Lesson 9: Oxidation Potential
Now let's turn our attention to how readily or easily chemicals gain and lose electrons. There are a number of ways this can be done.
One can look at how the position of elements on the periodic table influences how easily atoms will gain or lose electrons. This will help you relate these properties to atomic structure. One can also do experimental testing of elements and ions to see how readily they react with one another. You will do this in your lab work for this lesson.
These different approaches can be taken in any order, but I recommend the order in which they have been listed. In that order you can build a sense of why the elements are found where they are on the list.
Ease of Oxidation | Ease of Reduction | Lab Work
Ease of Oxidation
Let's relate the ease of oxidation of several metals to their position on the periodic table. You might want to have a periodic table handy while making these comparisons. Let's use sodium, potassium, calcium, magnesium and aluminum. (These comparisons are also shown in example 13 in your workbook.) |
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First let's compare sodium and potassium. Since potassium is lower on the periodic table, it loses electrons more easily. Thus, in ordering them from most easily oxidized to least easily oxidized, potassium goes at the top of the list and sodium goes below it. |
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Next let's compare calcium and magnesium. They are in the same group on the periodic table. Calcium is further down on the table, therefore its electrons are further away from the nucleus and more easily lost than the electrons of magnesium. |
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Next let's compare sodium, magnesium, and aluminum. They're in the same period on the periodic table. As you go from left to right it becomes more difficult to lose electrons because there are a greater number of protons. The additional protons are holding the electrons more tightly. Therefore, sodium, the one furthest to the left, is the most reactive; magnesium next most active; and aluminum the least reactive of these three. |
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Now let's put all of these comparisons together. Let's use the last set as our starting point. We also know that potassium is more reactive than sodium, so we will put potassium above sodium. We know that calcium is more reactive than magnesium so we will insert calcium in the list above magnesium. Now this leaves the question as to whether calcium should be above sodium or below sodium. Based on the periodic table, we don't really have a proper method of determining whether sodium or calcium should be higher, so I put them together. |
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Ease of Reduction
We can also relate the ease of reduction of nonmetals to position on the periodic table. Remember, reduction means gaining electrons. Let's use the halogens (fluorine, chlorine, bromine and iodine) as examples. |
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The most easily reduced of these is fluorine. It has the greatest tendency to gain electrons. Next would be chlorine, followed by bromine and iodine. This is also summarized in example 14 in your workbook. |
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Remember that reduction and oxidation are related processes. They are the opposites of one another. If you reduce fluorine, it become fluoride ion. If you reduce chlorine, it becomes chloride ion; reducing bromine turns it into bromide ion and reducing iodine turns it into iodide ion. These ions now have electrons which can be lost; that is, the ions can be oxidized. The reactions can be reversed. |
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We can rate the ease of oxidation for these negative ions by reversing the order of the ease of reduction of the atoms. So the most easily oxidized of those four ions would be iodide. The next most easily oxidized would be bromide; next, chloride; and the most difficult to oxidize would be fluoride ion. The difficulty of removing an electron from a fluoride ion matches the ease of adding an electron to a fluorine atom. An atom of fluorine has a very strong tendency to gain an electron. Once it has the electron and becomes an ion, the ion will have a very weak tendency to lose that electron. |
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We can add these ions to our ease of oxidation list by writing iodide, bromide, chloride and fluoride ions below aluminum. (As is shown in example 15.) Notice that I said the ions, not the elements. This is because this is an oxidation list and those elements gain rather than lose electrons. As you can see, a lot can be figured out from the periodic table. |
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F-
However, position on the periodic table is not the sole determiner of the ease of oxidation. This is especially true when it comes to the transition elements. Also, these reactions usually take place in solution and the tendency of water to bond to ions and thus stabilize them, varies from ion to ion. So that comes into play, also. The method for determining the actual order for ease of oxidation is experimental. We take up that aspect on the next page.
Lab Work
We got our original insight about trends that can be related to position on the periodic table by doing some experiments with the elements and seeing how their reactivity compared with one another (in CH 104). Let's do the same for oxidation-reduction reactions by having you do the experiment for this lesson. The instructions are in your workbook. When you are doing this experiment, be sure to use fresh cuts; if the material you are working with has already been cut, scrape the oxide film off of the metals so that the metal itself is exposed. Then look for changes occurring at that shiny surface.
If you are in the lab now, I suggest that you do the lab work now. After you have done that and answered the questions included there, then continue with the comments below. If you will be in the lab at a later time, you can come back to these comments after you have done the lab work or, if you prefer, look at the comments now as something to keep in mind when you do the lab work.
Comments
These comments won't follow directly along the line of the questions asked in the workbook, but they will touch upon some of the same things.
Which of these metals was most reactive, or most easily oxidized?
Start from the top of your table of observations. You should have noted that zinc metal reacted with two of these solutions. Magnesium metal reacted with all of them. Copper metal reacted with none of the solutions. Lead metal reacted with just one of the solutions.
Since all of these reactions involve the loss of electrons by the metal, we can say that magnesium metal is the most easily oxidized. Zinc is the next most easily oxidized. Lead is third. Copper is the least easily oxidized of these particular metals.
Expand List
So we could add these metals to our list (in example 7 in your workbook) by writing zinc, lead and copper down below magnesium. Then the next question is where to put aluminum since we didn't actually test that. Does aluminum go above or below or in the middle of these new additions? For now, let's put zinc, lead and copper below aluminum. |
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