What is an Endothermic Reaction?
An endothermic reaction is a chemical process that absorbs energy from the surroundings, usually as heat. The products have more energy than the reactants, so energy flows inward and the surroundings cool.
Endothermic reactions absorb energy (ΔH > 0) from the surroundings. Common examples: melting ice, evaporating water, photosynthesis, dissolving certain salts. The temperature of the surroundings decreases.
Step-by-step worked examples
Melting ice: H₂O(s) → H₂O(l). ΔH = +6 kJ/mol. Is this endothermic?
ΔH is positive (+6 kJ/mol). Positive ΔH means energy must be absorbed. This is endothermic.
Dissolving ammonium nitrate in water feels cold. Explain.
Dissolving NH₄NO₃ is endothermic (ΔH > 0). The reaction absorbs heat from the water and surroundings. Temperature drops → feels cold.
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. ΔH = +2800 kJ. Type?
ΔH is positive and large (+2800 kJ). The reaction requires energy input (sunlight). This is endothermic.
Flashcards
Quick quiz
Q1.Which is endothermic?
Q2.In an endothermic reaction, ΔH is
Q3.What happens to surroundings in endothermic?
Q4.Is evaporating water endothermic or exothermic?
The full card deck, worked steps and AI-tutor support for “What is an Endothermic Reaction?” are in Notek — study by hand before your exam.
Common mistakes
Endothermic reactions feel warm. — Correct: Endothermic reactions absorb heat → feel cold to touch.
Dissolving is always exothermic. — Correct: Most are exothermic, but some (e.g. NH₄NO₃, CaCl₂) are endothermic.
Melting and evaporation release heat. — Correct: Both absorb heat (endothermic). Freezing and condensation release heat (exothermic).
Endothermic means impossible or unlikely. — Correct: Endothermic just means ΔH > 0; many happen naturally (melting, evaporation).
FAQ
What is an endothermic reaction definition?
A chemical reaction that absorbs energy (usually heat) from the surroundings. ΔH > 0.
Endothermic reaction examples?
Melting ice, evaporating water, photosynthesis, dissolving ammonium nitrate, dissolving calcium chloride, cold packs.
How to tell if a reaction is endothermic?
ΔH > 0 (positive enthalpy change), or the surroundings cool down, or heat must be supplied.
Difference between endothermic and exothermic?
Endothermic: ΔH > 0, absorbs heat, surroundings cool. Exothermic: ΔH < 0, releases heat, surroundings warm.