Why chemical energy is doing the work on the electrons to move them from cathode to anode and not some kind of force?

Question

We say that in an electrolytic cell the redox reactions occuring on the electrodes are exothermic and that chemical energy does work on electrons to move them from cathode to anode and this work done is stored as energy in the electrons.

Doubt—> From the above concept it is clear that chemical energy did work on the electrons to move them from cathode to anode. But the electrodes create a potential difference and if there is a potential difference then there should be an electric field also going from cathode to anode therefore electrons coming into that electric field will experience a force towards cathode but to move them towards anode, one has to apply force in the opposite direction of electric field and that force should do the work done and that work done should be stored in electrons but this doesn't happen in reality. My question is why does this doesn't happen in reality, why there is no force acting on the electrons even though there is a potential difference and hence an electric field, why chemical energy is doing the work? Pls clear my doubt.

Answer 1

For electrolytic cells:

• The charge carriers are not electrons.
• The transport mechanism for conductive electrolytic solutions is not from the electric field generated between electrodes.

The mechanism for generating current in an electrolytic cell is very different than conduction in a wire. The biggest difference is that the charge carriers in wire are electrons. The charge carriers in electrolytes are not electrons. The charge carriers are the dissolved ions in solution. These ions act as a source/sink for electrons at the electrode surface. The ionization of these carriers occurs at the redox potential which is the energy to add or remove electrons from the ion. This reduction/oxidation reaction is the chemical energy and occurs at a given voltage. An IV plot of the reaction would show a current peak at the redox potential.

How do the ions in solution migrate to the electrode? An intuitive assumption( based on solid state physics) would be that the electric field generated by the potential difference between electrodes drives ions to the electrode surface for redox. But the electrolyte is a liquid ( the ions move around to redistribute the charge to minimize the field ) and conductive so the field generated by the electrodes is minimal and contributes in only a secondary manner. The primary transport mechanism is diffusion. As ions reach the electrode surface a reaction takes place and there is a depletion region near the electrode that is absent of ions. Ions diffuse into this region at a rate based on the concentration gradient. This occurs at both anode and cathode.

So there is no applied force to move the electrons through solution. It's done by diffusion.

Answer 2

Sometimes the same thing can be described by two different concepts.

If a force is applied to a particle, the particle speeds up and gains kinetic energy. A speeding particle can coast up a hill, losing kinetic energy and gaining potential energy. Gravity acts against the upward velocity component, so the particle slows down. You can describe what happens with either force or energy.

Suppose two blocks slide past each other. The force of friction opposes the direction of motion, so the blocks slow down. Under a microscope, atoms from the two different blocks collide with each other, and often gain kinetic energy. Atom bonds are electromagnetic forces that can be approximated as springs. So the atoms begin vibrating hard than they had been, trading kinetic and potential energy back and forth. From a macroscopic point of view, we do not track the motion of atoms. We characterize their average vibrational energy as heat. So you can describe this energy as either kinetic/potential energy or heat.

Chemical energy is like this. Quantum mechanics is needed for a good description, but atomic bonds are caused by forces between atoms. The forces are attractive when atoms are too far apart, and repulsive when too close. Atoms lose potential energy when they come together to form a bond. Sometimes they can lose even more energy by rearranging themselves into different bonds. From a macroscopic point of view, we describe the total energy gained and lost as chemical energy. From a microscopic point of view, it is potential energy.

An electrolytic cell contains charged ions. Electric forces cause them to be attracted to the electrodes, where they undergo reactions. You can see how many atoms react and describe this as chemical energy.

You can also measure the flowing charges that come out of these reactions. You can measure the energy by voltage (Joules per coulomb of charge). Energy is conserved. You find that the chemical energy lost is the same as the electrical energy gained.