Electrochemistry The study of the interchange of chemical and electrical energy.

Electrochemical Reactions All electrochemical reactions involve the transfer of electrons and are therefore, oxidation- reduction reaction. All electrochemical reactions involve the transfer of electrons and are therefore, oxidation- reduction reaction. Electrons are transferred from the reducing agent to the oxidizing agent. Electrons are transferred from the reducing agent to the oxidizing agent. Oxidation is a loss of electrons (increase in oxidation number)-”OIL” Oxidation is a loss of electrons (increase in oxidation number)-”OIL” Reduction is a gain of electrons (decrease in oxidation number)-”RIG” Reduction is a gain of electrons (decrease in oxidation number)-”RIG”

Types of Electrochemical Cells Galvanic or Voltaic Cells-those in which a spontaneous chemical reaction produces an electrical current that can be used to do work. Galvanic or Voltaic Cells-those in which a spontaneous chemical reaction produces an electrical current that can be used to do work. Electrolytic Cells-those in which electrical energy from an outside source causes a nonspontaneous reaction to occur. Electrolytic Cells-those in which electrical energy from an outside source causes a nonspontaneous reaction to occur.

Components of a Galvanic Cell Cell-the reacting system Cell-the reacting system Electrodes-surfaces where the electric current exits or enters Electrodes-surfaces where the electric current exits or enters 1) anode- electrode compartment in which oxidation occurs. “AN OX” 2) cathode-electrode compartment in which reduction occurs. “RED CAT” Salt Bridge- U-tube filled with an electrolyte or a porous disk in a tube connecting the two solutions. Salt Bridge- U-tube filled with an electrolyte or a porous disk in a tube connecting the two solutions. Wire-path by which the electrons flow from one compartment to the other. Wire-path by which the electrons flow from one compartment to the other. Electrons flow through the wire from the reducing agent to the oxidizing agent (from the anode to the cathode) Electrons flow through the wire from the reducing agent to the oxidizing agent (from the anode to the cathode)

Diagram of Galvanic Cell

Cell Potential Cell potential (E cell ) or electromotive force (emf) is the “pull” or driving force on the electrons. Cell potential (E cell ) or electromotive force (emf) is the “pull” or driving force on the electrons. The unit of electrical potential is the volt (V) which is defined as 1 joule/coulomb. The unit of electrical potential is the volt (V) which is defined as 1 joule/coulomb. Cell potential is measured with a voltmeter. Cell potential is measured with a voltmeter.

Standard Reduction Potentials Reactions in galvanic cells are broken down into half- reactions with each being assigned a reduction potential. Reactions in galvanic cells are broken down into half- reactions with each being assigned a reduction potential. All half reactions are assigned reduction potentials using the standard hydrogen electrode as the reference. (see page 796) All half reactions are assigned reduction potentials using the standard hydrogen electrode as the reference. (see page 796) The potentials are all given as reduction processes. The potentials are all given as reduction processes. If the process must be reversed (oxidation process), the sign for the potential is reversed. If the process must be reversed (oxidation process), the sign for the potential is reversed. Since reduction potential is an intensive process (doesn’t depend on the how many times the reaction occurs), the value of the reduction potential is not changed when a half-reaction is multiplied by an integer to balance an equation. Since reduction potential is an intensive process (doesn’t depend on the how many times the reaction occurs), the value of the reduction potential is not changed when a half-reaction is multiplied by an integer to balance an equation.

Standard Reduction Potentials (continued) The more positive the E o value for a half- reaction, the greater tendency for the half- reaction to occur. The more positive the E o value for a half- reaction, the greater tendency for the half- reaction to occur. The more negative the E o value for a half- reaction, the greater tendency for the half- reaction to occur in the opposite direction. The more negative the E o value for a half- reaction, the greater tendency for the half- reaction to occur in the opposite direction. If E o cell > 0 (positive), the forward reaction is spontaneous. If E o cell > 0 (positive), the forward reaction is spontaneous. If E o cell < 0 (negative), the forward reaction is not spontaneous and would have to be carried out in an electrolytic cell. If E o cell < 0 (negative), the forward reaction is not spontaneous and would have to be carried out in an electrolytic cell.

