Energy in Chemical Reactions: Exo- and Endothermic
Name: Period: Date:
REFSTUDY GUIDE: Big IdeasUse to answer form questions
1.In Cycle 7 you proved that atoms are conserved in every reaction (W2). This week you learn that energy is conserved too. The chemical potential energy stored in reactant bonds doesn't disappear — it either flows OUT as thermal energy (exothermic, products warm) or is pulled IN from the surroundings (endothermic, products cool).
2.Bonds are the energy bank account of a reaction. Breaking bonds COSTS energy (you have to pull atoms apart); forming bonds RELEASES energy (atoms fall into a more stable state). The NET of those two movements is what your thermometer measures: if forming releases more than breaking costs, the reaction is exothermic.
3.A reaction energy diagram tells the whole story on one chart. Activation energy (E_a) is the hill from reactants up to the peak — the minimum collision energy needed to start the reaction (W3 connection). The ΔE arrow between reactant and product levels tells you exo (products lower) vs endo (products higher). A catalyst lowers E_a but does NOT change ΔE.
DRAWKEY TERMSGoogle each → Images tab → sketch what you see
exothermic
Reaction that releases thermal energy — products feel warm; products store less PE than reactants
endothermic
Reaction that absorbs thermal energy — products feel cold; products store more PE than reactants
activation energy (E_a)
Minimum collision energy needed to break old bonds and start a reaction — the hill on an energy diagram
bond energy
Energy stored in a chemical bond — released when a bond forms, absorbed when a bond breaks
OTHER VOCAB: chemical potential energy · thermal energy · conservation of energy · reactants · products
WRITEFOCUS QUESTIONYour initial thinking
Open a hand-warmer (iron + oxygen) and the temperature climbs from 22 °C to ~50 °C — your hand feels hot. Burst a cold-pack (ammonium nitrate + water) and the temperature drops to ~2 °C — your hand feels cold. Both follow conservation of mass (W2). So why does one warm and one cool?
Where does the energy go when chemicals react?
I think...
WRITEAFTER HOOK FORMReflect before continuing
I learned that...
WRITEStation 1 — Hot vs Cold Reactions
I observed that the hand-warmer temperature ROSE from 22 °C to ~50 °C while the cold-pack temperature DROPPED from 22 °C to ~2 °C, which means:
This shows that thermal energy can flow in TWO directions:
The key concept that classifies a reaction as exo- or endothermic is:
CHECKDAY 1 CHECK-INBefore you leave
Q1:Based on today's hand-warmer vs cold-pack work, explain one key finding using evidence. Connect it to this week's focus question on where the energy goes when chemicals react.
Confidence:
Energy in Chemical Reactions: Exo- and Endothermic (Day 2)
Name: Period:
WRITEStation 2 — Where the Energy Hides: Bonds
The bond-energy accounting for an exothermic reaction (e.g. CH₄ + 2 O₂ → CO₂ + 2 H₂O) shows that breaking bonds COSTS energy while forming bonds RELEASES energy, with a net result of:
This connects to W2 (mass conservation) because energy, like mass, is conserved — meaning the chemical PE in reactant bonds doesn't disappear, it:
WRITEDesign a Hand-Warmer or Cold-Pack
My device GOAL (warm or cool, with target ΔT) is...
The reactants I'd choose and WHY (which bonds break, which form, expected net energy direction)...
I can test my design by measuring...
DRAWCONCEPT MODELAnnotated reaction energy diagram
Draw two reaction energy diagrams side-by-side: LEFT exothermic (products below reactants, ΔE arrow DOWN, products warm), RIGHT endothermic (products above reactants, ΔE arrow UP, products cold). Both must show the activation-energy hill above the reactant level. Label ≥3 word-bank terms across the two diagrams.
Word Bank: exothermic · endothermic · activation energy · bond energy · chemical potential energy · thermal energy · conservation of energy · reactants · products
CHECKTEST YOUR THINKINGRate each: FALSE ← 1-2-3-4-5 → TRUE · 4 new + 1 last week + 1 spiral
1. A cold-pack feels cold because the reaction DESTROYS energy.
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2. Breaking chemical bonds RELEASES energy (like an explosion).
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3. Only fires and explosions are exothermic — rusting and respiration are not.
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4. Activation energy (E_a) and net reaction energy (ΔE) are independent — a tall hill does NOT mean a large ΔE.
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5. [Last week] A catalyst added to an exothermic reaction makes it FASTER but does NOT change whether it is exo- or endothermic.
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6. [Spiral · C7W2 Conservation of Mass] In an exothermic reaction, energy LEAVES the system as heat — so MASS must leave the system too.
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Pick any TWO statements -- especially any you rated 2, 3, or 4. Justify your rating with evidence:
Statement #___ -- I rated it ___ because:
Statement #___ -- I rated it ___ because:
THINKSCIENTIST CIRCLE DEBRIEFReflect after your Scientist Circle discussion
AHA! Moment
The one I changed my thinking most about is...
Remaining Question
I still wonder...
WRITEFOCUS QUESTION REVISITFinal written response
Where does the energy go when chemicals react?
Word Bank: exothermic · endothermic · activation energy · bond energy · chemical potential energy · thermal energy · conservation of energy · reactants · products