2H₂ + O₂ → 2H₂O – sparking a hydrogen explosion
ChemistryReactions & RedoxAges 13–14
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Reactions & Redox
A mixture of H₂ and O₂ gas in a flask does not react at room temperature. Click Ignite (or tap the flask): the spark supplies energy, effective collisions form H₂O, and the exothermic reaction spreads rapidly into an explosion. A live graph of molecule counts over time, the 2:1 mole ratio, the limiting reactant and conservation of mass are all shown so you can check them against the chemical equation.
Lesson: Chemical reactions, chemical equations, conservation of mass
What it shows
Hydrogen and oxygen can stay mixed at room temperature without reacting, because collisions rarely have enough energy to overcome the activation energy. A spark supplies that energy; once some molecules react, the heat released lets further collisions succeed, so the exothermic reaction spreads through the mixture as an explosion. The molecule counts follow the equation 2H₂ + O₂ → 2H₂O: H₂ and O₂ are always used in a 2 : 1 ratio, one gas may be left in excess, and the total mass of the particles stays constant.
How to use
Set the Number of H₂ molecules and the Number of O₂ molecules, and turn off Auto-ignite after 2 seconds so students first see that nothing happens. Then click Ignite (spark) or tap the flask and watch the graph of molecule counts. Try mixtures that are not in a 2 : 1 ratio to find the limiting reactant, and raise the Initial temperature to show that faster molecules alone do not start the reaction.
Parameters you can change
- Number of H₂ molecules 4–80
- Number of O₂ molecules 2–40
- Initial temperature 0–600 °C
- Auto-ignite after 2 seconds
Questions to explore
- Starting with 30 H₂ and 12 O₂ molecules, which gas is left over, and how many molecules remain?
- Why can a mixture of H₂ and O₂ stay at room temperature without reacting?
- Does the number of atoms of each element change during the reaction? What does this show about mass?
