The formation of water (H₂O) from its constituent elements, hydrogen (H₂) and oxygen (O₂), is an exothermic reaction. The process can be represented by the chemical equation:
\(2 \text{H}_2(g) + \text{O}_2(g) \rightarrow 2 \text{H}_2\text{O}(l)\)
In this reaction, the bonds between the hydrogen atoms and the oxygen atom are broken in the diatomic molecules (H₂ and O₂), and new bonds are formed to create water molecules (H₂O). The overall process releases more energy than it absorbs, resulting in an exothermic reaction. Here's why this reaction is exothermic:
1. Bond Formation Releases Energy: The formation of water involves the creation of new chemical bonds between hydrogen and oxygen atoms. Breaking the bonds in H₂ and O₂ requires energy input, but the energy released during the formation of new H-O bonds is greater than the energy required to break the H-H and O=O bonds. This net release of energy contributes to the exothermic nature of the reaction.
2. Stability of Water Molecules: Water molecules are highly stable compared to the diatomic molecules of hydrogen and oxygen. Stability is associated with lower potential energy, and when stable molecules form, excess energy is released. Water molecules have a strong covalent bond between hydrogen and oxygen, resulting in a lower overall energy state.
3. Thermodynamics: The reaction is thermodynamically favorable, as it leads to a decrease in the enthalpy (ΔH is negative). The negative ΔH indicates that the system releases heat to the surroundings.
In summary, the exothermic nature of the formation of water is a result of the favorable energy changes associated with breaking and forming chemical bonds. The released energy contributes to the overall stability of water molecules.
