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How can I represent enthalpy in a potential energy diagram?

Answer 1

In the context of a potential energy diagram, there is an enthalpy of bond formation at the equilibrium bond distance. It is not entirely obvious, but it is the potential energy shown on the y-axis.

The equilibrium bond distance is known as the most stable proximity in space for each atom to be before a chemical bond can be made.

Side note: with only two atoms bonding, it doesn't matter at all where they are in the second or third dimensions---they will always be aligned in one dimension. So, the bond distance is in the "radial" direction, called r.

You can see that the optimal bond distance is #"74 pm"# for #"H"_2#, but at longer distances, the capacity to bond is of course too low, and at shorter distances, perhaps less obviously, there is too much nuclear repulsion and the atoms want some more breathing room.

The enthalpy of the formation of the #"H"-"H"# bond is about #"-432 kJ/mol"#. That means it releases #"432 kJ"# of energy in order to make a #"mol"# of #"H"-"H"# bonds.

This enthalpy is the difference in energy between the nonbonded state and the bonded state, and in the diagram, is therefore in reference to the zero-point energy. In this case, the zero-point energy represents no bonding interactions.

(You can define the zero-point energy anywhere, technically, but typically we just put that on the x-axis for simplicity.)

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Answer 2
Kai Bowman
Enthalpy is represented in a potential energy diagram by the vertical axis. The enthalpy change of a reaction corresponds to the difference in potential energy between the reactants and products. Typically, the products are positioned at a lower potential energy than the reactants, indicating an exothermic reaction, while an endothermic reaction would show the products at a higher potential energy level.
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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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