Question

\(\mathrm{CO}_{2}(\mathrm{g})\) can be removed from confined quarters (such as a spacecraft) by allowing it to react with an alkali metal hydroxide. Show that this is a Lewis acid-base reaction. For example, $$\mathrm{CO}_{2}(\mathrm{g})+\mathrm{LiOH}(\mathrm{s}) \longrightarrow \mathrm{LiHCO}_{3}(\mathrm{s})$$

Short answer

In this reaction, CO2 acts as a Lewis acid because it accepts an electron pair and LiOH acts as a Lewis base because it donates an electron pair. This fulfills the criteria for a Lewis acid-base reaction.

Step-by-step solution

Determine the Lewis acid and base

A Lewis acid is a molecule or ion that can accept an electron pair, and a Lewis base is a molecule or ion that can donate an electron pair. In this reaction, \(\mathrm{CO}_{2}\) is acting as a Lewis acid and \(\mathrm{LiOH}\) as a Lewis base as hydroxide ion (\(\mathrm{OH^{-}}\)) donates an electron pair to \(\mathrm{CO}_{2}\) molecule, resulting in the formation of \(\mathrm{LiHCO}_{3}\).

Describe the Lewis acid-base interaction

Once the \(\mathrm{OH^{-}}\) ion donates an electron pair to the \(\mathrm{CO}_{2}\) molecule, a bond is formed. Thus, a transfer of electron pair occurs from \(\mathrm{LiOH}\) (Lewis base) to \(\mathrm{CO}_{2}\) (Lewis acid), implementing a Lewis acid-base reaction.

Key concepts

Understanding Acid-Base Chemistry

Acid-base chemistry is fundamental to understanding a plethora of reactions in chemistry, including those necessary for life itself. At its simplest, an acid is a substance that can release a proton (a hydrogen ion, H+) in a solution, whereas a base is a substance that can accept a proton. However, in the broader Lewis definition, an acid is an electron pair acceptor and a base is an electron pair donor. This definition expands the scope of acid-base reactions to include those without the transfer of protons, such as the reaction between carbon dioxide (CO2) and an alkali metal hydroxide in the exercise.

The interaction between acids and bases often results in the formation of a salt and usually water in the classical Arrhenius context. In the Lewis context, acid-base reactions result in the formation of a coordinate covalent bond where both electrons in the bond come from the same atom, the Lewis base.

Role of Alkali Metal Hydroxide in Reactions

Alkali metal hydroxides, such as lithium hydroxide (LiOH), are strong bases that are commonly used in acid-base reactions. They consist of an alkali metal cation and the hydroxide anion OH-. When dissolved in water, these compounds readily dissociate to provide OH- ions, which are very effective electron pair donors. In solid form, as used in the spacecraft application presented in the exercise, they still can react with gases like CO2 due to the availability of OH- ions.

Given their high reactivity, alkali metal hydroxides are useful in neutralization reactions where they react with acids to form water and a salt. They are also applied in various industrial processes, including the sequestration of carbon dioxide.

Electron Pair Donation in Lewis Acid-Base Theory

Electron pair donation is at the heart of the Lewis theory of acids and bases. A Lewis base donates an electron pair, while a Lewis acid accepts this electron pair, leading to the formation of a coordinate covalent bond. This type of bond formation is crucial in understanding the behavior of numerous chemical species in a variety of reactions beyond the traditional scope of acid-base chemistry.

For instance, in the reaction between CO2 and LiOH, the hydroxide ion from LiOH donates a pair of electrons to the electron-deficient carbon atom in CO2. This results in the formation of lithium hydrogen carbonate (LiHCO3), illustrating a typical Lewis acid-base interaction.

Carbon Dioxide Sequestration Chemistry

Carbon dioxide sequestration is a process that captures CO2 emissions, from sources like power plants, preventing them from entering the atmosphere and contributing to climate change. One of the methods of sequestration involves reacting CO2 with alkali metal hydroxides, forming stable carbonates or bicarbonates, as shown in the exercise.

This process is not just vital for reducing greenhouse gas emissions, but it also serves an essential function in controlled environments such as spacecraft, submarines, or any closed-loop life support system. By converting CO2 into a solid form, such as lithium hydrogen carbonate (LiHCO3), we effectively remove the gas from the environment, improving air quality and mitigating the harmful effects of CO2 accumulation.