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Endothermic and exothermic

A cold pack and a hand warmer are the same idea running in opposite directions: energy moving between a system and its surroundings. Get the direction and the sign of ΔH right, and the rest of thermodynamics lines up.

§1

Which way does energy flow?

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In thermodynamics we split the world into the system (what we study) and the surroundings (everything else). Energy flows between them, and we track it from the system's point of view.

A process is endothermic if the system absorbs energy from the surroundings; its enthalpy change is positive (ΔH > 0), and the surroundings cool (a cold pack). It is exothermic if the system releases energy; ΔH is negative (ΔH < 0), and the surroundings warm (a hand warmer).

Energy is conserved: it is transferred between system and surroundings, never created or destroyed, and never simply 'used up.'

UNIT 6 TOPIC 6.1 • ENDOTHERMIC AND EXOTHERMIC PROCESSES ENERGY FLOW ENDOTHERMIC PROCESS System absorbs energy from surroundings ΔH > 0 energy in enthalpy reaction progress reactants products ΔH > 0 EXOTHERMIC PROCESS System releases energy to surroundings ΔH < 0 energy out enthalpy reaction progress reactants products ΔH < 0 ENERGY LEDGER RULE Breaking attractions or bonds absorbs energy · Forming attractions or bonds releases energy Dissolving may be endothermic or exothermic, depending on relative interaction strengths. TAKEAWAY Sign of ΔH describes heat flow for the system: positive = energy in, negative = energy out. AP Chemistry · Unit 6 · Thermodynamics
Fig. 6.1.1 Energy flows between a system and its surroundings. An endothermic process absorbs energy (ΔH > 0); an exothermic process releases energy (ΔH < 0). The sign of ΔH is defined from the system's point of view, and energy is transferred, never used up.
§2

Assigning the sign of ΔH.

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Take the system's view and follow the energy.

  1. Define the system. Decide what you are tracking; everything else is the surroundings.
  2. Ask which way energy flows. Into the system (absorbed) or out of it (released)?
  3. Assign the sign. Absorbed → endothermic, ΔH > 0. Released → exothermic, ΔH < 0.
  4. Check the surroundings. Endothermic cools the surroundings; exothermic warms them — a good consistency check.
§3

The pieces you'll meet.

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A few terms anchor every energy problem.

system
System
What we study; ΔH is defined from its viewpoint.
surroundings
Surroundings
Everything outside the system, exchanging energy with it.
endo
Endothermic
Absorbs energy; ΔH > 0; surroundings cool.
exo
Exothermic
Releases energy; ΔH < 0; surroundings warm.
ΔH
Enthalpy change
The heat exchanged at constant pressure; its sign is the direction.
conserve
Conservation
Energy is transferred, never created, destroyed, or used up.
§4

Worked example: cold pack versus hand warmer.

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Cold pack. Dissolving the salt absorbs energy from the surroundings. The system absorbs, so it is endothermic, ΔH > 0, and the pack feels cold because it drew heat from your hand.

Hand warmer. The reaction inside releases energy to the surroundings. The system releases, so it is exothermic, ΔH < 0, and it feels warm.

Consistency. Endothermic cools the surroundings; exothermic warms them — matching what you feel.

Key point. In neither case is energy destroyed; it simply moves between the system and its surroundings.

§5

Mistakes that cost real points.

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Pitfall · 01

"Exothermic means a positive ΔH."

It is the reverse: exothermic (energy released) has a negative ΔH, and endothermic (energy absorbed) has a positive ΔH. Inverting the sign convention flips every energy conclusion.

Fix. Fix the convention: released energy → ΔH < 0 (exothermic); absorbed energy → ΔH > 0 (endothermic).

Pitfall · 02

"The energy is used up during the process."

Energy is conserved — it is transferred between the system and the surroundings, not consumed. Exothermic energy goes to the surroundings; endothermic energy comes from them. Nothing is destroyed.

Fix. Track energy as moving between system and surroundings; it is never used up or lost from existence.

Pitfall · 03

"Only chemical reactions have enthalpy changes."

Physical processes have enthalpy changes too — dissolving, melting, and boiling all absorb or release energy. A cold pack works by a physical dissolving process, not a reaction. Enthalpy changes accompany physical and chemical processes alike.

Fix. Apply the endo/exothermic idea to any process, physical or chemical, that transfers energy.

§6

Skill Check.

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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

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