L30. Reaction Energy

Chemical Reactions

R-report

L30. Reaction Energy

Why do some reactions feel hot while others feel cold?

Central Idea:

Chemical reactions move energy when bonds break and when new bonds form. Breaking bonds requires energy input; forming bonds releases energy to the surroundings. If released energy > energy absorbed, the reaction is exothermic and surroundings get warmer (example: an iron oxidation in a hand‑warmer gives heat). If absorbed energy > energy released, the reaction is endothermic and surroundings get cooler (example: many cold‑pack dissolutions absorb heat). Write this idea as a simple energy balance: ΔE = Energy released by bond formation − Energy required to break bonds. Temperature change of surroundings shows the direction of heat flow and magnitude.

How to observe:

Observe temperature change during a reaction. Measure initial temperature, mix reactants, then record final temperature. Note whether the mixture warms (exothermic) or cools (endothermic). Smell, color, bubbling, or precipitate are additional observations but separate from heat evidence. Example: activate a hand‑warmer and record the temperature rise (iron powder + oxygen). Example: squeeze a cold‑pack and note temperature drop (salt dissolving in water absorbs heat). Explain: breaking bonds in reactants used energy; forming bonds in products released energy; compare totals to tell which effect wins. Record units (°C) and repeat the test twice for reliable comparison and average results when possible.

Compare and test:

To compare reactions and diagnose whether energy is released or absorbed, ask a short checklist: Did the temperature of surroundings rise, fall, or stay the same? Was there a gas, light, or precipitate that indicates a chemical change? Can you reverse the change easily (physical changes reverse, many chemical ones do not)? Simple falsification tests: measure temperature with units and see if repeated trials show the same direction; check mass if possible to detect lost gases. Compare two examples by listing bond changes: which bonds break, which form, and which process involves stronger bonds (stronger bond formation releases more energy). Quick practice: a sealed pack warms by 8 °C when opened and mixed — is this exothermic or endothermic? Explain in one sentence. If surroundings temperature changes, consider heat capacity of container and solution; subtract baseline experiment. Use pictograms to mark heat flow direction and bond diagrams to support claim.

Synthesis:

Link observations to micro‑level language: use terms like exothermic, endothermic, bond energy, heat flow, surroundings. Explain by counting bonds: energy to break reactant bonds versus energy released forming product bonds. Example exothermic reaction (balanced): 2H2 + O2 → 2H2O (net energy released). Example endothermic process (dissolution): NH4NO3(s) → NH4+(aq) + NO3−(aq) (absorbs heat). Always show atoms and charge conserved in your equations. Label temperature changes with units and state the heat direction and magnitude also.

Before finalizing, check these quickly: include units (°C or K) and time if relevant; balance any reaction and confirm atoms and charge are conserved. Tell the atom‑and‑bond level story: which bonds broke, which formed, and where energy moved. Match visible evidence (temperature change, gas, precipitate, color) to your bond‑level explanation. If something disagrees, repeat measurements, check apparatus, and try a control or blank trial to falsify your hypothesis. Then repeat steps for the next problem.