Electrodes 101 - Voltage and current in galvanic cells and electrolysis cells
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Lauth, Jakob Günter (SciFox)
(PhysChemBasics#12) The combination of an electronic conductor with an eletrolytic conductor is referred to as an electrode. An electrode may be quantitatively described by the electrode reaction and by the electrode potential. The electrode potential depends on the concentration, which can be described according to Nernst. The amount of material converted at an electrode is described by Faraday's law. We may combine 2 electrodes to build up a galvanic cell. The open circuit voltage or EMF of these cells corresponds to the difference between the electrode potentials.
 Batteries convert chemical energy into electrical energy. But whether you find voltage and current in a chemical reaction, to answer these questions we need to take a closer look at electrodes. Welcome to FISCAM Basics, our topic today electrodes. How much energy can be stored in a battery? How can we calculate current and voltage in electrochemical reactions? If we set up an electrical circuit which includes electrolyte, charge has to be transferred from the electronic conductor, the metal, to the electrolytic conductor. Electrons have to move from the metal into the electrolyte on one terminal. We call the electrode on this terminal the cathode. The transfer of electrons from the electronic conductor into the electrolytic conductor, or vice versa, is called the electrode reaction or rather half reaction. We always express this electrode reaction in the way that the oxidized species and the electrons are on the left hand side and the reduced species on the right hand side. With a cathode current, the electrode reaction proceeds from left to right as a reduction. Therefore reduction is what occurs on a cathode, easy to remember, red cat. In an electrolysis cell the cathodic reaction is forced, thus the cathode has a more negative potential, the cathode is the negative terminal. In the case of a battery, or generally a galvanic cell, on the other hand the cathodic reaction takes place voluntarily. Hence the cathode is the positive terminal. On the other terminal of the circuit, electrons have to move from the electrolyte into the metal. The electrode reaction runs from right to left and this terminal is called the anode. Thus at the anode there is oxidation, easy to remember, and ox. In electrolytic cells the anode reaction is forced, for example the oxidation of chloride to chlorine. The anode is therefore the positive terminal of an electrolytic cell. In galvanic cells however the anode reaction takes place voluntarily. Here the anode is the negative terminal, for example the zinc electrode in the zinc carbon battery. As for any other chemical reaction the laws of stoichiometry apply to the electrode reaction. Therefore we can do stoichiometric calculations using the total conducted charge. With this consideration Michael Farrowday was able to formulate his laws of electrolysis. The amount of material produced or consumed during an electrochemical reaction is directly proportional to the average current multiplied by the total time n equals i times t over nu e times f. If a current of one amp passes through several electrolytic cells for one day, 0.9 moles of electrons will flow. These can produce 0.45 moles of copper from copper chloride or 0.9 moles of silver from silver nitride electrolyte. Farrowday's law applies equally to anode and cathode. It applies to electrolytic and galvanic cells. The electrode reaction which takes place at the metal electrolyte phase boundary can be formulated as an equilibrium reaction. Copper is a noble metal so with the copper electrode the equilibrium lies to the right. When copper metal is immersed into a copper chloride solution at first the cathodic reaction will be predominant and copper ions will be consumed. So after having reached equilibrium the copper will be positively charged and the electrolyte is negatively charged. As in any dynamic equilibrium forward and reverse reaction anode and cathode current will be equal. The difference in electrical potential between metal and electrolyte in equilibrium is called the individual electrode potential E subredox. In standard state the potential of the copper electrode is 0.34 volts. If we were to repeat the immersion experiment with zinc metal and zinc chloride solution we would get a negative electrode potential. Zinc is easily oxidized the equilibrium of the electrode reaction lies to the left. The potential of an electrode depends both on the species involved and in the potential of an electrode depends both on the species involved in the electrode reaction and on their concentrations. In a one-olus solution of copper chloride the electron potential of copper will be 0.34 volts for a 0.1 molar solution it is only 0.31 volts. The concentration dependence of the electrode potential can be calculated using the Nernst equation. The Nernst equation for an individual electrode consists of the individual standard potential E naught and a concentration dependent term. The standard potential E naught are tabulated in the electrochemical series. Note that each and every parameter that occurs in the electrode reaction also does appear in the Nernst equation. The number of electrons converted, NU sub E can be found in the Nernst factor RT over NU sub EF. The concentrations of all oxidized species can be found in the numerator and the concentration of all reduced species can be found in the denominator in the quotient in the argument of the logarithm. The electrochemical series is an arrangement of redox epilebial in