Definition of Redox Process and Intervening Agents

Candela Rocío Barbisan | Jan. 2022
Chemical engineer

Every redox process involves two classic reactions: reduction and oxidation. In reduction, one species is able to reduce its oxidation state due to the fact that it is able to accept electrons from another species. In oxidation, a species is able to donate electrons and thus increase its oxidation state.

Let's look at the following example to clarify the issue:

no_(s) → Na+_(ac)+1e^-

Cl_{2 g)}+2e^- → 2Cl^-_(ac)

We observe two reactions, the first being an oxidation reaction, where sodium increases its oxidation state becoming a charged species (a cation) after losing a electron. Instead, molecular chlorine lowers its oxidation state by gaining an electron. Each of them is called half-reactions or hemi-reactions, since the complete reaction occurs when both occur simultaneously and would be the following:

2Na_(s)+Cl_{2 g)}+2e^- → 2Na⁺_(ac)+2e^-+ 2Cl^-_(ac)

The oxidizing and reducing agents

There are two intervening agents that are fundamental in the redox process: the oxidant and the reducer. The species that is reduced is capable of generating the oxidation of another species, therefore, it is called an oxidizing agent. While the species that is oxidized is capable of promoting the reduction of another species, for this reason, it is called a reducing agent.

If we see the above case, sodium increased its oxidation state from 0 to +1, so, it got oxidized, then Na is a reducing agent. In the case of Cl₂, was reduced by gaining electrons, it went from oxidation state 0 to -1, so it is an oxidizing agent.

These reactions are exploited industrially in electrochemical cells. In them, you enter a electric current that allows the flow of electrons through a circuit and, therefore, a redox reaction can occur. If the redox reaction that occurs is spontaneous, then that cell it is nothing more than a pile like the ones we know from our homes. Now, if a redox process occurs in the cell, spontaneous, that is, the current is used to form in a certain direction the reaction the cell unit is known as electrolytic.

This makes us think that it is necessary to understand redox processes comprehensively. For this we will study how is its spontaneity. When a reaction occurs naturally, without the need to form a certain current necessary for it to occur, the redox process is spontaneous. Such is the case of the following process:

2Ag⁺_(ac)+ Cu_(s) → Cu⁺²_(ac)+ 2Ag_(s)

In this case, if a sheet of the metal solid copper inside a solution containing Ag+ ions (silver cations), upon reaching the Balance, it is observed that the copper sheet has a whitish coating, product of the solid silver deposit on its surface.

Observing this we understand that Ag⁺ (silver cation) is reduced to solid silver, therefore, it is an oxidizing agent. Whereas, solid copper is a reducing agent that oxidizes to the Cu+ species that will be found in solution. Then, with the passage of time, the presence of silver cations in solution decreases and the concentration of Cu+2 cations increases. This occurs in this sense since a spontaneous redox process has taken place.

Now, if to the same copper sheet of the experiment above we immerse it in a solution containing zinc ions (instead of silver ions) we will not observe solid deposits on the copper sheet and the concentrations of Cu ions⁺² in solution and Zn⁺² in solution do not vary. This is because a certain current circulating through the electrochemical cell is required for the reaction to take place in that direction.

So, summarizing the above cases, the reaction between Cu and Ag⁺ could be carried out in a cell, while the reaction between Cu and Zn⁺² to produce solid Zn it should be carried out in an electrolytic cell.