# Entropy of half reaction

## Main Question or Discussion Point

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 !

Bystander
Homework Helper
Gold Member
My problem is the following: how to define the entropy of hydrogen ion H+ and electron ?
You don't have to define it. It is conventionally taken as 0 at "STP."

You don't have to define it. It is conventionally taken as 0 at "STP."
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 ?

Bystander
Homework Helper
Gold Member
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.

Bystander
Homework Helper
Gold Member
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.

I understand know ! Thank you !