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Defining Acids and Bases

  1. May 1, 2015 #1
    I've written the following brief history on how the definition of acids and bases change over time.
    Lavoisier came up with his definition circa 1776 in that 'p is acidic iff p contains much oxygen' and that held sway for 30 years[1] until it was falsified by Davy in 1810[2] who demonstrated that the acid H2S lacked oxygen among others, though he offered no countertheory. Then in 1836 Liebig surmised that 'p is acidic iff p is a hydrogen-containing substance in which the hydrogen could be replaced by a metal' and this theory was accepted as orthodox for a good 50 years.[3] In 1884 Arrhenius came up with 'p is acidic iff p increases the concentration of hydrogen ions (H+) in an aqueous solution' and 'p is basic iff p increases the concentration of hydroxide ions (OH-) in an aqueous solution. (6) In 1923 Bronstead and Lowry independently came up with the definition 'p is acidic and q is basic iff p donates a hydrogen ion (H+) to q.' (11) In 1938 Lewis posited that 'p is acidic and q is basic iff q donates an electron pair to p.' In the same year Usanovich hypothesized that 'p is acidic and q is basic iff q donates positive species to p.'(2) In 1947 Flood proposed that 'p is acidic and q is basic iff q donates an oxide ion (O2-) to p.

    I cannot figure out then when scientists propose a definition which is meant to put all acids and bases into a class because acids all share a common property what that property is. What is that property of acids which scientists agree they must have.

    For example, here is a chart of the Lewis' acids
    http://en.wikipedia.org/wiki/Lewis_acids_and_bases#/media/File:Lewis_Acids_and_Bases.jpg
    When you construct a definition there are certain beliefs you have about the class which you are not willing to give up. For example you come up with a list of 30 acids because they all have a certain property, then you try to figure out why they have that property by formulating that definition. Well, what is that property that acids have? The best I've been able to come up with is when an acid reacts with an alkali it forms a metal salt and water but of course that only applies to a special type of bases.



    [1] Miessler, G.L. Tarr, D.A., Inorganic Chemistry (1991) p 166.

    [2] Hall, Norris F. (March 1940). Systems of Acids and Bases. J. Chem. Educ. 17 (3) p 124-128

    [3] Meyers, R. (2003). The Basics of Chemistry. Greenwood Press. p 156.
     
  2. jcsd
  3. May 1, 2015 #2
    Hello gamow99,
    I will try to answer your question.

    I guess one could call that the "gamow99 definition of and acid". And you are wise to admit that it is incomplete description, in line with some of the other definitions of an acid ( and bases ) that have been proposed over the years.

    People like to classify things and put them in categories. So along with that premise, certain types of substances are are listed as belonging to the group "acid" or "base", with an accompanying list of similar features. One feature in the list could be the chemical reaction type, such as listed here,
    http://chemistry.about.com/od/chemicalreactions/a/reactiontypes.htm
    although you would be quick to notice there is some overlap, but it can be noted that obviously some substances do not fit the description of being and acid or base.
    Perhaps this is better,
    http://misterguch.brinkster.net/6typesofchemicalrxn.html

    Your "history" goes much much farther back than 1776. We can see from recorded history the labelling of stuff was just as much as rampant as it is today, and I think it is important to investigate the origins of the categories of acid and base to gain some perspective.

    Here we see where acid ( tastes sour ie like vinegar ) or base (slippery ) came about, and how additions to the so called definition progressed from ancient times. Alchemists, in the general sense of the word, in their never ending pursuit of the perfect magical elixir, were in a good part responsible for the progression towards modern day chemistry, by their tasting, weighing, scratching, mixing, measuring. I suppose one could surmise that they at first when tasting something as sour could have put it into the "sour tasting" category which later translated into the "acid" category.
    https://view.officeapps.live.com/op/view.aspx?src=http://www1.umn.edu/ships/modules/chem/acids.doc

    So, with all these sour tasting substances, someone would have to have come along eventually and ask the question "What makes it sour?" or "What do all these sour tasting substances have in common?"
    Lavoisier is credited I guess for coming up first with a formal defintion of an acid. Others followed suit over the years with more research, questions, and noted the exceptions, and provided another description to what is an acid. Are all the definitions the same?- yes and no, and you could do some research to determine if an Arrhenius acid is a Bronstead and Lowry acid, or vice-versa. Same thing with the other definitions and their compatability.

    Hopefullt that has at least answered part of your inquiry.
     
  4. May 2, 2015 #3
    Thank you for the very detailed response. I really appreciate you taking time to help me out on this issue.
     
  5. May 8, 2015 #4
    On (possibly) a related note.
    I can see how the Lewis definition, is a general case of the bronsted lowry definition for an acid/base. Can the Bronsted-lowry be a general case of Arrhenius?
    I've attached a picture. Does anyone agree or disagree with this?
     

    Attached Files:

  6. May 9, 2015 #5

    Borek

    User Avatar

    Staff: Mentor

    That's more or less the idea, each new theory attempts to generalize the previous one.
     
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