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
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
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.