can be obtained: 0 RT E E ln a a 6 F Cl H Introducing the mean activity of the ions defined by a±=±m one can obtain 2RT 2RT E E 0 ln a E 0 ln m 7 F F 2RT 2RT E ln m E 0 ln 8 F F 1/2 According to the Debye-Hückel theory, in dilute solutions ln ±=Am , where A is a constant. Hence the equation becomes: Lodz University of Technology, Faculty of Chemistry, Institute of Applied Radiation Chemistry, Laboratory of Chemistry ELECTROCHEMISTRY 2RT 2RTA E ln m E 0 ln m1/ 2 9 F F If the quantity on the left is plotted against m1/2, and extrapolated to m=0, the intercept at m=0 gives value of E0.