Distinguishing bronsted acids vs bases

Moderators: Chem_Mod, Chem_Admin

Sophia Fox 4B
Posts: 29
Joined: Fri Sep 28, 2018 12:27 am

Distinguishing bronsted acids vs bases

Postby Sophia Fox 4B » Thu Dec 06, 2018 11:47 am

When given an equation (for ex: HNO2 + HPO4 ^2- <=> NO2^- + H2PO4^-), how can you tell which one is going to be the bronsted acid and which one will be the bronsted base? Especially if you weren’t given the products? I know that a bronsted acid is the proton donor and a bronsted base is a proton acceptor but I’m not sure how to apply that.

Hanlin Zhu 4G
Posts: 30
Joined: Fri Sep 28, 2018 12:28 am

Re: Distinguishing bronsted acids vs bases

Postby Hanlin Zhu 4G » Thu Dec 06, 2018 11:55 am

In the example you give, HNO2 + HPO4 ^2- <=> NO2^- + H2PO4^-, HNO2 is the acid, because it loses (donates) a proton to HPO4^2-. HPO4 ^2- is the base, because it accepts the H+ from HNO2. You can distinguish bronsted acids and bases by finding their roles in proton transfer.

Erin Nash - 4G
Posts: 29
Joined: Fri Sep 28, 2018 12:27 am

Re: Distinguishing bronsted acids vs bases

Postby Erin Nash - 4G » Thu Dec 06, 2018 3:42 pm

A Bronsted acid is one that donates protons and a Bronsted base is one that accepts protons.

David S
Posts: 54
Joined: Fri Sep 28, 2018 12:15 am

Re: Distinguishing bronsted acids vs bases

Postby David S » Thu Dec 06, 2018 4:22 pm

HPO4^2- is an amphiprotic derivative of H3PO4. We know that with polyprotic acids, after the first proton is lost, subsequent amphiprotic acids formed are substantially harder to deprotonate (removing an H+ from an increasingly negative anion molecule is harder); the increase in pKa with each additional deprotonation is quantitative evidence of this. Instead, subsequently formed apmhiprotic molecules are more likely to act like a base and take a proton back. This is especially true for amphiprotic anions like HPO4^2-. For that reason, I'd wager that HPO4^2- will act as the base, and HNO2 will act as an acid. Comparing the pKa of HNO2 (~3.4 from online results) with pKa of HPO4^2- (~12.3-12.6 from online results) supports this.


Return to “Bronsted Acids & Bases”

Who is online

Users browsing this forum: Google [Bot] and 0 guests