# During formation and breaking of bonds, can the terms potential energy and bond energy be used interchangeably?

My chemistry professor uses these terms interchangeably and I was wondering if they refer to the same thing. From what I understand, bond energy is the energy required to break one mole of covalently bonded gaseous molecules. While potential energy refers to the energy gained or lost during bond formation which can be further characterised into endothermic or exothermic.

• You always gain energy when you form a bond (unless you had to break other bonds in the process). What is endothermic or exothermic is a reaction where multiple bonds might be formed or broken. You can estimate whether a reaction is endothermic or exothermic by evaluating the bond energies (technical term: bond dissociation energies) of bonds made and broken. Jan 29 '19 at 14:09
• Yes you can use them interchangeably. You may then refine the usage to be a defined in a particular way, such as bond energy. The other form of energy is kinetic energy such as in the vibrational and rotational motion of a molecule. Jan 29 '19 at 17:07
• @KarstenTheis Hmmm, nearly the opposite of what I write here: chemistry.stackexchange.com/questions/108697/… Jan 29 '19 at 20:29
• @Try_Hard: Yes, that might have been confusing. The "You" in "You always gain energy" would be the environment, so bond formation is exothermic. Maybe "it takes energy to break bonds" is the less confusing statement. Jan 29 '19 at 20:36

The answer is a qualified yes. Specifically, the "bond dissociation energy" (often shortened to "bond energy") is a positive number that is the amount of energy one must add to the system to completely break a mole of equivalent bonds. A completely broken bond by definition has 0 potential energy, so bond formation results in a negative potential energy. Thus, the potential energy of the bond is equal in magnitude but opposite in sign to the bond dissociation energy. If we are only talking about the magnitude (which is often the case), they're interchangeable.

In addition to the chemical bonds, a system can have other sources of potential energy such as gravitational potential energy or electrostatic potential energy, so in that sense potential energy is not synonymous with bond energy, but generally it's pretty clear from context when we mean only the potential energy of bonding.

The potential energy increases at the expense of the kinetic energy during the breaking of bonds, that the kinetic energy transforms to potential energy, so the definition of bond energy as a change of internal energy,$$\Delta{E}$$, which associates the breaking bond.

Because of the work $$\mathrm{P\Delta{V}}<<\Delta{E}$$ during breaking a bond, so: bond energy equal change in enthaphy$$\Delta{H}$$which associates with the breaking covalent bond in the molecule at gaseous state to produce neutral particles in the gaseous state.

e.g:Bond energy of hydrogen molecule$$~\ce{H-H}=436\pu{K.J/mol}~\ce{H2}$$

$$\ce{H-H_\mathrm{(g)} ~-> 2H\mathrm{(g)}} \quad\left(\Delta{H}=+436\pu{K.J/mol}~\ce{H2}\right)$$ so,$$\Delta{H_f^o}~\ce{H_\mathrm{(g)}}=\frac{436}{2}=218\pu{K.J}$$ to produce one $$\pu{mol}$$ of hydrogen atoms at gaseous state $$\ce{H_\mathrm{(g)}}$$. $$\text{Bond energy} =\Delta{H}= 2\times{(\text{Potential Energyof}~\ce{H_\mathrm{(g)}})}-\text{Potential Energyof}~\ce{(H-H)_\mathrm{(g)}}=+436\pu{K.J/mol}~\ce{H2}$$

• @Try Hard Your comment is correct, I edited it. Jan 30 '19 at 12:03