Residual Enthalpy

[Michelle 1E]

What is residual enthalpy?

How can you predict which compound would most likely have the residual enthalpy at T= 0 K
(Question 1 on Achieve Week 5 and 6)

[Giselle_Arreguin_2K]

Residual enthalpy, , is defined as the difference between the actual enthalpy and the ideal enthalpy.

[Christian_Lee_2K]

Residual Enthalpy is the amount of enthalpy remaining in a substance after it has undergone a phase change.

We can predict the relative amounts based off IM forces.

Stronger IM forces (ie covalent bonds) = higher residual enthalpy
- This is because we need more energy in the form of enthalpy to break these bonds into a more ordered crystalline at 0K
Weaker IM forces (ie LDF) = lower residual enthalpy
- This is because they're more likely to remain as individual atoms in a spatially uniform distribution. With no strong forces to form a crystalline, there would be be minimal residual enthalpy associated with the substance.

I hope this helps!

[KevinG_1G]

You can think of residual entropy as being the entropy of a compound at 0K resulting from position disorder. So less symmetrical molecules will have more residual entropy than more symmetrical molecules.

So for question 1 on Achieve, compounds like Cl2 and CO2 are likely to have little residual entropy since they are symmetrical compounds (Cl-Cl and O=C=O will look unchanged no matter how you change the atoms in it). But compounds like NO and NO2 are most likely to have the highest entropy since you can change the orientation of the atoms in them. (N=O —> O=N, O=N-O —> O-N=O for example).