polyprotic acids

[Kamille Kibria 2A]

ok so when there is an acid that can donate more than one H, when are we allowed to ignore the 2nd Ka value? like i know if the 2nd Ka value is significantly smaller than the first you just ignore it but is there a certain number/cutoff? or is there like a a certain difference between the 1st and 2nd Ka value we should know or something ?

[Sid Panda 3A]

ok so when there is an acid that can donate more than one H, when are we allowed to ignore the 2nd Ka value? like i know if the 2nd Ka value is significantly smaller than the first you just ignore it but is there a certain number/cutoff? or is there like a a certain difference between the 1st and 2nd Ka value we should know or something ?

Hi Kamille,

the book says if the 2nd deprotonation Ka is less than a thousandth of the 1st deprotonation Ka, you can ignore the 2nd deprotonation reaction.

Hope this helps.

[arisawaters2D]

Thank you! So just to clarify the second deprotonation will always have a smaller K value than the first?

[Sabrina Galvan 3J]

Thank you! So just to clarify the second deprotonation will always have a smaller K value than the first?

Yes! This is because it is more difficult for a molecule to ionize into H+ ions even further, and therefore it has to become less acidic.

[Khoa Vu 3l]

Sorry for being late to the discussion! I would like to add on how you can generally ignore the Ka2 approximation if it is again a thousandth of the Ka1 value, meaning the exponent for the Ka1 value - the exponent for the Ka2 value > 3 (e.g. Ka2 = 1.8 *10^-10 and Ka1 = 1.2 *10^-5, the difference between the Ka values is five and greater than 3, so the difference is magnitude is greater than a thousand and your approximation should be justified). You can also always check your approximation using the five percent rule if the exponent difference is relatively close to 3 and you are unsure about your approximation. If the percent ionization for the second deprotonation is greater than five, then your approximation is unfortunately not valid and you would have to use the quadratic formula.

[Kristina Krivenko 3I]

Adding on to what has been said above, according to the textbook, we can ignore second deprotonation for all polyprotic acids we will encounter in this class, except for H2SO4. While it is still good to compare Ka1 and Ka2 values to double check that we can ignore second deprotonation, I think what was stated in the textbook can be used as a good guideline. Hope this helps :)

[Ryan_Kien_1L]

Thank you! So just to clarify the second deprotonation will always have a smaller K value than the first?

Yes, and the third Ka, if there is one, will be smaller than the second as well, because they become less acidic.

[Bella Bursulaya 3G]

I believe you can ignore the Ka2 if it is << Ka1, and this is true for most acids EXCEPT H2SO4, so just use Ka1 and Ka2 for that acid. This is because H2SO4 is strong, so it's first Ka essentially goes to completion, while the other acids are weak, so H+ does not equal HA. And the Ka2 is smaller because HSO4- has a negative charge, meaning it's going to try to hold onto that H+ more which is why it dissociates less.

[josephspindler2H]

t=Thank you for asking this question! I was wondering when we were supposed to ignore the effect of the second or third ka values.

[Jack_Pearce_2H]

You can ignore it only when the second Ka value (Ka2) is at least 1/1000 smaller than the first Ka value. Also, depending on the acid, yes there most definitely can be K(n) protonations based on how many hydrogens the acid has (H(n)A).

[Naomi Hernandez-Ramirez 1J]

If the 2nd deprotonation of Ka is less than a 1/1000 of the 1st deprotonation of Ka, ignore the 2nd deprotonation reaction