Indirect redox reactions (with electrodes)

Apart from the direct, there are also indirect redox reactions;
this means: there is no direct contact between the particles of the oxydator and the reductor. The transfer of the electrons is realised via-via, mostly via metal wiring or other conductive material.
Exteriorly the conducting wires take care for the electron transport from RED to OX, without direct contact between the reactants. They do not meet; they do not collide.

The half reactions take place at the surface of electrodes (mostly a metal or graphite).

There are two kinds of indirect redox reactions:
  1. spontaneous indirect redox reactions, when strong substances react to produce weaker ones. These reaction mostly ar exothermic.
  2. forces indirect redox reactions, when weak substances must (are forced to) react to produce strong ones. These reaction mostly are endothermic.
In both cases, that means: in all indirect redoxreactions, electrodes are applied.

Indirect redox reactions only take place if in between the electrodes is an environment that is conductive for electric current (meaning that charged particles are present that can freely move around, like the ions of dissolved or molten salts).
You could also say that the circuit in this system must be a closed circuit. Part of the circuit is made by conductive metal wires an electrodes with free electrons, and the other part is made of a solution of molten or dissolved electrolyte with free ions.
Those free ions can freely move between the positive and the negative electrode.

To make this movement possible, the electrode compartments must have contact.
This contact will sometimes be realised with a (semi)porous barrier, of else - if the two compartments are separated - those compartments must be connected with an ion bridge, or better: a salt bridge. That is a kind of tube filled with salty gel (gel with ions).