Chapter 17 Electrochemistry The study of the interchange of chemical and electrical energy. Primarily concerned with two processes: Oxidation-reduction reactions. The generation of an electric current or the use of current to produce chemical change.

Review of Terms Oxidation-reduction (redox) reaction: involves a transfer of electrons from the reducing agent to the oxidizing agent. Oxidation: loss of electrons (an increase in oxidation number) Reduction: gain of electrons (a decrease in oxidation number)

Half-Reactions The overall reaction is split into two half-reactions, one involving oxidation and one reduction. 8H+ + MnO4 + 5Fe2+  Mn2+ + 5Fe3+ + 4H2O Fe2+ is oxidized and MnO4- is reduce. Electrons are transferred from Fe2+ (reducing agent) to MnO4- (oxidizing agent) Reduction: 8H+ + MnO4 + 5e  Mn2+ + 4H2O Oxidation: 5Fe2+  5Fe3+ + 5e The balanced overall reaction is the sum of the half-reactions.

Galvanic Cell A device in which chemical energy is changed to electrical energy. Electrons flow through the wire from reducing agent to oxidizing agent, and ions flow from one compartment to the other through salt bridge (or porous disk) to keep the net charge zero.

Figure 17.1 Schematic of a Method to Separate the Oxidizing and Reducing Agents of a Redox Reaction

Figure 17.2 Galvanic Cells

Anode and Cathode The reaction in an electrochemical cell occurs at the interface between the electrode and the solution where the electron transfer occurs. The electrode compartment in which OXIDATION occurs is called the ANODE. The electrode compartment in which REDUCTION occurs is called the CATHODE.

Cell Potential A galvanic cell consists of an oxidizing agent in one compartment that pulls electrons through a wire from a reducing agent in the other compartment. The “pull” or driving force on the electrons is called the cell potential (Ecell) or the electromotive force (emf) of the cell. The unit of electrical potential is the volt (v) which is defined as 1 joule of work per coulomb of charge transferred

Figure 17.3 An Electrochemical Process Involves Electron Transfer at the Interface Between the Electrode and the Solution

Figure 17.4 Digital Voltmeters

Standard Reduction Potentials The E values corresponding to reduction half-reactions with all solutes at 1M and all gases at 1 atm are called standard reduction potentials. Zn + Cu2+  Zn2+ + Cu Cu2+ + 2e  Cu E = 0.34 V vs. SHE Zn2+ + 2e-  Zn E = -0.76 V vs. SHE Eocell = Eooxidation + Eoreduction = 0.76V + 0.34 V = 1.10 V

Figure 17.5 A Zn/H Galvanic Cell

Figure 17.6 A Zn/Cu Galvanic Cell

Manipulations for combining half-reactions One of the half reactions must be reversed which means that the sign of the potential for the reversed half reaction also must be reversed. The half reactions must be multiplied by integers as necessary to achieve the balanced equation but the value of Eo is not changed when a half reaction is multiplied by an integer. Standard reduction potential is an intensive property. The potential is not multiplied by the integer.

Line Notation Handy line notation used to describe electrochemical cells. In this notation the anode compartments are listed on the left and the cathode compartments are listed on the right separated by double vertical lines indicting salt bridge or porous disk. Mg(s)|Mg2+(aq)||Al3+(aq)|Al(s) In this notation a phase difference is indicated by a single vertical line. The substance constituting the anode is listed at the far left and the substance constituting the cathode is listed at the far right.

Figure 17.7 A Schematic of a Galvanic Cell

Complete Description of a Galvanic Cell A complete description of a galvanic cell usually includes four items: The cell potential (always positive for a galvanic cell) and the balance cell reaction The direction of electron flow, obtained by inspecting the half reactions and using the direction that gives a positive Ecell Designation of the anode and cathode The nature of each electrode and the ions present in each compartment. A chemically inert conductor is required if none of the substances participating in the half reaction is a conducting solid. Fe(s)|Fe2+(aq)||MnO4-(aq), Mn2+(aq)|Pt(s)

emf and Work The driving force (the emf) is defined in terms of a potential difference (in volts) between two points in the circuit. A volt represents a joule of work per coulomb of charge transferred: 1 joule of work is produced or required (depending on the direction) when 1 coulomb of charge is transferred between two points in the circuit that differ by a potential of 1 volt.

Free Energy and Cell Potential G = nFE n = number of moles of electrons F = Faraday = 96,485 coulombs per mole of electrons This equation states that the maximum cell potential is directly related to the free energy difference between the reactants and the products in the cell. A positive Ecell value corresponds to a negative G value, which is the condition for spontaneity.

Concentration Cell Cell potential depends on concentration. Concentration Cell is a cell in which both compartments have the same components but at different concentrations. The difference in concentration is the only factor that produces a cell potential and the voltages are typically small.

Figure 17.9 A Concentration Cell That Contains a Sliver Electrode and Aqueous Silver Nitrate in Both Compartments

The Nernst Equation G = Go + RT ln(Q) G = -nFE and Go = -nFEo We can calculate the potential of a cell in which some or all of the components are not in their standard states. G = Go + RT ln(Q) G = -nFE and Go = -nFEo -nFE = -nFEo + RT ln(Q) Dividing each side by –nF E = Eo – (RT / nF)ln(Q)

Ion Selective Electrodes Electrodes that are sensitive to the concentration of a particular ion are called ion selective electrodes. Example: Glass electrode for pH measurement. Glass electrodes can be made sensitive to such ions as Na+, K+, or NH4+ by changing the composition of the glass. The pH meter has a standard electrode of known potential, a glass electrode that changes potential depending on the concentration of H+ ions, and a potentiometer that measures the potential between the electrodes and electronically converted to pH of the solution.

Figure 17.12 A Glass Electrode for Measuring pH

Calculation of Equilibrium Constants for Redox Reactions For a cell at equilibrium, Ecell = 0 and Q = K. Nernst equation at 25oC, E = Eo – (0.0591/n)log(Q) O = Eo – (0.0591/n)log(K)

Batteries A battery is a galvanic cell or, more commonly, a group of galvanic cells connected in series, where the potentials of the individual cells add to give the total battery potential. Batteries are a source of direct current and have become an essential source of portable power in our society.

Figure 17.13 One of the Six Cells in Storage Battery a 12-V Lead Storage Battery

Figure 17.14 A Common Dry Cell Battery

Fuel Cells . . . galvanic cells for which the reactants are continuously supplied. 2H2(g) + O2(g)  2H2O(l) anode: 2H2 + 4OH  4H2O + 4e cathode: 4e + O2 + 2H2O  4OH

Figure 17.16 Schematic of the Hydrogen-Oxygen Fuel Cell

Corrosion Corrosion involves oxidation of the metal. Since corroded metal loses its structural integrity and attractiveness, this spontaneous process has great economic impact. Some metals, such as copper, gold, silver and platinum, are relatively difficult to oxidize. These are often called noble metals.

Figure 17.17 The Electrochemical Corrosion of Iron

Electrolysis The process of electrolysis involves forcing a current through a cell to produce a chemical change for which the cell potential is negative. An electrolytic cell uses electrical energy to produce chemical change. Electron flows in the opposite direction and anode and cathode are also reversed between galvanic cell and electrolytic cell.

Figure 17.19 (a) A Standard Galvanic Cell (b) A Standard Electrolytic Cell

Stoichiometry of Electrolysis How much chemical change occurs with the flow of a given current for a specified time? current and time  quantity of charge  moles of electrons  moles of analyte  grams of analyte