CIE IGCSE Chemistry Electrolysis PDF

Head to www.savemyexams.com for more awesome resources CIE IGCSE Chemistry Your notes Electrolysis Contents Electrolysis Principles Electrolysis of Molten Compounds Electrolysis of Aqueous Sodium Chloride &...

Head to www.savemyexams.com for more awesome resources CIE IGCSE Chemistry Your notes Electrolysis Contents Electrolysis Principles Electrolysis of Molten Compounds Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid Electrolysis of Aqueous Solutions Ionic Half Equations Page 1 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Electrolysis Principles Your notes Electrolysis: general principles 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 Covalent compounds cannot conduct electricity hence they do not undergo electrolysis Ionic compounds in the solid state cannot conduct electricity either since they have no free ions that can move and carry the charge Particles in ionic compounds are in fixed position in the solid state but can move around when molten or in solution Key terms used in a simple electrolytic cell Electrode is a rod of metal or graphite through which an electric current flows into or out of an electrolyte Electrolyte is the ionic compound in a molten or dissolved solution that conducts the electricity Anode is the positive electrode of an electrolysis cell Anion is a negatively charged ion which is attracted to the anode Cathode is the negative electrode of an electrolysis cell Cation is a positively charged ion which is attracted to the cathode Page 2 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The basic set-up of an electrolytic cell Metals and hydrogen form positively charged ions and so either a metal or hydrogen gas is formed at the cathode Non-metals form negatively charged ions and so non-metals (except hydrogen) are formed at the anode Exam Tip Use the PANIC mnemonic to remember which electrode is the positive and which is the negative: Positive (is) Anode Negative Is Cathode Page 3 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Electrolysis: charge transfer Extended tier only Your notes During electrolysis, current needs to flow around the circuit In order for this to occur, charge must be transferred around the circuit (current is a measure of the rate of flow of charge) by charge carriers The power supply provides the cathode with a supply of electrons, causing it to become negatively charged Positive ions (cations) in the electrolyte move towards the cathode where they gain electrons Negative ions (anions) in the electrolyte move towards the anode where they lose electrons The electrons move from the anode back towards the power supply So, in a complete circuit: Electrons are the charge carriers in the external circuit Ions are the charge carriers in the electrolyte Page 4 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Diagram showing the direction of movement of electrons and ions in the electrolysis of NaCl Your notes Page 5 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Electrolysis of Molten Compounds Your notes Electrolysis of molten compounds A binary ionic compound is one consisting of just two elements joined together by ionic bonding When these compounds undergo electrolysis they always produce their corresponding elements To predict the products made at each electrode, first identify the ions 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 Electrolysis of molten lead(II) bromide Method: Add lead(II) bromide into a beaker and heat it 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 each loses one electron 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 a grey lead metal which deposits on the surface of the electrode Page 6 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Diagram showing the electrolysis of lead(II) bromide Worked example Identify the product formed at the anode and cathode during the electrolysis of molten potassium chloride. Answer: The ions present are potassium (K+) and chloride (Cl-) The chloride ions are attracted to the anode and form chlorine gas The potassium ions are attracted to the cathode and form potassium metal Page 7 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Exam Tip Your notes Remember: Electrodes need to be inert such as graphite or platinum so that they don’t participate in a side reaction with the electrolyte. Page 8 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid Your notes Electrolysis of aqueous sodium chloride & dilute sulfuric acid Electrolysis of aqueous sodium chloride Brine is a concentrated solution of aqueous sodium chloride It can be electrolysed using inert electrodes made from platinum or carbon / graphite The ions in brine are: Na+ and Cl– ions from the brine / aqueous sodium chloride H+ and OH– ions from the water When electrolysed, it produces bubbles of gas at both electrodes The gases chlorine and hydrogen are produced Sodium hydroxide solution is the product remaining in the electrolysis chamber / container These substances all have important industrial uses: Chlorine is used to make bleach Hydrogen is used to make margarine Sodium hydroxide is used to make soap and detergents Product at the negative electrode: The H+ ions are discharged at the cathode as they are less reactive than sodium ions The H+ ions gain electrons to form hydrogen gas Product at the positive electrode: The Cl– ions are discharged at the anode They lose electrons and chlorine gas forms The Na+ and OH– ions remain behind and form the NaOH solution Page 9 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Diagram showing the products of the electrolysis of aqueous sodium chloride Electrolysis of dilute sulfuric acid Dilute sulfuric acid can be electrolysed using inert electrodes made from platinum or carbon / graphite The ions in dilute sulfuric acid are: H+ and SO42– ions from the sufuric acid H+ and OH– ions from the water When electrolysed, it produces bubbles of gas at both electrodes The gases oxygen and hydrogen are produced Page 10 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Electrolysing dilute sulfuric acid in a Hoffman voltameter shows that twice as much hydrogen is produced, compared to oxygen Product at the negative electrode: H+ ions are attracted to the cathode The H+ ions gain electrons to form hydrogen gas Product at the positive electrode: OH- ions are attracted to the anode They lose electrons and form oxygen gas and water Page 11 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Exam Tip Your notes When a gas is produced during electrolysis, you should be able to give the appropriate gas test: Oxygen - if a glowing splint is dipped into a sample of the gas, then the splint will relight Hydrogen - if a lit splint is dipped into a sample of the gas, then a squaeky pop will be heard Chlorine - if damp litmus paper is dipped into a sample of the gas, it will turn red and then bleach to a white colour Page 12 