CHM 1046
General Chemistry II
Dr. Michael Blaber


Electrochemistry

Voltaic Cells


A spontaneous redox reaction involves the transfer of electrons

The development of the voltaic cell begins as follows...

A spontaneous redox reaction that we have previously discussed involves the metals Zinc (Zn) and Copper (Cu)

Zn(s) + Cu2+(aq) ® Zn2+(aq) + Cu(s)

The redox half-reactions for the above reaction would be:

Oxidation: Zn(s) ® Zn2+(aq) + 2e-

Reduction: Cu2+(aq) + 2e- ® Cu(s)

Thus, there would appear to be a movement, or flow, of electrons from the Zinc metal to the Copper ions

According to the above equation, if Zinc metal is placed in an aqueous solution containing Cu2+ ions (e.g. a solution of copper sulfate salt), the following will occur:

In the above reaction, the zinc and copper metal/ions are in direct contact with each other and that is why the reduced form of the copper (i.e. the metal) builds up on the surface of the zinc

The reaction will not occur because there is no way for the electrons released by the oxidation of Zinc metal to get over to the Copper ions and reduce them to Copper metal

Now that the electrons have a path to the Cu/Cu2+ side it would appear that the reaction can proceed
However, we now have another problem ...

We need a way to neutralize the charge build-up in the solutions due to the change in soluble ion concentration

What if we had a tube filled with aqueous solution that connected the two redox reactions?

As the reaction proceeds, although movement of anion allows the overall charges in solution to remain neutral, the net movement of anion produces a concentration gradient across the two solutions. In other words, after a while the net concentration of ions in the zinc oxidation side will be greater than in the coper reduction side. This concentration gradient will oppose movement of anion

Actually, this is not really a problem that we will see. What it means is that we have to realize that charge neutralization can occur by either anions moving to the left or cations moving to the right

The connecting tube of solution is called a Salt Bridge

Summary of the movement of ions, electrons and the redox half-reactions in a voltaic cell:

The two solid metals in the different half-reactions are called electrodes


2000 Dr. Michael Blaber