About this experiment
Choose hydrogen and oxygen molecule counts, then react them one batch at a time. Each batch consumes two hydrogen molecules and one oxygen molecule to form two water molecules. Consumed molecules fade into the background while products appear, making excess reactant and theoretical yield visible without hiding leftover particles.
Learning goals
- Use balanced coefficients to determine the limiting reactant
- Predict theoretical product count and leftover reactants
- Verify atom conservation rather than conservation of molecule count
- Explain the difference between the limiting reactant and the reactant in excess
How to use it
- 1With 12 hydrogen and 8 oxygen molecules, predict which reactant limits the yield before reacting.
- 2Advance one batch and count the decrease in each reactant and the increase in water.
- 3React all available batches and compare water formed with theoretical yield.
- 4Try the exact-ratio preset, then try an odd hydrogen count such as 13. Explain any leftover hydrogen.
- 5Set 6 hydrogen and 10 oxygen molecules. Predict the water formed and the leftover reactant, then react all to check.
The model
Assumptions
- Particle counts are integers. A complete batch requires exactly two H2 molecules and one O2 molecule.
- All available complete batches react with 100% model yield. Side reactions, equilibrium, activation energy and actual reaction rates are omitted.
- Animation duration is chosen for teaching and does not represent a chemical rate. Molecular drawings are schematic, including bond angles.
- The model counts molecules and atoms, not a measured gas volume or sample mass. Coefficients can separately be interpreted as molar ratios.
Try this in class
- Q1Why can oxygen remain when hydrogen is the limiting reactant?
- Q2For 13 hydrogen and 8 oxygen molecules, why is the theoretical water yield 12 rather than 13?
- Q3How many hydrogen and oxygen atoms exist before and after one complete reaction batch?
- Q4In a car engine, which reactant is usually in excess: fuel or oxygen? What happens if it is the other way round?
- Q5Scale the reaction up to moles: 4 mol of H₂ react with 3 mol of O₂. How many moles of water form, and what is left over?
Frequently asked questions
Why does the molecule count change although matter is conserved?
Atoms are rearranged into different molecules. One batch begins with three molecules and ends with two, but it still contains four hydrogen atoms and two oxygen atoms.
What is the difference between theoretical and actual yield?
Theoretical yield is the maximum allowed by the supplied reactants and balanced equation. Real measured yield may be lower. This model assumes every available complete batch reacts successfully.
Is the reactant with fewer molecules always the limiting one?
No. You must compare the amounts with the coefficients in the balanced equation. Here hydrogen runs out first whenever there are fewer than twice as many hydrogen molecules as oxygen molecules, even if there are more hydrogen molecules in total.