Chemistry · 40 minutes
Suggested grades 10–11 · Unit 2: Reactions and Energy · Lesson 2 of 2
Why Breaking Bonds Is Not the Whole Energy Story
Calculate a simplified energy change and critique a common explanation.
Mr. S Teaches · Original starter lesson · HS-CHEM-04 · Grade range is planning guidance.Watch, notice, discuss
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Energy & Chemistry: Crash Course Chemistry #17
By CrashCourse
Distinguish energy transfers from matter changes before calculating the lesson’s net bond-energy estimate.
Open on YouTube: Energy & Chemistry: Crash Course Chemistry #17 (opens a new tab) ↗Enthalpy: Crash Course Chemistry #18
By CrashCourse
Compare energy absorbed in breaking bonds with energy released on formation; state-function detail is an optional extension.
Open on YouTube: Enthalpy: Crash Course Chemistry #18 (opens a new tab) ↗What idea from the video helps you use supplied bond-energy values to calculate net change and distinguish bond breaking from bond formation? Identify one useful example and one limit of the video's model or explanation.
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Learning objective
Use supplied bond-energy values to calculate net change and distinguish bond breaking from bond formation.
Before you begin
- Use signed numbers and recognize molecular bonds.
Gather your materials
- Paper or notebook
- Pencil; calculator optional for numerical work; all task data supplied below
Language to know
- endothermic
- Describing a process with net energy transfer into the reaction system.
- exothermic
- Describing a process with net energy transfer out of the reaction system.
- bond energy
- A model quantity describing energy needed to break a specified bond.
Understand the concept
Model account: Breaking the selected reactant bonds requires 500 kJ per modeled batch; forming product bonds releases 650 kJ. Define ΔE = energy required for breaking − energy released on forming. Then ΔE is −150 kJ, indicating net transfer out of the reaction system under this convention. Breaking a bond always requires energy in this account; net release depends on all changes together. These are invented aggregate values, not a laboratory recipe or measured bond energies for a named reaction.
Teach it step by step
- Translate the text into separate upward energy-required and downward energy-released arrows.
- Calculate ΔE with its sign and state the chosen system boundary.
- Sketch relative initial and final energy levels consistent with the net change.
- Critique “Breaking the fuel bonds releases all the energy” using both required and released values.
Make the thinking visible
Worked example or model response
500 − 650 = −150 kJ. The products are represented 150 kJ lower in energy than the reactants for the defined batch. The negative net result does not make the bond-breaking contribution negative.
Guided practice
Together calculate a second model requiring 700 kJ to break bonds and releasing 550 kJ on formation; describe the sign in words.
Independent practice
Draw both energy diagrams and explain the different net directions. State why the simple diagram does not tell how fast either reaction proceeds.
Quick assessment
Does a negative net energy change imply that a reaction needs no initial energy input or proceeds instantly?
Answer key & assessment criteria
Answers and teaching guidance
- Guided: 700 − 550 = +150 kJ, so the second system has net energy transfer into it under the stated convention.
- Independent: First products lower by 150; second higher by 150. Rate and initial activation requirements are not specified by the net energy difference.
- Quick check: No. Net change is different from the pathway’s activation requirement or reaction speed.
Look for: Assess the stated objective using the concrete criteria and explanations in the answer key. Invite an alternative supported solution; distinguish a model’s prediction from an established observation.
Make it work for your learner
More support
Provide signed arrow cards and explicitly label “required” versus “released” before calculating.
Extension challenge
Add a hypothetical energy barrier to each diagram and compare how equal net changes could still involve different rates.
Verified references · Selected contributions
Standards connections
Selected NGSS high school-band contribution; full performance requires additional evidence and instruction.
Subject-content standards
These records identify the part of a subject standard practiced here. Course placement is instructional guidance; the source’s category or band remains authoritative.
Contributes to NGSS performance expectation
HS-PS1-4
NGSS · High school band
Model reaction energy changes using bond breaking and bond forming.
Read the official performance expectation ↗The paper model contributes selected bond-energy reasoning. It uses aggregate invented values and omits detailed molecular structures, measured bond energies and reaction pathways.
Verified 2026-11-04. Selected contribution, not full attainment.
Supporting literacy connections
These records apply to the specified reading, writing or communication task. They do not establish subject-content alignment.
Supporting Common Core literacy connection
RST.9-10.7
Common Core · Grades 9–10
Translate technical prose into visuals and explain mathematical or visual information.
Read the official standard ↗The activity explicitly analyzes the provided technical text, table or model. It contributes selected literacy practice rather than complete grade-band reading coverage or independent subject-content alignment.
Verified 2026-11-04. Selected contribution, not full attainment.