ELECTROLYSIS | MECHANISMS

Some chemical reactions are non-spontaneous; they will not take place unless forced, by inputting energy or applying external factors. A process where electric current is used in order to make a reaction (electrolyte) occur is called Electrolysis. Here, electric energy is converted to chemical energy. Electrolysis is conducted in an electrolytic cell.

An electrolytic cell consists of a container carrying an electrolyte, with a pair of electrodes dipped into the electrolyte, and also a battery which serves as a source of current and the wires usually connecting the battery to the electrodes.

Some Terms and Definition:

• Electrolysis – A process whereby chemical reaction/decomposition occurs when an electric current is passed through an electrolyte.
• Electrolyte – An ionic or polar compound in its molten (fused) state or aqueous state (dilute or concentrated solution). Examples NaCl, Al2O3, CaCl2 etc.
• Electrode – A pole-like structure made up of conductor (a metal), through which the current enters and leaves the electrolytes. There are two types of electrodes; the anode and the cathode.
• Anode – The electrode connected to the positive side of the battery. Current (electrons) from the battery enters the electrolyte through the anode.
• Cathode – the electrode connected to the negative side of the battery. Current (electrons) from the battery leaves the electrolyte through the cathode. Electrolytes can conduct electricity i.e allow electric current to pass through them because they are in there molten (fused) or aqueous state and thereby form mobile ions. NaCl for example, will not conduct electricity if not in its molten or aqueous state.

MECHANISMS:

Normally during electrolysis, the compound (electrolyte) dissociates to form two different ions. The cations (positive ions) formed move to the cathode electrode, while the anions (negative ions) formed move to the anode electrode, where they are all liberated as neutral atoms. Sodium Chloride, NaCl in its molten state forms Sodium ions, Na+ and Chlorine ions, Cl-.

Once Electrolysis starts, electrons (electric current) from the battery move through the negative side of the battery to cathode. The cathode gains these electrons and thus, becomes electron-rich and negatively charged. Since unlike (opposite) charges attracts, and like (same) charges repel, the cations (positive ions) in the electrolyte which are the sodium ions migrate to the negatively charged cathode. These sodium ions gain electrons from the cathode since it has more electrons (electron-rich), to form its neutral atoms, sodium metal.

Na+(aq) + e- ➡️ Na(s)

This process is called Reduction. In electrolysis, reduction occurs at the cathode; electrons are gained from the cathode.

Reduction is the process of gaining electrons, and here, sodium ions gained electrons at the cathode.

In other case, the anode electrode becomes electron-poor/deficient, therefore positively charged, since the electrons (electric current) from the battery flowed to the cathode. This attracts the anions (negative ions) in the electrolyte which are the chlorine ions, to the positively charged anode, where they get oxidized; loss electrons to form its neutral atoms, chlorine gas.

Cl-(aq) ➡️ Cl(g) + e-

Since chlorine, Cl is diatomic in nature, its atoms combine in pairs to form chlorine molecules, Cl2, i.e.

Cl(g) + Cl(g) ➡️ Cl2(g)

So the overall anode reaction is written this way;

2Cl-(aq) ➡️ Cl2(g) + 2e-

This process is called Oxidation; process of losing electrons. It occurs at anode. Here, chlorine ions lost electrons at the anode.

In electrolysis, the reaction that occurs at the cathode and the anode respectively happens at the same time and therefore the number of electrons lost at the anode must be the same with the number of electrons gained at the cathode, that is to say that oxidation and reduction processes are complementary.

Since two electrons have being lost by Cl- , then two electrons must be gained by the Na+ in order to balance the two half cell reactions. Thus the overall cathode equation is written;

1. 2Na+(aq) + 2e- ➡️ 2Na(s)

Net Equation of reaction: 2Na+(l) + 2Cl-(l) ➡️ 2Na(s) + Cl2(g)

Generally, ionic or polar compounds in its molten (fused) state tend to produce only two ions; one type of cation and one type of anion like in molten NaCl which forms Na+ as it only cation, and then Cl- as it only anion. Thus, sodium atoms will be deposited at the cathode and chlorine atoms liberated at the anode.

But differently, ionic or polar compounds in its aqueous state (whether in dilute or concentrated solution) tend to produce only four ions; two types of cations, and then two types of anions. In this scenario, it is only one type of cation is deposited at the cathode, and only one type of anion is liberated at the anode.

But how do we know the ion to be discharged at a particular electrode among the different ions migrating to that electrode?

When different ions of one-type migrate to the supposed electrode, only one ion is selectively discharged, and this is determined by three factors;

• Position of the ion in the electrochemical series. The cation to be discharged at the cathode depends on its ability to accept electrons at the cathode. The anion to be discharged at the anode depends on its ability to lose electrons.

Consider the electrolysis of aqueous NaCl (NaCl dissolved in H2O), where four ions (Na+, Cl-, H+, and OH-) are formed.

At the cathode, H+ is discharged preferentially because it readily accepts electrons than Na+.

At the anode, OH- is discharged preferentially because it readily loses its electrons to the anode, than Cl-.

• Relative concentration of the ions in the electrolyte. In the electrolysis of a concentrated solution of any electrolyte, where an halogen ion (anion) is present in the electrolyte, it is selectively discharged to the anode because it tends to have a higher concentration than other anions present. For instance in the electrolysis of concentrated NaCl (Brine), two different anions, Cl- and OH- are formed, and migrate to the anode electrode, but Cl- is selectively discharged.
• Nature of the electrodes. When the anode is made up of a metal in which its (the metal) salt is the electrolyte, the anode tends to dissolve in the electrolyte, thereby altering the ions to be discharged.

Also, when mercury is used as the cathode electrode in the electrolysis of Brine (concentrated NaCl), Na+ and H+ migrates at the cathode, and instead of H+ being discharged because of its reducing ability than Na+, Na+ is preferentially discharged because the hydrogen gas formed from H+ will create a high over-voltage.

Electrolysis is also used to separate a compound into its original elements which makes up the compound.