[dontwanttofail]

I just don't understand what is the difference between absolute and relative acidity?  To outline the situation, I am in a modeling course where I use Spartan to model quantum chemical calculations.

Here is the issue.  I want to use isodesmic reactions when i am faced with homolytic bond cleavage problems because the methods to calculation isodesmic reactions are much faster than the ones that can handle bond making/breaking.

So for example the porblem might say
find deltaH for:

absolute: HR --> H+ + R-

can use

HR + A- --> -HA + R- (isodesmic, so can model and get quick energies)
-HA --> H+ + A- (can look up in a table somewhere assuming A is well experiemented)

Now can use Hess's Law and combine the predetermined values with my modeled ones and find the deltaH

but what is the meaning of relative acidity/absolute acidity?  Does it just mean I am calculating acidity relative to something else? 

Please help

I can use the same thing for a hydrogenation reaction and to find Heat of formation

Thanks

[BluRay]

I just don't understand what is the difference between absolute and relative acidity?  To outline the situation, I am in a modeling course where I use Spartan to model quantum chemical calculations.

Here is the issue.  I want to use isodesmic reactions when i am faced with homolytic bond cleavage problems because the methods to calculation isodesmic reactions are much faster than the ones that can handle bond making/breaking.

So for example the porblem might say
find deltaH for:

absolute: HR --> H+ + R-

can use

HR + A- --> -HA + R- (isodesmic, so can model and get quick energies)
-HA --> H+ + A- (can look up in a table somewhere assuming A is well experiemented)

Now can use Hess's Law and combine the predetermined values with my modeled ones and find the deltaH

but what is the meaning of relative acidity/absolute acidity?  Does it just mean I am calculating acidity relative to something else? 

Please help

I can use the same thing for a hydrogenation reaction and to find Heat of formation

Thanks

The relative acidity of the compound HA is...relative to this reaction:

HA + H₂O <--> H₃O⁺ + A^-

The acidity constant Ka(HA) is then defined as:

Ka = [H₃O⁺]*[A^-]/[HA]

([H₂O ] doesn't appear in the denominator because we assume here the species different than water to be in low concentrations, so [H₂O ] stays constant and is included in the Ka.)