Why isn't bond dissociation energy/bond enthaply measured in Newtons?

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Discussion Overview

The discussion centers around the concept of bond dissociation energy and its relationship to force and distance, specifically questioning why bond dissociation energy is not measured in Newtons. Participants explore the theoretical underpinnings of bond energy, equilibrium, and the models used to represent atomic interactions.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that bond dissociation energy could be thought of as the force needed to break a bond multiplied by the bond length.
  • Others argue that bond dissociation energy is actually the energy required to move atoms infinitely far apart, emphasizing that force is the gradient of energy.
  • It is noted that at the equilibrium bond length, energy is at a minimum, resulting in zero net force, which is consistent with the concept of equilibrium.
  • Some participants challenge the idea of infinite separation, suggesting that practical dissociation occurs at distances greater than a few angstroms.
  • There is a discussion about the harmonic oscillator model, with some stating it is an accepted representation of diatomic atoms, while others contend that it may not be sufficient for quantum mechanical calculations.
  • Participants mention that molecular dynamics often use Hooke's law to model bonds, indicating a reliance on classical approximations in certain contexts.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between bond dissociation energy, force, and distance. There is no consensus on the interpretation of these concepts, and multiple competing models and perspectives are presented throughout the discussion.

Contextual Notes

Some limitations are noted regarding the assumptions made about equilibrium and the applicability of models like the harmonic oscillator in quantum mechanical contexts. The discussion also highlights the practical aspects of bond dissociation in small volumes.

adf89812
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TL;DR
Why isn't average bond dissociation energy/bond enthalpy measured in units of force/Newtons (kg*m/s^2)?
I understand every bond chemically has a length and energy to break, and energy is Newton*meters.
Is the Bond enthaply/Bond disassociation energy equivalent to the force needed to break the bond * the bond length?

Why don't we say, to break the bond from O to H we need to put magnets on left of the O and right of the H and apply some pulling force of XYZ?
 
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adf89812 said:
Is the Bond enthaply/Bond disassociation energy equivalent to the force needed to break the bond * the bond length?
No, it’s the energy required to move the atoms infinitely far apart. Think of it this way: force is the gradient of energy (derivative of energy with respect to distance)
$$F = \nabla E \left(=\frac{dE}{dx}\right)$$
At the equilibrium bond length, energy is at a minimum, meaning that the gradient (and therefore the force) is zero—this makes sense because a system at equilibrium has no net force acting on it.
 
TeethWhitener said:
No, it’s the energy required to move the atoms infinitely far apart. Think of it this way: force is the gradient of energy (derivative of energy with respect to distance)
$$F = \nabla E \left(=\frac{dE}{dx}\right)$$
At the equilibrium bond length, energy is at a minimum, meaning that the gradient (and therefore the force) is zero—this makes sense because a system at equilibrium has no net force acting on it.
It's an accepted model to represent atoms diatomic as ball attached to spring attached to ball so equilibrium is false. They can't be infinitely far apart because I can disassociate hydroxide in a small vial of very small size.
 
adf89812 said:
It's an accepted model to represent atoms diatomic as ball attached to spring attached to ball so equilibrium is false. They can't be infinitely far apart because I can disassociate hydroxide in a small vial of very small size.
I have no idea where you’re getting this from. I think you’re trying to say that bonds can be modeled as harmonic oscillators. And of course harmonic oscillators have an equilibrium point. It’s at the bottom of the potential well.

Also, the bond dissociation energy is a limit as the distance between atoms goes to infinity. Practically, with most bonds, once atoms are separated by more than a few angstroms they’re essentially dissociated.
 
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adf89812 said:
It's an accepted model to represent atoms diatomic as ball attached to spring attached to ball so equilibrium is false. They can't be infinitely far apart because I can disassociate hydroxide in a small vial of very small size.
The harmonic oscillator is an approximation. And you can in principle model a molecule as a balls on sticks that oscillate like a spring, but it's usually not sufficient for quantum mechanical calculations. Molecular dynamics force fields often model bonds with Hooke's law.
 

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