Can sodium bicarbonate be considered an acid?

I was reading about acids and bases today and finally decided to question the statement that "baking soda is a base." 


Let's start with the dissolving of baking soda, $\ce{NaHCO3}$. The equation to represent its dissolution in water is $\ce{NaHCO3 + H2O <=> Na+ + H3O+ + CO3^{-2}}$.


In this "reaction," baking soda is a proton donor, creating hydronium ions and a conjugate base, $\ce{CO3^{-2}}$. Doesn't this make baking soda an acid?


If what I have written here is correct, then baking soda will create a base when dissolved, but baking soda itself is not the base. Is this accurate?




The statement you read that "baking soda is a base" comes from the fact that a solution of sodium bicarbonate (baking soda) and water has a pH of around $8.3$. 


However, sodium bicarbonate is *amphoteric* with respect to Brønsted–Lowry acid/base theory, which means that it can act as either an acid or a base. More specifically, the bicarbonate ion is *amphiprotic*, meaning that it can either donate a proton, acting as an acid, or accept a proton, making it act as a base. The reason that a sodium bicarbonate solution is slightly basic is that it's ability to take a proton from a water molecule is greater than its ability to donate a proton to a water molecule. In other words, the chemical reaction causing the solution to be basic is: 


$$\ce{HCO3- + H2O <--> H2CO3 + OH-}$$ 


The other equilibrium reaction at play, the reaction "trying" to make the system more acidic, is: 


$$\ce{HCO3- + H2O <--> CO3^2- + H3O+}$$ 


Again, in an otherwise pure aqueous system, it is the first reaction that dominates, and thus giving the observed slightly basic solution. 


Note that the $\ce{H2CO3}$ produced in the first reaction is also part of yet another equilibrium process: 


$$\ce{H2CO3 <--> CO2 ^ + H2O}$$ 


Where the $\ce{CO2}$ is liberated as a gas. This is the cause of the well-known "fizzing" when baking soda is added to vinegar (acetic acid). 


 9

Sodium bicarbonate can indeed act as an acid. You just need a strong enough base with which it can react.


Take 0.1 mol sodium hydroxide and add it to a 100 mL distilled water sample, then add 0.1 mol sodium bicarbonate. The pH goes down in the second step as the solutes are combined to make the almost-but-not-quite strong base sodium carbonate. Compare this reaction:


$\ce{NaHCO3 + NaOH -> Na2CO3 + H2O}$


with this one where we see the same proton transfer process:


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


 5