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Homework Help: Stronger bond= lower energy? why?

  1. Sep 29, 2010 #1
    Stronger bond= lower energy? why???

    "In an exothermic reaction, some of the potential energy stored in the chemical bonds is being converted to thermal energy via kinetic energy(heat) In this exothermic process, the bonds in the products are stronger than those of the reactants."

    This does not make sense to me at all. If there is less potential energy in the products shouldn't the bonds be weaker. I visualize a strong bond as having a higher potential energy
    since the bonds are stronger and thus have more energy between the two atoms. Can someone please clarify because what seems logical is not true.

    I am equally confused on endothermic reactions. The bonds should be stronger as heat energy is flowing into the system.

    Yet the textbook clearly states :"In an endothermic reaction the bonds of the products will be on average, weaker."
     
  2. jcsd
  3. Sep 29, 2010 #2
    Higher potential energy equals weaker bonds?

    1. The problem statement, all variables and given/known data

    "In an exothermic reaction, some of the potential energy stored in the chemical bonds is being converted to thermal energy via kinetic energy(heat) In this exothermic process, the bonds in the products are stronger than those of the reactants."

    "In an endothermic reaction the bonds of the products will be on average, weaker."

    2. Relevant equations

    This does not make sense to me at all. If there is less potential energy in the products shouldn't the bonds be weaker. I visualize a strong bond as having a higher potential energy
    since the bonds are stronger and thus have more energy between the two atoms. Can someone please clarify because what seems logical is not true.

    I am equally confused on endothermic reactions. The bonds should be stronger as heat energy is flowing into the system.
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  4. Sep 29, 2010 #3
    Re: Stronger bond= lower energy? why???

    its the other way around,
    visualize potential energy as something that is there but is not used (which makes it pointless for this specific reason). this means that if an object has a greater potential energy then it would have less kinetic energy, thus making it weaker.
    SO you need to have more kinetic energy for the bond to be stronger.
    physics would help you understand that even more: consider a ball on a top of a cliff. potential energy lets say is 100 which makes kinetic energy 0. as the ball starts to drop, the kinetic energy will start to increase and the potential energy will (obvi) decrease.
    (i hope i answered you question, but i tried :) )
     
  5. Sep 29, 2010 #4
    Re: Stronger bond= lower energy? why???

    I completely understand the difference between potential and kinetic energy. But if you were to add heat energy to a system in an endothermic reaction shouldn't the atoms gain potential energy and kinetic energy. Thus making the bonds stronger so if they were to react again it would be able to give off its energy in the form of heat. If the molecules don't have stronger bonds then how is it possible that they have more energy.

    I guess I am not getting it yet...
     
  6. Sep 29, 2010 #5

    Ygggdrasil

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    Re: Stronger bond= lower energy? why???

    Consider the plot here of the interaction energy between two atoms (y-axis) as a function of the distance between the two atoms (x-axis)

    As the two molecules approach each other, the interaction energy between them decreases (becomes more negative, at least until the molecules get too close and start to repel). This is in line with other systems with potential energy. For example, as Maz88 noted, as a ball rolls down a hill (i.e. moves towards the Earth's center of mass), the ball's potential energy decreases. As a negatively charged object moves toward a positively charged object, the Coulomb potential decreases. This reflects the important principle that things in the universe tend to act to minimize their energy.

    Why do molecules that lose more potential energy in bond formation have stronger bonds? The answer has to do with the conservation of energy. If your atoms release 20 kJ of energy when they form a bond, then it must necessarily require 20 kJ of energy to break the bond.
     
  7. Sep 29, 2010 #6

    Ygggdrasil

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    Re: Stronger bond= lower energy? why???


    Basically, bond formation releases heat energy while bond breaking requires heat energy. The more heat energy you release when forming a bond, the more heat energy is required to break the bond. Let's say that two atoms with no bonds between them has zero potential energy. When the atoms form a bond, they release 20 kJ of heat energy, so because of conservation of energy, we know that the bond must have a potential energy of -20 kJ. In order to break the bond, we need to separate the atoms again. Since we said that the atoms have 0 kJ of potential energy when separated, breaking the bond would require the addition of 20 kJ of heat energy. Therefore, we say that the bond strength is 20 kJ.

    Let's say that we add 5 kJ of heat into the solution to rearrange the atoms. Well, let's do some math to add the heat energy to the potential energy of our molecules: 5 kJ + (-20 kJ) = -15 kJ. Now, our rearranged molecule has a bond strength of only 15 kJ. Therefore, adding heat energy to the molecule via an endothermic reaction weakened the bond.
     
  8. Sep 29, 2010 #7
    Re: Stronger bond= lower energy? why???

    Ok, I think I understand.

    In an endothermic reaction, the system gains potential energy in the form of heat. Because the atoms have more kinetic energy and are moving faster they have the ability to get further away from each other. Because they are further away from other the coulomb force between the two atoms is less, thus they have weaker bonds.

    Is that correct. Sorry, I really need to understand things completely before I move on.
     
    Last edited: Sep 29, 2010
  9. Sep 29, 2010 #8

    fzero

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    Re: Higher potential energy equals weaker bonds?

    It's a little bit counterintuitive because of conventions. A bound state is a lower potential energy configuration compared to that of the unbound constituents. However, bond energy is defined to run in the opposite direction, namely it's the energy that's released in going from the unbound state to the bound state.

    The main point is that lower potential energy configurations are in a real sense more stable than higher potential energy configurations. For a simple example, consider a H-atom in the 1s state vs a 2p state. The 1s state is the ground state, the lowest potential energy configuration. The 2p state is a higher potential energy state and is unstable against decay to the 1s state via photon emission.

    A stronger bond is a lower potential energy configuration, in accord with it being a more stable configuration. A reaction that leads to a stronger bond will release an amount of energy equal to the difference in bond energies as heat. If a reaction consumes a net amount of heat, we've raised the potential energy of the system and can conclude that the bonds in the product are not as strong as those in the reactants.
     
  10. Sep 30, 2010 #9
    Re: Higher potential energy equals weaker bonds?

    Ok, after about 2 hours I think I have my brain wrapped around this. So as potential energy goes up the distance between the atoms is greater and thus have weaker bonds.
     
  11. Sep 30, 2010 #10

    berkeman

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    Re: Stronger bond= lower energy? why???

    (2 threads merged)
     
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