Energy in Chemical Reactions: Exo- and EndothermicANSWER KEY
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...every chemical reaction this cycle followed the SAME pattern: atoms rearrange (not destroy or create), mass is conserved, and the SPEED of rearrangement depends on temperature, concentration, surface area, and catalysts. From rusting nails to cooking eggs, the rules are identical.
WRITEAFTER HOOK FORMReflect before continuing
I learned that...the 5 evidences of reaction (color change, gas, temperature change, precipitate, light) tell us WHEN a reaction happens. Conservation of mass tells us WHAT can come out (only what went in). Collision theory tells us HOW FAST it happens. Together they explain almost every chemistry phenomenon.
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: across the cycle's investigations, every reaction's reactant mass equalled product mass (when gases were trapped). Burning steel wool GAINED mass because oxygen joined; a sealed soda lost no mass even after bubbling.
This shows that thermal energy can flow in TWO directions: atoms aren't created or destroyed — they REARRANGE. The Law of Conservation of Mass (Lavoisier, 1789) holds in every reaction. When mass seems to disappear (burning wood) or appear (rusting iron), the 'missing' mass is in gases or absorbed from the air.
The key concept that classifies a reaction as exo- or endothermic is: Conservation of mass is the foundation: balanced chemical equations must have the same number of each atom on both sides. If you start with 2 H + 1 O, you must end with 2 H + 1 O — just rearranged into H2O.
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.
The big ideas of Cycle 7 chemistry connect through PARTICLES. Conservation of mass (atoms rearrange, never disappear) sets the foundation. Collision theory explains rate: reactions need enough-energy collisions in the right orientation. Temperature, concentration, surface area, and catalysts each change either how many collisions happen per second OR what fraction clear the activation-energy hurdle. Same chemistry, same products — conditions just change how fast you get there. This unifies cooking, rusting, digestion, and combustion.
Confidence:
Energy in Chemical Reactions: Exo- and Endothermic (Day 2)ANSWER KEY
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: reaction rate depends on four factors: temperature (more KE → more high-energy collisions), concentration (more particles → more collisions), surface area (powder reacts faster than chunks), and catalysts (lower the activation-energy hill).
This connects to W2 (mass conservation) because energy, like mass, is conserved — meaning the chemical PE in reactant bonds doesn't disappear, it: everyday life: refrigeration slows food spoilage by lowering temperature; we chew food (surface area) before swallowing; saliva's amylase (a catalyst) starts breaking down starch instantly. The same rules govern factories, fires, and your own body.
WRITEDesign a Hand-Warmer or Cold-Pack
My device GOAL (warm or cool, with target ΔT) is...matter, mass, and rate all link through PARTICLES. Reactions happen when particles collide with enough energy and proper orientation. Faster particles (heat) or more particles (concentration) or more exposed particles (surface area) all mean more reactions per second — without violating conservation of mass.
The reactants I'd choose and WHY (which bonds break, which form, expected net energy direction)...the patterns are universal: factories use catalysts to save energy (same idea as enzymes in your gut); refrigerators use temperature to slow spoilage (same idea as why food cooks faster on hot days). Once you know the rules, you can predict chemistry anywhere.
I can test my design by measuring...predicting which reactions will be fast or slow (e.g., grinding antacid tablets dissolves them faster than swallowing whole), reading nutrition labels with chemistry eyes, and understanding why combustion engines need spark plugs (activation energy).
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
Central box: 'Chemical Reaction = atoms rearranged'. Four arrows radiating out to four factor boxes: (1) TEMPERATURE — particles drawn with longer motion lines; (2) CONCENTRATION — more dots in same volume; (3) SURFACE AREA — chunk vs. powdered version; (4) CATALYST — energy diagram with two Ea hills, lower one labeled 'with catalyst'. Below the central box: equation balance — '2 H2 + O2 → 2 H2O' with atom count: 'Left: 4 H, 2 O = Right: 4 H, 2 O'.
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...the same particle-collision rules explain refrigerator chemistry, body chemistry, and factory chemistry. Once you see particles colliding, you can predict almost any reaction's speed.
Remaining Question
I still wonder...If catalysts come out unchanged, why do industrial catalysts eventually need to be replaced? What actually 'wears them out' over thousands of cycles?
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
CLAIM: Cycle 7's concepts connect through particles: chemical reactions rearrange atoms (conservation of mass), and the rate depends on how often and how energetically particles collide (collision theory).
EVIDENCE: Burning steel wool GAINED mass (oxygen joined); sealed reactions never lost mass. Raising temperature 10 °C ~doubles reaction rate. Powdered antacid dissolves faster than tablets. Catalysts speed reactions WITHOUT being consumed (enzymes in your body).
REASONING: Atoms are conserved because reactions only rearrange them — they never create or destroy atoms. Rate depends on collision frequency (concentration, surface area) and collision energy (temperature, lowered by catalysts). These rules govern every chemistry phenomenon we saw this cycle and connect chemistry in your body to chemistry in a factory.