Thermodynamics of dehydrogenation

[mess1n]

Hi guys, I have a topic which I'm finding very tricky regarding the thermodynamics of dehydrogenation. Hopefully you can help me out!

I have the following passage in my text which I'm finding it difficult to get my head around:

"The PEM fuel cell operates at a temperature of about 90^{o}C, and this is the temperature at which the storage system can be operated. The hydride should supply a hydrogen pressure of at least around 0.5MPa under these conditions which means that at room temperature an equilibrium pressure around 0.1MPa is reasonable.

Since the entropy of the dehydrogenation reaction is dominated by the entropy of the gaseous hydrogen (about 40kJ/mol) and all other contributions can be neglected to a first approximation, the enthalpy of the decomposition of a hydride needs to be 40kJ/mol H₂ in order to result in a free enthalpy of about zero, i.e. an equilibrium pressure of 0.1MPa."

What I think they're trying to say is:

1) If at RT (Room Temperature), the gas has pressure 0.1MPa, then at 90^{o}C it will have a pressure of approximately 0.5MPa. This is the pressure required from the hydride.

2) Entropy of H₂(gas) is far greater than the entropy of the hydride, so all other entropy values can be dismissed to a first approximation.

After this, I'm quite confused! By 'free enthalpy' do they mean 'gibbs free energy'? If so, shouldn't the enthalpy be scaled by the temperature in order to obtain a free energy of zero? (dG = dH - TdS). They say that enthalpy of decomposition must equal the entropy change of the reaction.

Why does a 'free enthalpy of about zero' retain an equilibrium pressure? Is it because there's no change in internal energy, and no volume change, therefore pressure remains the same?

I've thought about this for a long while but I think I lack understanding of the subject somewhere and I'm getting confused. Any help would be greatly appreciated.


Andrew

 

[Mapes]

Free enthalpy is indeed equivalent to Gibbs free energy. When the Gibbs free energy of reactants and products are the same (i.e., \Delta G=0), the reaction is at equilibrium. Does this help?