Week 4: Energy in Chemical Reactions: Exo- and Endothermic

Grade 8 Science | Rosche

MS-PS1-6 Undertake a design project to construct, test, and modify a device that either releases or absorbs thermal energy by chemical processes.

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The Phenomenon: Energy in Chemical Reactions: Exo- and Endothermic

Anchoring Context & Focus Question

Where does the energy go when chemicals react?

Methane combustion diagram showing reactants (CH4 + 2 O2) rearranging into products (CO2 + 2 H2O) with thermal energy released to the surroundings
Methane combustion. Same atoms enter and leave (mass conserved, Week 2) β€” but the products are warm because the reaction releases ENERGY to the surroundings. That stored energy is what this week is about.
Animated explanation of the week's focus question. Watch then reason about why this happens.
Animated explanation of the week's focus question. Watch then reason about why this happens.
Key Vocabulary (9 terms) — Practice Tool

Cognate Strategy: Many science words look similar in English and Spanish β€” use your Spanish to learn science!

Term Spanish Definition
exothermic exotΓ©rmico Reaction that releases thermal energy to the surroundings; products store less chemical potential energy than reactants
endothermic endotΓ©rmico Reaction that absorbs thermal energy from the surroundings; products store more chemical potential energy than reactants
activation energy energΓ­a de activaciΓ³n Minimum energy a collision needs to break old bonds and start a reaction
bond energy energΓ­a de enlace Energy stored in a chemical bond; released when a bond forms, absorbed when a bond breaks
chemical potential energy energΓ­a potencial quΓ­mica Energy stored in the arrangement of atoms in molecules
thermal energy energΓ­a tΓ©rmica Energy of random particle motion β€” what we feel as 'heat'
conservation of energy conservaciΓ³n de la energΓ­a Energy cannot be created or destroyed β€” only transferred or transformed between forms
reactants reactivos Starting substances on the left side of a chemical equation
products productos Substances formed on the right side of a chemical equation

Hook – Why Glow Sticks Cool Down

Where does the energy come from β€” and where does it go?
Methane combustion (CH4 + 2 O2 β†’ CO2 + 2 H2O) β€” atoms conserved, thermal energy released. Reused as Hook anchor to bridge Week 2 conservation framing to this week's energy framing.
Hook anchor: combustion releases energy that was stored in the methane bonds.

Same atoms in, same atoms out β€” but different temperature

In Cycle 7 Week 1 you watched a glow stick fade. In Week 2 you proved that mass is conserved when chemicals react β€” atoms aren't created or destroyed. In Week 3 you learned what controls the SPEED of a reaction. This week we ask the next question: where does the energy go when chemicals react? A burning match heats your hand. A cold-pack chills your knee. Same chemistry rule (atoms are conserved) β€” but the energy story is very different. By the end of this week, you'll be able to explain both β€” and design a device that releases or absorbs energy on purpose.

Two Demonstrations β€” Notice & Wonder

  1. Hand-warmer: open a commercial hand-warmer (iron filings + air). Temperature climbs from room temperature to ~50 Β°C. Hand feels hot. Reaction: 4 Fe + 3 Oβ‚‚ β†’ 2 Feβ‚‚O₃.
  2. Cold pack: burst an instant cold-pack (ammonium nitrate dissolving in water). Temperature drops to ~2 Β°C. Hand feels cold. Reaction: NHβ‚„NO₃(s) + Hβ‚‚O β†’ NH₄⁺(aq) + NO₃⁻(aq).

Both follow conservation laws. So why does one warm and one cool? The Hook form (5 questions, 12 pts) starts you on the answer.

COMPLETE THE HOOK FORM

Hook activity: where does the energy go when chemicals react?

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Worked Example – Reading an Energy Diagram

Two side-by-side reaction energy diagrams: exothermic (products lower than reactants, Ξ”E down) and endothermic (products higher than reactants, Ξ”E up). Both diagrams show an activation-energy peak between reactants and products.
Worked example: exothermic vs endothermic energy diagrams.

