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$\ce{CO_2 + Ca(OH)_2 -> CaCO_3 + H_2O}$

I was wondering why the reaction between carbon dioxide and calcium hydroxide is considered an acid-base reaction when there are no hydrogens which are donated or received (at least that's what I can see). It looks like a double displacement reaction to me.

I will be very thankful if anyone can clear up my confusion.

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    $\begingroup$ CO2 can act as Lewis acid... BTW are you talking about aqueous solution here? $\endgroup$
    – Mithoron
    Oct 21, 2019 at 21:49
  • $\begingroup$ You could imagine this reaction taking place in the two steps $\ce{2OH^- -> H_2O + O^{2-}},\ce{O^{2-} + CO_2->CO_3^{2-}}$, then bring the calcium in to pair with the carbonate. That just isn't how it actually proceeds for the most part, which should make some sense since this reaction involves $\ce{OH^-}$ acting as an acid with another $\ce{OH^-}$, so it is analogous to the autodissociation of water. Instead, initially $\ce{OH^-}$ wants to act as a base (which is probably familiar) while the $\ce{CO_2}$ wants to act as a Lewis acid (which might not be familiar). $\endgroup$
    – Ian
    Oct 22, 2019 at 16:14

3 Answers 3

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My guess is that your confusion stems from the fact that there are 3 main definitions of acid/base.

In introductory chemistry, we cover the concept of Arrhenius acid/base, where the acid is a proton donor and the base is a hydroxide donor.

$$\ce{HCl + NaOH -> H2O + NaCl}$$

As we learn more, we're introduced to the concept of Brønsted-Lowry acid/base, where the acid is a proton donor and the base is a proton acceptor.

$$\ce{HCl + NH3 -> NH4+ + Cl-}$$

Finally, we're introduced to the concept of Lewis acid/base, where the acid is an electron-pair acceptor and the base is an electron-pair donor.

Your example is confusing probably because it is one of these cases. Hydroxide has a lone pair (Lewis base). That lone pair interacts with an empty $\pi*$ orbital in carbon dioxide (a Lewis acid) to create a new bond (bicarbonate). The bicarbonate then reacts with water in a more familiar way, so technically, there are multiple acid/base interactions happening even in your simple example.

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What about this ?

$$\ce{CO2(aq) + H2O <=> H2CO3}$$

$$\ce{H2CO3 <=> H+ + HCO3-}$$

$$\ce{HCO3- <=> H+ + CO3^2-}$$

$$\ce{Ca(OH)2 <=> Ca(OH)+ + OH-}$$

$$\ce{Ca(OH)+ <=> Ca^2+ + OH-}$$

$$\ce{H+ + OH- <=> H2O}$$

$$\ce{Ca^2+ + CO3^2- <=> CaCO3 v}$$

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An acid-base reaction is a reaction where a base is destroyed by an acid. Here the base OH- is present in the molecule Ca(OH)2. And it is destroyed by CO2, which in solution is acid as it produced the acid H2CO3 with water. So the reaction CO2 + Ca(OH)2 is an acid-base reaction

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