Chemistry - Why are acetic acid and sodium bicarbonate mixed to unblock a drain?

Solution 1:

Quoting text from this source under the section "Why could baking soda and vinegar clean clogged drains?" :

Bicarb soda and vinegar react because of the acid-base reaction. Bicarb soda is bicarbonate $(\ce{NaHCO3})$ and vinegar is acetic acid $(\ce{CH3COOH})$.

When bicarb soda and vinegar react, they fizzle and sizzle and they expand. This is why they are able to remove clogs from drains. The pressure from the expanding product shoves the clog down as it moves along. The vinegar solution (vinegar plus water) can remove soap residue that clings to the walls of the pipes.

So it seems like your conjecture is justified, that it is not actually the reactivity of these two substances with the waste that is responsible for majority of the cleansing, but simply the mechanical action of the expansion of the carbon dioxide gas and the forming product that pushes the waste down the drain, and leads to unclogging.

The relevant chemical equation is:

$$\ce{CH3COOH (aq) + NaHCO3 (s) -> CH3COONa (aq) + CO2 (g) + H2O (l)}$$

This is similar to the mechanism by which baking powder makes a cake fluffy on addition of a little vinegar as explained here.

Solution 2:

$$\ce{NaHCO3(s) + CH3COOH(aq) -> CO2(g) + H2O(l) + CH3COONa (aq)}$$

From what I can see in the reaction, an amount of water and carbon dioxide is produced which has the potential to move around blockages in the drain similar to using club soda to remove stains from a shirt or table cloth. The resulting sodium acetate is a weak conjugate base and water is too weak to react with anything blocking a drain to begin with. By deduction, I think your hypothesis of the drain cleaning being an action of the gases produced is correct.

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Solution 3:

The exothermic reaction of the acetic acid on $\ce{NaHCO3}$ provides heat and I do agree that the vigorous nature of the reaction forming $\ce{CO2}$ is likely instrumental in clearing the drain.

Also, I suspect employing an excess of NaHCO3 is probably beneficial as on warming it releases CO2 and creating more alkaline Na2CO3, which will also attack grease.

Reaction:

$\ce{2 NaHCO3 -> Na2CO3 + H2O + CO2 (g)}$

Note: Baking Soda is a common ingredient employed in dishwashers owing to its ability, especially on warming, to dissolve fats/grease (see discussion here).