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Homework Help: What exactly happens when a molecule breaks down due to heat

  1. Dec 11, 2015 #1
    • Please post this type of questions in HW section using the template and showing your work.
    Let's say a molecule of hydrogen ends up in a blast furnace. It breaks into atoms, which then might combine with oxygen or some other element. What killed it?

    a. Internal vibration shook it apart.
    b. Another molecule smacked it right between the atoms and cleaved it.
    c. It absorbed a photon emitted by another atom, which increased its energy level or ionized it, and then I don't know what happens. I know that ionized molecules can still hold together, but I don't know the limit of that.
    d. All of the above[, and more]. How frequent would each process be?

    My understanding of how bonds work is... improving. This is part of my attempt to understand them.
  2. jcsd
  3. Dec 12, 2015 #2
    Easiest to understand is photodissociation: a molecule absorbs a photon by the transition from a bonding orbital to an anti-bonding orbital; electron density goes from between the atoms to the outside. The nuclei feel eachother's repulsion, the molecule is then on a dissociating energy surface, and the atoms fly apart. But that is typically in the ultraviolet, does not play a role for thermal dissociation.

    Thermally, collisions transfer translational energy to internal vibrations. Sometimes the vibrational energy will become higher than the binding energy and the atoms fly apart.
  4. Dec 12, 2015 #3
    A. Most probable for hydrogen molecules in a "blast furnace." Ambient thermal energy kT is greater than the bond energy and the diatomic hydrogen easily overcomes the energy barrier for diossociation.
    B. unlikely. Diatomic hydrogen is very small, and the idea that the location of impact of molecules matters seems to be at odds with the electrostatic repulsion and cloud-like nature of molecules.
    C. this is what PK is talking about.
  5. Dec 12, 2015 #4
    In a furnace, kT might be 0.1 or 0.2 eV, much less than molecular binding energies. But vibrational states get populated, also some of the ones close to the dissociation threshold, with probabilities given by the Boltzmann factor.
  6. Dec 12, 2015 #5
    highest calculated vibrational state for diatomic hydrogen: 5481 cm-1 = 0.68 eV
    Boltzmann distribution at kT = 0.2 eV (2000 Celsius) :: exp(-0.68/0.2)=0.032
    diatomic hydrogen dissociation energy: 426 kJ/mol = 4.5 eV per molecule.
  7. Dec 12, 2015 #6
    This is what the energy of the binding state of a diatomic molecule looks like as a function of distance between the atoms (modeled as a Morse potential):
    Vibrational levels are indicated, each of them also has series of rotational states.
    The Boltzmann factor to reach the dissociation energy is very small, but collisional frequencies are high, so the lifetime of the bound state goes down at high temperatures.
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