We all know that a typical cell has two electrodes, a salt bridge, a voltmeter or just a device it provides energy to. But do you know how does it actually work?
To begin with, you must know about the electrode. An electrode consists of a metal plate , salt solution of same metal.Technically the energy produced is not like producing energy from nothing, god forbid I can’t say that energy is created or destroyed (the soul of Joules will come and haunt me!) . But where are we getting energy from, if we are not giving any? Or are we?
The Gibbs Free energy of the chemical species, yes we are aware of it but does it feel like a glass of water next to you have some energy stored in it,aah science I tell you.But how are we converting the chemical energy into electrical energy?
The metal(M) dipped in the solution of its own salt have ions say Mn+ and when both are present in a solution a pair of reaction starts
M → Mn+ + ne– (metal dissolve in the solution)
Mn+ + ne– → M (ion from the solution deposite on electrode metal)
If you would have closely looked for any electrode plate or a car battery, inverter when you remove the metal plate you see the part outside the solution is smooth but the portion inside is porous , as the metals ions continuously attach and detach from the plates. There is an equilibrium between the above two and is represented as:
Mn+ + ne– <——> M
Reduction potential – The tendency to get reduced.Now every metal has the tendency to reduce and oxidize but one is higher and other is lower (we consider it equal in case of hydrogen). So, from there only we get to know a metal having more reduction tendency are having positive value of electrode potential which on having a higher oxidation potential have a positive oxidation potential but a negative reduction potential.
E0Red = – E0oxi
By convention we use the reduction potential, and scale with respect to hydrogen’s tendency to be zero.
Due to all these this happening there are produce free electron (more or less depending on the tendency of metal) and this is a single electrode.
Combining two electrodes
we have two electrodes with different tendencies so number of electrons are different, now when we connect we have a flow of current side of high density of e- to one having lesser the number.
Naming them properly, we have the side with higher number of e-,one which have a higher oxidation tendency, that electrode is called anode.
while electrode having one with the opposite tendency to reduce, having lower no of electrons on metal surface is called cathode.
Since electron is flowing from anode to cathode the current flow from cathode to anode giving another convention +ive sign is assigned to the cathode and -ive to the anode.
All electrochemical cells work the same way,with slight modifications in reaction mechanism. I end this piece with a question: if this is how cell works, Why do we need a Salt Bridge?
Abhishek Kumar Jha
(Chemistry at Utkarshini)
Notes –1-101-electrochemistry-mechanism-of-working-cell , Video link – https://www.youtube.com/watch?v=AkHY6RuEj6c&t=1s