6.1 Endo/Exo Processes

Endothermic vs Exothermic Explorer

A glowing ball traces exothermic and endothermic energy profiles for 6 real reactions. Surroundings particles speed up or slow down to show heat flow direction.

Energy ProfileΔH SignSurroundings6 Reactions
Topic 6.1

Endothermic and Exothermic Processes

Explain the relationship between experimental observations and energy changes associated with a chemical or physical transformation.

Temperature changes in a system indicate energy changes. That is the experimental handle on all of thermochemistry: you cannot see enthalpy, but you can read a thermometer.

Energy changes are described as endothermic or exothermic, and the classification applies to heating or cooling a substance, phase changes, and chemical transformations alike.

EK 6.1.A.3 states the bookkeeping precisely:

  • Exothermic reaction: the energy of the system decreases. Energy lost by the reacting species (the system) is gained by the surroundings, as heat transfer from — or work done by — the system. ΔH < 0.
  • Endothermic reaction: the energy of the system increases. The system gains energy from the surroundings by heat transfer to it, or work done on it. ΔH > 0.

The observational rule that follows: in an exothermic process, energy released by the reaction warms the surroundings, so the flask feels hot and the measured temperature rises. In an endothermic process the reaction pulls energy out of the surroundings, so the flask feels cold and the temperature falls.

This is where sign errors are born. The temperature of the solution goes up in an exothermic reaction, but the enthalpy change of the reaction is negative. The solution is the surroundings, not the system.

EK 6.1.A.4 adds dissolution: forming a solution may be exothermic or endothermic, depending on the relative strengths of the intermolecular/interparticle interactions before and after the dissolution process. Instant cold packs (NH₄NO₃) and hand warmers (CaCl₂) are the two everyday demonstrations.

Key points

  • Exothermic: system loses energy, surroundings warm, ΔH < 0.
  • Endothermic: system gains energy, surroundings cool, ΔH > 0.
  • The solution in a calorimeter is the surroundings — its temperature change has the opposite sign to the reaction’s enthalpy change.
  • Dissolving can be either endothermic or exothermic depending on which interactions dominate.

Common mistakes

  • Sign confusion. Temperature of the solution rises ⇒ reaction is exothermic ⇒ ΔH is negative.
  • Calling something exothermic because it feels cold. Cold means the reaction absorbed energy — endothermic.
  • Assuming exothermic means fast. Rate is Unit 5; enthalpy says nothing about it.
  • Assuming exothermic means favorable. Entropy matters too (Unit 9).

Worked example

A student dissolves NH₄NO₃ in water in an insulated cup. The temperature drops from 22.0 °C to 16.5 °C. Classify the process, give the sign of ΔH, and explain at the particulate level.

Classification: endothermic. ΔHsoln > 0.

Reasoning: the temperature of the water dropped, meaning the water (the surroundings) lost energy. Since energy is conserved and the cup is insulated, that energy must have been absorbed by the dissolution process (the system). The system gained energy, so ΔH is positive.

Particulate explanation: dissolving requires separating NH₄⁺ and NO₃⁻ ions from the crystal lattice, which costs energy, and separating water molecules from one another to make room, which also costs energy. Energy is released when ion–dipole interactions form between the ions and water. For NH₄NO₃ the energy released by hydration is less than the energy required to break apart the lattice and the water network, so the process absorbs energy overall and the solution cools.

Full notes for topic 6.1 →