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Thread: Bases and Acids

  1. #1 Bases and Acids 
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    Is there any way to identify bases or acids by the struture formula, or do I have to memorize them?


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  3. #2  
    Him
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    If you master the basic chemical principles you can reason which substance act as acids and other as bases.
    If you have a molecule HA you need to compare two situations, which one is more stable:
    H2A+ or A-.

    For HCl it is Cl- so act as an acid
    For NH3 it is NH4+

    But as you can imagine this not so simple, a whole bunch of factors come into play.
    -electronegetivity
    -possible electron translocation
    -octet structures
    (and now I am in thinking about it the activation energy (not sure it is the correct term in English) will also influence if the reaction will take place at all)

    So I must disappoint you no easy answers here , or maybe just one :P if no H is present on the molecule it will NOT be and acid. But who says it will be a base… :wink:

    Maybe someone else has a better idea.


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  4. #3  
    Forum Freshman kestasjk's Avatar
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    If I remember year 12 chem right acidity is a measure of the concentration of H+ (protons). (This is what the H in pH is) If it gives protons away when dissolved in water it's acidic, if it takes protons when dissolved in water it's basic.
    Dissolve HCl in water and you get lots of H+ and lots of Cl-, that's very acidic.
    Dissolve NH4+ (ammonium) in water and not all of it dissolves, you get some H+ and some NH3, but mostly it stays NH4+, that's weak acid.
    Dissolve NH3 in water and it may take some protons, becoming NH4+, but mostly stays NH3, that's a weak base.
    Dissolve NaOH in water and it makes lots of Na+ and lots of OH-, and OH- takes as many protons as are available, becoming H2O, that's a strong base..

    So it all depends on whether it gives or takes protons, and that's about as simple as it can be boiled down to I think. Anyway shouldn't your teacher be telling you all this?
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  5. #4  
    Universal Mind John Galt's Avatar
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    As HIM said there are no easy answers. This is quite a good site for learning more about Acids and Bases
    http://www.chem.ubc.ca/courseware/pH/launch.html
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  6. #5  
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    Quote Originally Posted by kestasjk
    If I remember year 12 chem right acidity is a measure of the concentration of H+ (protons). (This is what the H in pH is) If it gives protons away when dissolved in water it's acidic, if it takes protons when dissolved in water it's basic.
    Dissolve HCl in water and you get lots of H+ and lots of Cl-, that's very acidic.
    Dissolve NH4+ (ammonium) in water and not all of it dissolves, you get some H+ and some NH3, but mostly it stays NH4+, that's weak acid.
    Dissolve NH3 in water and it may take some protons, becoming NH4+, but mostly stays NH3, that's a weak base.
    Dissolve NaOH in water and it makes lots of Na+ and lots of OH-, and OH- takes as many protons as are available, becoming H2O, that's a strong base..

    So it all depends on whether it gives or takes protons, and that's about as simple as it can be boiled down to I think. Anyway shouldn't your teacher be telling you all this?
    That I know, those substances are the common reffered to bases and acids, I was wondering if there is a way to absolutely determine the acidity/baseity of every substance.

    Im not in school anymore, theres no teacher.
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  7. #6 How about oxidation states? 
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    Can we also tell if something is going to acidic or basic given the oxidation state. How do you determine the oxidation state of your acid or base?
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  8. #7  
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    Oxidation state as in the oxidation number or the entire molecule? Thats the charge of the molecule.
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  9. #8  
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    I don't even know how people assign oxidation numbers to atoms let alone whole molecules! Tell me, how do you find out what is oxidation state of anything?
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  10. #9  
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    There are a few oxidation states which are more or less constant:

    F always has -1, being the most electronegative.

    O usually has -2, -1 in peroxids and +1 with F, having the second highest EN

    Alcali metalls always have +1

    Earth-Alcali metalls always have +2

    Using these rules you can find out the oxidation numbers using algebra based on the principle, that the sum of all numbers is 0 in an uncharged molecule and equal to its charge in a charged one.

    example: glucose

    C6H12O6, to find out the oxidation number of C --->

    6x + 12(-1) + 6(-2) = 0

    6x = 24

    x = 4

    number of oxidation of carbon in glucose is +4
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  11. #10  
    Him
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    Quote Originally Posted by Ultrashogun

    number of oxidation of carbon in glucose is +4
    Your method is somewhat correct but you’ve used the wrong the numbers.

    H is mostly +1 in oxidation state (exception hydrogen gas were it is zero).

    Meaning the combined oxidation status of O and H is zero so the global oxidation of C must also be zero. If one looks at the structure of glucose one can calculate the different oxidation states of each C atom

    being

    +1 for C1

    0 for C2 until C4

    and

    -1 for C6

    Determining the oxidation state of an atom in a molecule is a pretty straightforward process. You should manage by googling ‘oxidation state’.

    Keep in mind that oxidation is the loss of electrons, so whenever an atom loses an electron his oxidation state raises.

    Oxidation states you can decipher using the structure of a molecule.

    To know if a molecule is base or acid there is the pKa value. With some experience you can rationally ‘guess’ its pKa from the structure but you can not call this exact science …

    pKa values are determined experimentally (again google will help you out).
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  12. #11  
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    i love chemistry, too bad its so hard.
    acids and bases, is tricky, but above sums it up well.

    i doing a chemistry class, and one of our exam questions wil be a acid base reaction, invloving the H ion thingy, if your given a reaction it should be easier.

    my two cents.

    oh.. why is it i always say acid base? not base acid> abd's.. 1 2 3's me thinks, mor eusless infor :<
    Stumble on through life.
    Feel free to correct any false information, which unknown to me, may be included in my posts. (also - let this be a disclaimer)
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