Exothermic and Endothermic Reactions
Exothermic reactions release heat; endothermic absorb it.
About this topic
An exothermic reaction releases energy to the surroundings, so the temperature rises and the enthalpy change ΔH is negative: combustion, neutralisation and respiration. An endothermic reaction absorbs energy, so the surroundings cool and ΔH is positive: photosynthesis, thermal decomposition and dissolving ammonium nitrate.
Energy changes arise from bonds: breaking bonds requires energy, and forming bonds releases it. If more energy is released forming new bonds than is needed to break old ones, the reaction is exothermic. On an energy profile diagram, an exothermic reaction has products lower than reactants, with the difference being ΔH, and an endothermic reaction has products higher. Both types need activation energy to get started. Temperature changes can be measured with a thermometer or calorimeter, using q = m c ΔT.
Examples: a hand warmer using iron oxidation is exothermic and a cold pack is endothermic. Burning methane releases about 890 kJ per mole. Photosynthesis stores energy from sunlight in glucose, and respiration releases it. Cooking an egg requires energy input continuously.
A misconception is that exothermic reactions need no starting energy; a match must be struck first. Another is that endothermic means cold: the reaction mixture feels cold because it takes energy from its surroundings. Make sure the sign of ΔH is described from the system's point of view.
Key ideas
- ΔH negative for exothermic
- Combustion is exothermic
- Photosynthesis is endothermic
Quick check
Use cases: where Exothermic and Endothermic Reactions helps
- Homework: review the key idea behind Exothermic and Endothermic Reactions before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Exothermic and Endothermic Reactions without notes.
- Visual learning: play with the interactive lab to see how Exothermic and Endothermic Reactions changes when you adjust its inputs.
- Teaching a friend: explain Exothermic and Endothermic Reactions in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Exothermic and Endothermic Reactions?
Exothermic reactions release heat; endothermic absorb it.
What will I learn in Exothermic and Endothermic Reactions?
ΔH negative for exothermic. Combustion is exothermic. Photosynthesis is endothermic.
Is Exothermic and Endothermic Reactions part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Exothermic and Endothermic Reactions lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Exothermic and Endothermic Reactions?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.