Oxidation-reduction (redox) reaction is one type of chemical reaction that involves the transfer of electrons, with oxidation and reduction referring to the loss and gain of electrons, respectively.
Voltaic pile was the first practical electrochemical system (or battery) that could continuously provide electrical current. A Voltaic pile consists two electrodes, copper (or silver) and zinc discs, with a layer of salt water soaked paper stacked in between. When the two electrodes are connected with a wire, electricity is obtained.
There are different physical processes occurring inside the Voltaic pile system:
- Oxidation (electrochemical reaction) at zinc: Zn -> Zn2+ + 2e– (anode)
- Reduction (electrochemical reaction) at copper (or silver): Cu2+ + 2e– -> Cu (cathode)
- Ions travel through the salt water (electrolyte) from copper (or silver) to the zinc.
- Electrons travel through the wire from zinc to copper (or silver) due to the chemical potential (voltage) difference between these two metals. Zinc: lower potential, copper (or silver): higher potential.
The driving force for the reactions are determined by the thermodynamic properties of electrodes and electrolyte. Given the driving force, the reaction rate at the electrode-electrolyte interface depends on the kinetic rate parameters. The reaction is also related to how fast reactants are transported through the electrolyte and how fast electrons are traveled through electrodes. The total resistance inside the electrochemical system is a combined effect of all the above processes.
A Voltaic pile is a good example of Voltaic cell or Galvanic cell, which can convert the chemical energy of spontaneous redox reactions into electrical energy.