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Electrochemistry Tricky

Electrochemistry Tricky

by Shashank Todwal -
Number of replies: 5

If the pressure of hydrogen gas is increased form 1 atm to 100 atm,keeping the temp. and hydrogen ion conc. same,the voltage of hydrogen half cell will change by:

a)0.059V                              b)0.59V

C)0.118V                              d)0.0295V

I don't want answer I just want you all to tell me that while applying E=E0-RT/nF ln(Q) , this Q is Qc or Qp.In other words,should we Consider[H+] or  [H2] in this eq.

In reply to Shashank Todwal

Re: Electrochemistry Tricky

by Manish Verma -
Here we are dealing with the thermodynamic form of reaction quotient. In this Q, concentrations of solutes in liquid solutions are expressed in molarities whereas concentrations of gases are expressed in partial pressures.

Q = QC when reactions involve only solutes in liquid solutions
Q = QP when reactions involve only gases

For, \(H_2\rightleftharpoons 2H^+ +2e\)

\(E_{RP}={{0.059}\over 2}\log {{[H^+]^2}\over {P_{H_2}}}\)
In reply to Manish Verma

Re: Electrochemistry Tricky

by Shashank Todwal -

Dear Sir,

While taking logQ we either express everything in partial pressure or everything in concentration.We never express one qty.in conc. and other in partial pressure.

So here how can we take Q=[H+]/P(H2)?.What is this Qc or Qp?

In reply to Shashank Todwal

Re: Electrochemistry Tricky

by Shashank Todwal -
Sir,need ur help badly.
In reply to Shashank Todwal

Re: Electrochemistry Tricky

by Manish Verma -
It seems that you have indeed put in some efforts to dig into it further. Actually, in previous reply it was stated that here we are talking about thermodynamic constant (emphasis on the word thermodynamic). Nerst's equation just makes use of the following while deriving:

\(\Delta G = \Delta G^ \circ + RT\ln Q\)

Here Q is thermodynamic quotient.

To arrive at Nerst equation, one uses this as starting point and uses \(
\Delta G=- nFE_{cell}\) subsequently.

Let me again emphasise on the word thermodynamic.

To understand it more deeply you need to read standard texts on thermodynamics. One text that comes to my mind is Ebbing that discusses thermodynamic quotient to some extent in the chapter on Thermodynamics and Equilibrium (chapter on Chemical Equilibrium is different).