C5: Electricity and Chemistry

Electrolysis

Electrolysis is the breakdown of an ionic compound when molten or in aqueous solution by the passage of electricity
It cannot be done for covalent compounds or solid ionic compounds as they do not have a free flow of electrons or ions (with the exceptions such as graphite)
Generally, metals and hydrogen form around the cathode, and non-metals (with the exception of hydrogen) form around the anode
The ions that are less reactive with be discharged first

There are different equipment needed to run electrolysis

Inert electrode: An electrode that does not interfere with the chemical reaction
Electrode: A metal or graphite rod which electricity enters through the electrolyte or exits into the electrolyte and does not
Electrolyte: A dissolved or molten ionic compound conducting electricity
Anode: The positively charged electrode which attracts negative ions/anions
Cathode: The negatively charged electrode which attracts positive ions/cations

In electrolysis, separation happens as ions are attracted to the electrodes through opposite charges with the electrodes.

Cations will gain electrons from the cathode (negatively charged electrode) to become normal atoms without a charge
Anions will lose electrons to the anode (positively charged electrode) to become normal atoms without a charge

Therefore, the components in the ionic compounds will become neutral atoms with some surrounding the cathode and some surrounding the anode

This is a diagram of the general setup of electrolysis

Let's look at 3 examples of electrolysis using inert electrodes (graphite or platinum)

Molten lead bromide is the electrolyte and two graphite rods connected to a battery are being used
Negative bromine ions (Br^-) move to the anode and each lose their electron. Bromine molecules (Br2) are formed
The balanced equation happening at the anode is: 2Br- —> Br2 + 2e-
Positive lead ions (Pb²⁺) move the the cathode and gain electrons. Lead (Pb) is formed
The full balanced electrolysis equation is: PbBr₂ = Pb + Br₂

Sodium chloride contains 4 ions, as it is aqueous, meaning that aside from the ions Na⁺ and Cl^-, there is also H⁺ and OH^- from water
As chloride ions(Cl^-) are the less reactive out of the two types of negative ions, it loses its electrons and becomes chloride gas at the anode
This is the half equation at the anode: 2Cl^- - 2e^- —> Cl₂
As hydrogen ions (H⁺) are less reactive than sodium, it gains electrons and becomes hydrogen gas at the cathode
This is the half equation at the cathode: 2H⁺ + 2e^- —> H²
Na⁺ and OH^- remains, causing an aqueous solution of sodium hydroxide to be formed

Sulfuric acid (H₂SO₄) contains 3 ions. Sulfuric acid has H⁺ and SO₄^2-, there is H⁺ and OH^- from water. As H⁺ repeat, there will only be one positive ion to choose from
As the hydroxide (OH^-) ions are less reactive that SO₄^2-, it loses electrons and will become water and oxygen
This is the half equation at the anode: 4OH^2- - 4e^- —> 2H₂O +O₂
As hydrogen (H²) ions are the only positive ion, it gains electrons and becomes hydrogen gas at the cathode
This is the half equation at the cathode: 2H⁺ + 2e^- —> H₂

Electroplating with copper

An appliance for using electrolysis is to coat an object with metals
We replace the negative electrode/cathode with an object that we want to coat
As metals are attracted to the negatively charged side, they will form around the object
For example, if we replaced the cathode with a spoon, we could coat the cathode with a metal
If we were using a compound with copper as the least reactive cation, we would coat an object like a spoon with copper

Manufacturing techniques

Aluminium oxide is used to extract aluminium. However, its melting point is too high, which will lead to high costs to try to melt the aluminium oxide.
Therefore, we use cryolite. Cryolite does not interfere with the chemical reaction. It acts as an impurity and lowers the melting temperature of aluminium to lower costs
A steel tank with carbon lining is used and electrolysis occurs. Graphite is used as the anode, and carbon lining in the tank is used as the cathode
Aluminium oxide only has two ions, Al³⁺ and O^2-
At the anode, oxygen ions will lose electrons and form oxygen gas. The equation will be 2O^2- - 4e^- —> O₂
At the cathode, the aluminium ions gain electrons and form liquid aluminium metal. The equation will be Al³⁺ + 3e^- —> Al
The aluminium liquid will sink to the bottom of the cell and the oxygen will react with the graphite anodes to from carbon dioxide gas. This means that the anodes will be corroded from the reaction and need to be replaced

Chlorine, hydrogen and sodium hydroxide from concentrated aqueous sodium chloride (repeat of earlier sodium chloride point)

Sodium chloride contains 4 ions, as it is aqueous, meaning that aside from the ions Na⁺ and Cl^-, there is also H⁺ and OH^- from water
As chloride ions(Cl^-) are the less reactive out of the two types of negative ions, it loses its electrons and becomes chloride gas at the anode
This is the half equation at the anode: 2Cl^- - 2e^- —> Cl²
As hydrogen ions (H⁺) are less reactive than sodium, it gains electrons and becomes hydrogen gas at the cathode
This is the half equation at the cathode: 2H⁺ + 2e^- —> H²
Na⁺ and OH^- remains, causing an aqueous solution of sodium hydroxide to be formed
Therefore, chlorine, hydrogen, and sodium hydroxide are manufactured