The minimum amount of heat energy required to cause complete dissociation of $n$ moles of $H_2$ gas at constant temperature $T$ is? Do I have to do anything with binding energy out here in this question,but my book puts it under thermodynamics, how is it so? I think that after dissociation the kinetic energy of the system increases doesn't it? Does that affect the required answer?


closed as off-topic by John Rennie, Chris, Jon Custer, stafusa, Kyle Kanos Feb 2 '18 at 12:59

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  • $\begingroup$ I think you omit some part of your question $\endgroup$ – MatMorPau22 Feb 1 '18 at 12:59

Bond strength is the energy required to dissociate this diatomic molecule. It’s just 432 kJ/mol. Since the question said at constant temperature, that must be what it is after. However, it would take further energy to give the liberated atoms the kinetic energy expected at that temperature, since dissociation also raises the number of degrees of freedom from five to six.

Be aware that the amount of heat needed to raise the temperature to the point where hydrogen will be 50% dissociated is a very different issue. The equilibrium equation for dissociation is $${{[H]}^{2}}/[{{H}_{2}}]=K=\exp (-\Delta G/kT)$$ so the answer would depend on the starting number of molecules per unit volume, and $\Delta G$ will have an important contribution from $\Delta S$ as well. (Use Gibbs free energy for constant pressure, Helmholtz for constant volume.)


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