Reduction potential is measured in volts V or millivolts mV. Each species has its own intrinsic reduction potential. The standard reduction potential is defined relative to a standard hydrogen electrode SHE reference electrode, which is arbitrarily given a potential of 0. Since the reduction potential measures the intrinsic tendency for a species to undergo reduction, comparing standard reduction potential for two processes can be useful for determining how a reaction will proceed.
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. The two may be explicitly distinguished by using the symbol E 0 r for reduction and E 0 o for oxidation. Boundless vets and curates high-quality, openly licensed content from around the Internet.
This particular resource used the following sources:. Skip to main content. Search for:. Standard Reduction Potentials. Learning Objective Recall that a positive reduction potential indicates a thermodynamically favorable reaction relative to the reduction of a proton to hydrogen.
Key Points A reduction potential measures the tendency of a molecule to be reduced by taking up new electrons. The standard reduction potential is the reduction potential of a molecule under specific, standard conditions. Standard reduction potentials can be useful in determining the directionality of a reaction. The reduction potential of a given species can be considered to be the negative of the oxidation potential.
Show Sources Boundless vets and curates high-quality, openly licensed content from around the Internet. Licenses and Attributions. CC licensed content, Shared previously.In a galvanic cell, current is produced when electrons flow externally through the circuit from the anode to the cathode because of a difference in potential energy between the two electrodes in the electrochemical cell.
Hence electrons flow spontaneously from zinc to copper II ions, forming zinc II ions and metallic copper. Just like water flowing spontaneously downhill, which can be made to do work by forcing a waterwheel, the flow of electrons from a higher potential energy to a lower one can also be harnessed to perform work. Because the potential energy of valence electrons differs greatly from one substance to another, the voltage of a galvanic cell depends partly on the identity of the reacting substances.
Thus we can conclude that the difference in potential energy between the valence electrons of cobalt and zinc is less than the difference between the valence electrons of copper and zinc by 0. The measured potential of a cell also depends strongly on the concentrations of the reacting species and the temperature of the system.
To develop a scale of relative potentials that will allow us to predict the direction of an electrochemical reaction and the magnitude of the driving force for the reaction, the potentials for oxidations and reductions of different substances must be measured under comparable conditions.
Corrections for non ideal behavior are important for precise quantitative work but not for the more qualitative approach that we are taking here. Measured redox potentials depend on the potential energy of valence electrons, the concentrations of the species in the reaction, and the temperature of the system.
It is physically impossible to measure the potential of a single electrode: only the difference between the potentials of two electrodes can be measured this is analogous to measuring absolute enthalpies or free energies ; recall that only differences in enthalpy and free energy can be measured.
We can, however, compare the standard cell potentials for two different galvanic cells that have one kind of electrode in common. This allows us to measure the potential difference between two dissimilar electrodes.
This implies that the potential difference between the Co and Cu electrodes is 1. In fact, that is exactly the potential measured under standard conditions if a cell is constructed with the following cell diagram:.
All tabulated values of standard electrode potentials by convention are listed for a reaction written as a reduction, not as an oxidation, to be able to compare standard potentials for different substances Table P1.
In contrast, recall that half-reactions are written to show the reduction and oxidation reactions that actually occur in the cell, so the overall cell reaction is written as the sum of the two half-reactions. The overall cell reaction is the sum of the two half-reactions, but the cell potential is the difference between the reduction potentials:.
Although it is impossible to measure the potential of any electrode directly, we can choose a reference electrode whose potential is defined as 0 V under standard conditions. The standard hydrogen electrode SHE is universally used for this purpose and is assigned a standard potential of 0 V.
Protons are reduced or hydrogen molecules are oxidized at the Pt surface according to the following equation:. One especially attractive feature of the SHE is that the Pt metal electrode is not consumed during the reaction. When the circuit is closed, the voltmeter indicates a potential of 0. Thus the hydrogen electrode is the cathode, and the zinc electrode is the anode. The diagram for this galvanic cell is as follows:. The half-reactions that actually occur in the cell and their corresponding electrode potentials are as follows:.
