Question 1

[admin]

robert0e23

Joined: 18 Feb 2008
Posts: 5

Posted: 31 Mar 2008 06:51 pm Post subject: Q#1

---------------------------------------------------What should happen when a piece of copper is placed in 1M HCl?

The relevant reaction is Cu + 2H+ -> Cu2+ + H2
Using the data in Table 1, E = -0.34 + 0.00 = -0.34 V
Since E < 0, the reaction is not spontaneous.

My question:
Why are you able to ignore Chlorine Cl2 + 2e- ↔ 2Cl having a potential 1.36 V


---------------------------------------------------


The relevant reaction is Cu + 2H+ -> Cu2+ + H2

Using the data in Table 1, E = -0.34 + 0.00 = -0.34 V

Since E < 0, the reaction is not spontaneous.

On the other hand, if you are considering the reaction: Cl2 + 2e- ↔ 2Cl- 1.36 V

That could only occur in reverse (ie, you are starting with HCl not with Cl2 gas).

In other words, you end up with -1.36 which is even less likely to occur than the ionization of copper due to hydrogen.

[robert0e23]

thank you so much

[klee92181]

For the MCAT, do you always assume strong acids (i.e HCl, HBr, etc) can be replaced by 2H+ whenever doing a redox reaction because I am not understanding how you can write the equation Cu + 2H+ --> Cu2+ + H2. I would think you would write it Cu + HCl --> CuCl2 + H2 because that is the way the question worded it.

Thanks

[nathan.blu4013]

I would also like to know why not Cu + HCL --> CuCl2 +H2

...this question confused me...

[admin]

Certainly Cl is involved in the redox reaction but adding the Cl half reaction is not relevant to determine spontaneity since:

Cl2 + 2e- ↔ 2Cl- +1.36 V

Thus

2Cl- ↔ Cl2 + 2e- -1.36 V


HCl can be considered H+; unlike for reactions involving enthalpy, because Eo values are based on standard conditions including 1m concentration, whether you refer to H+ or 2H+, it does not change the Eo value. Thus you use a stoichiometric value to balance the relevant equation.

[nathan.blu4013]

just to be sure I understand your explanation (this may already be obvious to some):

-Eo is not affected by stoichiometry
-enthalpy is?

[admin]

That is correct.

[maxsati7049]

Does this mean if Chlorine (Cl2) and Silver metal (Ag) reacted, the Chlorine will dissolve the metal away? and would there be any gas produced if no acid was involved?

[aali112821093]

I do not understand the answer or the explanations you have written.
Why is Cu written in reverse since it is more reduced than the hydrogen, so shouldn't it remain as it is provided in the table as 0.34. and the hydrogen reaction should be written in reverse, and the overall E should be positive and the overall reaction spontaneous.

[jellywing_2058]

The standard reduction potential table gives the half reaction values for all reduction potentials.

In this question, we are comparing the copper half cell potential with a standard hydrogen electrode (SHE) of 0.00 V.
As such, the standard electrode potential of copper can be obtained using a galvanic cell with a copper electrode and with a SHE with comparison to the SHE electrode.
Thus, the copper electrode would be the cathode consistent with
Cu2+ + 2E- > Cu(s)
and the overall cell would be = + 0.34 V as SHE is = 0.00 V.

Therefore,
E°cell = E°H2/H+ + E°Cu2+/Cu
which is 0.34 V = 0.00 + E°Cu2+/Cu

So, the standard potential of copper is = +0.34 V and the standard oxidation potential would be -0.34 V.

The +0.34 V is the half potential in comparison to a standard SHE half cell and not another half cell. The SHE cell is a common cell in which all other cells are compared to in order to obtain all their strengths as reducing agents (or oxidizing agents).

[Guest]

still lost