Problem 9 - Solution

                                            

The proton on the hydroiodic acid is the most acidic proton.  Spartan Calculations indicate that this proton has the largest positive electrostatic potential suggesting that it is the most acidic proton.  It is followed by hydrochloric acid and hydrofluoric acid is the least acidic of the three.  This is consistent with the experimental pKa for these three compounds.

 

 

H-F

H-Cl

H-I

pKa= 3.1

pKa = -3.9

pKa= -10.4

Potential = +31.0

Potential= +51.5

Potential= +83.3

 

 

 







Furthermore, other trends can prove the acidity by looking at bond lengths one is able to tell that the longer the bond length the weaker the bond and the stronger the acid. 

 

Hydrofluoric acid has the shortest bond length and therefore is the weakest acid. Then compared to the hydrochloric and hydroiodic acid the bonds are longer, so the halide can be pulled off more easily.  

             

Hydrochloric acid has a bond length that is longer than hydrofluoric, but shorter than hydroiodic.  Therefore, this makes hydrochloric acid a stronger acid than hydrofluoric.  

              

Hydroiodic acid has the longest bond length because iodine is a large atom.  This causes the bond length to be long and weak; so breaking the bond is less energetically costly making hydroiodic acid the strongest of the three acids.

               

 

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last modified 16 November 2003 by princece@jmu.edu