With three electrons consumed in the reduction and two produced in the oxidation, the overall reaction is not balanced. Recall, however, that standard potentials are independent of stoichiometry. When the compartments are connected, a potential of 3.
- Standard Electrode Potentials!
- Quality of Life and Work in Europe: Theory, Practice and Policy.
- Immota Manent.
We can use this procedure described to measure the standard potentials for a wide variety of chemical substances, some of which are listed in Table P2. These data allow us to compare the oxidative and reductive strengths of a variety of substances. The strongest oxidant in the table is F 2 , with a standard electrode potential of 2.
This high value is consistent with the high electronegativity of fluorine and tells us that fluorine has a stronger tendency to accept electrons it is a stronger oxidant than any other element. This fact might be surprising because cesium, not lithium, is the least electronegative element. The apparent anomaly can be explained by the fact that electrode potentials are measured in aqueous solution, where intermolecular interactions are important, whereas ionization potentials and electron affinities are measured in the gas phase.
Lithium metal is therefore the strongest reductant most easily oxidized of the alkali metals in aqueous solution. Species that lie below H 2 are stronger oxidizing agents. Any species on the left side of a half-reaction will spontaneously oxidize any species on the right side of another half-reaction that lies below it in the table. Conversely, any species on the right side of a half-reaction will spontaneously reduce any species on the left side of another half-reaction that lies above it in the table.
The black tarnish that forms on silver objects is primarily Ag 2 S. The half-reaction for reversing the tarnishing process is as follows:. Given: reduction half-reaction, standard electrode potential, and list of possible reductants. Asked for: reductants for Ag 2 S, strongest reductant, and potential reducing agent for removing tarnish. Determine which species is the strongest reductant.
Given: redox reaction and list of standard electrode potentials Table P2. B Adding the two half-reactions gives the overall reaction:. The standard cell potential is quite negative, so the reaction will not occur spontaneously as written. B The two half-reactions and their corresponding potentials are as follows. The standard potential for the reaction is positive, indicating that under standard conditions, it will occur spontaneously as written.
Hydrogen peroxide will reduce MnO 2 , and oxygen gas will evolve from the solution. To answer these questions requires a more quantitative understanding of the relationship between electrochemical cell potential and chemical thermodynamics. When using a galvanic cell to measure the concentration of a substance, we are generally interested in the potential of only one of the electrodes of the cell, the so-called indicator electrode , whose potential is related to the concentration of the substance being measured.
How do you calculate electrochemical cell potential?
To ensure that any change in the measured potential of the cell is due to only the substance being analyzed, the potential of the other electrode, the reference electrode , must be constant. You are already familiar with one example of a reference electrode: the SHE. The potential of a reference electrode must be unaffected by the properties of the solution, and if possible, it should be physically isolated from the solution of interest.
To measure the potential of a solution, we select a reference electrode and an appropriate indicator electrode. Whether reduction or oxidation of the substance being analyzed occurs depends on the potential of the half-reaction for the substance of interest the sample and the potential of the reference electrode. The potential of any reference electrode should not be affected by the properties of the solution to be analyzed, and it should also be physically isolated. There are many possible choices of reference electrode other than the SHE.
The SHE requires a constant flow of highly flammable hydrogen gas, which makes it inconvenient to use. Consequently, two other electrodes are commonly chosen as reference electrodes.
10.3: Half Cells and Standard Reduction Potentials
One is the silver—silver chloride electrode , which consists of a silver wire coated with a very thin layer of AgCl that is dipped into a chloride ion solution with a fixed concentration. The cell diagram and reduction half-reaction are as follows:. If a saturated solution of KCl is used as the chloride solution, the potential of the silver—silver chloride electrode is 0.
That is, 0. A second common reference electrode is the saturated calomel electrode SCE , which has the same general form as the silver—silver chloride electrode. The SCE consists of a platinum wire inserted into a moist paste of liquid mercury Hg 2 Cl 2 ; called calomel in the old chemical literature and KCl.
