[MightyMan]

So, i basically have trouble getting this concept,
but I'll just ask you guys one of my homework questions and if you guys could help out, I'd really appreciate.

Question: Which of the following cells would produce a spontaneous reaction?  Justify each answer, using cell potential:

C(s) | Cr(NO3)2 (aq) | C(s)

What I have so far:

The things involved in this cell are:  NO3[1-], Cr[2+], and water     C is inert, so no effect

I have that the Cr[2+] is the Strongest Oxidizing Agent (using a table in my textbook) therefore it will under go reduction

Cr[2+] + 2e -----> Cr(s)     (E = -0.91)

I found that water is the Strongest Reducing Agent, therefore it will oxidize

2H20  -----> 4e  +  4H  +O2     (E = 1.23)

Now since i have my half reactions, i can calculate the change in E

E(cathode) - E(anode)
= -0.91 - 1.23
= -2.14

therefore, this is not spontaneous because energy is needed to be put in.

the problem is, the answer in the back of the textbook says -0.50V...   

[Borek]

C(s) | Cr(NO3)2 (aq) | C(s)

0.00 IMHO. Please check if it is correct scheme of the cell. Looks like you have just a solution with two electrodes put in. If there will be any redox reaction it will happen in the solution, not on the electrodes.

[MightyMan]

The scheme is right (unless im blind).

Argg, im confused..

Won't the electrodes act as an anode/cathode?
Causing Cr to build up on the cathode and hydrogen gas bubbling around the anode?

It says its a "Type 2 Electrolytic Cell" in my note.

[Borek]

For the electrodes to act as anode and cathode they have either to be connected to some external source of voltage (which will force the to be anode/cathode), or they have to be put in different solutions (then solution compositions will define which one is anode and which one is cathode).

No idea where does -0.5V come from in the answers. There is no potential difference if there is only one half cell. Potential difference is just that - a difference, between one and another. If you have two numbers - like 3 and 2 - the difference is 3-2. But if you have one number - what is difference between 3?

[MightyMan]

uh-oh...
i hope i havent learnt it wrong...

heres a copy of my note, i think its the same type of cell, just in a different solution

http://img.photobucket.com/albums/v692/sp00g/chemnote.jpg

thanks again