phatty925

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A strong acid has a weak conjugate base and a weak acid has a strong conjugate base. when trying to figure out if something is a strong acid or weak acid and i try to figure this out by looking at the acid's conjugate base, how do i know whether the conjugate base is "STRONG" or not? in other words, what constitues strong? for ex, HCN is a weak acid. how can we tell that CN- is a strong conjugate base?
thanks.
 

SuzyQ

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Stong acids and bases will have no basic or acidic properties for their conjugates. To decide if a conjugate base is strong you have to look at the acid in which it came from. If the acid is weak,(there should be list of strong acids, and if the acid you are looking for isnt on the list, its classified as weak......the list is HI, HBR, HCl, HClO4, HClO3, H2SO4, HNO3) then you will have a strong conjugate base. The same goes with bases....if you have a conjugate acid, look at the base....if the base is weak, your conj acid is strong. List of strong bases is LiOH, NaOH, KOH, RbOH, CsOH, there are some other ones but i cant remember. Hope this helps.
 
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souljah1

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Look at the pka's and pkb's.

Generally, conjugate bases of strong acids are extemely low in basicity (H2CO3, HCO3-), (H3PO4, H2PO4-). Conversely, weak acids may have very strong conjugate bases (Acetic Acid and Acetate), (H2O and OH-), etc..


I haven't had ochem for three years, but I think I am atleast on the right track.
 

Diogenes

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Well, assuming that the pKa's are not available, a good general rule is to try to determine how stable the conjugate base or acid would be. For example, we know that NaCl dissolves in water well. So we know that Cl- is very stable in water. So HCl should be a strong acid, and it is. Deprotonated carboxylic acids can share the extra electron via its resonance structures. Therefore it is pretty stable -- but not as much as some other things like sulphuric acid, which is more acidic, that has more possible resonance structures. Drawing resonance structures can give you a very, very rough idea of how acidic those oxide acids are, I think. But it's been 4 years since gen chem, so I don't really know.
 
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phatty925

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thanks for all of your input!
sorry, but i think i worded my original queston poorly.
i guess my main question is: besides memorizing the list of strong acids and bases in a book, is there a way you can figure out whether an acid or base is strong or weak? do any of you have any quick methods that work? i seem to freeze when i get an acid-base question.

thanks.
 

Mr. Z

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diogenes hit the nail on the head.

To determine the strenght of an acid look at the stability of the conjugate base. The more stable the conjugate base the stronger the acid. The oxyacids are good examples, the conjugate base of perchloric acid (HClO4) has many electonegative oxygens which will each share the negative charge on the conjugate base, this has the effect of stabilizing the base and making for a very strong acid. In contrast, hypochlorous acid (HOCl) only has one oxygen to share the negative charge with, so it is less stable and a weaker acid.
 
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phatty925

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mr. z--
thanks for the explanation...that was really helpful.
i think it's easier to see with electron withdrawing groups such as oxygen, but how do we figure out for ones that are a little less obvious? for ex, HCN is a weak acid...how would you figure this out using the method you just discussed?
 

Diogenes

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Originally posted by phatty925
mr. z--
thanks for the explanation...that was really helpful.
i think it's easier to see with electron withdrawing groups such as oxygen, but how do we figure out for ones that are a little less obvious? for ex, HCN is a weak acid...how would you figure this out using the method you just discussed?
Think of a nitrile as similar to a carboxylic acid that doesn't give up a proton from the OH. The N has 3 bonds to the C and withdraws electrons. But CN- isn't very "happy" in that there are no resonance structures, no way to delocalize that negative charge. Therefore HCN won't want to give up its proton very much. Thus it is a weak acid. Think of it this way -- could you pull the proton off of the carbon atom in formic acid (HCOOH)? Not very easily.
 

Mr. Z

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to determine the strenght of an acid you need to consider the stability of the conjugate base (we already talked about that), the strenght of the bond between the H+ and the molecule, and the polarity of the bond.

The more polar the bond between the H and the molecule, the easier it will be to remove the H ion. For example HCl is a strong acid due to the polarity of the H-Cl bond. In your example of HCN, the bond is not all that polar and therefore it makes for a weaker acid because the H+ will not be easily removed. Be careful using this as your guide, it doesn't always work, you need to consider bond strength and conjugate base stability as well.
Always remember a strong bond makes for a weak acid.
 

wgu

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Whether it is acid/conj-base or base/conj-acid, whichever of the pair is more stable will be the usually considered weak. Weak and strong are relative terms though. Besides memorization and experience you can use the periodic table for trends. I'm sure an orgo book will explain this much more clearly than me but here's some trends: HX where X is a halogen is usually a strong acid (thus X- is the weak conjugate base) . The stronger acids in that column are closer to the bottom. A larger atomic radius implies a proton is not stable with that halogen so the halogen will tend to lose that proton (so it is more acidic). If without the proton the halogen anion is relatively stable, thus making it a weak base. Same rule applies with sulfur versus oxygen. A proton by a electronegative atom is usually going to fall off more easily (less stable so strong acid)- the exception to that rule I've just mentioned w/ the halogens. Besides that there's also resonance which you'll really need an orgo book w/ lots of schemes and pictures to explain
 

Mudd

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As an acid gets stronger, its conjugate base gets weaker. Also, the conjugate base of a strong acid is so weak, it does not hydrolyze to any detectable extent in water. Likewise, the conjugate acid of a strong base is so weak, it does not dissociate to any detectable extent in water.

The facts are good starting points, but the reality is that you really do need to know (read 'memorize') some common strong acids and some common pKa values.

Typical strong acids are:

  • Sulfuric acid (H2SO4)
    Nitric acid (HNO3)
    Perchlroic acid (HClO4)
    Hydrochloric acid (HCl)
    Hydrobromic acid (HBr)
    Hydroiodic acid (HI)

A good rule to follow is that a strong acid has a pKa less than zero, while a weak acid has a pKa greater than zero.

Typical weak acids are:

  • Phosphoric (H3PO4, pKa1 = 1.8)
    Carbonic acid (H2CO3, pKa1 = 6.4)
    Acetic acid (H3CCO2H, pKa = 4.7)
    Hydrofluoric acid (HF, pKa = 3.2)
    Phenols (C6H5OH, pKa = 10)
    Ammonium and Protonated amines (RNH3+ pKa = 9-11)

Bases follow a similar set of guidelines, but the emphasis is on pKb rather than pKa and hydrolysis rather than dissociation.
 
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