Electrolysis: the process of decomposing an ionic compound into its elemental form by passing a direct current through the compound in its molten state. When an electric current is passed through a molten ionic compound the compound decomposes or breaks down The process also occurs for aqueous solutions of ionic compounds Liquids and solutions that are able to conduct electricity are called electrolytes Covalent compounds cannot conduct electricity hence they do not undergo electrolysis An electrolytic cell is the name given to the set-up used in electrolysis and which consists of the following: o Electrode: a rod of metal or graphite through which an electric current flows into or out of an electrolyte o Electrolyte: ionic compound in molten or dissolved solution that conducts the electricity o Anode: the positive electrode o Anion: negatively charged ion which is attracted to the anode o Cathode: the negative electrode o Cation: positively charged ion which is attracted to the cathode The basic set-up of an electrolysis cell Exam Tip Use the PANIC mnemonic to remember which electrode is the positive and which is the negative: Positive (is) Anode Negative Is Cathode. Electrical Conductivity of Ionic Compounds Ionic compounds in the solid state cannot conduct electricity since they have no free ions that can move and carry the charge The ions must be able to move and can only do so in the molten state or when dissolved in a solution, usually aqueous When the cell is turned on and an electric current is passed through an electrolyte the ions in the solution start to move towards the electrodes Electrical Conductivity of Ionic Compounds Particles in ionic compounds are in fixed position in the solid state but can move around when molten or in solution Exam Tip Cations are attracted to the cathode and anions are attracted to the anode. Electron flow in electrochemistry occurs in alphabetical order as electrons flow from the anode to the cathode. Movement of Ions During electrolysis the electrons move from the power supply towards the cathode Electron flow in electrochemistry thus occurs in alphabetical order as electrons flow from the anode to the cathode Positive ions within the electrolyte migrate towards the negatively charged electrode which is the cathode Negative ions within the electrolyte migrate towards the positively charged electrode which is the anode Diagram showing the direction of movement of electrons and ions in the electrolysis of NaCl Electrolysis of Simple Ionic Compounds Lead(II) bromide is a binary ionic compound meaning that it is a compound consisting of just two elements joined together by ionic bonding When these compounds are heated beyond their melting point, they become molten and can conduct electricity as their ions can move freely and carry the charge These compounds undergo electrolysis and always produce their corresponding element To predict the products of any binary molten compound first identify the ions present The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode Therefore the cathode product will always be the metal and the product formed at the anode will always be the non-metal Diagram showing the electrolysis of lead (II) bromide Method: Add lead(II) bromide into a crucible and heat so it will turn molten, allowing ions to be free to move and conduct an electric charge Add two graphite rods as the electrodes and connect this to a power pack or battery Turn on the power pack or battery and allow electrolysis to take place Negative bromide ions move to the positive electrode (anode) and lose two electrons to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off Positive lead ions move to the negative electrode (cathode) and gain electrons to form grey lead metal which deposits on the bottom of the electrode Electrode Products: Anode: Bromine gas Cathode: Lead metal Choosing the Method of Extraction The position of the metal on the reactivity series determines the method of extraction Higher placed metals (above carbon) have to be extracted using electrolysis as they are too reactive and cannot be reduced by carbon Lower placed metals can be extracted by heating with carbon which reduces Electrolysis is very expensive as large amounts of energy are required to melt the ores and produce the electrical current The reactivity series of metals is shown below with the corresponding method of extraction Metal Extraction Methods Summary Table Exam Tip Questions on this topic often ask you to explain why in some cases electrolysis is used and in other cases reduction by heating with carbon is used. Make sure you can explain when each process is used and why. The Extraction of Aluminium Aluminium is extracted on a large scale using electrolysis: Diagram showing the extraction of aluminium by electrolysis Raw Materials: Aluminium Ore (bauxite) Cryolite (sodium aluminium fluoride) Explanation: The bauxite is first purified to