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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.
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.
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.
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