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Strong Acid Reactions

  1. Sep 13, 2014 #1
    For a reaction like HCl + H2O → Cl- + H3O+, why is this reaction heavily favoured to the right in water?

    I've heard explanations that the products on the right are stabilized by the polar water molecules (considering aqueous solution). I'm just struggling to fully understand this because the reactants on the left seem much more stable since they are all neutral and have stable octets (or 1s orbitals full for the hydrogens). I just don't quite see why the products that are charged would be favoured since charge leads to a molecule in a more reactive and higher energy form...

    Any explanations to help better understand this concept would be greatly appreciated!

    Also, just to confirm, would this reaction be more favoured to the left given a nonpolar solvent?

    I've heard relative acidities always stay the same (i.e. HCl will always be more acidic than CF3H). Is this statement true and absolute for all compounds and mediums? I can't help but think that there are exceptions where the solvent could possibly react with various compounds differently, giving different acidic properties.
     
  2. jcsd
  3. Sep 13, 2014 #2
    For a reaction like HCl + H2O → Cl- + H3O+, why is this reaction heavily favoured to the right in water?

    I've heard explanations that the products on the right are stabilized by the polar water molecules (considering aqueous solution). I'm just struggling to fully understand this because the reactants on the left seem much more stable since they are all neutral and have stable octets (or 1s orbitals full for the hydrogens). I just don't quite see why the products that are charged would be favoured since charge leads to a molecule in a more reactive and higher energy form...


    I like to think of it more as in that the H2O is polar (has dipole) so the O has a partial negative charge and the H has a partial positive charge. These dipoles make water strong at taking apart compounds with charge.

    In HCl, the H holds a partial positive charge and the Cl holds a partial negative charge.

    As a result of these two cases, the O in H2O pulls the H from Cl and the H in H2O pulls the Cl from the H. H+ combines with H2O and forms H3O because the H+ ion is highly reactive. Since the pull of all the water molecules surrounds the Cl- ions, equilibrium tends to the right. The HCl covalent bond is also quite weak so it is much easier to pull the HCl apart than it is to reform it. This is why almost 100% of HCl is ionized.

    Note that all the compounds in your equation have octets.


    Any explanations to help better understand this concept would be greatly appreciated!
    You should read on Bronsted Acid and Bases, Lewis acid and Arrhenius Theory. You may also want to look at equilibrium constants.


    Also, just to confirm, would this reaction be more favoured to the left given a nonpolar solvent?
    No, if the solvent is non polar then there is nothing causing the covalent bond between H and Cl to break.

    I've heard relative acidities always stay the same (i.e. HCl will always be more acidic than CF3H). Is this statement true and absolute for all compounds and mediums? I can't help but think that there are exceptions where the solvent could possibly react with various compounds differently, giving different acidic properties.

    *I am not too sure on this one, but here is my take!*

    Yes this is always the case because acids are organized by how readily it "donates" a proton and HCl will always be better at it than CF3H. It doesn't matter what compound it is in because you are comparing the acidity relative to each other unless you are talking about a entirely different kind of reaction which would make the acidity part pretty much irrelevant. The more acidic a acid is, the more hydronium ions there will be and this is dependent on the ability for the acid to give up protons.


    Hopefully I answered this well enough, haven't done chemistry in a while :biggrin:
     
  4. Sep 13, 2014 #3
    Just to confirm, don't you mean yes? To clarify, the HCl and water will remain in their current, uncharged forms.

    Yep, that's more or less what I've heard. I just can't help but think there are special solvents and compounds where the molecular size and structure make the acid more permeable to the solvent to make dissociation easier compared to another one which has a more closed structure in the specific medium. I am still learning about molecular structures, but maybe something to the effect of the actual conformation also changing at different temperatures or pH, as to allow ionization to occur more readily in that environment.

    Your insight is appreciated!
     
  5. Sep 13, 2014 #4
    For the non polar solvent, if it is non polar, it doesn't pull on the HCl and won't do anything to separate them into ions, so I mean no. Non polar -> no partial charges. Water is not a non polar solvent, HCl and water (the original equation) will move towards the right side because of water being polar and what I said earlier. In context of a acid reaction, there would be none, no favoring, nothing changes.
    ex. (just an example in terms of acid reaction, not sure if this is what happens but you get the gist of it)

    HCl + C8H18 -> No reaction

    This is mainly because when you talk about HCl and water, the water is a base in this case and can accept H+ and is polar because of the O-H bonds.

    The only thing that would really make acids stronger would be having a more polar solvent than water from what I can think of. The only thing I can think of straight off the top of my head is this (Somewhat of what you are thinking of):
    http://en.wikipedia.org/wiki/Piranha_solution#Mechanism_of_action
    Changing temperature would really only make the reaction happen faster unless you got to a point where it changed the structure.


    Thanks :smile:
     
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