Why is CO2 + Ca(OH)2 an acid-base reaction? [closed]








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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.




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.


 3

What about this ?


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


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


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


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


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


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


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


 2

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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