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Standard Reduction Potentials: Definition & Example

Instructor: Amanda Robb

Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

In this lesson we'll be investigating what the standard reduction potential is. Here, we'll explain how to determine this value experimentally and use it to calculate standard cell potentials.

What Is a Standard Reduction Potential?

What do you think of when you hear 'reduction'? Many of us associate the word reduction with getting smaller, but in the world of chemistry, reduction actually means to gain something, electrons specifically. Reduction is the gaining of electrons by an atom. It is part of a type of reaction called redox reaction. The other atom in the reaction is oxidized, meaning it loses electrons. To help you remember this, consider the following acronym: LEO says GER. Lose electrons, oxidize. Gain electrons reduce.

So, what does this have to do with the standard reduction potential? The standard reduction potential is the likelihood of a particular molecule or atom to be reduced, or gain electrons. The standard reduction potential is expressed in volts at standard conditions, a temperature of 298 K, a pressure of 1 atmosphere, and with 1 M solutions. The more positive the standard reduction potential, the more likely it is that the molecule will gain electrons and become reduced. If the reduction potential is negative, that molecule is more likely to give up electrons and become oxidized.

Examples

Let's look at some examples. Fluorine gas is extremely electronegative and wants to acquire electrons to complete its outer shell. Because of this, fluorine has a relatively high reduction potential of +2.87V, indicating it is likely to receive electrons and become reduced.

Reduction potentials are written in half reactions for the molecule that is being reduced. The table here shows the values for some common standard reduction potentials.

Reaction Reduction Potential (volts)
Cl2+ 2e- ---> 2 Cl- +1.36
O2 + 4H +4e- ---> 2H2 +1.23
Ag+ e- ---> Ag +0.80
Cu2 + 2e- ---> Cu +0.34
2H + 2e- ---> H2 0
Fe2 + 2e- ---> Fe -0.44
Zn2 + 2e- ---> Zn -.076
Mn2 + 2e- ---> Mn -1.18
Li + e- ---> Li -3.04

Determining the Standard Reduction Potential

So, how do scientists come up with these numbers for the standard reduction potential? Any standard reduction potential can be determined experimentally. First, a scientist will create a galvanic cell, which can conduct electricity, using a standard hydrogen electrode (SHE). This is the reference electrode, and the standard reduction potential is set as zero for SHE. Then, the other side of the galvanic cell is set up with the material in question, for example copper.

Next, a voltmeter is used to measure the flow of electrons between the SHE and the copper electrode. The voltmeter indicates the standard reduction potential of the copper, or whatever material is being used. In our example, copper tends to be reduced and the voltmeter will read a standard reduction potential of 0.337V.

Calculating Standard Cell Potential

Once the standard reduction potentials for the components of a cell are determined, the standard cell potential can be calculated. If you've ever used batteries, you're already familiar with how the standard cell potential can be useful. The standard cell potential measures essentially how much energy is in the cell, or battery. The energy available comes from the movement of electrons during the redox reactions between the two chemicals in the battery.

The potential energy in a battery can be calculated using the standard cell potential
batteries

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