Why do carbon dioxide and sodium hydroxide not form sodium oxide and carbonic acid?
$\ce{CO2 + 2NaOH -> Na2O + H2CO3}$
as opposed to:
$\ce{CO2 + 2NaOH -> Na2CO3 + H2O}$
This is in the context of fractional distillation of liquified air, before which carbon dioxide must be removed using sodium hydroxide.
Answer
Sodium oxide is not a base in either the Bronsted-Lowry or Arrhenius senses. It is technically a base anhydride, meaning that it can be hydrated to yield a base:
- $$\ce{Na2O + H2O -> 2NaOH}$$
Carbon dioxide can similarly be viewed as an acid anhydride, meaning it can be hydrated to form an acid.
- $$\ce{CO2 + H2O <=> H2CO3}$$
The difference is that the hydration of sodium oxide is essentially irreversible. One program I used estimated $K_{eq}=10^{39}$ for sodium oxide hydration. In contrast the hydration of carbon dioxide is reversible, with $K_{eq}\approx1.5\times10^{-3}$.
The reaction that you are wondering about, $\ce{CO2 +2NaOH⟶Na2O +H2CO3}$, is essentially the dehydration of sodium hydroxide by $\ce{CO2}$ to form sodium oxide and carbonic acid, that is, it is reaction 2 minus reaction 1. But the equilibrium constants I just mentioned show that sodium oxide has an "affinty" for water that is about 42 orders of magnitude greater than carbon dioxide's affinity for water. So that reaction is very unfavorable. The reverse reaction would be extremely favored: Sodium oxide would be a powerful reagent for $\ce{CO2}$ removal prior to air liquefaction, at least as powerful as sodium hydroxide.
If you are curious, here is the R code I used to estimate the equilibrium constant of sodium oxide hydration, using the package CHNOSZ:
> subcrt(c("Na2O","H2O","NaOH"),c(-1,-1,2),T=25)
info.character: found H2O(liq), also available in gas
subcrt: 3 species at 298.15 K and 1 bar (wet)
$reaction
coeff name formula state ispecies
2031 -1 sodium oxide Na2O cr 2031
1 -1 water H2O liq 1
1157 2 NaOH NaOH aq 1157
$out
logK G H S V Cp
1 39.01742 -53229.29 -57143.24 -13.24728 -36.04979 -40.8745
No comments:
Post a Comment