Chapter 3 · Matter and Energy
3.9Energy and Chemical and Physical Change
9 min · two checks
Predict
If a reaction releases heat to the surroundings, what happened to the chemical energy of the system?
The idea
- Label processes exothermic or endothermic from the system’s point of view.
- Apply conservation of energy to a chemical change.
Track energy from the system’s point of view, usually the chemicals in the beaker. An exothermic process releases energy to the surroundings: the beaker warms, and the products store less chemical energy than the reactants. Burning, and the freezing of water, are exothermic. An endothermic process absorbs energy: the beaker cools unless you supply heat, and the products store more chemical energy. Melting ice and photosynthesis are endothermic.
Energy is conserved. What the system loses, the surroundings gain, and the reverse. A cold pack does not destroy heat; the dissolving process absorbs energy from your hand. Physical changes have energy changes too. You already pay an energy cost to pull liquid particles apart into a gas, which is why evaporation cools the liquid left behind.
Keep these
- Exothermic: system releases energy. Endothermic: system absorbs energy.
- The surroundings warm in an exothermic process if the heat stays nearby.
- Energy conserved means “moved,” not “gone.”