[alanjz]

I'm having trouble solving this problem. I just don't know where to begin. I think it uses Nernst equation but I can't seem to figure it out.

39. The standard reduction potential for H⁺ (aq) is 0.00 V.
What is the reduction potential for a 1×10^-3 M HCl solution?
(A) 0.355 V (B) 0.178 V
(C) –0.178 V (D) –0.355 V

The answer is C.

[Stovn0611]

Yes, the Nernst equation would be used to solve this problem.

The first step is to identify the reaction (reduction of HCl)

Cl is in its lowest oxidation state at -1 already so only H can be reduced from +1 to 0

2H+ + 2e- -> H2

Now use the Nernst equation

E = 0.00 V - (RT)/(nF)ln(Q)

R = 8.3145 J/mol*K (constant)
T = 298 K (standard conditions)
F = 96485 C/mol (Faraday constant)
n = number of electrons transferred
Q = equilibrium expression of reduction reaction

[Borek]

Problem is, there is no mentioning of the partial pressure of H₂, so question is not a straightforward as it may look.

The only way to get an answer is to assume standard hydrogen pressure (1 atm).

[alanjz]

would I use 273 or 298 for the temperature and why is the answer negative? I get that it's positive when I was plug into the equation.

[Borek]

would I use 273 or 298 for the temperature

Make your pick - this is one of the things that are not stated in the question. I would go with 25°C (298K).

and why is the answer negative? I get that it's positive when I was plug into the equation.

Most likely you are confusing sign conventions.

[alanjz]

Could you explain a little more? I plugged in 8.314 for R, 96500 for F, 273 for T, 2 for n and 1E-6 for Q and I got a positive value for E.

[Borek]

Show the exact equation you used.

[alanjz]

E = 0.00 V - (8.314)(273)/(2*96500)ln(1e-6)

[Borek]

Please write it using symbols, so that it will be clear what is your Q.

[alanjz]

What do you mean by symbols? I just used Nernst equation and my Q was 10^-6 M because the HCl dissociates

[Borek]

Symbols as opposed to values.

[alanjz]

E = 0.00 V - (RT)/(nF)ln(Q)
this? where Q is [H] [Cl]

[Borek]

Write explicitely your Q. It should reflect the reaction.

[alanjz]

ah I think I got it: the equation is H⁺ + Cl^- --> H₂+Cl₂ so Q would be 1/(10^-3*10^-3) which makes the final E negative. Lemme know if this is right. Thanks for your patience with me

[Borek]

No, this is not the correct reaction. Question asks about the reduction of H⁺ - what is H⁺ reduced to?