The number of ammonia molecules formed is not an input. It follows from molecules pairing up in the coefficient (mole) ratio.
First predict, then change the molecule numbers and observe, and finally check yourself.
Key summary: (1) The coefficients in N₂ + 3H₂ → 2NH₃ give the ratio in which molecules react (1:3:2). (2) When one reactant runs out, the reaction stops and the other reactant is left over. (3) Set the numbers in the box, press [Run reaction], and see which reactant runs out first.
Learning objectives After this activity you can …
explain, with a reason, whether the limiting reactant is decided by the number of molecules or by the coefficient ratio.
identify the limiting reactant from the 1:3 mole ratio for given numbers of N₂ and H₂.
calculate the NH₃ formed and the leftover (excess) reactant from the coefficient ratio.
Entry check Before you start — prerequisites and key terms
In the equation N₂ + 3H₂ → 2NH₃, how many H₂ molecules does one N₂ molecule need to react completely?
123
Key terms
Mole: a counting unit for particles (here we compare numbers of molecules directly). ·
Coefficient ratio (mole ratio): the ratio of the coefficients in the equation, N₂:H₂:NH₃ = 1:3:2. ·
Batch: how many complete reaction sets a reactant can supply (number ÷ its coefficient). ·
Limiting reactant: the reactant that runs out first relative to the coefficient ratio and stops the reaction. ·
Excess reactant: the reactant that is not used up and is left over.
Predict Think first, before you observe
You put 2 N₂ and 3 H₂ molecules into the box (N₂ + 3H₂ → 2NH₃). Which is the limiting reactant?
Nitrogen, N₂Hydrogen, H₂Both run out at the same time
ConfidenceMedium
NH₃ = —Shown after you lock in a prediction
Move the sliders for N₂ and H₂. The limiting reactant, the NH₃ formed and the leftover reactant are calculated live.
The explanation opens after you lock in a prediction
010
030
Atom labels
N₂ (nitrogen, blue)H₂ (hydrogen, white)NH₃ (ammonia, green = product)
The explanation opens after you lock in a prediction.
Key idea 1 · Limiting reactant = the one that is short relative to the coefficient ratio
The limiting reactant is not simply the one with fewer molecules. Compare the reactants after dividing by the coefficients (N₂:H₂ = 1:3).
Example: with N₂ = 2 and H₂ = 3, N₂ gives 2 batches and H₂ gives 3 ÷ 3 = 1 batch. H₂ gives fewer batches, so H₂ is the limiting reactant and N₂ is left over.
N₂ + 3H₂ → 2NH₃
Batches: N₂ batches = N₂ | H₂ batches = H₂ ÷ 3 → the smaller one is limiting
Key idea 2 · NH₃ formed and leftover
N₂ that reacts = min(N₂, H₂÷3 rounded down). One N₂ gives two NH₃, so
NH₃ formed = 2 × (N₂ that reacted). After the reaction, the limiting reactant is 0 and the other reactant is left over (excess).
The same idea is used for nitrogen fertilizer: ammonia made from N₂ and H₂ is the starting point of most nitrogen fertilizers.
NH₃ = 2 × min(N₂, floor(H₂/3))
Limiting reactant used up to 0 | excess reactant left over
Check Formative check — pass all items to complete
Q1You add 2 N₂ and 3 H₂. Which is the limiting reactant? (N₂ + 3H₂ → 2NH₃)
Nitrogen, N₂Hydrogen, H₂No limiting reactant
Hint
Divide each reactant by its own coefficient (N₂÷1, H₂÷3) to get its batches, then compare. The smaller one is limiting.
Q2 · Hands-on taskAdjust the sliders and press [Run reaction] to reach the target state Then press [Check].
NH₃ formed now: — molecules
Q3For the given numbers of N₂ and H₂, how many NH₃ molecules form? (enter a whole number)
Hint
Compare the batches.
Q4 · Fill in the blankFill in the blank: The reactant that runs out first relative to the coefficient ratio and stops the reaction is called the [_____].
Hint
A two-word term: an adjective + "reactant".
Q5 · Apply (higher-order, not required for completion)You add 4 N₂ and 9 H₂ and let the reaction finish. Which pair of limiting reactant and leftover reactant is correct?
H₂ limiting · 1 N₂ left overN₂ limiting · H₂ left overH₂ limiting · nothing left over
Hint
Batches: N₂÷1 = 4, H₂÷3 = 3. Which is smaller? Then count how many N₂ react.
Reflect Reflect — metacognition · completion
My prediction: Not predicted yet
In one sentence, explain why the limiting reactant is not just "the one with the smaller amount" (key words: limiting reactant, mole ratio / coefficient ratio):
Going further (optional)
Set N₂ and H₂ to exactly 1:3 (for example N₂ 3, H₂ 9). Both are used up, with no limiting reactant and nothing left over.
Then NH₃ formed = 2 × N₂. In the industrial Haber–Bosch process, H₂ is usually added in excess so that as much N₂ as possible reacts.
Apply Use the idea in a new context (not required for completion)
Transfer task: one sandwich needs 2 slices of bread and 1 patty. You have 10 slices of bread and 3 patties.
How many sandwiches can you make, and what is left over? Explain in one sentence using the idea of limiting reactant and coefficient ratio.
COMPLETE · Completed — all checks passed.
Worksheet — Limiting reactant: N₂ + 3H₂ → 2NH₃
Name ________ Class ________ Date ________
1. Predict — With 2 N₂ and 3 H₂, which is the limiting reactant? (N₂ / H₂ / none)
2. Observe — For each case, fill in the batches, the limiting reactant, the NH₃ formed and the leftover.
"There are fewer N₂ molecules. So why does H₂ run out first?"
"Which reactant decides the number of NH₃ molecules formed?"
"The reaction has stopped. Why is one kind of molecule still there?"
Common misconceptions
MC-lim-01: "Limiting reactant = the one with fewer molecules" → it is the one with fewer batches after dividing by the coefficient. With N₂=2, H₂=3, H₂ is limiting.
MC-lim-02: "Product formed = amount of limiting reactant" → use the coefficient ratio. NH₃ formed = 2 × (N₂ that reacted).