Electrolysis
Electrolysis is the process of breaking apart an ionic compound using electricity - it’s used to extract metals from their ores. It’s only used for the most reactive metals though, since the less reactive ones can be extracted by reduction with carbon which is much cheaper and uses less energy.
The process of electrolysis
The electrolysis set-up consists of two electrodes: a positive electrode called the anode and a negative electrode called the cathode. You know that an ionic compound consists of positive and negative ions. When an ionic compound is dissolved in water or molten, these ions move apart and can move through the liquid. The positive ions (called cations) will be attracted towards the negative electrode (the cathode). The reason the cathode is negative is because an electric current is running from the anode towards the cathode, causing a build-up of electrons on the cathode. A positively charged ion will happily take an electron (or two) from the cathode to become neutral. On the other hand, a negatively charged ion (an anion) will deposit an electron, or two, onto the positive anode to become neutral. This is summarised below:
Anode: the positive electrode attracts negatively charged ions (anions), which will lose electrons to become oxidised
Cathode: the negative electrode attracts positively charged ions (cations) which will gain electrons and become reduced.
Remember OILRIG: oxidation is loss, reduction is gain.
Electrolysis of molten ionic compounds
Ionic compounds that have been melted are described as molten. When a molten ionic compound is electrolysed using inert electrodes, the metal is produced at the cathode and the non-metal is produced at the anode.
For example, the electrolysis of molten lead bromide will produce lead at the cathode and bromine at the anode.
Using electrolysis to extract metals
Aluminium is more reactive than carbon so needs to be extracted from its ore by electrolysis
The technique used to extract a particular metal from its ore depends on its position in the reactivity series. Metals that are less reactive than carbon can be extracted by reacting it with carbon. However, metals that are more reactive than carbon need to be extracted using electrolysis. Electrolysis requires large amounts of energy for melting the compound and for the electricity used during electrolysis, which makes the process expensive. This is why extraction of more reactive metals, such as aluminium, is more expensive than the extraction of less reactive metals, such as tin.
Aluminium is extracted from molten aluminium oxide by electrolysis. Since aluminium oxide has a very high melting point and would require a lot of heat energy to melt, it is first mixed with a substance called cryolite. This brings the melting point down so less energy is needed to convert aluminium oxide into a molten compound. The positive electrode (anode) is made of carbon and needs to be continually replaced since the carbon reacts with oxygen to form carbon dioxide. This means that the carbon anode gets smaller and smaller over time.
Electrolysis of aqueous solutions
An aqueous ionic solution is simply where we have an ionic compound dissolved in water. The solution will contain the following ions:
- The ions which make up the ionic compound (such as sodium ions (Na+) and chloride ions (Cl-).
- The ions which make up water: hydrogen ions (H+) and hydroxide ions (OH-).
At the cathode
- Hydrogen (from the H+ in water) is produced unless the positive ion from the ionic compound is less reactive than hydrogen.
- If the metal ion is less reactive, this will be produced instead. The only metals less reactive are copper, silver, gold and platinum.
At the anode
- Oxygen (from the OH- in water) is produced unless the negative ion is one of the halogens.
- Remember that the halogens are what we call group 7 of the periodic table and include the elements fluorine, chlorine, bromine and iodine.
- If there are halide ions, the halogen will be produced instead of oxygen.
Example: aqueous sodium chloride solution
- We know that at the cathode (negative electrode), either hydrogen (from H+) or sodium (Na+) will be produced. The rule is that it will be the least reactive of the two elements. In this case, hydrogen is the least reactive so hydrogen gas will form at the anode.
- We know that at the anode (positive electrode), oxygen is formed in the absence of a group 7 element. Here, chloride ions are in our solution, which is a group 7 element so chlorine gas forms at the cathode instead of oxygen.
Half equations
We can represent the gain or loss of electrons during electrolysis using half equations. Remember that the electrons should be written on the left hand side of the equation when they are gained by an ion (reduction) and on the right when being lost (oxidation). A good trick to make sure you’ve written the half equation properly is to make sure the charges on the left and right hand sides are equal.
Did you know…
Before electrolysis was developed as a method of extracting aluminium from its ore, it was one of the most expensive elements on earth and more costly than gold. The US placed a small aluminium pyramid on top of the Washington Monument to signify its prosperity and aluminium cutlery was used by royalty. Once electrolysis was invented, the price of aluminium plummeted.
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