I am currently studying stoichiometry (high school level) and we came across the following equation in school:

$\ce{2H2 + 1O2 -> 2H2O}$

which is clear to me. We learned in school that reactions in chemistry only happen if there are enough atoms/molecules to react (so every molecule/atom can react), therefore the following reaction won't do anything:

$\ce{1H2 + 1O2 -> nothing happens}$

So now my first question: Why does the following not happen:

$\ce{1H2 + 1O2 -> 1H2O + 1O}$

And, as a follow up question, why does the following not work?

$\ce{H2 + O2 -> H2O + 1/2O2 (Left) }$

What do I do wrong? (maybe I simply misunderstood our teacher)

Are there any explanations for this? I'm especially interested in the answer of my example with $1$ mole of $\ce{H2}$ and $1$ mole of $\ce{O2}$. According to our teacher nothing will happen at all. I would assume that at least all $\ce{H2}$ molecules react with half of the oxygen molecules to $\ce{H2O}$ - leaving half a mole of $\ce{O2}$ left over. (otherwise this would mean that all molecules "know" each other so they are "aware" that not all molecules/atoms can react)

Can somebody help me? Or point out what I am doing wrong?

  • $\begingroup$ You may also want to have a look at this question for this reaction. Your intuition is quite right. $\endgroup$ Commented Mar 31, 2014 at 6:42

1 Answer 1


As you have said, you are studying stoichiometry at High School Level. From this I can guess, that you have probably not studied the concept of Limiting Reagent yet.

What is Limiting Reagent?

In a chemical reaction, the limiting reagent, also known as the "limiting reactant", is the substance which is totally consumed when the chemical reaction is complete. The amount of product formed is limited by this reagent since the reaction cannot proceed further without it. The other reagents may be present in excess of the quantities required to react with the limiting reagent.

As per your question, $\ce{H2}$ is limiting reagent. Hence it will get totally consumed when reaction happens and 1/2 Moles of $\ce{O2}$ will be left.

1 mole of H2 and 1 mole of O2 won't make 1 mole of H2O and 1/2 mole O2.

1 mole of $\ce{H2}$ and 1 mole of $\ce{O2}$ will make 1 mole of $\ce{H2O}$ and 1/2 mole $\ce{O2}$ will be left.

$\ce{H2 + O2 -> H2O + 1/2O2 (Left) }$

And here is an advice for you. If your teacher is saying that this will not happen at all, tell him to be sure that reactant in reaction are single molecules, not 1 mole.

Read this to be more clear on concept of limiting reagent.

  • 1
    $\begingroup$ Wow! Thanks for the great answer :) I'll definitely share your answer with my class mates! $\endgroup$ Commented Sep 16, 2013 at 19:40
  • $\begingroup$ I've said it before, but your edits are always appreciated! (I was trying to figure out a way to accept that one anyway, but I figured it would be good to point out the rationale, so please don't take it personally!) $\endgroup$
    – jonsca
    Commented Sep 16, 2013 at 20:59

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