What Are Endothermic Reactions?
When chemical reactions occur, energy is always involved. Some reactions release energy to the surroundings, while others absorb energy from the surroundings. Endothermic reactions are the type that take energy in.
A chemical reaction that absorbs energy from the surroundings, causing the temperature of the surroundings to decrease.
The word endothermic comes from the Greek roots:
- endo = within/inside
- thermic = heat
So endothermic literally means "heat going inside" — energy flows from the surroundings into the reaction.
Think of an endothermic reaction like a sponge soaking up water. Instead of soaking up water, the reaction soaks up energy from its surroundings. As a result, the area around the reaction gets colder — just like a sponge leaves the surface drier after absorbing water.
A key observable sign of an endothermic reaction is that the temperature of the surroundings drops. If you hold a test tube containing an endothermic reaction, it will feel cold.
Energy and Chemical Bonds
To understand why some reactions are endothermic, we need to think about what happens to chemical bonds during a reaction.
Every chemical reaction involves two processes:
- Breaking bonds in the reactants — this step always requires energy (endothermic step)
- Forming bonds in the products — this step always releases energy (exothermic step)
Whether the overall reaction is endothermic or exothermic depends on the balance between these two:
- If more energy is needed to break bonds than is released when forming bonds → the reaction is endothermic (net energy absorbed)
- If more energy is released when forming bonds than is needed to break bonds → the reaction is exothermic (net energy released)
In endothermic reactions, the products have more stored (chemical potential) energy than the reactants. This extra energy has been absorbed from the surroundings.
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