Question About Breaking Bonds in Crystal Lattices

[NullusSum]

TL;DR Summary

How do we calculate the bond dissociation energy of compounds in crystal lattices?

I am trying to understand how certain endothermic reactions work as a layman. I found the idea of reducing a rock to separate atoms fascinating, like what you would see in a comic book. However, I struggled to wrap my head around what that process would be like. I did some preliminary research, so I hope I was able to frame the question appropriately.

Question:​

How does the bond dissociation energy of compounds like silicon dioxide (SiO$_2$) and aluminum oxide (Al$_2$O$_3$) differ when they are embedded in a crystal lattice, such as in granite, compared to their isolated molecular forms? What additional interactions (e.g., lattice energy, covalent/ionic bonding) need to be considered when calculating the energy required to break these bonds in the lattice context?

 

[Borek]

Broadly speaking there is no such thing as a "bond dissociation energy of a compound". Bond energy typically refers to bond between two atoms (they can be part of something larger, each atom can be bonded to more than one other atom). We sometimes write more complicated compounds in the oxide form (like Na₂O⋅SiO₂ instead of Na₂SiO₃), but that's mostly notational trick (it does reflect some underlying chemical properties of atoms involved, but doesn't mean "compound is a mixture of oxides"). There are some compounds in which it is possible to "show" separate molecules (and we can calculate energy of forces keeping them together), but these are not very common (and often classifying them as "compounds" blurs the difference between a compound and a mixture).

That being said for every atom energy required to remove it from the lattice is a sum of interactions with all surrounding atoms, so it doesn't matter much if you want to remove Si from SiO₂ or Na₂SiO₃ - calculation will follow the same general principles.

What you ask about can be defined through the process "remove atoms, then create a compound out of them" (that's what the Hess's law is about) - but I don't remember seeing it used in this sense (even if it could be potentially useful in some contexts).

 

[NullusSum]

Broadly speaking there is no such thing as a "bond dissociation energy of a compound". Bond energy typically refers to bond between two atoms (they can be part of something larger, each atom can be bonded to more than one other atom). We sometimes write more complicated compounds in the oxide form (like Na₂O⋅SiO₂ instead of Na₂SiO₃), but that's mostly notational trick (it does reflect some underlying chemical properties of atoms involved, but doesn't mean "compound is a mixture of oxides"). There are some compounds in which it is possible to "show" separate molecules (and we can calculate energy of forces keeping them together), but these are not very common (and often classifying them as "compounds" blurs the difference between a compound and a mixture).

That being said for every atom energy required to remove it from the lattice is a sum of interactions with all surrounding atoms, so it doesn't matter much if you want to remove Si from SiO₂ or Na₂SiO₃ - calculation will follow the same general principles.

What you ask about can be defined through the process "remove atoms, then create a compound out of them" (that's what the Hess's law is about) - but I don't remember seeing it used in this sense (even if it could be potentially useful in some contexts).

Thank you for the detailed answer. My presumption was that because a compound has an enthalpy of formation, it must also have a specific enthalpy of dissociation. Based on your answer, this seems to be an erroneous way of thinking.

So, if we took a simple SiO$_2$ molecule, where the Si atom is covalenty bonded to two O atoms (O=Si=O), would the energy to remove the Si atom be approximately the dissociation energy of an O-Si bond (~799.6 kJ/mol) multiplied by 2? And if SiO$_2$ is a compound in a lattice, would we have to take it a step further and identity every other atom bonded to the Si and O atoms by analyzing the lattice itself?