# Electrochemistry: Basic Concepts

## A REDOX Example

1. One would predict a spontaneous reaction between….
2. What is the coefficient in front of Fe2+ in the balanced reaction?
3. What is E° for the total reaction?

## ΔG, E and K Example

Eo for the following reaction (as written) is 0.736 volts. Calculate ΔG and K for the reaction.

$\ce{MnO4- + 8H+ + 5Fe^2+ ⇌ Mn^2+ + 4H2O + 5Fe^3+}\nonumber$

## Calculating Potential of a Cell (Nernst equations)

Calculate the potential for the following cell:

$\mathrm{Pt \mid Sn^{2+}(0.400\: M),\: Sn^{4+}(0.500\: M) \mid\mid NO_3^-(0.100\: M),\: H^+(pH=2.00),\: HNO_2(0.400\: M) \mid Pt}\nonumber$

## Potentiometry – Cells (line notation with REF)

Calculate the cell potential of the following cell:

## Bringing in Ksp

Calculate the cell potential of the following cell:

$\mathrm{Ag \mid AgCl_{(s)} \mid AgCl_{(aq)}(sat’d),\: Cl^- (1.00\: M) \mid\mid \textrm{reduction half cell} \mid Pt}\nonumber$

Couldn’t one use the Ag+ + e- ⇌ Ag(s) half cell at Eo = 0.799?

When you “add” chemical reactions, you multiply the K values and/or add the E values.

Given the following Eo values:

$\ce{Ag+ + e- ⇌ Ag_{(s)}} \textrm{ (0.799 V vs SHE) and}\nonumber$

$\ce{AgCl_{(s)} + e- ⇌ Ag_{(s)} + Cl-} \textrm{ (0.222 V vs SHE)}\nonumber$

Calculate the Ksp for AgCl.

## “Mixing” Problems

30.00 mL of 0.120 F NaNO3 is mixed with 20.00 mL of 0.150 F SnCl2 and the solutions is buffered at pH=2.00. (NOTE: no gas or solid is observed)

1. Calculate Eo and K for the spontaneous reaction that will occur.
2. Calculate the solution potential of the resulting solution.

## Potentiometry Problems (mixing, then measuring)

Calculate the cell potential, as measured at a Pt/SCE electrode pair, for a solution prepared by mixing 30.00 mL of 0.120 F NaNO3 and 20.00 mL of 0.150 F SnCl2. The solution is buffered at pH=2.00. (except for SCE, this is the same problem as the previous exercise)

## Potentiometry Problems, II (mixing, then measuring, EP)

Calculate the cell potential, as measured at a Pt/SCE electrode pair, for a solution prepared by mixing 25.00 mL of 0.120 F NaNO3 and 20.00 mL of 0.150 F SnCl2. The solution is buffered at pH=2.00.

KEY: Reactant conc’ns are “Not really zero, just really small….AND in a defined ratio” …as indicated by coefficients in balanced eqn!

## Formal Potential (Eo’)

Derive an equation for the pH-dependent formal potential for the dichromate/ chromium III half reaction, and calculate the Eo’ at pH 7.00.

$\ce{Cr2O7^2- + 14H+ + 6e- ⇌ 2Cr^3+ + 7H2O} \hspace{30px} \mathrm{E^o} = \textrm{1.33 V vs SHE}\nonumber$

## Redox Titration

Consider the titration of 50.00 mL of 0.100 M Cr2O72- with 0.300 M Fe2+ as monitored with a Pt/SCE electrode pair...calculate the measured cell potential and construct the expected titration plot. The pH is maintained at 0.00.

$\ce{Cr2O7^2- + 14H+ + 6e- ⇌ 2Cr^3+ + 7H2O} \hspace{30px} \mathrm{E^o} = \textrm{1.33 V vs SHE}\nonumber$

$\ce{Fe^3+ + e- ⇌ Fe^2+} \hspace{30px} \mathrm{E^o} = \textrm{0.771 V vs SHE}\nonumber$