order of their standard electrode potentials or redox potentials. At the top of the list the redox half reactions with the highest electron affinity, the highest hunger for electrons can be found, for example the noble metals. At the bottom redox reactions that easily donate electrons like zinc and magnesium are listed. So the redox half reactions at the top are strongly oxidizing, the half reactions on the bottom are strongly reducing. If you combine two half reactions from the electrochemical series the electrons will only flow voluntarily from bottom to top. Let's set up the Nernst equation for a copper electrode. Let's formulate the electrode reaction first. In the electrochemical series we will find the corresponding E0 in the Nernst factor. The number of electrons NU sub E is 2. The numerator of the argument of the logarithm shows the concentration of the oxidized species that is copper 2 plus ions. The denominator of this fraction depicts the concentration of the reduced species that is the copper metal. According to the concentration conventions in thermodynamics solid and liquid species have to be quantified with their mole fraction. Gaseous species with their partial pressure in bar and dissolved species with the molarity. The Nernst factor RT over F times logarithm of 10 corresponds to approximately 59 millivolts. So a change in potential of about 30 millivolts of a copper electrode means that the concentration of copper ions have changed by a 10-fold. Let's discuss the oxygen electrode. Those electrodes are common in zinc abotaries or in fuel cells. In the electrochemical series we will find the electrode reaction. The standard potential of 1.23 volts tells us that we are dealing with a very strongly oxidizing agent. The Nernst factor does have to include the four exchanged electrons. In the argument of the logarithm we find the proton concentration to the power of 4 and the oxygen concentration in the numerator. In the denominator we find the water concentration squared. The proton concentration must be given in moles per liter. The oxygen concentration is simply the partial pressure in bar. For water the mole fraction would have to be used but this can be set to 1 in good approximation. The potential of the oxygen electrode is thus strongly affected by pH. A change of one unit on the pH scale results in a potential change of 59 millivolts. We can combine any two electrodes to a galvanic cell or a battery. A classical battery is the Daniel's cell. It consists of a copper and a zinc electrode. We formulate the electrode reactions of both electrodes and calculate their potential according to the Nernst equation. The electrode with the lower potential is the anode here. The oxidation takes place, anox. The electrode with the higher potential is the cathode. Here the reduction takes place, red cat. The open circuit voltage of the galvanic element can be calculated as a difference between the individual electrode potential E sub cathode minus E sub anode. So for the standard Daniel's cell we get an open circuit voltage, OCV for short, or EMF, electron-motive force, of 0.34 minus negative 0.76 equals 1.1 volts. We may as well combine zinc and copper ions without a galvanic cell. With this reaction taking place spontaneously the heat release corresponds to the reaction enthalpy delta H, in this case negative 270 kJ. As with any spontaneous reaction no useful work is involved. If the same reaction takes place in a Daniel's cell oxidation and reduction are spatially separated. Electrons have to flow through an external circuit and electrical work can be obtained. Ideally, in so-called reversible operation, we get a maximum of negative 212 kJ of useful work and this corresponds to Gibbs free energy delta G. There's also heat involved in the reversible process. There's also heat involved in the reversible process. It is only slightly exothermic, releasing negative 6 kJ. We may calculate this heat using entropy of reaction and temperature. The ratio between free energy delta G and total energy delta H is called the efficiency of the galvanic cell. While the entire energy or other enthalpy is released as heat with spontaneous processes, we are able to obtain a very large fraction of this enthalpy as useful work with reversible processes. As just discussed at the boundary of an electronic conductor and an electrolytic conductor, an electrical potential occurs, the electrode potential. The combination of two electrolytic conductors may also result in an electrical potential. Consider two solutions with different pH values that are separated by a membrane. The membrane is permeable to protons. Brotons will move from the more concentrated solution through the membrane into the more valued solution and charge this side positively. We end up with a so-called membrane potential, which can be calculated with an equation similar to the Nernst equation. Membrane potentials are important in biochemistry and in electrochemical and analytical devices like glass electrodes. Let's summarize. The combination of an electronic conductor with an electrolytic conductor is referred to as an electrode. An electrode may be quantitatively described by the electrode reaction and by the electrode potential. The electrode potential depends on the concentration, which can be described according to Nernst. The amount of material converted at an electrode is described by Faraday's law. We may combine two electrodes to build up a galvanic cell. The open circuit voltage or EMF of these cells correspond to the difference between the electrode potentials. More information about the topic will be found in the book and in the lecture. Thanks for watching.
