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Topic Overview

Redox Chemistry

Modern Methods

Standard Electrode Potentials

The Standard Hydrogen Electrode

Standard Conditions

Predicting a Reaction

Electrode Potential Charts

Standard Electrode Potentials

When a metal electrode is placed in a solution of its ions, e.g. Cu in a solution containing Cu²⁺ ions, a potential difference (voltage) is set upbetween the two.  This can occur in two ways:

1. the metal atoms tend to form positive ions leaving the metal with a surplus of electrons:
      Cu(s)      Cu²⁺(aq)   +   2e^-
2. the aqueous ions tend to gain electrons from the metal leaving the electrode with a net positive charge:
      Cu²⁺(aq)   +   2e^-      Cu

We cannot be quite certain which of these processes occurs in a particular case because we cannot measure the potential difference between electrode andsolution (known as electrode potential).

Measurement would require an electrical connection to be made between a voltmeter and the solution by inserting a metal wire or strip, which would inevitablyact as another electrode with its own electrode potential.

That is why we can only measure the sum of two electrode potentials, never a single electrode potential.  To overcome this difficulty, wearbitrarily assign an electrode potential of zero to one particular half-cell and compare all other half-cells with this standard.  By internationalagreement, the standard hydrogen electrode has been chosen as the standard reference electrode.

The Standard Hydrogen Electrode

The diagram below represents a standard hydrogen electrode connected by a salt bridge to another electrode, or half-cell, to make a cell.  This cellgenerates a potential difference, known as the e.m.f., which can be measured by a high resistance voltmeter (one which allows no current to pass).

The general cell can be represented by a short-hand diagram, where M represents any metal and its ions.

The e.m.f.:    E   =   ER  -  EL

ER  =  standard electrode potential of right-hand electrode.

EL  =  standard electrode potential of left-hand electrode.

By convention, the sign of the e.m.f. is the polarity of the right hand electrode in the cell diagram, and the hydrogen electrode is always written on theleft.

Standard Conditions

The electrode potential developed in a half cell depends on:

1. the metal,
2. the concentration of ions in solution,
3. the temperature of the solution,
4. the pressure of any gaseous component.

Standard conditions are:

1. concentration 1 mol dm^-3,
2. temperature 298K (25^oC),
3. pressure 101,325Nm^-2 (1 atmosphere).

The diagram below shows two metals inserted into solutions of their ions.  The two solutions are joined by a salt bridge, and the two metal electrodes areconnected by an external circuit.  The cell has an e.m.f. which is equal to the difference between the standard electrode potentials of the two metals,and a current flows through the external circuit.

This cell can be represented as:

The convention is to always write the half-cell with the more positive electrode potential on the right-hand side.

Predicting a Reaction

From a cell diagram it is possible to predict which way a chemical reaction will go.

In the above cell the overall chemical reaction is:

Zn(s)   +   Cu²⁺(aq)    Zn²⁺(aq) +  Cu(s)

If the e.m.f. of the cell is positive as drawn, then in general, in a cell

the equation for the reaction is:

A   +   C      B   +  D

Example

Calculate the e.m.f. of the cell:

Write the overall chemical equation for the reaction.

As the e.m.f. is positive the reactants are Ni(s) and Ag⁺(aq) and the products are Ni²⁺(aq) and Ag(s).

If the members of a half-cell are in the same phase they are separated only by a comma.  An inert electrode (Pt) is shown next to the more reduced species in the half cell in which it is used.

Electrode Potential Charts

Electrode potential charts provide a useful way of displaying and using data.  They can be used to make predictions about the direction of a particularredox reaction.  In constructing electrode potential charts:

• half-reactions are all written with electrons on the left-hand side,
• the most positive potential is placed at the bottom of the chart,
• if two half-cells are connected electrons will flow through the external circuit to the half-cell with the more positive potential.

Problem.  What reaction will occur between the MnO₄^- / Mn²⁺ system and the Fe³⁺ / Fe²⁺system?  Write an equation for the overall reaction.

1. The system with the less positive E supplies electrons, i.e.
   
   Fe²⁺(aq)    Fe³⁺(aq)  +  e^-
2. The system with the more positive E accepts electrons, i.e.
   
   MnO₄^-(aq)   +   8H⁺(aq)   +  5e^-      Mn²⁺(aq)   +   4H₂O(l)
3. Balance electrons by multiplying the first equation by five
   
   5F²⁺(aq)      5Fe³⁺(aq)   +  5e^-
4. Add the two equations together
   
   MnO₄^-(aq)   +   8H⁺(aq)   +  5Fe²⁺(aq)      Mn²⁺(aq)   +   4H₂O(l)  +   5Fe³⁺(aq)

Hence MnO₄^- reacts with Fe²⁺.

NB. The use of electrode potentials tells you what is feasible.  In practice the reaction may not take place because the activation energy for thereaction is too high.  Hence it tells you nothing about the rate of the reaction.

The electrode potentials refer to standard conditions.  If a reaction is not feasible under standard conditions, it may be possible to change theconditions which will alter the values of the electrode potentials.  The reaction may then take place.

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