Bond dissociation energy and enthalpy

[pepocho]

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[Czarcasm]

So quick question about BE and deltaH

A very stable bond like oh idk, C-C on benzene would have a high BE and a low deltaH right? Or am I completely wrong?

BDE is the energy required to break a bond homolytically. The stronger the bond, the more energy it would require, and therefore, the greater the value. deltaH deals with relative stabilities of products - reactants, so really you'd need a reference point to make that comparison. But what you can say, is if you're going from some unstable, higher energy compound to a lower energy, more stable compound, than the free energy would certainly be negative, releasing a whole lot of energy in the process.

 

[Chrisz]

Actually bond dissociation energy is another perspective of enthalpy. a molecule is made of bonds. Delta H can be calculated bond dissociation energy. When a bond is formed, energy is released, and when a bond is broken, energy is added. Bond dissociation energy is measured by taking the average value of similar bonds broken in different compounds. It is not the actual amount of energy required to break a bond, for example, a C-H single bond. So, pretty much it is an estimate of energy released or added when a bond is formed or broken respectively. So, it can be used to estimate the enthalpy change of a reaction. Pretty much, bde is a microscopic view change in energy (bonds formed or broken), while Delta H of a reaction is the macroscopic view of a chemical reaction, (the entire system as a whole).

 

[Czarcasm]

Actually bond dissociation energy is another perspective of enthalpy. a molecule is made of bonds. Delta H can be calculated bond dissociation energy. When a bond is formed, energy is released, and when a bond is broken, energy is added. Bond dissociation energy is measured by taking the average value of similar bonds broken in different compounds. It is not the actual amount of energy required to break a bond, for example, a C-H single bond. So, pretty much it is an estimate of energy released or added when a bond is formed or broken respectively. So, it can be used to estimate the enthalpy change of a reaction. Pretty much, bde is a microscopic view change in energy (bonds formed or broken), while Delta H of a reaction is the macroscopic view of a chemical reaction, (the entire system as a whole).

You're right, it can be used to calculate enthalpy, but to do so, you'd need to know what bonds are being broken and formed (ie. the reactant and products), which brings it back to needing some sort of reference point. In other words, deltaH describes a reaction process, not necessarily a reactant is the point I was trying to make.

 

[Chrisz]

You're right, it can be used to calculate enthalpy, but to do so, you'd need to know what bonds are being broken and formed (ie. the reactant and products), which brings it back to needing some sort of reference point. In other words, deltaH describes a reaction process, not necessarily a reactant is the point I was trying to make.

what is a molecule made of? Answer: atoms connected by bonds. BDE serves two major purposes, 1 measure of bond strength, 2 estimation of change change in potential energy before and after a reaction. I give much more weight to the later one. As it is commonly known, chemistry is the study of chemical reactions. Chemistry focus on whether a reaction is gonna take place, how fast it is gonna take place. This is where thermodynamic and kinetics comes into play. So there is nothing wrong to call BDE a microscopic view. I believe estimation of enthalpy change and what is going to happen is more of an interest to a chemist than simply knowing the bond strength. I totally agree with your def of BDE and enthalpy. Actually, I dont think there is any essential difference between your interpretation and mine