 Batteries convert chemical energy into electrical energy.  But whether you find voltage and current in a chemical reaction,  to answer these questions we need to take a closer look at electrodes.  Welcome to FISCAM Basics, our topic today electrodes. How much energy can be stored in a  battery? How can we calculate current and voltage in electrochemical reactions?  If we set up an electrical circuit which includes electrolyte, charge has to be transferred from  the electronic conductor, the metal, to the electrolytic conductor. Electrons have to move  from the metal into the electrolyte on one terminal. We call the electrode on this terminal the cathode.  The transfer of electrons from the electronic conductor into the electrolytic conductor,  or vice versa, is called the electrode reaction or rather half reaction.  We always express this electrode reaction in the way that the oxidized species and the electrons are  on the left hand side and the reduced species on the right hand side. With a cathode current,  the electrode reaction proceeds from left to right as a reduction. Therefore reduction is what  occurs on a cathode, easy to remember, red cat. In an electrolysis cell the cathodic reaction is  forced, thus the cathode has a more negative potential, the cathode is the negative terminal.  In the case of a battery, or generally a galvanic cell, on the other hand the cathodic  reaction takes place voluntarily. Hence the cathode is the positive terminal. On the other terminal  of the circuit, electrons have to move from the electrolyte into the metal. The electrode reaction  runs from right to left and this terminal is called the anode. Thus at the anode there is  oxidation, easy to remember, and ox. In electrolytic cells the anode reaction is forced, for example the  oxidation of chloride to chlorine. The anode is therefore the positive terminal of an electrolytic cell.  In galvanic cells however the anode reaction takes place voluntarily. Here the anode is the  negative terminal, for example the zinc electrode in the zinc carbon battery. As for any other chemical  reaction the laws of stoichiometry apply to the electrode reaction. Therefore we can do stoichiometric  calculations using the total conducted charge. With this consideration Michael Farrowday was able  to formulate his laws of electrolysis. The amount of material produced or consumed during an  electrochemical reaction is directly proportional to the average current multiplied by the total time  n equals i times t over nu e times f. If a current of one amp passes through several  electrolytic cells for one day, 0.9 moles of electrons will flow. These can produce 0.45 moles  of copper from copper chloride or 0.9 moles of silver from silver nitride electrolyte. Farrowday's  law applies equally to anode and cathode. It applies to electrolytic and galvanic cells.  The electrode reaction which takes place at the metal electrolyte phase boundary can be formulated  as an equilibrium reaction. Copper is a noble metal so with the copper electrode the equilibrium lies  to the right. When copper metal is immersed into a copper chloride solution at first the cathodic  reaction will be predominant and copper ions will be consumed. So after having reached equilibrium  the copper will be positively charged and the electrolyte is negatively charged. As in any  dynamic equilibrium forward and reverse reaction anode and cathode current will be equal. The  difference in electrical potential between metal and electrolyte in equilibrium is called the individual  electrode potential E subredox. In standard state the potential of the copper electrode is 0.34 volts.  If we were to repeat the immersion experiment with zinc metal and zinc chloride solution  we would get a negative electrode potential. Zinc is easily oxidized the equilibrium of the  electrode reaction lies to the left. The potential of an electrode depends both on the species involved  and in the potential of an electrode depends both on the species involved in the electrode reaction  and on their concentrations. In a one-olus solution of copper chloride the electron potential of  copper will be 0.34 volts for a 0.1 molar solution it is only 0.31 volts. The concentration dependence  of the electrode potential can be calculated using the Nernst equation. The Nernst equation for an  individual electrode consists of the individual standard potential E naught and a concentration  dependent term. The standard potential E naught are tabulated in the electrochemical series.  Note that each and every parameter that occurs in the electrode reaction  also does appear in the Nernst equation. The number of electrons converted,  NU sub E can be found in the Nernst factor RT over NU sub EF. The concentrations of all  oxidized species can be found in the numerator and the concentration of all reduced species  can be found in the denominator in the quotient in the argument of the logarithm.  The electrochemical series is an arrangement of redox epilebial in order of their standard  electrode potentials or redox potentials. At the top of the list the redox half reactions  with the highest electron affinity, the highest hunger for electrons can be found, for example the  noble metals. At the bottom redox reactions that easily donate electrons like zinc and magnesium  are listed. So the redox half reactions at the top are strongly oxidizing, the half reactions on  the bottom are strongly reducing. If you combine two half reactions from the electrochemical series  the electrons will only flow voluntarily from bottom to top.  