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Thread: Is HSO4- an acid or a base?

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    Question Is HSO4- an acid or a base?

    i know that Cl- is the conjugate base of HCl
    but is HSO4- a conjugat acid / base of H2SO4???
    is HSO4- acidic or basic? what about SO4 2-??
    please... im so confused
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    Re: Is HSO4- an acid or a base?

    HSO4- is actually amphiprotic. It has a free proton that it can lose to become a sulfate in a basic environment but its negative charge can attract another proton to become sulfuric acid in an acidic environment.

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    Re: Is HSO4- an acid or a base?

    HSO4- is the conjugate base of H2SO4.
    HSO4- is an acidic anion because it tends to donate protons when in pure water.
    H2SO4 --> H+ + HSO4- <--> 2H+ + SO4 2-

    Notice how the first arrow is not an eqilibrium - it fully dissociates and the second only partially so some of the HSO4- will donate protons. Only in a very acidic environment would HSO4- act as a base.

    SO4 2- is basic but is sometimes treated as neutral in HSC.
    Last edited by someth1ng; 15 Jul 2012 at 12:04 AM.
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    Re: Is HSO4- an acid or a base?

    So when diprotic acid, such as H2SO4 ionise in water, for HSO4- and SO4 2-
    Would the first conjugate base always be acidic? That is, will it be acidic in water?
    What about the second conjugate base?

    What about triprotic acid such as H3PO4?
    Great thanks
    Last edited by HeyJes; 15 Jul 2012 at 3:06 PM.
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    Re: Is HSO4- an acid or a base?

    whattt?
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    Re: Is HSO4- an acid or a base?

    what's your question ? lol

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    Re: Is HSO4- an acid or a base?

    HSO4- is amphiprotic as previously said. The Bronsted-Lowry acid/base definition defines an acid as a proton donator, and a base as a proton acceptor. HSO4- has the capacity to both accept or donate a proton depending on whether the environment it is in is acidic or basic.
    ~

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    Re: Is HSO4- an acid or a base?

    Agreeing with previous comments, its because it still has the ability to give or take a proton
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    Re: Is HSO4- an acid or a base?

    How can HSO4- accept a proton when it's in water if H2SO4 totally dissociates?
    Last edited by someth1ng; 16 Jul 2012 at 1:12 PM.
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    Re: Is HSO4- an acid or a base?

    That's what im asking about.
    I'm so confused when diprotic acid dissociate in water, would the products be acidic or basic in water?
    if HSO4- is acidic, then what about SO4 2-? is it acidic or basic in water?
    is this a general rule for diprotic acid?
    then what about H3PO4?
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    Re: Is HSO4- an acid or a base?

    Quote Originally Posted by HeyJes View Post
    That's what im asking about.
    I'm so confused when diprotic acid dissociate in water, would the products be acidic or basic in water?
    if HSO4- is acidic, then what about SO4 2-? is it acidic or basic in water?
    is this a general rule for diprotic acid?
    then what about H3PO4?
    Technically, it's possible for HSO4- to accept a proton in certain conditions. HCl is a stronger acid than H2SO4 - this means that HCl will tend to give off protons more than H2SO4, if concentrated enough, you can have a solution of H2SO4 that is not dissociated.

    It depends totally on the ion.
    The stronger the acid (H2SO4), the weaker the conjugate base - this means that HSO4- is a very weak conjugate base and in solution (eg NaHSO4) is slightly acidic.
    The stronger the base, the weak the conjugate acid - this means that NH3 (weak base), when it accepts a proton (NH4+), is weakly acidic.
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    Re: Is HSO4- an acid or a base?

    Quote Originally Posted by HeyJes View Post
    That's what im asking about.
    I'm so confused when diprotic acid dissociate in water, would the products be acidic or basic in water?
    if HSO4- is acidic, then what about SO4 2-? is it acidic or basic in water?
    is this a general rule for diprotic acid?
    then what about H3PO4?
    Also, SO4 2- can evidently not be acidic as it does not have a proton to donate. SO4 2- is a weak base because it is the conjugate base of the weak acid HSO4 -.

    H3PO4 is a weak acid, so it does not fully ionise in water. Hence, H2PO4- can be treated as a weak, base as it is the conjugate base of a weak acid. HOWEVER! for examples of water testing to test for a phosphate ion, we need to have the phosphate ion on its own in solution. Hence, we make the solution basic.

    H3PO4 <---> 3H+ and PO4 3-.

    By le chateliers principle, a change in the concentration of products (OH- reacting with H+ reducing H+ concentration) will result in an opposing shift in equilibrium, shifting to the right and forming more individual phosphate ions. Then after this has occurred we can test for phosphate ions by precipitation. So as can be seen by this experiment, certain acids act differently under different conditions due to le chateliers principle.
    ~

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    Re: Is HSO4- an acid or a base?

    In the HSC you are told it completely ionises for the purpose of explaining it is a strong acid. I believe if you refer to the Jacaranda textbook it explains that it only partially ionises an. Cause H2SO4 is a 2 step acid. H2SO4 -> HSO4- -> SO4-2
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