General Chemistry II
Dr. Michael Blaber
A spontaneous redox reaction involves the transfer of electrons
- In principle, this transfer or movement of electrons can be used to do a type of work - specifically, electrical work
- A type of device that makes use of redox reactions to produce electron flow, and to allow electrical work, is known as a Voltaic Cell (named after Count Alessandro Volta, 1745-1827, an Italian physicist)
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)
- If you recall the Activity Series for metals and hydrogen ion:
- When comparing Zinc with Copper in the activity series, we expect that Zinc should be easier to oxidize (i.e. should give up electrons easier than Copper)
- The spontaneity of the reaction between zinc and copper ion reflects the Activity Series relationship:
Zn(s) + Cu2+(aq) ® Zn2+(aq) + Cu(s)
- The reaction as shown is spontaneous. The Zinc metal is oxidized, and the Copper ion is reduced
- The reverse reaction, the oxidation of Copper and the reduction of Zinc ion, is not spontaneous
The redox half-reactions for the above reaction would be:
Oxidation: Zn(s) ® Zn2+(aq) + 2e-
Reduction: Cu2+(aq) + 2e- ® Cu(s)
- The oxidation of a Zinc atom releases 2 electrons
- The reduction of a Copper ion is achieved by the acceptance of 2 electrons
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:
- At the surface of the Zinc metal, the oxidation of Zinc atoms will be coupled to the reduction of Cu2+ ions
- Electrons from the oxidation of the zinc will reduce the copper ions to elemental copper on the surface of the zinc
- Over time, the zinc metal will dissolve (as solid zinc is oxidized to zinc ion) and zinc ion will build up in the solution.
- Also over time, the solution will no longer contain copper ions, an the copper will be present in elemental form
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
- What if we separated the Cu(s)/Cu2+ from the Zn(s)/Zn2+?
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
What if we provide a path of conductance for the electrons released by the oxidation of the zinc to get over to the copper?
Now that the electrons have a path to the Cu/Cu2+ side it would appear that the reaction can proceed
At the beginning of the redox reaction we have a neutral aqueous salt solution, e.g. ZnSO4 (i.e. the concentration of cation equals the concentration of anion)
As the redox reaction proceeds we build up Zn2+ ions in the solution where the Zinc is being oxidized. Conversely, we remove Cu2+ ions from solution where the Copper ion is being reduced. The anion concentration (sulfate ion in this case) does not change.
Thus, we are building up a net positive charge in the zinc solution, and a net negative charge in the copper solution
These charges will oppose the flow of electrons. The positive charge in the zinc solution will make it harder for the negative electrons to leave. Likewise, the negative charge in the copper solution will repel the electrons that are trying to come over from the zinc side
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?
- This would allow anion to move from the copper reduction side to the zinc oxidation side and keep the charges in solution neutral
- In turn, this would now allow the electrons to flow
- However, as the reaction proceeds, we have yet another problem...
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
- In consideration of keeping the overall concentration of ions in balance between the two sides, cations will also be moving to the right:
- Thus, in the connecting tube of solution we have net movement of both types of ions:
- anions are going into the oxidation side
- cations going into the reduction side
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
- The metal in the half-reaction where oxidation is occurring is called the Anode
- The metal in the half-reaction where reduction is occurring is called the Cathode
- The cathode is often labeled with a "+"; "this electrode attracts electrons"
- The anode is often labeled with a "-"; "this electrode repels electrons"
2000 Dr. Michael Blaber