🔥thermodynamics
Exothermic vs Endothermic
Exothermic vs Endothermic
Classification of reactions based on energy flow. Exothermic reactions release energy to surroundings. Endothermic reactions absorb energy from surroundings.
Comparison Table
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy Flow | Released to surroundings | Absorbed from surroundings |
| Delta H Sign | Negative (-) | Positive (+) |
| Temperature Effect | Surroundings get warmer | Surroundings get cooler |
| Energy Diagram | Products lower than reactants | Products higher than reactants |
| Bond Energy | Bonds formed > bonds broken | Bonds broken > bonds formed |
| Spontaneity | Often spontaneous | May need energy input |
| Examples | Combustion, neutralization | Photosynthesis, melting ice |
| Feels Like | Hand warmers, fire | Cold packs, evaporation |
Key Differences
- →Exothermic: system loses energy, surroundings gain energy
- →Endothermic: system gains energy, surroundings lose energy
- →Exothermic reactions have negative delta H; endothermic have positive delta H
- →Exothermic reactions release heat; endothermic reactions absorb heat
- →Bond making releases energy; bond breaking requires energy
When to Use Exothermic
- ✓Describing combustion reactions
- ✓Explaining why reactions feel hot
- ✓Reactions that produce heat spontaneously
- ✓When products are more stable than reactants
When to Use Endothermic
- ✓Describing dissolution of some salts
- ✓Explaining why reactions feel cold
- ✓Photosynthesis and similar reactions
- ✓When products are less stable than reactants
Common Confusions
- !Confusing exothermic/endothermic with spontaneous/non-spontaneous
- !Forgetting that delta H sign is from the system perspective
- !Thinking all spontaneous reactions are exothermic (entropy matters too)
- !Not considering that catalysts affect rate but not delta H
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Common questions about this comparison
Yes! Spontaneity depends on Gibbs free energy (delta G), not just enthalpy. If delta S (entropy increase) is large enough, an endothermic reaction can be spontaneous: delta G = delta H - T(delta S). Ice melting at room temperature is an example.
delta H = sum of bonds broken - sum of bonds formed. Breaking bonds requires energy (+), forming bonds releases energy (-). If more energy is released forming bonds than required to break them, the reaction is exothermic.