Standard Oxidation Potentials

Standard Reduction Potentials
Using the SOP List
Calculating Voltages
SOP List
SRP List

Standard Oxidation Potentials

Over the years chemists have made comparisons and measurements of the ability of chemicals to lose electrons. Those comparisons have been compiled into a standard oxidation potential list. One such list is given in example 16 in your workbook and on the SOP List page in this section. If you to take a look at the one in your workbook you will be able to refer to it while you read this page.

Comparison with Expectations

This list contains much information, but for now see how the list we made using the periodic table and lab work compares to this standard list. (An abbreviated SOP list is shown here.) Near the top of this list are potassium, calcium, sodium, magnesium, aluminum and zinc. Then go about half-way down the list to find lead. (Lead is listed in a couple places. Skip the lead plus sulfate because we didn't test lead plus sulfate. We tested lead changing to lead ion by itself.) Further on down is copper. We also had iodide ion. it is right below copper; then a little further down is bromide, and then chloride, and then fluoride way down at the bottom.

Abbreviated SOP List

K⁺ + e^- 2.93
Ca

Ca²⁺ + 2e^- 2.87
Na

Na⁺ + e^- 2.71
Mg

Mg²⁺ + 2e^- 2.37
Al

Al³⁺ + 3e^- 1.66
Zn

Zn²⁺ + 2e^- 0.76
Pb

Pb²⁺ + 2e^- 0.13
Cu

Cu²⁺ + 2e^- -0.34
2 I^-

I₂ + 2e^- -0.54
2 Br^-

Br₂ + 2e^- -1.07
2 Cl^-

Cl₂ + 2e^- -1.36
2 F^-

F₂ + 2e^- -2.87

So you can see that our rating of the ease of oxidation for these chemicals was not bad. The uncertain order of sodium and calcium is established. (The reason for calcium being higher is primarily that the calcium ions have a higher charge and smaller size than sodium ions and thus form stronger more stable bonds to water molecules.) The uncertain position of aluminum is also established. You can see that the general trend for the ease of losing electrons fits in pretty well with what can be deduced about the ease of oxidation and reduction from the position of an element on the periodic table.

Potentials

This oxidation potential list does more than just list the chemicals in order of their ease of oxidation. On the right hand side of the list there is a numerical rating (or measure) of their ease of oxidation, EMF or E^o. It is measured in volts compared to an arbitrary standard.

That standard, right in the middle of the chart, is hydrogen becoming hydrogen ion and giving off two electrons. That reaction is used as a comparison standard so it's given an arbitrary rating of 0 volts and everything else is compared to that in terms of being easier to oxidize or not as easily oxidized as hydrogen. The chemicals that can be oxidized are listed in the left hand column.

Abbreviated SOP List

K⁺ + e^- 2.93
Ca

Ca²⁺ + 2e^- 2.87
Na

Na⁺ + e^- 2.71
Mg

Mg²⁺ + 2e^- 2.37
Al

Al³⁺ + 3e^- 1.66
Zn

Zn²⁺ + 2e^- 0.76
Pb

Pb²⁺ + 2e^- 0.13
H₂

2 H⁺ + 2e^- 0.00
Cu

Cu²⁺ + 2e^- -0.34
2 I^-

I₂ + 2e^- -0.54
2 Br^-

Br₂ + 2e^- -1.07
2 Cl^-

Cl₂ + 2e^- -1.36
2 F^-

F₂ + 2e^- -2.87

This standard oxidation potential list is far from a complete list. There are many, many reactions. This is just a sampling.

You should notice in the list in your workbook that some lines include more than one chemical. If the oxidation rating is based on the presence of some other chemical in the reaction, that chemical is listed there, too. Two examples are that lead is more easily oxidized in the presence of sulfate ion and gold is more easily oxidized in the presence of chloride ion. (That is part of the reason why aqua regia can dissolve gold.)

The phrase standard oxidation potential deserves a bit of explanation.

Standard simply means that these measurements and comparisons are made under standard conditions for redox reactions. It is under these conditions that the voltage is measured.

	the concentration of all soluble chemicals is 1 M,
	the temperature is 25^oC, and
	the pressure is 1 atm for any gaseous chemicals.

Oxidation is the type of half-reaction shown here.

Potential means the voltage which is associated with the tendency for that reaction to occur.

If the concentrations, temperature or pressure are different , then the voltage associated with that particular half-reaction will be different.

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