Which reactant runs out first?
You will be able to: Determine the limiting reactant and calculate product and excess remaining.
Which reactant runs out first?
Five sandwich tops do not make five sandwiches if only three bottoms are available. In a reaction, compare usable reaction groups rather than simply picking the smaller starting mass or mole amount.
A useful starting point: How does a balanced equation connect two masses? →
Words and symbols before equations
- Limiting reactant
- Reactant exhausted first under the complete-reaction assumption.
- Excess reactant
- Reactant remaining when the limiting reactant is exhausted.
- Reaction extent, ξ
- Amount of reaction in mol; each species changes by its coefficient times ξ.
What this picture assumes
Theoretical complete conversion for N₂ + 3H₂ → 2NH₃. Industrial ammonia synthesis is equilibrium-limited; this model computes a stoichiometric maximum, not equilibrium yield.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- Extent min(2/1, 3/3) = 1 mol. NH₃ 2 mol; N₂ left 1 mol; H₂ left 0 mol.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
For N₂ + 3H₂ → 2NH₃, divide each initial mole amount by its coefficient. The smaller result is the maximum reaction extent.
With 2.0 mol N₂ and 3.0 mol H₂, the ratios are 2.0/1 and 3.0/3. H₂ limits at ξ = 1.0 mol, even though it has the larger initial mole amount.
Product is 2ξ = 2.0 mol NH₃. Nitrogen consumed is ξ = 1.0 mol, leaving 1.0 mol N₂. Hydrogen consumed is 3ξ = 3.0 mol, leaving zero.
This stoichiometric maximum assumes complete conversion. Real ammonia synthesis is equilibrium-limited; the illustration is a bookkeeping model, not a claim about industrial yield.
A worked example, step by step
For 2H₂ + O₂ → 2H₂O, start with 3.0 mol H₂ and 2.0 mol O₂. Find the limiting reactant and final amounts.
- Compare 3.0/2 = 1.5 mol reaction with 2.0/1 = 2.0 mol reaction.
- H₂ limits; take ξ = 1.5 mol.
- Water formed is 2ξ = 3.0 mol. O₂ consumed is ξ = 1.5 mol.
- O₂ remaining = 2.0 − 1.5 = 0.5 mol; H₂ remaining is zero. All amounts are nonnegative.
The smaller initial amount is not always limiting. Compare amount divided by coefficient.
What if both n/coefficient ratios are equal?
Compare with an explanation
The reactants are in exact stoichiometric proportions and both are exhausted in the complete-reaction model.
Predict. Change one thing. Explain.
Hold N₂ at 2 mol. Change H₂ from 3 to 6 to 9 mol. Predict when product stops increasing and identify the leftover species at each setting.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Extent min(2/1, 3/3) = 1 mol. NH₃ 2 mol; N₂ left 1 mol; H₂ left 0 mol.
Theoretical complete conversion for N₂ + 3H₂ → 2NH₃. Industrial ammonia synthesis is equilibrium-limited; this model computes a stoichiometric maximum, not equilibrium yield.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using conserved atoms/charge, reaction ratios, particle identity or electron/proton transfer. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionFor N₂ + 3H₂ → 2NH₃, assume complete conversion of 1.5 mol N₂ and 6.0 mol H₂. Determine the limiting reactant, product and both leftovers.
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Compare with the answer and four-point rubric
- 1 point: Compare 1.5/1 = 1.5 with 6.0/3 = 2.0; N₂ limits.
- 1 point: NH₃ formed = 2 × 1.5 = 3.0 mol.
- 1 point: H₂ consumed = 4.5 mol, leaving 1.5 mol.
- 1 point: N₂ is exhausted; this is a theoretical stoichiometric result, not an equilibrium prediction.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What quantity should you compare?
Initial moles divided by stoichiometric coefficient.
RECALL 2What happens to excess reactant?
Only the required amount is consumed; the remainder stays.
RECALL 3Does adding excess reagent always increase yield?
Not when the other reagent is already limiting.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Which reactant runs out first?
- ξmax = min(nA/a, nB/b) for aA + bB → cC.
- nC = cξ; nA,left = nA − aξ.
Remember: The smaller initial amount is not always limiting. Compare amount divided by coefficient.
Conditions: Theoretical complete conversion for N₂ + 3H₂ → 2NH₃. Industrial ammonia synthesis is equilibrium-limited; this model computes a stoichiometric maximum, not equilibrium yield.
Refresh Kid · AP Chemistry Unit 4 · Objectives 4.5.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 4.5, objective 4.5.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 4: Chemical Reactions, Topics 4.1–4.9. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Solubility facts for sodium, potassium, ammonium and nitrate salts are included; other precipitation cases give the needed information. Lewis acid-base theory and the labels oxidizing/reducing agent are not treated as required exam content. Quantitative pH, equilibrium and electrochemical potentials are developed in later units. Stoichiometric models state complete-reaction assumptions; they are not mechanisms or equilibrium simulations.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
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