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Acids And Bases

  1. Sep 11, 2004 #1
    Hello People,
    I am a new member and i really think this forum is very helpful....
    I got a question...
    The pH values of 0.100moldm-3 solutions of hydrocloric acid and ethanoic acid are 1 and aproximately 3 respectively. Explain the difference.

    Thanks already....
     
  2. jcsd
  3. Sep 11, 2004 #2
    Hi Jack,

    I suppose that your question is: why are different values of pH if both of them are monoprotic acids in the same concentration? rigth?

    Well, they simply have different pKas. Both of them are monoprotic acids (they only have one acid hydrogen) but hydrocloric acid is more susceptible of "loosing" it´s acid proton H+. So a solution of hydrocloric acid wolud have bigger concentration of free H+ than an ethanoic acid solution of the same initial concentration. Then the pH of hydrocloric acid solution would be lower.

    The reason of why some acids are stronger than others is the molecular structure, it dictates the "easyness" of loosing the proton. When a molecule looses an acid proton it transforms in a negative ion. If the stability of that ion is enhanced by that concrete molecular structure we say that the substance is a strong acid (it looses easily that proton).
     
  4. Sep 11, 2004 #3

    chem_tr

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    Science Advisor
    Gold Member

    Hello

    Altered-gravity is right. Even very powerful monoprotic acids like HNO3 and HCl have very slight differences in terms of losing proton to solution, but it is very small, so we can omit it and call both as very powerful acids with 100% ionization in nearly all cases (for aqueous systems).

    I want to add something to what Altered-gravity said: the less the overall electronegativity of the atom binding hydrogen decreases, the faster the hydrogen is lost. For example, alcohols (alkyl-OH) are weaker acids than thiols (alkyl-SH). HCl is essentially a gas, and used as aqueous solutions by passing this gas from water. Water molecules make hydrogen bonding with chlorine atom by Cl...H bonds, so the overall electronegativity decreases a bit. You may conclude that HCl solution prepared with aprotic solvents having a low dielectric constant has a lower acidity than that with protic, high dielectric-constant-showing solvents. I think this assumption is not wrong.

    For acetic acid, the proton is not that free to be lost to water; it is strongly bonded by two very electronegative oxygen atoms. The electronegativity of oxygen is even stronger due to the positive inductive effect of the alkyl group. But when you substitute some hydrogens with electronegative atoms such as chlorine, the acidity increases due to decreasing overall electronegativity of oxygens. But there is a lucky thing that acetic acid molecules prefer to occur as dimers in solution; it makes dissociation easier. So the difference is not very high, higher homologs of acetic acid have very low dissociation constants and it even becomes impossible for them to be dissolved in water for some members.

    I hope I did not cause any confusion; for most reasons, Altered-gravity's answer is sufficient. But if you wonder it very much, you will have to go into more detail, mine may help you then.

    Regards
    chem_tr
     
  5. Sep 12, 2004 #4
    Thanks altered gravity your answer really helped ,although im turkish too chem_tr:) your answer caused me some confusion but after i read it more carefully i could understand it .Thanks Guys !!!
     
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