endothermic reaction
Chemical reaction
Endothermic reactions are chemical reactions that absorb energy instead of releasing it.
This energy is usually taken as heat, but can also be light or electrical energy in rare cases.
Because energy goes inward, the products have more energy than the reactants.
These reactions often feel cold because the energy is being pulled from the surroundings.
2. Reactants
Reactants in an endothermic reaction require extra energy to break their bonds.
Without this added energy, the reaction will not start or will stop halfway.
Endothermic reactants are usually very stable, so they need more force to break apart.
The energy absorbed becomes stored in the products.
3. Temperature
Temperature drops in the surroundings because heat energy is taken into the reaction.
The more energy the reaction absorbs, the bigger the decrease in temperature.
This temperature drop is the main way to identify endothermic reactions.
Ice packs used in injuries work using this cooling effect.
4. Heat
Heat is absorbed continuously throughout the reaction.
In some reactions, the heat absorption is slow, while in others it is sudden and strong.
When heat is removed from the surroundings, objects around the reaction feel cooler.
Because heat moves inward, endothermic reactions never produce flames or glow.
5. Energy Flow
Energy enters from the surroundings and goes into the reactants.
This is why energy is written on the left side (reactant side) of the chemical equation.
The products end up with higher stored energy, making the reaction energy-positive.
Energy flow direction: surroundings → reaction.
6. Examples
Photosynthesis: Plants absorb sunlight to make sugar—classic endothermic reaction.
Dissolving ammonium nitrate: Used in instant cold packs because it absorbs lots of heat.
Melting ice: Ice needs heat to become water, so it takes it from its surroundings.
Evaporation: Liquid absorbs heat to turn into gas, cooling surfaces (like sweating).
Boiling: Heat must be constantly added to keep water boiling.
7. Environment Effect
The surroundings lose heat, causing coldness or a cooling effect.
This can be felt on the skin, on containers, or in the air around the reaction.
Large-scale endothermic processes can create noticeable drops in temperature.
Endothermic reactions are used in industry for refrigeration and cooling processes.
8. Molecules
Particles absorb energy to move faster or spread apart.
This energy breaks strong bonds between particles so the reaction can continue.
Because particles move further apart, the reaction needs constant energy input.
Many endothermic reactions cause expansion of particles, like when solids melt.
9. State Changes
Melting: Solid → liquid (requires heat to loosen tight particles).
Evaporation & Boiling: Liquid → gas (particles need energy to escape).
Sublimation: Solid → gas (needs even more energy).
All these changes are endothermic because no state change can occur without absorbing heat.
10. Reaction Speed
Endothermic reactions are often slow because they depend on continuous energy supply.
If heat supply stops, the reaction slows immediately or stops completely.
This is the opposite of exothermic reactions, which often speed up as they continue.
Some endothermic reactions, like photosynthesis, only occur under special conditions (light, temperature, enzymes).