Hello, Please answer fully and explain as well! Very important to explain to get

Question

Hello,

Please answer fully and explain as well!  Very important to explain to get full rating.

The purpose here is to learn.  Thanks in advance!

Q: Malonic acid is a diprotic carboxylic acid. the first acidic proton has pKa ~ 3, and the 2nd acidic proton has pKa ~ 6.

(Malonic acid => (HO2CCH2CO2H)

1) Why is malonic acid stronger than acetic acid which has pKa ~ 5?

2) Why is the anion of malonic acid less acidic than malonic acid? (Why is the 2nd acidic proton less of an acid?)

Please explain fullly and diagrams/pics are encouraged to help illustrate idea/concept

Explanation / Answer

pKa is the value that you need to use to judge which acid is stronger.

Smaller the pKa value is, stronger the acid is.

1) the pKa is 3 when Malonic acid lose its first proton, which is smaller than pKa of 5 for acetic acid. Thus, Malonic acid is stronger than acetic acid.

2) However, when Malonic acid in its anionic form, it need lose its 2nd proton to exhibit its acidity. However, it is harder. pKa of this step is 6, which is much smaller than pKa = 3, which is for Malonic acid losing its first proton. Thus, anionic malonic aicd is a weaker acid than malonic acid itself.

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