The potential of a half-reaction measured against the SHE under standard conditions is called the standard electrode potential for that half-reaction. Because electrical potential is the energy needed to move a charged particle in an electric field, standard electrode potentials for half-reactions are intensive properties and do not depend on the amount of substance involved. In this cell, the copper strip is the cathode, and the hydrogen electrode is the anode. In Section 4.
Oxidation numbers were assigned to each atom in a redox reaction to identify any changes in the oxidation states. Here we present an alternative approach to balancing redox reactions, the half-reaction method, in which the overall redox reaction is divided into an oxidation half-reaction and a reduction half-reaction, each balanced for mass and charge. This method more closely reflects the events that take place in an electrochemical cell, where the two half-reactions may be physically separated from each other.
We can illustrate how to balance a redox reaction using half-reactions with the reaction that occurs when Drano, a commercial solid drain cleaner, is poured into a clogged drain.
Drano contains a mixture of sodium hydroxide and powdered aluminum, which in solution reacts to produce hydrogen gas:. The overall redox reaction is composed of a reduction half-reaction and an oxidation half-reaction.To browse Academia. Skip to main content.
Standard Reduction Potentials
Log In Sign Up. Standard Reduction Potential Table. Koushik Mondal. The more positive the potential is the more likely it will be reduced. The standard cell potential is the potential difference between the cathode and anode. For more information view Cell Potentials.
P1: Standard Reduction Potentials by Element
The standard potentials are all measured at K, 1 atm, and with 1 M solutions. It is written in the form of a reduction half reaction. It is the tendency for a species to be oxidized at standard conditions. Therefore, when the standard reduction and oxidation potential of chemical species are measured, it is actually the difference in the potential from hydrogen.
By using a galvanic cell in which one side is a SHE, and the other side is half cell of the unknown chemical species, the potential difference from hydrogen can be determined using a voltmeter. Standard reduction and oxidation potentials can both be determined in this fashion. When the standard reduction potential is determined, the unknown chemical species is being reduced while hydrogen is being oxidized, and when the standard oxidation potential is determined, the unknown chemical species is being oxidized while hydrogen is being reduced.
The following diagrams show how a standard reduction potential is determined. The standard reduction cell potential and the standard oxidation cell potential can be combined to determine the overall Cell Potentials of a galvanic cell.
This can be done using an activity series. The species at the top have a greater likelihood of being reduced while the ones at the bottom have a greater likelihood of being oxidized. Therefore, when a species at the top is coupled with a species at the bottom, the one at the top will become reduced while the one at the bottom will become oxidized.
Below is a table of standard reduction potentials. Petrucci, Harwood, Herring, and Madura. General Chemistry: Principles and Modern Applications. Zhumdal, Zhumdal. Hydrogen has oxidation potentials of 0. The standard oxidation potential is not much like the standard reduction potential. The standard reduction cell potential and the standard oxidation cell potential can never be combined.
True 2. False: the standard oxidation potential is much like the standard reduction potential 3. What does the standard reduction potential measure?In this worksheet, we will practice using standard reduction potentials to identify the reduction and oxidation reactions in an electrochemical process. Using the standard electrode potentials shown in the table, determine which of the following metals is the most prone to corrosion.
Using the standard electrode potentials shown in the table, determine which of the following ions is the strongest oxidizing agent in acidic aqueous solutions.
Q2: Using the standard electrode potentials shown in the table, determine which of the following ions is the strongest oxidizing agent in acidic aqueous solutions.Standard electrode potentials are a measurement of equilibrium potentials. The position of this equilibrium can change if you change some of the conditions e.
It is therefore important that standard conditions be used:. It is possible to measure the potential of an electrode and electrolyte. It is not a simple process however, and the value obtained will depend on the concentration of the electrolyte solution, the temperature and the pressure. A way to remove these inconsistencies is to compare all electrode potentials to a standard reference electrode. These comparisons are all done with the same concentrations, temperature and pressure.
