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Electrolysis - using an electrical current (direct current) to cause a reaction.
- During Electrolysis, a current is passed through an electrolyte (Liquid/solution that can conduct electricity).
- It involves a cathode (negative electrode) and anode (positive electrode)
- Make sure to remember that the cations (positive ions) will be attracted to the cathode so therefore reduced and anions (negative ions) will be attracted to the anode so therefore oxidised.
Reduction- Gain of electrons
Oxidation- Loss of electrons
Electrolysis of Molten Ionic Solids forms elements
- An ionic solid cannot be electrolysed as the ions cannot move and therefore cannot conduct electricity.
- This is why molten ionic compounds are used as they can be electrolysed because the ions can move freely and therefore conduct electricity.
- Metal ions are always reduced in electrolysis.
- Non-metal ions are always oxidised in electrolysis.
Some metals can be extracted from their ores using electrolysis.
- A good example is aluminium that is extracted from the ore bauxite by electrolysis. Bauxite contains aluminium oxide, but because it has a very high melting point, it is mixed with cryolite to lower the melting point.
- The mixture is molten so can conduct electricity.
Electrolysis of Aqueous Solutions
- Aqueous = A solution containing H2O/water.
- When you electrolyse an aqueous solution, you also have to consider the ions in the water.
- Make sure you know the difference between the electrolysis of molten ionic solids and aqueous ionic compounds in solution before I/GCSE Chemistry exams. 👨🏫The rules only apply for ionic compounds in aqueous solution:
First Rule of electrolysis of aqueous solutions:
- If the metal in the compound is more reactive than Hydrogen, hydrogen gas will be produced at the cathode from the H+ ions in the water.
- If the metal in the compound is less reactive than Hydrogen, then the metal will be produced at the cathode.
- Memorise this reactivity series for help, it is very useful in I/GCSE Chemistry exams! 👨🏫
- As you can see, only copper is less reactive than hydrogen so, in the electrolysis of aqueous solutions, only copper ions can be electrolysed to produce copper metal, the rest will produce hydrogen.
Second Rule of electrolysis of aqueous solutions:
- If the non-metal ion within the compound is a halogen (group 7 element) like fluorine, chlorine, bromine, iodine... Then the non-metal element will be produced (fluorine, chlorine, bromine, iodine...) at the anode.
- If the non-metal ion within the compound is not a halogen, then oxygen and water will be produced at the anode from the OH- ions in the water.
A good example of this is the electrolysis of copper sulfate solution and sodium chloride solution:
- The Copper is ionically bonded to the Sulfur as ionic bonds only can be formed between a metal and a non-metal (the bonding between two non-metals is covalent).
- Copper has 2 electrons in the outer shell and Sulfur has 6 in the outer shell.
- Copper wants to loose the 2 electrons to have a full outer shell of electrons, so it ionically bonds with the Sulfur, giving it 2 electrons and therefore also making
- Sulfurs outer shell full (the shells can only hold up to 8 electrons except for the first shell which containing only 2 so 6+2=8).
- The Copper lost 2 electrons so now has a full outer shell of electrons and the Sulfur gained 2 electrons and also now has a full outer shell. This is the basis of ionic bonding.
- Sodium chloride has the same concept, it is also bonded ionically to the chlorine (metal and non-metal).
- Sodium metal has 11 electrons overall so therefore it contains 1 electron in its outer shell. The chlorine has 17 electrons so 7 in the outer shell. It is easier for the Sodium to lose the electron and it is easier for Chlorine to gain an electron so they bond ionically and the Sodium gives the chlorine the electron.
- Now the Sodium lost one electron so has a + charge as electrons have a - charge so if one is lost from a neutral atom, the overall charge of the atom becomes positive. The Chlorine gained an electron so therefore now has an overall charge of - .
References:
https://quizlet.com/372963475/reactivity-series-diagram/
https://quizlet.com/372963475/reactivity-series-diagram/
This is the end of the topic!
Drafted by Cherry (Chemistry)