How to Read a Reaction Energy Diagram

Every reaction you'll meet this week can be drawn the same way β€” energy on the y-axis, reaction progress on the x-axis. Three landmarks tell the whole story: where the reactants start, how tall the activation-energy peak is, and where the products end up. Compare reactant height to product height and you instantly know exothermic vs endothermic.

Step 1: Find the Reactants and the Products

Reactants are the FLAT level on the LEFT β€” before the climb. Products are the FLAT level on the RIGHT β€” after the descent. Don't confuse the peak with either; the peak is the activation-energy threshold, not a stable state. Tip: trace the curve with your finger left β†’ right. You start at the reactant level and end at the product level.

Step 2: Measure Activation Energy (E_a)

E_a is the VERTICAL distance from the reactant level UP to the peak. This is how much energy a collision needs to start the reaction (break the old bonds). A taller hill means a slower reaction (Week 3 connection). A catalyst lowers this hill.

Step 3: Decide Exothermic vs Endothermic (Ξ”E)

Compare product level to reactant level. If products are BELOW reactants β†’ Ξ”E is negative β†’ EXOTHERMIC (energy released, products feel warm). If products are ABOVE reactants β†’ Ξ”E is positive β†’ ENDOTHERMIC (energy absorbed from surroundings, products feel cold). The arrow you draw between the two levels IS the energy that left or entered as thermal energy.

Step 4: Connect to Energy Conservation (CCC-5)

Energy is never created or destroyed (conservation of energy). The chemical potential energy stored in reactant bonds doesn't disappear β€” it becomes thermal energy of the surroundings (exo) or it pulls thermal energy in from the surroundings to climb to a higher PE (endo). The bookkeeping is exact.
Two side-by-side reaction energy diagrams. Left panel labeled exothermic shows products lower than reactants with delta-E released arrow pointing down. Right panel labeled endothermic shows products higher than reactants with delta-E absorbed arrow pointing up. Both panels show an activation-energy peak above the reactant level.
Two reaction energy diagrams: exothermic (left, products below reactants) vs endothermic (right, products above reactants). Activation-energy peak in both.
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Station 1 – Hot vs Cold Reactions

Distinguish exothermic from endothermic reactions using temperature evidence.
Hand-warmer panel (left, red) showing iron filings reacting with oxygen, thermometer rising from 22 to 50 Β°C, heat-out arrows; cold-pack panel (right, blue) showing ammonium nitrate dissolving in water, thermometer dropping from 22 to 2 Β°C, heat-in arrows. Data tables compare starting T, ending T, and energy direction.
Station 1: side-by-side hand-warmer (exothermic) vs cold-pack (endothermic) with temperature data.

Station 1 β€” Hot vs Cold Reactions (20 points)

Two demo reactions, two thermometers. You'll collect (or analyze sample data for) temperature-over-time for an exothermic reaction (iron oxidation in a hand-warmer) and an endothermic reaction (ammonium nitrate dissolving in a cold-pack). Same starting temperature (~22 Β°C). Very different endings. Use the data to classify each reaction by what the THERMOMETER tells you.

Data Recap

  • Hand-warmer (Fe + Oβ‚‚ β†’ Feβ‚‚O₃): 22 Β°C β†’ ~50 Β°C over 4 hours. Temperature rose β†’ energy left the reaction and warmed the surroundings β†’ EXOTHERMIC.
  • Cold-pack (NHβ‚„NO₃ + Hβ‚‚O): 22 Β°C β†’ ~2 Β°C over 20 minutes. Temperature dropped β†’ energy entered the reaction from the surroundings β†’ ENDOTHERMIC.

Connect to Earlier Weeks

Temperature change was already on your W1 evidence-of-reaction list. This week you go deeper: a temperature change is not just evidence that a reaction happened β€” it's also a measurement of how much energy moved into or out of the reaction. The rate of that temperature change connects to W3 (faster reaction β†’ faster Ξ”T).