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Electrolysis of Aqueous Solutions Your notes Electrolysis of aqueous solutions Extended tier only Aqueous solutions always have water, H2O, present 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 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 Concentrated and dilute solutions of the same compound give different products For anions, the more concentrated ion will tend to get discharged over a more dilute ion Positive electrode - anode Negatively charged OH– ions and non-metal ions are attracted to the positive electrode If halide ions (Cl–, Br–, I–) are present, the halogen is produced at the anode The halide ions lose electrons and forms the halogen (chlorine, bromine or iodine) If there are no halide ions but OH– ions are present, oxygen is produced at the anode The hydroxide ions lose electrons and forms oxygen gas (and water) In both cases, the other negative ion remains in solution How concentration affects products at the anode The concentration of the solution affects the ion being discharged: If a concentrated halide solution is being electrolysed, the halogen forms at the anode If a dilute halide solution is being electrolysed, oxygen forms at the anode For example: For concentrated barium chloride solution : Cl– ions are discharged more readily than the OH– ions So, chlorine gas is produced at the anode For dilute barium chloride solution: Only OH– ions are discharged So, oxygen is produced at the anode 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 metal will be produced Page 13 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources If the metal is above hydrogen in the reactivity series: The ions of the more reactive metal remain in solution This causes the less reactive hydrogen ions, H+, to be discharged Your notes So, hydrogen will be produced and bubbling will be seen at the cathode If the metal is below hydrogen in the reactivity series: The less reactive metal ions are discharged So, the metal is produced and this will be seen plating onto the cathode The reactivity series of metals including hydrogen and carbon Electrolysis of aqueous copper sulfate Page 14 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Aqueous copper sulfate contains the following ions: Cu2+ and SO42– from the copper sulfate H+ and OH– from the water Your notes Using graphite electrodes: Apparatus for the electrolysis of copper(II)sulfate using inert / passive graphite electrodes Product at the cathode: Cu2+ and H+ will both be attracted to the cathode but the less reactive ion will be discharged In this case, copper is less reactive than hydrogen Copper ions are discharged at the cathode They gain electrons and are reduced to form copper metal Page 15 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The half equation for the reaction at the electrode is: Cu2+ + 2e– → Cu Your notes Product at the anode: SO42– and OH– are both attracted to the anode OH– ions lose electrons more readily than SO42- OH– lose electrons and are oxidised to form oxygen gas The half equation for the reaction at the anode is 4OH– ⟶ O2 + 2H2O + 4e– Using copper electrodes: Apparatus for the electrolysis of copper(II)sulfate using active copper electrodes Observations at the anode and cathode: The cathode increases in mass This is because copper atoms are oxidised at the anode and form copper ions, Cu2+ Page 16 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The anode decreases in mass This is because copper ions, Cu2+, are reduced at the cathode and form copper atoms The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode Your notes Therefore, the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode This implies that the concentration of the Cu2+ ions in the solution remains constant Products formed for common aqueous solutions Aqueous solution - ions present Product at the anode Product at the cathode Concentrated sodium chloride, Chlorine gas Hydrogen gas NaCl Dilute sodium chloride, NaCl Oxygen gas Hydrogen gas Concentrated aqueous copper(II) Oxygen gas Copper sulfate, CuSO4 Dilute sulfuric acid, H2SO4 Oxygen gas Hydrogen gas Page 17 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Ionic Half Equations Your notes Ionic half equations Extended tier only Electrochemistry is concerned with the transfer of electrons This is why the definitions of oxidation and reduction are in terms of losing or gaining electrons, not oxygen Oxidation is the loss of electrons Reduction is the gain of electrons As the ions come into contact with the electrode: Electrons are lost or gained The ions form neutral substances These substances are discharged as products at the electrodes At the anode, negatively charged ions lose electrons So, oxidation occurs at the anode At the cathode, the positively charged ions gain electrons So, reduction occurs at the cathode Ionic half-equations only show half of what is happening in a reaction involving electron transfer The ionic half-equation for oxidation shows the loss of electrons The ionic half-equation for reduction shows the gain of electrons Ionic half-equations must have the atoms and charges balanced Writing ionic half-equations Metals Metals are positive ions, e.g. Li+, Cu2+, Al3+ If a metal is produced during electrolysis: The metal ions will gain electrons to form metal atoms The metal ions will be reduced The charge on the metal ion indicates the number of electrons that will be gained For example: Li+ + e– → Li Cu2+ + 2e– → Cu Al3+ + 3e– → Al Non-metals Non-metals are typically negative ions, e.g. Cl–, Br–, OH– Page 18 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The hydrogen ion, H+, is an exception to this Hydrogen ion / hydrogen half-equation: Your notes If hydrogen is formed during electrolysis: Two hydrogen ions will gain 2 electrons to form hydrogen, H2 Hydrogen ions are reduced 2H+ + 2e– → H2 Halide ion / halogen half-equation: If a halogen is formed during electrolysis: Two halide ions will lose 2 electrons to form the halogen Halide ions are oxidised 2X– → X2 + 2e– Hydroxide ion / oxygen half-equation: The ionic half-equation for hydroixde ions forming oxygen is more challenging to balance Hydroxide ions lose electrons to form oxygen, O2, and water, H2O Hydroxide ions are oxidised 4OH– → O2 + 2H2O + 4e– Table of reduction and oxidation reactions at the electrodes Electrolysis of Anode reaction Cathode reaction Molten lead(II) bromide, PbBr2 2Br– → Br2 + 2e– Pb2+ + 2e– → Pb Concentrated aqueous sodium 2Cl– → Cl2 + 2e– 2H+ + 2e– → H2 chloride, NaCl Dilute sulfuric acid, H2SO4 4OH– → O2 + 2H2O + 4e– 2H+ + 2e– → H2 Exam Tip To help you remember the definitions of oxidation and reduction, use OIL RIG Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons) To help you remember where oxidation and reduction take place, use a RED CAT and AN OX REDuction at the CAThode ANode for OXidation Page 19 of 19 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers