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Bond Energy

  1. May 17, 2016 #1
    "When we supply bond energy to two molecules that have bonded, the potential energy of the molecules increase causing the molecules to break bonds." -Is this the correct explanation regarding bond energy? If not, kindly explain this process to me, it's very confusing.
     
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  3. May 18, 2016 #2

    Simon Bridge

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    The bond energy is the amount of work you have to do to break the bond.
     
  4. May 18, 2016 #3
    Doesn't the bond energy get converted to potential energy? If no, what does it get converted to?
     
  5. May 18, 2016 #4

    Simon Bridge

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    The work that was done breaking the bond was stored as potential energy, yes.

    A simple case would be the way everyone is bound to the earth by gravity.
    A 1kg mass on the surface of the earth has -125.440MJ in gravitational potential energy ... notice the minus sign?
    Any force you use to pull the mass away from the earth will need to add at least +125.44MJ to get the mass completely free.
    More work than that and the 1kg will also have some kinetic energy after it is free.
    If the force only did, say, 25MJ work, the new potential energy would be -100.44MJ (and the mass would be higher up).
    The 25MJ of energy went into the gravitational field between the earth and the mass... you can get it back by letting the mass fall.

    chemical bonding is sort of like that - only it uses electromagnetism instead of gravity and it happens on smaller scales (and there's quantum stuff.)
    That help?
     
  6. May 18, 2016 #5
    The OP asked about bond energy between two molecules not two atoms. These would be Van der Waal's forces, not covalent bonding. Am I misreading, or did the OP mean covalent bonds?
     
  7. May 18, 2016 #6
    Yes that helped very much. :D Just one last question sir, can I say that the potential energy is helping the molecule to move further away and stay there instead of banging into the other molecule?
     
  8. May 18, 2016 #7
    Hey, I was talking about the intermolecular forces.
     
  9. May 18, 2016 #8

    Simon Bridge

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    Van der Walls forces are electromagnetic in nature... with quantum stuff. ;)
    https://en.wikipedia.org/wiki/Van_der_Waals_force.
    "Van der Waals forces include attraction and repulsions between atoms, molecules, and surfaces, as well as other intermolecular forces. They differ from covalent and ionic bonding in that they are caused by correlations in the fluctuating polarizations of nearby particles (a consequence of quantum dynamics.)"

    That would be like saying that gravitational potential energy helps falling objects avoid banging into the ground.

    Objects that are bound to each other sort of hang around together.
    At the molecular level - they are like dancers at a party: jiggling and twirling around all over the place but still associated with each other.
    The jiggling is thermal - basically it is kinetic energy that holds objects, otherwise attracted to each other, apart.
    There are also repulsive intermolecular forces (see link above).

    This is also the scale where objects stop having surfaces the way we are used to ... so they cannot strike each other in the way that pool balls do.
     
  10. May 18, 2016 #9
    So what force during state transition pulls molecules away from each other and then gets converted to potential energy? What basically is this work? There must be some force doing this work, what do we call it?
     
    Last edited: May 18, 2016
  11. May 18, 2016 #10

    Simon Bridge

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    You can identify the forces at play yourself... it's a good exercise since you need to get used to thinking about this.

    The most common example of a state-change where intermolecular bonds are broken is the vaporization of water.
    You've done it lots! So what do you do to make water go from liquid to vapour state?
    What force are you applying?
     
  12. May 19, 2016 #11
    Let me put it like this: An Ice cube is given enough heat to break down its bonds, this heat initially turns to kinetic energy which drags the molecules away from each other and then it quickly turns into potential or chemical energy. Now these molecules are further apart and the bonds have vanished due to the distance.
    Does that make sense or am I still getting it all wrong? :p
     
  13. May 19, 2016 #12

    Simon Bridge

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    That's not bad. I'll just tweak it a bit ... note: heat is a form of kinetic energy.
    When you heat something on an element, you are transmitting heat by conduction and some radiation to the ice (via the pan) ... the heat itself is probably provided by electricity and heat moves from hot places to cold places by molecular jostling: which works by the electromagnetic force.

    molecules of the solid ice start out in a crystal lattice structure - all juggling about in place.
    The jiggling motion is heat - kinetic energy is constantly being exchanged for electromagnetic potential energy.
    The more heat the more jiggling you get. The average speed of the jiggling about is the temperature.

    From here you can see that too much jiggling will break the solid apart.
     
  14. May 19, 2016 #13
    So because this heat turns into electromagnetic potential energy, the temperature of the whole thing does not rise. Whereas when we supply heat to the unbonded molecules most of this kinetic energy on being used up turns to heat again and causes the temperature to rise. Right?

    And because the attraction is continuous the molecules firstly have to gain enough force against the attraction to break the bond, and when this energy gets converted to potential energy the molecules are a good distance apart from each other to bond again. Right?

    Can I say that the kinetic energy initially gets converted to potential energy because the other molecule tries to restrict its movement?
     
    Last edited: May 19, 2016
  15. May 19, 2016 #14

    ogg

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    I don't agree with most of the responses here. Kinetic Energy is the energy involved in motion and vibration is not simply motion (since with it, velocity varies periodically with time). Vibration is both kinetic and potential energy and usually the SUM of the two is the (constant) vibrational energy. Just as a planet's orbital energy has both potential and kinetic terms. Or just as two opposite charges have both P.E. and (probably) K.E. (depending on the system). Heat is a very difficult term, generally its definition is either formal or vague/murky. Mostly, from a practical perspective, heat is the flow of energy. Or even more practical, heat is what a thermometer measures changes of. Perhaps you can see by this that both heat and P.E. are not specific types of energy? So using them to explain only works as long as you're willing to keep the explanation quite superficial. I also have a serious problem understanding the original post: 1) if two molecules have bonded then there is one molecule, right? If the product molecule required the addition of more energy, (and that energy wasn't returned as kinetic, electronic, or vibrational (rotational, etc.) energy) then the product is less stable than the reactants. Some chemical reactions (eg simple acid-base reactions) nearly instantaneously form product, but many reactions (especially the more "interesting" ones) require some more energy to form a transition state (which means a state of higher energy, bond energy to be specific (bond energy being the electromagnetic interactions between all atoms, which involves quantum mechanical interactions)) which then rearranges to form product(s). The over-all reaction may cause cooling or heating and may be slow or fast (but generally chemical reaction steps are very very fast compared to what we can perceive with our natural senses). Energy may come from bond energy or kinetic (thermal) energy (as well as rotational, vibrational, and conformational energy) and be "used" to change atoms' positions or to change the electron clouds surrounding atoms or groups of atoms (usually both). Do bonds break "because of" P.E.? Well, sure - in fact one way we determine whether two atoms are bonded is by measuring their distance apart. While it varies depending on the atoms and electronic states involved, most if not all (I can't think of an example where this is not true) bond breaks require the atoms to separate (distance wise) which therefore requires motion, hence k.e. and that has to come from somewhere. It can come from k.e. (collisions) but as soon as the energy is transferred from velocity (deceleration) it goes somewhere (and we call that P.E. unless some other object rebounds with more k.e.) or it can come from one or more of the many types of P.E. (which include electrostatic, conformation, vibration, and rotation (and by the way, there are several kinds of rotation)). The OP seems to assume that the product molecule is in its lowest (P.E.) energy state, and that is NOT given in the original problem. A molecule may spontaneously decay (if it isn't the lowest energy for the system), so saying added P.E. "causes" a bond to break is only true in certain situations. But bond energy is P.E. by definition, so any change in bonds is a change in P.E. (even if the total P.E. remains constant for the entire system). BTW, the OP also has a problem in the wording: Two molecules form one molecule which then breaks up into (two?) other molecules (or ions). Both the bond (Van der Waals "bonds" are generally not considered "bonds" or "chemical bonds" even though they are bonds between chemicals..but that depends on the context.) making and bond breaking involves changes in P.E. but what the TOTAL P.E. of the 3 states is, is not stated and could be anything A>B>C or A<B<C or A>B<C or A=B<C ..I think that makes for 9 possibilities....
     
    Last edited: May 19, 2016
  16. May 19, 2016 #15
    I had concerns with bonding such that in ice molecules which is broken and the ice turns into water.
    Also, I've few queries:
    1) Does bonding force reduce with distance?
    2) Does the kinetic energy get converted to potential energy because the molecule it was bonded to is opposing its motion?
     
    Last edited: May 19, 2016
  17. May 19, 2016 #16

    Ygggdrasil

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    The potential energy of intermolecular bonds is often modeled as a Lennard-Jones potential, which has the following shape:
    =Figure_B.jpg
    (note that the interaction between water molecules involves hydrogen-bonding so it is probably more complicated than an L-J potential)

    Remember that force (F) is given by the equation: ##F = –\frac{dV}{dr}##, so the force is given by the derivative (slope) of the potential energy function (V). At distance ##r_{eq}## where V is minimum, the atom experiences no net force (because attractive and repulsive forces cancel out). As the distance between the atoms increases from ##r_{eq}##, the atom experiences an attractive force that increases to some point, but then begins to decrease again to zero as the two atoms become very far apart.

    Unbound atoms have a higher potential energy (zero in the diagram above) than bound atoms (-ε in the diagram above). Therefore, heat gets converted to potential energy when you break an intermolecular bond.
     
  18. May 19, 2016 #17
    Had these molecules been unbound, this latent heat would have been absorbed by the molecules and got converted to heat energy thereby increasing the temperature. But because this heat energy is absorbed by bonded molecules it gets converted to potential energy, therefore the temperature does not rise.
    Is that right?
     
  19. May 19, 2016 #18

    Ygggdrasil

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    Yes, I'd say that's fairly accurate (though the term latent heat is misused).

    Normally, when you add heat to a substance, it increases the kinetic energy of the molecules in that substance. At some point, you reach a point where you cannot add more kinetic energy without disrupting bonds between molecules. At this point a phase transition occurs and added heat goes to breaking the intermolecular bonds between molecules in the substance (increasing their potential energy). During the process of the phase transition, the added heat is converted to potential energy—not kinetic energy—so the temperature does not rise. The total amount of heat that needs to be added to fully effect the phase transition is called the latent heat and is essentially equal to the potential energy difference between the bound molecules before the transition and the unbound molecules after the transition.
     
  20. May 19, 2016 #19
    So this heat energy initially gets converted to kinetic energy and then potential energy, isn't that potential energy somewhere keeping the molecule away from the attractive force of the other molecule?
     
  21. May 19, 2016 #20

    Ygggdrasil

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    Yes. In order to separate two bound molecules, you must perform work in order to overcome the attractive force between the molecules. Energy from heat is used to overcome the attractive force and break the intermolecular bond.
     
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