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# Energy from Hydrogen

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Calculate the minimum energy required to produce molecular hydrogen from gas-phase water. Then compare this to the energy need to produce water electrochemically or photoelectrochemically. Finally, compare this to the energy needed to make this on a zirconia surface. Comment on why there are differences.

##### Solution Summary

Energy from hydrogen is examined. The expert calculates the minimum energy required to produce molecular hydrogen from gas-phase water.

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a. 2H2O (g) --> 2H2 (g) + O2 (g)

To calculate energy required, we can use the standard enthalpy of formation for each compound in the gas phase and determine enthalpy of reaction. If it's endothermic, then this amount would be the energy required to form the gases from water vapor.

dHreaction = [2 x dHf(H2) + dHf(O2) ] - [ 2 x dHf(H2O) ] = (0 + 0) - (-483.64 kJ)

dHreaction = 483.64 kJ

This is for production of 2 mol of H2.

b. To produce H2 electrochemically, we consider the electrolysis of H2O. The reaction is the same as written above except we have water instead of water vapor. Then we use standard reduction potentials ...

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