What exactly is 'bond energy'

[johndoe3344]

I've been reading through several prep books, namely EK and Kaplan, and I seem to be confused on the concept of bond energy.

From EK, I quote "A sigma bond has the lowest energy and is the most stable form of covalent bond."

But bond energy is the amount of energy required to break a bond, so shouldn't it make sense that a higher bond energy means more stable?

Can someone explain this?

EDIT: Would it be correct just to memorize for all cases "The lower the bond strength, the lower the bond energy?"

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[kobe200LATE]

Correct me if I'm wrong, but I think you're getting "bond energy" and "bond dissociation energy" confused

More stable bonds have lower bond energy because more stability refers to less energy

However, these more stable bonds also have higher bond DISSOCIATION energy because they require more energy input to break/dissociate the bonds

 

[johndoe3344]

So given this question,

As bond length between a given pair of atoms increases:

...what would you say happens to bond energy (not bond DISSOCIATION energy)?

From your logic, more stable bonds --> lower bond energy. As bond length increases, bonds become LESS stable --> HIGHER bond energy.

But that is not the correct answer.

 

[Dr Gerrard]

I think it is safe to say that you can always equate less stable to higher potential energy. In this case, potential energy is represented by bond energy.

It is true that the smaller the bond length, the greater the stability and thus the greater energy required to break the bond. However, this is obviously limited by the fact that if the bond length was close to 0, then the two nuclei would be nearly touching each other which would also be a very high energy level state.

Maybe if you can post the exact question, we can tell you the correct answer though.

 

[johndoe3344]

Exact question:

As bond length between a given pair of atoms increases:

bond strength and bond energy decrease
bond strength and bond energy increase
bond strength increases and bond energy decreases
bond strength decreases and bond energy increases

The correct answer is a), as has been discussed on a previous thread. This leads me to believe that what is indeed correct is "The lower the bond strength, the lower the bond energy"

 

[Dr Gerrard]

All right, I was getting bond energy/bond dissociation energy confused with potential energy of the bond.

As bond length increases, the potential energy of the bond increases. The ideal bond length of the compound is when the potential energy is the most negative. Thus, it is at the lowest energy level. Since the potential energy is the most negative, it needs the most energy inputted in order to break the bond, or create kinetic energy.

Bond energy is the energy required to break a bond. It is NOT the energy level of the bond. Thus, as a bond increases in size, the energy level of the bond is increasing, because it is approaching zero, but the bond energy is decreasing because you need less energy to put into the compound in order to break the bond (since the potential energy is lower).

Hope that helped.

Maybe if you think about it this way. Lets say you have C-C. This is a very stable bond. However, lets say you increase the bond length to C---------C. Obviously, since the two carbons are further apart, you will need to put much less energy into the bond in order to break it. Thus, the bond energy is lower.

However, tying this back into my potential energy stuff. The C-C has a very negative potential energy, because this represents a state of much stability. As stability decrease, the amount of potential energy increases.

And finally, just in case you are having trouble correlating potential energy with stability, think about a ball. If the ball is on the ground, it is very stable, is not going to go anywhere, and has a potential energy of 0. However, if you lift the ball 10 ft into the air, it is much less stable and quickly wants to move to reduce this potential energy.

 

[johndoe3344]

Much, much clearer. Thanks a lot.

 

[Unlearner]

so can you clarify the difference between bond energy and bond dissociation energy?

 

[Dr Gerrard]

I think they are very closely related.

Bond dissociation energy is the amount of energy required to break one bond in a compound.

Bond energy is the average of all the bond dissociation energies of a particular compound.