Slaked lime: Why does calcium hydroxide have to be used in conjunction with sodium hydroxide? Why can't it be used alone? [closed]

To absorb carbon dioxide. I haven't been able to find an answer to the above anywhere.

The overall reaction for a solid soda lime scrubber according to the Wikipedia page here 1 is:

$$\ce{CO2 + Ca(OH)2 -> CaCO3 + H2O + {heat (in the presence of water)}}$$

Each mole of $\ce{CO2}$ (44 g) reacting with calcium hydroxide produces one mole of water (18 g).

The reaction can be considered as a strong-base-catalysed, water-facilitated reaction.

The reaction mechanism of carbon dioxide with soda lime can be decomposed in three elementary steps:

1. $\ce{CO2 (g) -> CO2 (aq)}$
($\ce{CO2}$ dissolves in water - slow and rate-determining)
2. $\ce{CO2 (aq) + NaOH -> NaHCO3}$
(bicarbonate formation at high pH)
3. $\ce{NaHCO3 + Ca(OH)2 -> CaCO3 + H2O + NaOH}$
(($\ce{NaOH}$ recycled to step 2) - hence a catalyst)

This sequence of reactions explains the catalytic role played by sodium hydroxide in the system and why soda lime is faster in chemical reactivity than calcium hydroxide alone. It reacts much more quickly and so contributes to a faster elimination of the $\ce{CO2}$ from the rebreathing circuit. The formation of water and the moisture from the respiration also act as a solvent for the reaction. Reactions in aqueous phase are generally faster than between a dry gas and a dry solid.

• But if the first step, the dissolving of CO2 is the rate determining step then the efficiency of steps 2 and 3 shouldn't make any noticeable difference? – Edward Garemo Jun 4 '18 at 4:43

I'm assuming that you are using a solution to trap the $\ce{CO2}$. It would seem that you're probably filtering out the $\ce{CaCO3}$.

The gist is that $\ce{CaOH2}$ has a low solubility in water (1.73 g/L at 20 °C). So adding $\ce{NaOH}$ increases the efficiency of trapping the $\ce{CO2}$ in bubbles of air as $\ce{CO3^{2-}}$ in the solution.

• Bad assumption. I think the OP is asking about solid soda lime scrubbers – Waylander Jun 3 '18 at 16:50