Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- The system is what you study (the reacting particles); the surroundings are everything else, usually the water and the container.
- Energy is conserved: what the system loses, the surroundings gain, and the other way round.
- Exothermic: the system’s energy decreases; energy flows out. Endothermic: the system’s energy increases; energy flows in.
- Energy crosses the boundary as heat (because of a temperature difference) or as work (for example, a gas pushing back the air).
- Temperature changes show energy changes. The thermometer usually reads the surroundings.
Key relationships
| Process | Endothermic or exothermic? | Why |
|---|---|---|
| Surroundings warm up | exothermic (for the system) | energy left the system |
| Surroundings cool down | endothermic (for the system) | energy entered the system |
| Heating a substance | endothermic for that substance | it gains energy |
| Melting, boiling, sublimation | endothermic | attractions between particles are overcome |
| Freezing, condensing, deposition | exothermic | attractions form as particles come together |
| Breaking a bond | always needs energy | — |
| Forming a bond | always releases energy | — |
| Dissolving | either | depends on the strength of attractions formed vs broken |
Assumptions behind the classification
- The thermometer is in the surroundings, and the cup is insulated well enough that little energy escapes to the room.
- The temperature change is bigger than the precision of the thermometer.
- Any energy moved as work is small compared with the energy moved as heat (true unless a lot of gas is made or used up).
Mistakes to avoid
- “Cold flows in.” Only energy moves. Cooling means energy left the surroundings.
- Reading the thermometer as the system. Warmer water means an exothermic system.
- “Breaking bonds releases energy.” It never does.
- No system named. Say “dissolving NaOH is exothermic”, not just “exothermic”.
- Dissolving = always endothermic. New solute–solvent attractions release energy too.
- Over-reading tiny changes. A change smaller than the thermometer’s precision is not evidence.
Quick self-check
- Frost forms on a car overnight. Endothermic or exothermic for the water? (Exothermic: deposition releases energy.)
- A solution cools from 20.4 °C to 17.1 °C as a salt dissolves. Classify the dissolving. (Endothermic; ΔT = −3.3 °C, energy moved from the water into the dissolving system.)
- When is dissolving exothermic? (When the solute–solvent attractions formed are stronger overall than the solute–solute and solvent–solvent attractions broken.)
Next: practice questions.