standard electrode potential

Examples of standard electrode potential in the following topics:

• Predicting Spontaneous Direction of a Redox Reaction

  • To figure this out, it is important to consider the standard electrode potential, which is a measure of the driving force behind a reaction.
  • The sign of the standard electrode potential indicates in which direction the reaction must proceed in order to achieve equilibrium.
  • What happens to the standard electrode potential when the reaction is written in the reverse direction?
  • Neither the relative strengths of the oxidizing or reducing agents nor the magnitude of the potential will change.
  • However, what will change is the sign of the standard electrode potential.

• Free Energy and Cell Potential

  • Electricity is generated due to the electric potential difference between two electrodes.
  • In electrochemistry, the standard electrode potential, abbreviated E°, is the measure of the individual potential of a reversible electrode at standard state, which is with solutes at an effective concentration of 1 M, and gases at a pressure of 1 atm.
  • Since the standard electrode potentials are given in their ability to be reduced, the bigger the standard reduction potentials, the easier they are to be reduced; in other words, they are simply better oxidizing agents.
  • For example, F2 has a potential of 2.87 V and Li+ has a potential of -3.05 V.
  • In the example of Zn2+, whose standard reduction potential is -0.76 V, it can be oxidized by any other electrode whose standard reduction potential is greater than -0.76 V and can be reduced by any electrode with standard reduction potential less than -0.76 V.

• Electrolytic Properties

  • In other systems, the electrode reactions can involve electrode metal as well as electrolyte ions.
  • In order to determine which species in solution will be oxidized and which will be reduced, the standard electrode potential of each species may be obtained from a table of standard reduction potentials, a small sampling of which is shown here:
  • Historically, oxidation potentials were tabulated and used in calculations, but the current standard is to only record the reduction potential in tables.
  • This is the standard reduction potential for the reaction shown, measured in volts.
  • Use a table of standard reduction potentials to determine which species in solution will be reduced or oxidized.

• Standard Reduction Potentials

  • Standard reduction potentials provide a systematic measurement for different molecules' tendency to be reduced.
  • The standard reduction potential is defined relative to a standard hydrogen electrode (SHE) reference electrode, which is arbitrarily given a potential of 0.00 volts.
  • The values below in parentheses are standard reduction potentials for half-reactions measured at 25 °C, 1 atmosphere, and with a pH of 7 in aqueous solution.
  • Historically, many countries, including the United States and Canada, used standard oxidation potentials rather than reduction potentials in their calculations.
  • These are simply the negative of standard reduction potentials, so it is not a difficult conversion in practice.

• Electrochemical Cell Notation

  • Recall that standard cell potentials can be calculated from potentials E0cell for both oxidation and reduction reactions.
  • A positive cell potential indicates that the reaction proceeds spontaneously in the direction in which the reaction is written.
  • Conversely, a reaction with a negative cell potential proceeds spontaneously in the reverse direction.
  • One beaker contains 0.15 M Cd(NO3)2 and a Cd metal electrode.
  • The other beaker contains 0.20 M AgNO3 and a Ag metal electrode.

• Predicting if a Metal Will Dissolve in Acid

  • A metal is soluble in acid if it displaces H2 from solution, which is determined by the metal's standard reduction potential.
  • The sign of this potential difference depends on the direction (oxidation or reduction) in which the electrode reaction proceeds.
  • Therefore, the half-cell potential for the Zn/Zn2+ electrode always refers to the reduction reaction:
  • These values can be determined using standard reduction potentials, which can often be looked up.
  • Using the standard reduction potentials of a reaction, one can determine how likely a given metal is to accept or donate electrons.

• Other Rechargeable Batteries

  • NiMH batteries use positive electrodes of nickel oxyhydroxide (NiOOH), as does the NiCd, but the negative electrodes use a hydrogen-absorbing alloy instead of cadmium.
  • The lithium-ion battery is a family of rechargeable batteries in which lithium ions move from the negative electrode to the positive electrode during discharge, and back when charging.
  • The negative electrode of a conventional lithium-ion cell is made from carbon.
  • The positive electrode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent.
  • The advantages of LiPo over the lithium-ion design include potentially lower cost of manufacture, adaptability to a wide variety of packaging shapes, reliability, and ruggedness.

• Dry Cell Battery

  • A fibrous fabric separates the two electrodes, and a brass pin in the center of the cell conducts electricity to the outside circuit.
  • In some more modern types of so-called "high-power" batteries that have a much lower capacity than standard alkaline batteries, the ammonium chloride is replaced by zinc chloride.

• Cathode Rays

  • If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite the negative electrode is observed to glow from electrons emitted from the cathode.
  • The early cold cathode vacuum tubes, called Crookes tubes, used a high electrical potential between the anode and the cathode to ionize the residual gas in the tube.
  • The voltage applied between the electrodes accelerates these low mass particles to high velocities.
  • In 1838, Michael Faraday passed a current through a rarefied air-filled glass tube and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end almost at the anode (positive electrode).
  • Connect the two electrodes to a high voltage source and see them produce cathode rays.

• Thermodynamics of Redox Reactions

  • The thermodynamics of redox reactions can be determined using their standard reduction potentials and the Nernst equation.
  • The Nernst equation allows the reduction potential to be calculated at any temperature and concentration of reactants and products; the standard reaction potential must be measured at 298K and with each solution at 1M.
  • This equation allows the equilibrium constant to be calculated just from the standard reduction potential and the number of electrons transferred in the reaction.
  • The relationship between the Gibbs free energy change and the standard reaction potential is:
  • Translate between the equilibrium constant/reaction quotient, the standard reduction potential, and the Gibbs free energy change for a given redox reaction