The simulation
Three trays sit on a bench. The left two hold your reactants, the right one holds the product. Two sliders in the bottom strip set how many particles of each reactant you start with, and the balanced equation sits alongside them.
Press React and the particles pair off according to the equation ratio. Reactants that get used shrink away and drift into the product tray, where new product particles grow in. Reactants that cannot find a partner stay exactly where they are, full size and gently pulsing, so the leftover is countable on screen.
Boards float above each tray showing the name and one big number. After the reaction they tag themselves LIMITING or EXCESS, and a banner announces the verdict.
Three reactions are available: hydrogen and oxygen, magnesium and oxygen, and the Haber process where hydrogen is consumed three times faster than nitrogen and catches pupils out.
A class activity: Beat the Balance
Put the simulation on the board and set both sliders yourself without pressing React. Pupils write down three predictions on whiteboards: which reactant will run out first, how much product will be made, and how much will be left over. Then press React and reveal.
Run it five or six times, escalating. Start with an obvious case, then switch to the Haber tab where the 3:1 ratio breaks their intuition. Finish by challenging them to find slider settings that leave nothing over at all, which forces them to reason backwards from the ratio rather than guessing.
For the plenary, ask them to explain in one sentence why doubling the excess reactant changes nothing about the yield.