working out the dissociation energy of HD molecule

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franniexo
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hi

I was given the dissociation energy for H2 molecule at zero point energy level which is 430 kJ mol-1, and vibrational frequency (We) which is 1.319 x10^(14) Hz. How do I calcultate the dissociation Energy for HD?
And what is the chemical significance? (I'm assuming this is to do with the fact that the D atom is heavier? therefore the dissociation energy would be lower?) i could be wrong :confused::confused:
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MexicanKeith
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(Original post by franniexo)
hi

I was given the dissociation energy for H2 molecule at zero point energy level which is 430 kJ mol-1, and vibrational frequency (We) which is 1.319 x10^(14) Hz. How do I calcultate the dissociation Energy for HD?
And what is the chemical significance? (I'm assuming this is to do with the fact that the D atom is heavier? therefore the dissociation energy would be lower?) i could be wrong :confused::confused:
So, dissociation involves, increasing the energy of the system from the zero point energy, up to zero, so the bond breaks.

the zero point energy is 1/2 hbar W for a harmonic oscillator.

W= (k/m)^1/2 where m is the reduced mass of the system

so for H2 you can work out what the ground state energy is in terms of k.

k is the bond force constant, and is the same for H2 and HD so jus tthe change in mass needs to be accounted for.

you'll find that HD has a smaller zero point energy and hence more energy is required to dissociate HD, this is the origin of the primary kinetic isotope effect!

hopefully that info will allow you to work out the answer
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franniexo
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(Original post by MexicanKeith)
So, dissociation involves, increasing the energy of the system from the zero point energy, up to zero, so the bond breaks.

the zero point energy is 1/2 hbar W for a harmonic oscillator.

W= (k/m)^1/2 where m is the reduced mass of the system

so for H2 you can work out what the ground state energy is in terms of k.

k is the bond force constant, and is the same for H2 and HD so jus tthe change in mass needs to be accounted for.

you'll find that HD has a smaller zero point energy and hence more energy is required to dissociate HD, this is the origin of the primary kinetic isotope effect!

hopefully that info will allow you to work out the answer
Thank you!!
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