# Identifying Lewis acid

What is the Lewis acid on the reactants side?

$$\ce{HNO2(aq) + HPO4^{2-}(aq) <=> NO2-(aq) + H2PO4-(aq)}$$

The book says it is the $$\ce{H+}$$ only, not the whole molecule of the nitrous acid, how so?

• That's a Bronsted not Lewis equilibrium. – Mithoron Mar 16 at 18:14
• @Mithoron the reaction can be interpreted in terms of the Lewis theory. But yes, when that theory identifies $\ce{H^+}$ as the acid, the Bronsted interpretation is usually simpler. – Oscar Lanzi Mar 16 at 22:16

What we have, in the Lewis theory, is an acid-base displacement reaction. The Lewis acid, $$\ce{H^+}$$, is initially combined with one Lewis base, $$\ce{NO2^-}$$. Then another Lewis base, $$\ce{HPO4^{2-}}$$, takes the $$\ce{H^+}$$ away displacing the $$\ce{NO2^-}$$.
Similarly when $$\ce{HCl}$$ is dissolved in water and dissociates, the water is displacing the chloride ion. The Lewis acid involved in this displacement is again $$\ce{H^+}$$.