MadSci Network: Chemistry |
In general, when chemists speak of acids they talk of the acid's ability to generate H+ (a proton) and not so much about how much OH- is present in solution (usually an aqueous solution). If a solution is acidic, then you will be measuring the H+ concentration as acids generate protons and not hydroxide ion (OH-). When dealing with basic solutions (such as an aqueous solution of sodium hydroxide), the amount of OH- becomes important. Strong acids are characterized by their ability to generate a lot of H+ and include hydrochloric acid, sulfuric acid, and nitric acid. Weak acids are able to generate H+, but not nearly as much as much as strong acids. Weak acids include acetic acid (vinegar), formic acid, and carbonic acid (which is present in soda). Why is a weak acid weak? It has to do with what happens when the proton leaves the rest of the molecule. The part of the molecule that remains after the proton leaves is called the conjugate base and usually has a negative charge, though it can be neutral in some cases. If the conjugate base is exceptionally stable in solution, then it is easier to dissociate. The hydrogen sulfate ion (HSO4-) is the conjugate base of sulfuric acid, and is extremely stable, making sulfuric acid very strong. The carboxylate ion (H3CCOO-) is the conjugate base of acetic acid and is not as stable in solution – it would rather be bound to its proton. The reasons for the stability or lack of stability involve something called "resonance" and is too advanced for our discussion here. There is some math and experimental work involved in determining whether an acid will be strong or weak, but it, too, may be more than necessary for this explanation. If you are interested, I would suggest reading an introductory general chemistry book or the following website, which has an excellent explanation of the basic math behind acid/base chemistry (http://en.wikipedia.org/wiki/Acid).
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