HS-PS1-4: Bond Energy and Why Reactions Get Hot or Cold
Also called NGSS HS-PS1-4 · Topic: Chemical Reactions · Physical Science
The Official Wording
Develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
Clarification statement: Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.
Assessment boundary: Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What HS-PS1-4 Means in Plain English
High school students build a model showing that a reaction gives off or takes in energy depending on the balance between the energy needed to break the old bonds and the energy released when new bonds form. Molecule drawings and energy diagrams both count.
Students reason about the overall balance without adding up bond energy values from a table. The idea builds on thermal energy devices in middle school (MS-PS1-6) and returns in biology when cells release energy from food (HS-LS1-7).
HS-PS1-4 I Can Statements
- I can explain that breaking bonds takes energy and forming bonds releases it.
- I can sketch an energy diagram for an exothermic reaction.
- I can tell from a diagram whether a reaction releases energy.
HS-PS1-4 Worked Example
A ball-and-stick kit session that turns a gas stove flame into a bond-by-bond story.
Where does the heat of a methane flame come from?
Set It Up: A ball-and-stick molecular model kit, a large sheet of paper for an energy diagram, and colored pencils.
- Build one methane molecule and two oxygen molecules.CH₄ + 2 O₂: 4 C–H bonds, 2 O=O bonds
- Pull every bond apart, noting that each break costs energy.Loose atoms: energy taken in
- Rebuild the atoms into carbon dioxide and two water molecules.CO₂ + 2 H₂O: 2 C=O bonds, 4 O–H bonds
- Draw the diagram: reactants, a hill for breaking, then products.Products end below reactants
- Label the gap between start and finish as the energy given off.Gap = heat and light of the flame
Forming the new bonds in carbon dioxide and water released more energy than breaking the old bonds took, so the reaction gave off heat.
Why it works: The total energy of the system is conserved, so any energy the bonds lose overall leaves as heat and light.
Common Mistakes with HS-PS1-4
Thinking breaking bonds releases energy
“The flame is hot because the methane bonds snap and let out energy.”
Try this: Breaking a bond always takes energy in. The heat comes from forming new, stronger bonds that release more energy than the breaking used.
Drawing products above reactants for a hot reaction
An energy diagram that climbs to finish higher
Try this: In a reaction that warms its surroundings, the products sit lower than the reactants, and the drop is the energy released.
Forgetting the system boundary
Saying a cold pack makes cold
Try this: Energy moves between the reaction and its surroundings. A cold pack feels cold because the reaction absorbs energy from your hand.
Where HS-PS1-4 Fits
Practice HS-PS1-4
Quick Check
- Does breaking a chemical bond release or absorb energy?
- On an energy diagram, the products are higher than the reactants. Is the reaction exothermic or endothermic?
- Where does the energy go in a reaction that releases it?