Reactants, Products and Leftovers (Part II) [03 May, 2021]

Reactants, Products and Leftovers

We discussed the concept of “Reactants, Products and Leftovers” from the scratch via PhET simulation in the Part I. We will extend our discussion here. :slight_smile:

Here our main focus is in the Level 2 and Level 3 of the Game section of the simulation.


  1. Looking the balanced chemical equation how one will predict the amount of products and leftovers for a given amount of reactant?
  2. What is limiting reagent? How the concept of limiting reagent is helpful in determining the amount of products?
  3. Given the balanced chemical equation, 4NH3 + 3O2 ----> 2N2 + 6H2O
    Now if 6 molecule of NH3 and 4 molecule of O2 is given as reactants, what will be the number of molecules of the products and leftovers? Which reactant will be limiting reagent in this case?
  4. How to determine the limiting reagent for a given reaction?
  5. Share your thoughts, questions and comments regarding the simulation and your experience during exploring the simulation.

We welcome all kind of suggestions and feedback if this was useful and how can be collectively make it better.

Collaborators in video resource creation: Prof Savita Ladage, Deepak Arora, Hanza George, Mursaleen Shaikh, Ravi Sinha, Ashish Pardeshi, Indrani Das, Swarnava Mitra

PhET Interactive Simulations
University of Colorado Boulder


@ravi312 asked Q3 in the discussion. We tried to find the limiting reagent in that case during the discussion session. But we won’t be able to come to any satisfactory answer.

I asked one of my friend @Hemant about the same. According to him, we will only call something limiting reagent when it completely vanished from the reaction medium. So for 2 component reaction if more that one component is left in the medium, the concept of limiting reagent won’t work there. Hence, it’s inconclusive in this case.

This seems reasonable to me. I will also urge others to look into it and put forward their comments on the same. @ravi312, @Ashish_Pardeshi @savital @mursaleen @indrani @deepak @hanza

1 Like

Hi !,
I would like to differ slightly, even if one of the reactant still does not vanish completely from the reaction, still the concept of limiting agent will hold. Example take this reaction-
1SO2 + 3H2 → 1H2S + 2H2O
and let us suppose, we have 6 molecules of SO2 and 4 molecules of H2O. So after the reaction, there will be 1of H2S, 2 of H2O formed as products and 5 of SO2 and 1of H2 remaining as leftovers.
Here, we see that the amount of H2 (which is less before the reaction) will determine the amount of products formed, although it is still remaining as a leftover. So, in this case, H2 is the limiting agent. (This is one of the example from Level 3 of phet simulation). We need at least 2 more of H2 for the next set of reaction to occur.

So, in bulk medium (not molecular level),which is the laboratory scale, you will not always be able to exactly say that one of the reactant is completely used up in a reaction, often that is determined by other factors such as the equilibrium constant value of that reaction, however, if the amount (or moles) of a reactant present in the system is less than the other (wrt to the stoichiometry of the reaction), then that reactant becomes the limiting reagent.