Although it sounds and looks complex, this cell is actually easy to prepare and maintain, and its potential is highly reproducible. The SCE cell diagram and corresponding half-reaction are as follows:. The interior cell is surrounded by an aqueous KCl solution, which acts as a salt bridge between the interior cell and the exterior solution.
- Navigation menu.
- The Elkhart Truth?
- Table of Standard Electrode Potentials.
- Ethics (Wicca 101 - Lecture Series)!
- Key Concepts!
- Standard Reduction Potentials.
- Standard cell potentials.
The glass membrane absorbs protons, which affects the measured potential. Although it can be measured, in practice, a glass electrode is calibrated; that is, it is inserted into a solution of known pH, and the display on the pH meter is adjusted to the known value. Once the electrode is properly calibrated, it can be placed in a solution and used to determine an unknown pH. These electrodes usually contain an internal reference electrode that is connected by a solution of an electrolyte to a crystalline inorganic material or a membrane, which acts as the sensor.
When fluoride ions in solution diffuse to the surface of the solid, the potential of the electrode changes, resulting in a so-called fluoride electrode. Similar electrodes are used to measure the concentrations of other species in solution. The flow of electrons in an electrochemical cell depends on the identity of the reacting substances, the difference in the potential energy of their valence electrons, and their concentrations. Only the difference between the potentials of two electrodes can be measured.
By convention, all tabulated values of standard electrode potentials are listed as standard reduction potentials. The potential of the standard hydrogen electrode SHE is defined as 0 V under standard conditions. The potential of a half-reaction measured against the SHE under standard conditions is called its standard electrode potential.
The standard cell potential is a measure of the driving force for a given redox reaction. So E o for the cell will be given by. A typical galvanic electrochemical cell can be constructed similar to what is shown in the diagram above. The electrons flow from the anode the electron source to the cathode the electron sink. The salt bridge allows for the flow of ions to complete the circuit while minimizing the introduction of a junction potential.
Measuring Standard Electrode Potentials
Patrick E. Electrochemical Cells Standard reduction potentials can be measured relative to the convention of setting the reduction potential of the Standard Hydrogen Electrode SHE to zero. Calculating Cell Potentials Using values measured relative to the SHE, it is fairly easy to calculate the standard cell potential of a given reaction.
Measuring the Voltage A typical galvanic electrochemical cell can be constructed similar to what is shown in the diagram above. The standard hydrogen electrode is assigned a potential of zero volts. The second hurdle is overcome by choosing standard thermodynamic conditions for the measurement of the potentials.
The standard cell potential is denoted by a degree sign as a superscript.
The values for the table entries are reduction potentials, so lithium at the top of the list has the most negative number, indicating that it is the strongest reducing agent. The strongest oxidizing agent is fluorine with the largest positive number for standard electrode potential. The link below takes you to a more extensive table. When an electrochemical cell is arranged with the two half-reactions separated but connected by an electrically conducting path, a voltaic cell is created.
Calculate the standard cell potential of a Al/Al 3+ |
The maximum voltage which can be produced between the poles of the cell is determined by the standard electrode potentials under the standard conditions under which those potentials are defined. Consider the historic Daniell cell in which zinc and copper were used as electrodes. The data from the table of standard electrode potentials is. Since the tabulated standard electrode potentials are reduction potentials, the one which is most negative will need to be reversed in sign to get its oxidation potential. When that is done, it is clear that the theoretical standard cell potential for the zinc-copper cell is 1.
In general, a real voltaic cell will differ from the standard conditions, so we need to be able to adjust the calculated cell potential to account for the differences. This can be done with the application of the Nernst equation. Presuming that there are two metal electrodes with their ions, the standard potential for a cell with anode potential volts and cathode potential volts is.
By numerically exploring the above calculation, you can confirm that large concentration differences between the anions and the cations can cause the voltage to differ significantly from the standard cell potential, and that such large concentration differences also make the cell potential more temperature dependent.
Related Determination of Standard Reduction Potential of Half-reaction
Copyright 2019 - All Right Reserved