Chemistry Help!?!? Please help!
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Chemistry Help!?!? Please help!

[From: ] [author: ] [Date: 12-04-25] [Hit: ]
Under these conditions, What is the iron(II) concentration on the rebar?Fe 2+ + 2e-= Fe E standard= -0.44 V-Hi,First step, you will writte the global equation.......
The extent of corrosion in the steel reinforcing rods (rebar) of concrete is measured by the galvanic cell shown in the diagram of the instrument. The half-cell of the probe is usually a AgCl/Ag redox couple:

AgCl + e-= Ag Cl- (1.0 M) E standard= 0.23 V

Corrosion is said to be severe if the cell potential is measured at greater than 0.41 V. Under these conditions, What is the iron(II) concentration on the rebar?

Fe 2+ + 2e-= Fe E standard= -0.44 V

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Hi,
I'll try to explain to you:
First step, you will writte the global equation.
We need a reaction to form Fe2+ (corrosion), then the half-reaction is:
Fe -----> Fe2+ + 2e- Eo = +44 It's positive because we taking the inverse reaction.
Now, we write the reduction reaction:
2 x (AgCl + e- -----------> Ag(s) + Cl-) Eo = 0,23.
See... we multiply only the reaction, but, we don't multiply the Eo, it is afinity!
The global equation is:

Fe + 2 AgCl ------------> Fe2+ + 2Ag(s) + 2 Cl- Eo = 0,23 + 0,44 = 0,67


Using the Nernst Equation:

E = Eo - 0,059 . (1/n) . LN (Fe2+/Ag+), were n is the number of electron (2 for this reaction).


But, What´s is the concentration of silver cation???
Answer: use the Kps = 1,0 . 10^(-10)
= (Ag+) . (Cl-)
were both are equal = S
Then
Kps = S^2 = 10^(-10)
and S = 10^(-5) solubility of silver in the solution.

Backing to the Eq. Nernst...
0.4 = 0.6 - 0.059 / 2 Ln (Fe2+ / 10^-5)

Solving it..
[Fe2+] = 0,067 mol/L


P.S. Sorry for my English.... I'm from Brazil

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If someone just tells you the answer without explaining, you wouldn't learn. Then when it came time too take the test, you wouldn't know how to solve the problem. I am sure your teacher would be able to help you, or a classmate.
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