Using the SOP List

A standard oxidation potential list can be used to do a variety of things. The most significant of these are discussed below.

Identify Most Easily Oxidized or Reduced Chemicals

You can use it to determine which of many substances has the greatest tendency to be oxidized or reduced. To do that you simply need to look for its position on the list and understand what that position represents.

Chemicals which can be oxidized are written on the left side of the arrows, and the ones which are highest on the left side have the greatest tendency to be oxidized; that is, they are most easily oxidized. In order for one of the chemicals on this list to be reduced, it must be on the right side of the arrows, and those that are furthest down have the greatest tendency to be reduced.

Of all the chemicals on this list, potassium has the greatest tendency to be oxidized and fluorine, F₂, has the greatest tendency to be reduced. Conversely, fluoride ion has the least tendency to be oxidized and potassium ion has the least tendency to be reduced. Again, you should be able to see the relationships between the position on this potential list and the tendency for the chemicals to be either oxidized or reduced.

Abbreviated SOP List

most easily oxidized

hardest to reduce

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

hardest to oxidize

most easily reduced

Practice

Take time now to make sure that you understand the importance of the position on this standard oxidation potential list as it relates to the tendency of these chemicals to be oxidized or reduced. After you have done that, check your answers below and continue with this lesson to learn some of the other things we can do with oxidation potential lists.

Which has the greater tendency to be oxidized, Al or Cr?
Which has the greater tendency to be reduced, I₂ or Fe³⁺?
Which has the greater tendency to be oxidized, Fe²⁺ or Sn²⁺?
Which has the greater tendency to be reduced, Fe²⁺ or Sn²⁺?

Answers

Which has the greater tendency to be oxidized, Al or Cr? Al is higher on the left.
Which has the greater tendency to be reduced, I₂ or Fe³⁺? Fe³⁺ is lower on the right.
Which has the greater tendency to be oxidized, Fe²⁺ or Sn²⁺? Sn²⁺ is higher on the left.
Which has the greater tendency to be reduced, Fe²⁺ or Sn²⁺? Sn²⁺ is lower on the right.

Identifying Oxidized and Reduced Forms of Chemicals

Using a standard oxidation potential list, you should also be able to identify the oxidized or reduced forms of chemicals. The chemicals listed on the right side are the oxidized form. Remember that the process of losing electrons is oxidation, so potassium can be oxidized. Potassium ion has been oxidized so is in the oxidized state. So you can distinguish between the oxidized and reduced forms of chemicals by whether they are on the left or the right side of the arrows.

There are few chemicals which are on both sides, like the Fe²⁺. About a third of the way down the list at the voltage of .44, Fe²⁺ is the oxidized form when compared to Fe. But if you go further down the list to -.77 volts, Fe²⁺ is the reduced form when compared to Fe³⁺.

Practice

Identify the oxidized and reduced forms of zinc, tin and bromine.

Answers

Element	Oxidized form	Reduced form
zinc	Zn²⁺	Zn
tin can have more than one oxidized state so multiple answers result	Sn⁴⁺ (and Sn²⁺ when compared to Sn)	Sn (and Sn²⁺ when compared to Sn⁴⁺)
bromine	Br₂	Br^-

Identifying Spontaneous Reactions

To determine which redox reactions will occur spontaneously by using the standard oxidation potential list, you must first realize that in order for a redox reaction to occur, something must lose electrons and something must gain electrons. So, if the chemicals are present on this list, one of the chemicals has to be on the left side of the arrows and the other chemical has to be on the right side of the arrows. In addition, the one on the left must be higher than the one on the right.

In the following examples be sure to locate the chemicals on your larger SOP list, even though the examples on this page will show the chemicals being used. The examples here will show only the chemicals used. You need to get experience finding them in the larger list and to see where they are in the larger context of the entire list.

Examples

Take a simple case off the top: Potassium metal and calcium ion. Those two chemicals will react spontaneously with one another. Potassium can lose electrons and the calcium ion can gain electrons.

K K⁺ + e^- 2.93
Ca Ca²⁺ + 2e^- 2.87

Let's look at a slightly different situation. Go down two more lines to check out sodium ion with magnesium metal. The sodium ion can gain electrons and magnesium metal can lose electrons. However, the tendencies are such that those two will not react spontaneously with one another. Electrons would have to be forced from magnesium metal to sodium ion.

Na Na⁺ + e^- 2.71
Mg Mg²⁺ + 2e^- 2.37

In order to determine that a reaction will occur spontaneously between two chemicals on this list, not only must one be on the left side of the arrows and the other on the right side, but the one that is on the left has to be higher on the list than the one on the right. So K will react with Ca²⁺ but Mg will not react with Na⁺.

Practice

Here are some additional combinations for practice. (These are also given in example 18 in your workbook.)

Will zinc metal and nickel metal react?
Will zinc ion react with nickel metal?
Will zinc metal react with nickel ion?

Answers

Will zinc metal and nickel metal react? No, they both need to lose electrons. In order for a reaction to occur, one has to lose electrons and one has to gain electrons.
Will zinc ion react with nickel metal? No, they are on the opposite sides of the arrows, but the one on the left is lower on the list than the one on the right. So that won't work.
Will zinc metal react with nickel ion? Yes, that will work and when the reaction takes place, the zinc becomes zinc ion and the nickel ion becomes nickel metal.

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