This means that these values can be used to calculate the potential difference between two electrodes. It also means that electrode potentials can be compared without the need to construct the specific cell being studied. This reference electrode can be used to calculate the relative electrode potential for a substance.
The reference electrode that is used is the standard hydrogen electrode Figure The standard hydrogen electrode is a redox electrode which forms the basis of the scale of oxidation-reduction potentials.
The solution e. As the hydrogen gas bubbles over the platinum electrode, the reaction is as follows:.
The standard hydrogen electrode used now is actually the potential of a platinum electrode in a theoretical acidic solution.
However, in order to use this as a reference electrode this value is set to zero at all temperatures so that it can be compared with other electrodes. In order to use the hydrogen electrode, it needs to be attached to the electrode system that you are investigating. For example, if you are trying to determine the electrode potential of copper, you will need to connect the copper half-cell to the hydrogen electrode; if you are trying to determine the electrode potential of zinc, you will need to connect the zinc half-cell to the hydrogen electrode and so on.
Let's look at the examples of zinc and copper in more detail. The solid zinc is more likely to form zinc ions than the hydrogen gas is to form ions.Electrochemistry: Using Standard Reduction Potential Values
A simplified representation of the cell is shown in Figure Figure There are lots of electrons on the zinc electrode. So a negative value means that the other element or compound has a greater tendency to oxidise, and a positive value means that the other element or compound has a greater tendency to be reduced. The voltmeter measures the potential difference between the charge on these electrodes.
The copper ions are more likely to form solid copper than the hydrogen ions are to form hydrogen gas.While it is impossible to determine the electrical potential of a single electrode, we can assign an electrode the value of zero and then use it as a reference. The electrode chosen as the zero is shown in [link] and is called the standard hydrogen electrode SHE.
Platinum, which is chemically inert, is used as the electrode. The reduction half-reaction chosen as the reference is. This voltage is defined as zero for all temperatures.
In cell notation, the reaction is. The minus sign is necessary because oxidation is the reverse of reduction. Using the SHE as a reference, other standard reduction potentials can be determined. Consider the cell shown in [link]where. The minus sign is needed because oxidation is the reverse of reduction. The SHE is rather dangerous and rarely used in the laboratory. Its main significance is that it established the zero for standard reduction potentials.
For example, for the cell shown in [link].
Standard reduction potentials for selected reduction reactions are shown in [link]. A more complete list is provided in Appendix L. Tables like this make it possible to determine the standard cell potential for many oxidation-reduction reactions. Identify the oxidizing and reducing agents. Solution Using [link]the reactions involved in the galvanic cell, both written as reductions, are. Galvanic cells have positive cell potentials, and all the reduction reactions are reversible.
The reaction at the anode will be the half-reaction with the smaller or more negative standard reduction potential. Reversing the reaction at the anode to show the oxidation but not its standard reduction potential gives:.
The reduction potentials are not scaled by the stoichiometric coefficients when calculating the cell potential, and the unmodified standard reduction potentials must be used. As the name implies, standard reduction potentials use standard states 1 bar or 1 atm for gases; 1 M for solutes, often at The reduction reactions are reversible, so standard cell potentials can be calculated by subtracting the standard reduction potential for the reaction at the anode from the standard reduction potential for the reaction at the cathode.
When calculating the standard cell potential, the standard reduction potentials are not scaled by the stoichiometric coefficients in the balanced overall equation. Is the reaction spontaneous at standard conditions? Assume the standard reduction for Br 2 l is the same as for Br 2 aq.
By the end of this section, you will be able to: Determine standard cell potentials for oxidation-reduction reactions Use standard reduction potentials to determine the better oxidizing or reducing agent from among several possible choices.These metrics are regularly updated to reflect usage leading up to the last few days.
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Table of Common Standard Reduction Potentials
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Table of Common Standard Reduction Potentials
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