Knowledge database: Chemical reactions: Limiting reagent

The limiting reagent is the reagent that determines to what extent will a reaction be completed, because of the fact that the amount of this reagent is too small to completely react will all the other reagents. In other words, the reagent of which there is less than the coefficients in the reaction equation request, is the limiting reagent.

Let's take a look at the following reaction: HCl + NaOH -> NaCl + H₂O. This is a very simple case in which all the reagents and products have the same coefficient (1). One mole of hydrochloric acid weighs about 36.5 grams, and one mole of sodium hydroxide weighs about 40.0 grams. If we take 40.0 grams of hydrochloric acid and 40.0 grams of sodium hydroxide, then we have a case in which there is more hydrochloric acid than is needed, since we added more than one mole of it, while the amount of sodium hydroxide equals one mole. Because of that, there is a deficit of sodium hydroxide, so some hydrochloric acid will remain unreacted. This is why sodium hydroxide is the limiting reagent in this case.

On the picture above we can see the result of a similar reaction between acetic acid and sodium bicarbonate. In this case, there was also a surplus of acid in the reaction, which was easily determined by using a universal pH indicator strip.