Chemical bond energy example

In the chemical bonds of a molecule the attractive electrical forces cause bound states to exist. That is, the atoms of the molecule cannot escape the molecule without a supply of external energy. Bound states imply a negative potential energy compared to the free atoms, so any chemical bond has associated with it a negative potential energy. The principle of conservation of energy can be used as an overall analysis tool for looking at chemical reactions involving changes in bonds.

在分子的化学键中，吸引力的电 forces 导致束缚态的存在。也就是说，分子中的原子在没有外部能量供应的情况下无法脱离分子。束缚态意味着与自由原子相比具有负的势能，因此任何化学键都伴随着负的势能。能量守恒原理可以作为整体分析工具，用于研究涉及键变化的化学反应。

Consider the combination of two molecules of H₂ with one molecule of O₂ to form two molecules of water, H₂O. Energetically, the process can be considered to require the energy to dissociate the H₂ and O₂, but then the bonding of the H₂O returns the system to a bound state with negative potential. It is actually more negative than the bound states of the reactants, and the formation of the two water molecules actually releases 5.7 electron volts of energy .

考虑两个H₂分子与一个O₂分子结合形成两个水分子H₂O的过程。从能量角度看，这个过程需要能量来解离H₂和O₂，但随后H₂O的形成使系统回到一个具有负势能的束缚态。实际上，这种束缚态比反应物的束缚态更负，两个水分子的形成实际上释放出5.7电子伏特的能量。

The balance of energy before and after the reaction can be illustrated schematically with the state in which all atoms are free taken as the reference for energy.

反应前后能量的平衡可以示意性地用所有原子均为自由态的状态作为能量参考来表示。

If the dissociation energies of the H₂ and O₂ and the energy release upon their reaction were measured, that would offer an experimental path to determining the total bond energy of the H₂O molecule.

如果测量了H₂和O₂的解离能以及它们反应时释放的能量，这将提供一条实验途径，以确定H₂O分子的总键能。

Available experimental parameters for sodium, chlorine and the NaCl molecule provide data for calculation of the dissociation energy of the molecule from conservation of energy. The steps toward forming the NaCl ionic molecule could be seen as (1) providing the ionization energy of 5.14 eV to ionize Na, then (2) the recovery of -3.62 eV from the electron affinity of Cl, then the binding energy of the electric attractive forces based on the known bond length of 0.236 nm. But this process does not match the measured dissociation energy of NaCl, 4.26 eV. This reveals the presence of another repulsive energy term called Pauli repulsion (+0.32 eV).

钠、氯和NaCl分子的实验参数提供了计算分子解离能的数据，该解离能基于能量守恒来计算。形成NaCl离子分子的步骤可以视为：(1) 提供5.14 eV的电离能以电离钠，然后(2) 从氯的电子亲和能恢复-3.62 eV，接着基于已知的键长0.236 nm计算基于静电吸引力的结合能。但这一过程与测量得到的NaCl解离能4.26 eV不一致。这表明还存在另一个排斥能项，称为Pauli排斥（+0.32 eV）。