Complete the practice problems on page 797. A V A V B V B V

Zinc-Copper Galvanic Cell

The Zinc-Copper Cell Example: Example: Zn(s) + Cu 2+ (aq)  Zn 2+ (aq) + Cu(s) Anode: Zn  Zn e - Anode: Zn  Zn e - Cathode: Cu e -  Cu Cathode: Cu e -  Cu E o cell = = 1.10 V E o cell = = 1.10 V Line Notation: Line Notation: Zn | Zn 2+ | | Cu 2+ | Cu (anode is written on the left side and the vertical line represents a phase difference or boundary)

Write the line notation for a galvanic cell consisting of copper (II) and chromium (III) Cr e -  Cr E o = V Cr e -  Cr E o = V Cu e -  Cu E o = V Cu e -  Cu E o = V Copper reduction occurs at the cathode Copper reduction occurs at the cathode Chromium oxidation occurs at the anode. Chromium oxidation occurs at the anode. Line Notation: Line Notation: Cr | Cr 3+ | | Cu 2+ | Cu

Cell Potential, Work, and Free Energy The work that can be accomplished when electrons are transferred through a wire depends on the “push” behind the electrons. The work that can be accomplished when electrons are transferred through a wire depends on the “push” behind the electrons. Potential difference (V) = work (J)/charge (C) Potential difference (V) = work (J)/charge (C) E = -w/q E = -w/q Work is viewed from the point of view of the system. (Work flowing out of a system is indicated by a minus sign). Work is viewed from the point of view of the system. (Work flowing out of a system is indicated by a minus sign). In any real, spontaneous process some energy is wasted due to frictional heating-the actual work realized is always less than the calculated maximum. In any real, spontaneous process some energy is wasted due to frictional heating-the actual work realized is always less than the calculated maximum.

Electrical Charge The charge on one mole of electrons is a constant called the faraday (F), which has the value 96,485 coulombs of charge per mole of electrons. The charge on one mole of electrons is a constant called the faraday (F), which has the value 96,485 coulombs of charge per mole of electrons. q = nF (n is the number of moles of e - ) q = nF (n is the number of moles of e - ) w (∆G) = -nFE max w (∆G) = -nFE max Solve example 17-3 on page 802. Solve example 17-3 on page 802.

The Nernst Equation The Nernst Equation is used to calculate electrode potential and cell potentials for concentrations and partial pressures other than standard-state values. The Nernst Equation is used to calculate electrode potential and cell potentials for concentrations and partial pressures other than standard-state values. E = E o – (2.303 RT/nF ) log Q E = E o – (2.303 RT/nF ) log Q E = potential under nonstandard conditions E = potential under nonstandard conditions E o = standard potential E o = standard potential R= R= T = temp in Kelvin T = temp in Kelvin n= number of moles of electrons transferred n= number of moles of electrons transferred F = 96,485 C/mole of e - F = 96,485 C/mole of e - Q=reaction quotient Q=reaction quotient

Electrolytic Cells In an electrolytic cell, an outside source of voltage is used to force a nonspontaneous redox reaction to take place. In an electrolytic cell, an outside source of voltage is used to force a nonspontaneous redox reaction to take place. Oxidation takes place at the anode and reduction takes place at the cathode just as it does in a galvanic cell. Oxidation takes place at the anode and reduction takes place at the cathode just as it does in a galvanic cell. The cell potential in an electrolytic cell < 0. The cell potential in an electrolytic cell < 0. Electrolytic cells are used in electroplating. Electrolytic cells are used in electroplating.

Electrolysis Problems I = q/t I = q/t I = current (amperes, A) I = current (amperes, A) 1 amp = 1C/sec 1 amp = 1C/sec q = charge (coulombs, C) q = charge (coulombs, C) t = time (sec) t = time (sec) Once the charge is known, solve the problem as a stoichiometry problem. Once the charge is known, solve the problem as a stoichiometry problem.

Practice Problem #1 How long must a current of 5.00 A be applied to a solution of Ag + to produce 10.5 g of silver metal? How long must a current of 5.00 A be applied to a solution of Ag + to produce 10.5 g of silver metal?

Practice Problem #2 What mass of Co can be produced from aqueous Co 2+ in 1 hour with a current of 15 A? What mass of Co can be produced from aqueous Co 2+ in 1 hour with a current of 15 A?

Practice Problem #3 A zinc-copper battery is constructed as follows at 25 o C: A zinc-copper battery is constructed as follows at 25 o C: Zn Zn2+ (0.1M) Cu2+ (2.50M) Cu Zn Zn2+ (0.1M) Cu2+ (2.50M) Cu The mass of each electrode is 200g. a. Calculate the cell potential when this battery is first connected. b. Calculate the cell potential after 10.0A of current has flowed for 10.0 h. c. Calculate the mass of each electrode after 10.0 h d. How long can this battery deliver a current of 10.0A before it goes dead?