produce aluminium oxide, Al 2O3 Aluminium oxide has a very high melting point so it is first dissolved in molten cryolite producing an electrolyte with a lower melting point, as well as a better conductor of electricity than molten aluminium oxide This reduces the costs considerably making the process more efficient The electrolyte is a solution of aluminium oxide in molten cryolite at a temperature of about 1000 °C The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell The cell operates at 5-6 volts and with a current of 100,000 amps The heat generated by the huge current keeps the electrolyte molten A lot of electricity is required for this process of extraction which is a major expense The overall equation is: 2Al2O3 (l) ⟶ 4Al (l) + 3O2 (g) Some of the oxygen produced at the positive electrode then reacts with the graphite (carbon) electrode to produce carbon dioxide gas: C (s) + O2 (g) ⟶ CO2 (g) This causes the carbon anodes to burn away, so they must be replaced regularly USE OF: Cryolite (Na3AlF6)- to reduce the melting point Fluorspar (CaF2)- to increase the conductivity Electrolysis of an Aqueous Solution Using Inert Electrodes Aqueous solutions will always contain water molecules (H2O) In the electrolysis of aqueous solutions, the water molecules dissociate producing H+ and OH– ions: H2O ⇌ H+ + OH– These ions are also involved in the electrolysis process and their chemistry must be considered We now have an electrolyte that contains ions from the compound plus ions from the water Which ions get discharged and at which electrode depends on the relative reactivity of the elements involved Note: The concentration of the solution can affect the products of electrolysis, however, this is beyond the scope of this course and you are not expected to know the specific details of this Exam Tip When answering questions on this topic, it helps if you first write down all of the ions present first. Only then you should start comparing their reactivity and deducing the products formed. Electrode Reactions Positive Electrode (anode) Negatively charged OH– ions and non-metal ions are attracted to the positive electrode If halide ions (Cl-, Br-, I-) and OH- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine) If no halide ions are present, then OH- is discharged at the anode, loses electrons and forms oxygen In both cases the other negative ion remains in solution Negative Electrode (cathode) Positively charged H+ and metal ions are attracted to the negative electrode but only one will gain electrons Either hydrogen gas or the metal will be produced If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode This is because the more reactive ions will remain in solution, causing the least reactive ion to be discharged Therefore at the cathode, hydrogen gas will be produced unless the positive ions from the ionic compound are less reactive than hydrogen, in which case the metal is produced The reactivity series of metals including hydrogen and carbon Worked example Predict the products formed at each electrode in the electrolysis of magnesium iodide solution Answer o Work out what ions are attracted to the cathode Hydrogen (H+) and magnesium (Mg2+) o Decide which element will be discharged: The less reactive element is usually formed Hydrogen is discharged at the cathode as it is less reactive than magnesium o Work out what ions are attracted to the anode Hydroxide (OH-) and iodide (I-) o Decide which element will be discharged: If a halide is present, the corresponding halogen is formed, otherwise, oxygen is formed Iodine is discharged at the anode as iodide ions are present Worked example Predict the products formed at each electrode in the electrolysis of copper sulfate solution Answer o Work out what ions are attracted to the cathode Hydrogen (H+) and copper (Cu2+) o Decide which element will be discharged: The less reactive element is usually formed Copper is discharged at the cathode as it is less reactive than hydrogen o Work out what ions are attracted to the anode Hydroxide (OH-) and sulfate () o Decide which element will be discharged: If a halide is present, the corresponding halogen is formed, otherwise, oxygen is formed Oxygen is discharged at the anode as no halide ions are present Determining what Gas is Produced The gas produced can be tested to determine its identity If the gas produced at the cathode burns with a ‘pop’ with a lighted splint then the gas is hydrogen If the gas produced at the anode relights a glowing splint dipped into the gas then the gas is oxygen If the gas produced at the anode turns damp blue litmus paper red and is then bleached white then the gas is chlorine The halogen gases all produce their own colours (bromine is red-brown, chlorine is yellowgreen) Electrolysis is used to electroplate objects. This may be for reasons such as appearance, durability and prevention of corrosion. It includes copper plating and silver plating.
