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Entropy of half reaction

  1. Dec 13, 2014 #1
    Hi everybody,

    I have some troubles to define the enthalpy and entropy of a half reaction. If we consider the 2 following reactions :

    Anode : H2 = 2H+ + 2e-
    Cathode : 2H+ + 2e- + 1/2O2 = H2O

    So the global reaction is :
    H2 + 1/2O2 = H2O

    We know the global reaction's entropy can be define as :
    ΔS H2O - ΔS H2 - 1/2*ΔS O2

    We can do the same thing for the half reactions
    Anode : 2*ΔS H+ + 2*ΔS e- - ΔS H2
    Cathode: ΔS H2O - 1/2*ΔS O2 - 2*ΔS H+ - 2*ΔS e-
    My problem is the following: how to define the entropy of hydrogen ion H+ and electron ? How can we define the entropy if half reaction ? I saw in many books people do the assumption to only consider the entropy at cathode side and so use the global reaction.
    Can u help me to solve this point ?

    Thank's !
  2. jcsd
  3. Dec 13, 2014 #2


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    You don't have to define it. It is conventionally taken as 0 at "STP."
  4. Dec 13, 2014 #3
    So for the half reaction at anode side I have to take account only the entropy of H2 ? At 293.13 K, for the Anode : 2*ΔS H+ + 2*ΔS e- - ΔS H2 we have ΔS=130.68 J/K/mol ? I just read in a thesis the author make the assumption ΔS at anode side is null and ΔG too (for the same reason you have mentioned), but the entropy of H2 is neglected. What is the most realistic assumption ?
  5. Dec 13, 2014 #4


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    The entropy of H2 at STP is defined as zero. That's just the starting point/benchmark for the measurements. There've been no absolute measurements of enthalpies, entropies, or free energies of formation of ionic species; they're all referred to a convention in which hydrogen is zero.
  6. Dec 13, 2014 #5
    There is something i don't really understand. I looked for the hydrogen's entropy at standard conditions, i found 130.7 J K-1mol-1
    In this following link :
    So even if we don't consider the entropy of H+ and e-, at the anode side at STP, we have ΔS ≠ 0 (only the entropy of H2) ?
  7. Dec 13, 2014 #6


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    You are talking about several different reactions:
    1) diatomic hydrogen dissociating into monatomic hydrogen;
    2) ionization of monatomic hydrogen;
    3) formation of a hydrogen ion in solution;
    4) solution of hydrogen in water;
    5) dissociation of dissolved hydrogen into monatomic hydrogen;
    6) ionization of dissolved hydrogen.

    Do not confuse them. "1, 2, 4, 5" are referred to diatomic hydrogen at STP. "3, 6" are impossible to measure, and are arbitrarily selected as zeros to which the enthalpies, entropies, and free energies of formation of all other aqueous cations and anions are referred.
  8. Dec 13, 2014 #7
    I understand know ! Thank you !
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