heat change of reactions

When a chemical bond is formed, the system becomes stable. A more stable system is always associated with a lower energy content of the system.

The following diagram shows the energy of a system containing two H atoms, where R is the internuclear distance between the two H atoms.

圖片參考：http://www.chemicool.com/img1/graphics/morse-potential-curve.gif

Refer to the diagram. The system has the lowest energy content when the distance between the two H atoms is 0.77 (7.7 nm), which is the bond length of H2­ molecule. In other words, the energy content is lowered when a chemical bond is formed.

When a chemical bond is formed, the energy content is lowered, and the excess energy is thus released. In other words, chemical bond formation is an exothermic process.
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