# Where is potential energy seen in chemistry?

Jun 1, 2016

The potential energy of a chemical bond is where one might see potential energy.

On the $y$ axis, we have the potential energy in $\text{kJ/mol}$, and on the $x$ axis, we have the distance $r$ between the left atom ($\text{H}$) and the right atom ($\text{H}$)'s nuclei.

• As the two hydrogen atoms move closer together, the attraction between hydrogen A's valence electron and hydrogen B's proton increases. Same with hydrogen A's proton and hydrogen A's valence electron.

• An energy of attraction is conventionally negative, so we move downwards on the potential energy curve.

If we move leftwards past the $\text{74 pm}$ mark on the graph:

• When the two hydrogen atoms are too close, each proton in each nucleus repels the other. This is a nuclear repulsion energy.

• This energy of repulsion is conventionally positive, so we move upwards on the potential energy curve.

A molecule wants to be as stable as possible; if it is unstable, it breaks apart.

So, the molecule prefers to be at the $\text{74 pm}$ mark, which balances the repulsive and attractive energies, giving us the potential energy of the $\text{H"-"H}$ single bond once the bond forms:

$- \text{436 kJ/mol}$

This potential energy is the energy leftover, stored in the bond, after making the $\text{H"-"H}$ bond and releasing excess energy.

If the bond breaks, the potential energy is absorbed back into the atoms as kinetic energy as the atoms move far apart.