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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?

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    $\begingroup$ That's a Bronsted not Lewis equilibrium. $\endgroup$ – Mithoron Mar 16 at 18:14
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    $\begingroup$ @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. $\endgroup$ – Oscar Lanzi Mar 16 at 22:16
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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 $\ce{H^+}$.

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