Which reactant amount determines the heat released?
You will be able to: Combine limiting-reactant reasoning with reaction enthalpy.
Which reactant amount determines the heat released?
A sandwich recipe cannot use all the bread if the filling runs out. A reaction likewise cannot release heat for reactant that remains unused.
A useful starting point: Review limiting amounts and leftovers →
Words and symbols before equations
- Limiting reactant
- Reactant exhausted first according to stoichiometry and assumed completion.
- Available extent
- Starting moles divided by the species coefficient.
- Excess reactant
- Material left after the limiting amount is consumed.
What this picture assumes
N₂+3H₂→2NH₃, initial N₂=0.20 mol and supplied ΔrH=−92 kJ/mol reaction. Assume complete conversion without side reactions. This is limiting-amount bookkeeping, not an industrial ammonia equilibrium prediction.
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.
- Complete-conversion model q=-9.2 kJ. H₂ limits.
- 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 each reactant calculate n/coefficient. The smallest available extent limits the reaction if it proceeds to completion.
Multiply this extent by the molar enthalpy for the equation as written. Do not use total starting moles of an excess reactant.
Check leftovers through stoichiometry. Heat changes linearly with added limiting material but eventually stops increasing when another reactant becomes limiting.
Completion is a stated approximation. Actual yield, side reactions, heat loss and equilibrium can change measured heat; this model does not predict an industrial yield.
A worked example, step by step
For N₂+3H₂ → 2NH₃, use supplied ΔrH=−92 kJ/mol reaction and assume completion. Start with 0.20 mol N₂ and 0.30 mol H₂. Find reaction heat and leftovers.
- Available extents: N₂ gives 0.20/1=0.20; H₂ gives 0.30/3=0.10 mol reaction.
- H₂ limits, so ξ=0.10 mol reaction.
- q=0.10(−92)=−9.2 kJ.
- N₂ consumed=0.10 mol, leaving 0.10 mol; H₂ is exhausted. These are complete-conversion bookkeeping results.
Starting amount of excess reactant is not the amount that reacted.
Why can extra excess reagent leave heat unchanged?
Compare with an explanation
The other reactant already caps the possible reaction extent under the completion assumption.
Predict. Change one thing. Explain.
Hold N₂ fixed and add H₂. Predict when heat release stops increasing and identify the reactant that becomes limiting.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Complete-conversion model q=-9.2 kJ. H₂ limits.
N₂+3H₂→2NH₃, initial N₂=0.20 mol and supplied ΔrH=−92 kJ/mol reaction. Assume complete conversion without side reactions. This is limiting-amount bookkeeping, not an industrial ammonia equilibrium prediction.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using heat-flow signs, energy conservation, phase changes, bond inventories or the stated thermochemical path. 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 A+2B → AB₂ with ΔrH=−80 kJ/mol reaction, start with 0.25 mol A and 0.30 mol B and assume completion. Identify limiter, extent, heat and leftover A.
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Compare with the answer and four-point rubric
- 1 point: B limits because 0.30/2<0.25/1.
- 1 point: ξ=0.15 mol reaction.
- 1 point: q=0.15(−80)=−12 kJ.
- 1 point: A left=0.25−0.15=0.10 mol.
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 should be compared?
Moles divided by coefficients.
RECALL 2Which amount produces heat?
The amount actually reacting, expressed as extent.
RECALL 3What is the main model assumption?
The limiting-reactant amount reacts completely by the specified equation.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Which reactant amount determines the heat released?
- ξ_max=min(n_i/ν_i) for supplied complete conversion.
- q=ξ_maxΔrH; leftovers=n_initial−νξ_max.
Remember: Starting amount of excess reactant is not the amount that reacted.
Conditions: N₂+3H₂→2NH₃, initial N₂=0.20 mol and supplied ΔrH=−92 kJ/mol reaction. Assume complete conversion without side reactions. This is limiting-amount bookkeeping, not an industrial ammonia equilibrium prediction.
Refresh Kid · AP Chemistry Unit 6 · Objectives 6.6.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 6.6, objective 6.6.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 6: Thermochemistry, Topics 6.1–6.9. 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. Technical enthalpy/internal-energy distinctions and formal state-function terminology are not assessed in the current AP framework. Constant-pressure heat, conservation, phase-specific capacities, reaction amounts and Hess sums are taught here with explicit conditions. Supplied rounded data and original molecular geometry are teaching models, not experimental measurements. A phase transition preserves molecular identity; a bond-energy accounting path is not an actual reaction mechanism.
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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