6.1 Endothermic and Exothermic Processes

Energy transfer between system and surroundings

In thermochemistry, we divide the universe into the system (the reaction/process studied) and the surroundings (everything else). Energy flows as heat (q) between them. If heat flows from system to surroundings, the process is exothermic; if heat flows from surroundings into the system, it's endothermic.

Exothermic vs. endothermic

Exothermic processes release energy — the surroundings feel warmer, and q is negative for the system. Endothermic processes absorb energy — the surroundings feel cooler, and q is positive for the system. Bond breaking requires energy input (endothermic), while bond formation releases energy (exothermic); the overall sign depends on which dominates.

Temperature change as evidence

Temperature change of the surroundings is used experimentally to classify a process. A rise in surrounding temperature indicates heat left the system (exothermic), while a drop indicates heat entered the system from the surroundings (endothermic). This is measured using calorimetry, where energy transferred equals q = mcΔT.

Key terms

System

The specific part of the universe (reaction or process) being studied for energy changes.

Surroundings

Everything outside the system that can exchange energy with it.

Exothermic process

A process that releases heat to the surroundings; q (system) is negative, surroundings warm up.

Endothermic process

A process that absorbs heat from the surroundings; q (system) is positive, surroundings cool down.

Calorimetry

A method for measuring heat transfer during a process using temperature change: q = mcΔT.

Try an AP-style question

Dissolving NH₄NO₃ in water makes the water colder. What does this show?

1. The water loses mass
2. No energy is transferred
3. The dissolving is endothermic
4. The dissolving is exothermic