What are Exothermic and Endothermic Reactions?
Chemical reactions either release energy to the surroundings (exothermic) or absorb it (endothermic). Burning, rusting, and freezing release heat; melting ice and photosynthesis absorb it.
Exothermic reactions release energy: products have lower energy than reactants (ΔH < 0). Endothermic reactions absorb energy: products have higher energy than reactants (ΔH > 0). The difference determines whether reactions feel hot or cold.
- •Products have lower energy
- •ΔH (enthalpy change) < 0 (negative)
- •Heat flows OUT to surroundings
- •Surroundings feel warmer
- •Examples: combustion, rusting, freezing, neutralization
- •Products have higher energy
- •ΔH > 0 (positive)
- •Heat flows IN from surroundings
- •Surroundings feel colder
- •Examples: melting, evaporation, photosynthesis, dissolving ammonium salts
Step-by-step worked examples
Classify: C + O₂ → CO₂ (releases 393 kJ/mol) as exothermic or endothermic.
Combustion reaction releases 393 kJ/mol to surroundings → Exothermic (ΔH = −393 kJ/mol < 0) You feel heat; surroundings warm up.
Is melting ice endothermic or exothermic?
Ice must absorb heat from surroundings to become liquid → Endothermic (ΔH > 0) Surroundings cool down (ice pack effect).
Burning wood releases 15 MJ. Is this exothermic?
Wood burning releases energy (negative ΔH) → Yes, exothermic You feel warmth; chemical energy → heat + light.
Flashcards
Quick quiz
Q1.Burning candle releases heat. This is…
Q2.Melting ice cream requires heat from surroundings. This is…
Q3.In an exothermic reaction, ΔH is…
Q4.Which is endothermic?
The full card deck, worked steps and AI-tutor support for “What are Exothermic and Endothermic Reactions?” are in Notek — study by hand before your exam.
Common mistakes
Thinking 'exo' means energy is absorbed. — Correct: 'Exo' = exit/outward → energy exits (released).
Confusing ΔH sign with actual energy flow. — Correct: ΔH < 0 = exothermic (releases). ΔH > 0 = endothermic (absorbs).
Believing endothermic reactions are rare. — Correct: Common: melting, evaporation, dissolving some salts, photosynthesis, ice packs.
Thinking all reactions with a positive ΔH won't happen. — Correct: Endothermic reactions can be spontaneous if ΔS > 0 (entropy effect).
FAQ
What is the difference between exothermic and endothermic?
Exothermic releases heat energy (ΔH < 0); endothermic absorbs heat (ΔH > 0). In exothermic, products are lower in energy; in endothermic, products are higher.
Is combustion always exothermic?
Yes — burning always releases heat and light energy.
Why does melting ice feel cold?
Melting is endothermic. Ice absorbs heat from surroundings to break bonds, cooling your hand.
Can an endothermic reaction happen spontaneously?
Yes, if entropy increase (ΔS) outweighs the energy absorption cost (Gibbs: ΔG = ΔH − TΔS).