COMPLETE THE STATION 1 FORM

Station 1 for Hot vs Cold Reactions.

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Station 2 – Where the Energy Hides: Bonds

Explain energy release/absorption in terms of bonds breaking and forming.
Four-step bond-energy story: reactant molecules with intact bonds β†’ bonds breaking (energy in, amber arrow up) β†’ atoms recombining as new bonds form (energy out, red arrow down) β†’ product molecules with new bonds. Summary box: net energy = energy in βˆ’ energy out determines exothermic or endothermic.
Station 2: bond-by-bond energy accounting β€” breaking costs energy, forming releases energy.

Station 2 β€” Where the Energy Hides: Bonds (20 points)

Zoom in on what's happening at the bond level. Energy stored in chemical bonds β€” chemical potential energy β€” is the bank account of a chemical reaction. Breaking bonds COSTS energy (you have to pay in to pull atoms apart). Forming bonds RELEASES energy (atoms 'fall together' into a more stable state). The net of those two movements is what your thermometer measured at Station 1.

The Net-Energy Rule

Net energy = Ξ”E_in (break bonds) βˆ’ Ξ”E_out (form bonds)

  • If forming bonds releases MORE energy than breaking bonds costs β†’ energy flows OUT β†’ EXOTHERMIC (reaction feels hot).
  • If breaking bonds costs MORE energy than forming bonds releases β†’ energy must be pulled IN β†’ ENDOTHERMIC (reaction feels cold).

Combustion of methane (CHβ‚„ + 2 Oβ‚‚ β†’ COβ‚‚ + 2 Hβ‚‚O) is exothermic because the new C=O and O–H bonds in COβ‚‚ and Hβ‚‚O are much stronger than the C–H and O=O bonds you started with. The difference comes out as heat and light.

Energy Is Always Conserved

Just like atoms (W2), energy doesn't disappear in a chemical reaction β€” it just changes address. Chemical potential energy in bonds β†’ thermal energy of moving particles β†’ warm hand. Trace it carefully and the total before and after the reaction is the same. This is the same conservation idea you used in W2 β€” applied to energy instead of mass.
COMPLETE THE STATION 2 FORM

Station 2 for bond-energy reasoning.

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Station 3 – Design a Hand-Warmer or Cold-Pack

Engineer a device that releases or absorbs thermal energy via chemical process.
Engineering schematic with two cutaway panels. Left: chemical hand-warmer with outer breathable pouch, inner iron filings plus salt plus activated carbon plus O2 inlet holes, red energy-OUT arrows, target 50 Β°C for 4 hr. Right: cold-pack with sealed outer pouch, inner water bag, solid ammonium nitrate granules outside, squeeze-to-burst indicator, blue energy-IN arrows, target 2 Β°C for 20 min. Comparison table below lists design criteria.
Station 3: engineering schematic comparing a chemical hand-warmer to a chemical cold-pack.

Station 3 β€” Engineering Design Brief (25 points)

This is the MS-PS1-6 performance task for the cycle. You will design a chemical device that EITHER releases thermal energy (hand-warmer) OR absorbs thermal energy (cold-pack), then justify your choice of reactants using everything you've learned this week.

Your Design Must Specify

  1. Goal: warm or cool (and the target Ξ”T β€” e.g., +25 Β°C or βˆ’15 Β°C).
  2. Reactants you'd choose and WHY (which bonds break, which form, expected net energy direction).
  3. Container/delivery: how the user starts the reaction (squeeze a bag, open to air, mix). How is it safe?
  4. Duration target: how long should the temperature stay on-target? (Hand-warmer: hours. Instant cold-pack: minutes.)
  5. Test plan: what would you MEASURE to confirm the design works? (Thermometer, time, dose of reactant.)