Let's set up the Nernst equation for a copper electrode. Let's formulate the electrode reaction  first. In the electrochemical series we will find the corresponding E0 in the Nernst factor.  The number of electrons NU sub E is 2. The numerator of the argument of the logarithm  shows the concentration of the oxidized species that is copper 2 plus ions. The denominator of  this fraction depicts the concentration of the reduced species that is the copper metal.  According to the concentration conventions in thermodynamics solid and liquid species have  to be quantified with their mole fraction. Gaseous species with their partial pressure in  bar and dissolved species with the molarity. The Nernst factor RT over F times logarithm of 10  corresponds to approximately 59 millivolts. So a change in potential of about 30 millivolts  of a copper electrode means that the concentration of copper ions have changed by a 10-fold.  Let's discuss the oxygen electrode. Those electrodes are common in zinc  abotaries or in fuel cells. In the electrochemical series we will find the electrode reaction.  The standard potential of 1.23 volts tells us that we are dealing with a very strongly oxidizing agent.  The Nernst factor does have to include the four exchanged electrons. In the argument of the logarithm  we find the proton concentration to the power of 4 and the oxygen concentration in the numerator.  In the denominator we find the water concentration squared. The proton concentration must be given  in moles per liter. The oxygen concentration is simply the partial pressure in bar. For water  the mole fraction would have to be used but this can be set to 1 in good approximation.  The potential of the oxygen electrode is thus strongly affected by pH. A change of one unit  on the pH scale results in a potential change of 59 millivolts. We can combine any two electrodes  to a galvanic cell or a battery. A classical battery is the Daniel's cell. It consists of a  copper and a zinc electrode. We formulate the electrode reactions of both electrodes and calculate  their potential according to the Nernst equation. The electrode with the lower potential is the anode  here. The oxidation takes place, anox. The electrode with the higher potential is the cathode.  Here the reduction takes place, red cat. The open circuit voltage of the galvanic element  can be calculated as a difference between the individual electrode potential  E sub cathode minus E sub anode. So for the standard Daniel's cell we get an open circuit  voltage, OCV for short, or EMF, electron-motive force, of 0.34 minus negative 0.76 equals 1.1 volts.  We may as well combine zinc and copper ions without a galvanic cell. With this reaction  taking place spontaneously the heat release corresponds to the reaction enthalpy delta H,  in this case negative 270 kJ. As with any spontaneous reaction no useful work is involved.  If the same reaction takes place in a Daniel's cell oxidation and reduction are spatially  separated. Electrons have to flow through an external circuit and electrical work can be obtained.  Ideally, in so-called reversible operation, we get a maximum of negative 212 kJ of useful work  and this corresponds to Gibbs free energy delta G. There's also heat involved in the reversible  process. There's also heat involved in the reversible process. It is only slightly exothermic,  releasing negative 6 kJ. We may calculate this heat using entropy of reaction and temperature.  The ratio between free energy delta G and total energy delta H is called the efficiency of the  galvanic cell. While the entire energy or other enthalpy is released as heat with spontaneous  processes, we are able to obtain a very large fraction of this enthalpy as useful work with  reversible processes. As just discussed at the boundary of an electronic conductor and an  electrolytic conductor, an electrical potential occurs, the electrode potential. The combination  of two electrolytic conductors may also result in an electrical potential. Consider two solutions  with different pH values that are separated by a membrane. The membrane is permeable to protons.  Brotons will move from the more concentrated solution through the membrane into the more valued  solution and charge this side positively. We end up with a so-called membrane potential,  which can be calculated with an equation similar to the Nernst equation. Membrane potentials are  important in biochemistry and in electrochemical and analytical devices like glass electrodes.  Let's summarize. The combination of an electronic conductor with an electrolytic conductor is  referred to as an electrode. An electrode may be quantitatively described by the electrode reaction  and by the electrode potential. The electrode potential depends on the concentration,  which can be described according to Nernst. The amount of material converted at an electrode  is described by Faraday's law. We may combine two electrodes to build up a galvanic cell.  The open circuit voltage or EMF of these cells correspond to the difference between the electrode  potentials. More information about the topic will be found in the book and in the lecture.  Thanks for watching.
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