Lesson 5: Formation of Acids & Bases
Formation - Oxides of Metals and Nonmetals
There are many ways that acids and bases can be made. For now, we'll just look at one of the simplest. If an element is oxidized by burning it in air, and then that oxide is dissolved in water, the resulting solution wil be a base or an acid depending on the type of element with which you started. With a few elements you would get an amphoteric product. Let's consider how this is done in the lab and then look at an explanation for it. We'll also use this knowledge to help recognize acids and bases by their formulas.
Lab Work
I'd like you to actually do this now or when you are next in the lab. Separately, take some sulfur and some magnesium. Burn each of these, dissolve the product in water, and then test that solution to find out whether the dissolved oxide is acidic or basic. The specific instructions for doing that are in exercise 19 in your workbook and the chemicals are over in the fume hood. Part c of exercise 19 lists the equations for the reactions that you'll be doing and has a place to record your results. Please pay attention to what is happening while you're performing the tests by comparing the observed chemical reactions with the equations shown in the example. If you are in the lab, please take some time now to do exercise 19, then we'll proceed with explaining why one of these becomes an acid and one becomes a base.
You should observe that when the oxide of magnesium is dissolved in water it gives a basic solution; when the oxide of sulfur is dissolved in water gives an acidic solution. Magnesium is a metal; sulfur is a nonmetal.
Explanation
In general, the oxide of a metal dissolved in water gives a base and the oxide of a nonmetal dissolved in water gives an acid. The oxides of some metalloids and transition metals give amphoteric compounds when dissolved in water.
Let's look at why it is that the oxides of metals give basic solutions and the oxides of nonmetals give acidic solutions.
When the oxides react with water they form compounds containing hydrogen as well as oxygen and the other element. In general, if that other element is a metal the compound is basic. On the other hand, if that other element is a nonmetal, the compound is generally acidic. |
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Mg(OH)2 base
Even though magnesium hydroxide is an ionic compound, it is represented here using an electron dot diagram. The Mg-O bond is ionic the O-H bond is covalent. H has a stronger attraction (bond) to the electrons around O than does Mg because H is more electronegative than Mg. Therefore, when the compound splits H remains attached to O and OH separates away from Mg. The electrons from Mg go with O giving Mg2+ and OH-. The OH- makes the solution basic. |
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Next we have H2SO3 which is a covalent compound. S and H are both nonmetals. Therefore, they both share electrons with O. S is more electronegative than H, so when splitting occurs it is the S that stays attached to the electrons on O and the H separates away from O, leaving its electrons behind and becoming an H+ ion. The H+ makes the solution acidic. The second H+ can also come off, but it is more difficult to remove because that involves pulling it away from something that already has a negative charge. Of course the H+ doesn't just leave, it is pulled off by water or some other proton acceptor. |
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Recognizing Formulas of Acids and Bases
With your knowledge of the interactions between H, O and other elements you should be able to tell from the formula of some compounds whether they are acids or bases. Those compounds are the ones that contain H, O and one other element. If that other element is a metal, the compound probably will be basic. If that other element is a nonmetal, the compound probably will be acidic. But keep in mind that not all acids and bases contain oxygen. |
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Also, note that ammonium hydroxide (NH4OH) is an important exception to this generality. If you remember that ammonium hydroxide is an ionic compound (two polyatomic ions) containing hydroxide ion it shouldn't be too hard to remember that it is basic.
Practice (Ex. 22)
For practice, identify the chemicals listed below and in exercise 22 as being acidic or basic. You can find the answers below.
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Answers to Exercise 22
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If you missed any of those, please check with your instructor to get squared away.