Bridge to Cycle 8

Your hand-warmer doesn't just have thermal energy β€” it TRANSFERS that energy to your hand by conduction (touch). Next cycle (C8) you'll measure those transfers. C8W3's Urban Heat Island week assesses combustion as a city-scale heat source β€” exactly the engineering logic you're using here, scaled up.
COMPLETE THE STATION 3 FORM

Station 3 β€” MS-PS1-6 engineering design.

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Exit Ticket – Energy in Reactions Integration

Synthesize the week's energy-in-reactions story; spiral on conservation (W2) and rate (W3).
Energy flow summary diagram: chemical potential energy in bonds β†’ bond breaking β†’ thermal energy β†’ useful work (lighting a bulb, warming a hand). Cycle 8 preview banner shows conduction, convection, and radiation.
Exit ticket: weekly synthesis + bridge to Cycle 8 thermal energy.

Exit Ticket β€” Energy in Reactions Integration (23 points)

Six questions, 23 points: 2 NEW (energy-diagram reading; exo/endo classification), 2 SPIRAL (W2 mass conservation; W3 reaction rate with a catalyst), and 1 INTEGRATION + SEP-6 question that asks you to construct an explanation about a real device using bond-energy reasoning. Energy is conserved; atoms are conserved. The chemistry hasn't changed β€” your model of it has just gotten one layer richer.

Left-to-right flow diagram: chemical potential energy in reactant bonds (green) β†’ bond breaking (amber) β†’ thermal energy released (red, with particle cloud) β†’ fans out to two examples of useful work: lighting a bulb and warming a hand. Footer banner previews Cycle 8 conduction, convection, and radiation.
Energy flow summary: chemical PE β†’ bond breaking β†’ thermal energy β†’ useful work, with a preview of Cycle 8 heat transfer.
COMPLETE THE EXIT TICKET

Demonstrate mastery of exothermic/endothermic reasoning.

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Enrichment & Extension
Optional content if you finish early or want to go deeper.

Scientist Spotlight: Marie Maynard Daly

Marie Maynard Daly (1921–2003) was the first Black woman in the United States to earn a Ph.D. in chemistry (Columbia, 1947). Her doctoral work focused on the chemistry of digestion β€” specifically the enzymes (biological catalysts) that release energy from the food you eat. Every meal you've ever digested is a series of carefully-controlled chemical reactions, most of them exothermic, and the energy released is what powers your body's thermal regulation, your muscles, and your brain. Daly went on to study cholesterol and high-blood-pressure chemistry at Albert Einstein College of Medicine. Her work links directly to this week's content: living organisms are walking exothermic reactors β€” we run on the same bond-energy bookkeeping you saw at Station 2, just at higher precision. Daly established a scholarship at Queens College for Black students in science, honoring her father who had to abandon his own science studies for lack of funds.

Environmental Justice: Combustion Heat Islands

The exothermic combustion reactions you studied this week β€” methane in stoves, gasoline in engines, diesel in trucks, natural gas in power plants β€” release thermal energy into urban air. In dense, low-income neighborhoods of St. Louis, Memphis, and across the Mississippi River corridor, the combustion is concentrated: more trucks, more power-plant exhaust, more fossil-fuel heating, and less tree canopy to absorb the heat. The local air is measurably hotter than wealthier neighborhoods just miles away β€” sometimes by 7–10 Β°F on summer afternoons. This is called the urban heat island effect, and it has real health consequences: higher heat-related ER visits, higher cooling costs, more childhood asthma. The chemistry is the same chemistry you learned this week β€” exothermic reactions transferring energy to the surroundings. The injustice is in who gets to breathe the cleanest air and stand under the most shade. Next cycle (C8W3 Urban Heat Island) you'll measure heat transfer at the surface level and design surface-treatment interventions.

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Week 4 Complete!

Next: Cycle 8 begins β€” thermal energy and heat transfer. You'll trace the energy your reactions released into surfaces, fluids, and waves.