Exothermic and endothermic
Chemical reactions proceed with conservation of mass and energy. Bonds are broken in reactants, and different bonds are formed in products. If the energy required for bond-breaking is different to the energy released in bond-breaking, energy will be transferred to or from the surroundings as heat.
Processes that release heat energy to their surroundings are said to be exothermic.
Because energy is conserved in all reactions, the products must be of lower energy than the reactants.
Processes that absorb heat energy from the surroundings are said to be endothermic.
Because energy is conserved in all reactions, the products must be of higher energy than the reactants.
As shown above: 2H2O(g)
2H2(g) + O2(g) is endothermic.
The reverse of an exothermic reaction is endothermic.
Processes that release heat energy to their surroundings are said to be exothermic.
Because energy is conserved in all reactions, the products must be of lower energy than the reactants.
As shown below, formation of H2O(g) from its elements is exothermic.
| high energy  low energy | heat released to surroundings | 2H2(g) + O2(g)  H2O(g) | heat absorbed from surroundings |
Processes that absorb heat energy from the surroundings are said to be endothermic.
Because energy is conserved in all reactions, the products must be of higher energy than the reactants.
As shown above: 2H2O(g)
2H2(g) + O2(g) is endothermic.The reverse of an exothermic reaction is endothermic.
As shown above:
Bond breaking (F-F 2F) is endothermic.
Bond formation (2F F-F) is exothermic.
Bond breaking (F-F
2F) is endothermic.Bond formation (2F
F-F) is exothermic.| high energy low energy | heat released to surroundings | 2F(g) F2(g) | heat absorbed from surroundings |