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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: keresturec on October 25, 2012, 02:37:29 AM

Title: Chloric acid (HClO3) strong or weak
Post by: keresturec on October 25, 2012, 02:37:29 AM

I had a midterm in chemistry and to my surprise I got a mark off for putting Chloric acid as a strong acid. Now in high school I've been taught that there are seven strong acids, chloric acid being one of them... can someone explain why chloric acid is not strong before I make a fool out of myself in front of my professor?

Title: Re: Chloric acid (HClO3) strong or weak
Post by: 408 on October 25, 2012, 03:13:16 AM

perchloric is strong
chloric is not

Title: Re: Chloric acid (HClO3) strong or weak
Post by: Borek on October 25, 2012, 04:17:06 AM

Chloric acid is quite strong. Depending on the source its pKa is given as either -1 or -2.7, which makes it either comparable to or stronger than nitric acid.

Acids and their pKa values, quoted from Kotrly&Sucha, Handbook of Chemical Equilibria in Analytical Chemistry:

HCl -7
HClO 7.53
HClO₂ 1.95
HClO₃ -2.7
HClO₄ -7.3

Title: Re: Chloric acid (HClO3) strong or weak
Post by: 408 on October 25, 2012, 09:18:10 AM

I stand corrected. 

Title: Re: Chloric acid (HClO3) strong or weak
Post by: Big-Daddy on October 25, 2012, 10:50:31 AM

The safest bet is I think hydroiodic acid (HI), with a pKa (from Wikipedia) of -9.3. The stronger acids are less well-known, the weaker common acids are less strong.

If the definition of "strong acid" is one which has a pKa higher than that of the hydroxonium ion (pKa=-1.74), chloric acid has inconsistent figures. Wikipedia lists -1 for the pKa of chloric acid, which would suggest it is not quite strong, and this may be why you lost the mark.

Title: Re: Chloric acid (HClO3) strong or weak
Post by: Borek on October 25, 2012, 11:03:36 AM

Quote from: Big-Daddy on October 25, 2012, 10:50:31 AM

If the definition of "strong acid" is one which has a pKa higher than that of the hydroxonium ion (pKa=-1.74)

Nitric acid pKa is consistently listed as below -1.74 yet it is considered a strong acid.

Title: Re: Chloric acid (HClO3) strong or weak
Post by: Big-Daddy on October 25, 2012, 12:20:14 PM

Quote from: Borek on October 25, 2012, 11:03:36 AM

Quote from: Big-Daddy on October 25, 2012, 10:50:31 AM

If the definition of "strong acid" is one which has a pKa higher than that of the hydroxonium ion (pKa=-1.74)

Nitric acid pKa is consistently listed as below -1.74 yet it is considered a strong acid.

What then is the definition of a strong acid? The extent to which a dissociation is complete is defined by the pKa ...

Title: Re: Chloric acid (HClO3) strong or weak
Post by: keresturec on October 25, 2012, 12:37:30 PM

thanks guys for your help, sadly my professor said chloric acid is not universally accepted as a strong acid.   :-\

Title: Re: Chloric acid (HClO3) strong or weak
Post by: Borek on October 25, 2012, 12:40:25 PM

There is no single, commonly accepted definition of the strong acid. That's why there will be always a gray area.

Title: Re: Chloric acid (HClO3) strong or weak
Post by: Borek on October 25, 2012, 12:42:32 PM

Quote from: keresturec on October 25, 2012, 12:37:30 PM

thanks guys for your help, sadly my professor said chloric acid is not universally accepted as a strong acid.   :-\

As it is a border case I am surprised by such a reaction, unless you were earlier given a list of acids that your prof considers to be the one you should follow. That sounds like completely arbitrary decision.

Title: Re: Chloric acid (HClO3) strong or weak
Post by: Big-Daddy on October 25, 2012, 01:00:47 PM

Quote from: keresturec on October 25, 2012, 12:37:30 PM

thanks guys for your help, sadly my professor said chloric acid is not universally accepted as a strong acid.   :-\

In my experience teachers are often tempted to save themselves having to change a student's marks by finding some way of justifying their marking system.

I would question whether HNO3 is universally accepted as a strong acid then. Wikipedia, for instance, rates it as "almost strong", not as strong.

Tend to choose among HClO4, HCl, HBr or HI for monoprotic strong acids. They are your surest bets.