# Why does Sodium Bicarb. Reduce Combustion Rate? [closed]

In many common solid-propellant model rocket engines, Sodium Bicarbonate, $$\ce{NaHCO3}$$, is used to predictably regulate/limit the rate of combustion of the fuel. My questions are as follows:

1. What is the mechanism for this (reaction?)?
2. How would I know how much $$\ce{NaHCO3}$$ to add to the fuel for a certain reaction delay time?
3. Is $$\ce{NaHCO3}$$ somewhat unique in it's ability to do this, or is this something that many common chemicals can do?

The use of dry NaHCO3 to extinguish a fire works in two ways:

First, the smothering of the flame with CO2 and water per the decomposition reaction:

$$\ce{2NaHCO3 ⟶ Na2CO3 + CO2 + H2O}$$

Second, the decomposition reaction is endothermic, so it lowers the temperature of the fire. To quote a source:

In an endothermic reaction heat must be continuously supplied and the reaction is not self-sustaining. Stop heating NaHCO3 and the decomposition will slow down and stop.

When heated, $$\ce{NaHCO_3}$$ is easily decomposed and produces much $$\ce{CO_2}$$ in the chemical reaction : $$\ce{2 NaHCO_3 -> Na_2CO_3 + CO_2 + H_2O}$$ and $$\ce{CO_2}$$ is a gas that tends to prevent Oxygen from maintaining the fire. As to your question 2, I have no idea how to solve it. And as to your question 3, $$\ce{NaHCO_3}$$ is by far the best compound and the cheapest. The other possibilities are the other alcaline bicarbonates, and apart from potassium bicarbonate, they are rather expensive.

• I guess that rules out francium bicarbonate! ;-) – Ed V Mar 30 at 19:42
• Which would vaporize from its own radiation anyway. I've read in Wikipedia that magnesium hydroxide works too, by endothermically evolving water vapor. – Oscar Lanzi Mar 30 at 19:47
• Cool! Thanks to Maurice & Oscar! – mpprogram6771 Mar 30 at 21:15