Question

Electrochemical Measurement of Concentration

An electrochemical cell consists of a standard hydrogen electrode and a copper metal electrode. What is the potential of this cell at 25°C if the copper electrode is placed in a solution in which [Cu2+] = 7.1×10-10 M?

1. 0.069 V 1 pts You are correct.

2. The copper electrode of the above cell is placed in a solution of unknown [Cu2+]. The measured potential at 25°C is 0.193 V. What is [Cu2+] (in mol/L)? (Assume Cu2+ is reduced.)

1.07×10-5 mol/L 1 pts You are correct.

3. A calibration curve to show how the cell potential varies with [Cu2+] may be constructed. A plot of cell potential (in V, with the same sense as in question 1) versus log10[Cu2+] should yield a straight line. What is the slope of this line?

*** I just need help with the 3rd question, I'm confused on how I should make the graph

Answer 1

ANSWER:

The cell is composed by the following two semi-reactions:

As Cu²⁺ has the higher Eº_red potential, this will be the semi-reaction of reduction and the semi-reaction of oxidation will occurs in the standard hydrogen electrode, then:

For any electrochemical cell where the concentration of solutions is different from 1 M, the potential of cell is defined by Nersnt equation:

The cell has a standard hydrogen electrode, where the pressure of H₂ is atm and the [H⁺] is 1 M. Therefore,

This could be rearranged as:

All values of R (8.314 J/mol.K), T (25 ºC = 298 K), n (2 e^- transfered) and F (96485 J.V.mol) are constant and Eº_cell = +0.34 V, then :

If you changes the [Cu²⁺], then the E_cell will proportionally change. As you can see, the Nernst equation has the form of a straight-line equation:

If you made a plot of E_cell vs. log [Cu²⁺] you will obtain a straight-line where y-intercept will be Eº_cell (+0.34 V) and the slope will be 0.0296 (this means that slope will be equal to 2.303RT/nF).

Now, you can made the plot. choose several [Cu²⁺] and calculate the E_cell for each concentration:

[Cu2+]	Ecell
1 M		+0.34 V
1x10-3 M		+0.251 V
1x10-6 M		+0.162 V
1x10-9 M		+0.074 V
1x10-12 M		-0.015 V

The graph will be:

As you can see:

• y-intercept = 0.3398 V = 0.34 V = Eº_cell
• slope = 0.0295 = 2.303 RT / nF