1.Factors affecting electrolysis Prepared by Janadi Gonzalez-Lord

2. Electrolysis summarized A brief overview of what electrolysis is and how it works 3. What is electrolysis? Electrolysis is the chemical separation of an ionic compound using a direct electrical current. 4. Key points in Electrolysis Ions (instead of electrons) conduct electrical current. Ionic compound in molten or aqueous state Needs power source Key points in electrolysis 5. The electrolytic cell The cathode is negatively charged. It attracts positively charged ions (cations) The anode is positively charged. It attracts negatively charged ions (anions) The power source provides electrical current that gives the electrodes their respective charges These are known as electrodes. They are usually a metallic (or in the case of graphite a non-metallic) conductor The electrolyte is the ionic compound in its molten or aqueous state. It provides mobile electrons that allow electrical conduction 6. How electrolysis works Anions move towards the positive electrode (anode). In doing so, they lose electrons to become a neutral element. nXn- - ne- -> Xn The cations move towards this negatively charged electrode (cathode). In doing so, they gain electrons to become an electrically neutral element. Yn+ + ne- -> Y The electrons from the anions then move along the circuit through the power source to the negative electrode These electrons then provide the negative charge for the negative electrode (cathode) 7. Factors affecting electrolysis What conditions affect what products are formed during and after electrolysis? 8. Key factors affecting products of electrolysis Type of electrolyte The electrochemical series Molarity/Concentration of Solution Type of Electrodes 9. Type of electrolyte A look at the role electrolytes play in determining the products of electrolysis 10. Electrolytes can be either Molten Pure Ionic compound Liquid form Solution Impure Mixture of ionic compounds or 11. Comparison of molten vs solution electrolytes Products limited to cation and anion present in binary ionic compound only High energy consumption due to high melting points of pure ionic compounds Multiple products possible based on cations/anions present in solution Lower energy consumption due to impurities Molten Solution 12. Electrolysis of molten lead (II) bromide The only ions here are Pb2+ and Br- This why lead is deposited and bromine gas is released So at the anode, bromide ions lose electrons to form bromine gas: 2Br- - 2e- -> Br2 (g) While at the cathode, lead ions gain electrons to form solid lead metal: Pb2+ + 2e- -> Pb (s) 13. Electrolysis of aqueous lead (II) bromide Water is slightly ionised: H2O H+ + OH- This means that in aqueous solutions, there are two set of cations and two sets of anions. However, only one set gets off at the electrodes. The ions that do so are said to be displaced from solution. Which ions get displaced is discussed in later parts of this presentation At the cathode, hydrogen is displaced (strangely enough!): 2H+ + 2e- -> H 2 (g) At the anode, hydroxide ions lose electrons to form water and oxygen gas: 2OH- - 4e- -> 2H2O (l) + O2 (g) 14. The electrochemical series A look at how the electrochemical series affects the products of electrolysis 15. What is the electrochemical series? This is a list of elements in order of their ability to be reduced. For cations, the higher the element in the series, the less likely it is that this will gain electrons (that is be reduced). For anions, the higher it is on the series the less likely will it lose electrons (that is be oxidized) 16. Reduction potential Why some ions have a greater ability to be reduced than others 17. What affects the ability to be reduced? Group to which ion belongs Ionic radii Whether a metal or non-metal Bond energies 18. Reduction potential - cations An overview of the factors affecting the ability of cations to be reduced. 19. For Group 1 elements Group 1 elements only need to lose 1 electron in order to achieve a stable octet of electrons This means that they would not easily gain back the electrons lost The ionic radii is usually very small and therefore Group 1 ions tend to bond very strongly with their anion counterparts The ionic bond has a low bond energy because it is very strong and would be energetically preferred that the metal alone. This is why they tend to be at the top of the electrochemical series 20. For Group 2 elements Group 2 elements need to lose 2 electron in order to achieve a stable octet of electrons However, it is more difficult to lose the 2nd electron than the 1st electron as you will be trying to remove an electron from an already positive ion The ionic radii is usually very small but larger than that of Group 1 elements in the same period. Therefore Group 2 ions tend to bond very strongly with their anion counterparts but not as strongly when compared to Group 1 ions The bond energy would be lower than that of the metal alone but higher than that of Group 1 elements They would be more likely to gain electrons than Group 1 metals This is why they tend to be at the top of the electrochemical series but under Group 1 elements generally 21. For Group 3 elements Group 3 elements need to lose 3 electron3 in order to achieve a stable octet of electrons However, it is more difficult to lose the 3rd electron than the 2nd and 1st electrons as you will be trying to remove an electron from an already positive ion The ionic radii is usually small but larger than that of Group 1& 2 elements in the same period. Therefore Group 3 ions tend to bond strongly with their anion counterparts but not as strongly when compared to Group 1 & Group 2 ions The bond energy would be lower than that of the metal alone but higher than that of Group 1 & Group 2 elements They will be more likely to gain electrons than Group 1 or 2 metals This is why they tend to be at the top of the electrochemical series but under Group 2 elements generally 22. For Transition elements Transition elements need to lose a varying number of electrons in order to achieve a stable octet as they are in the 4th period. The ionic radii is usually larger than that of other elements in the same period. Therefore transition element ions tend to not bond strongly with their anion counterparts The bond energy would be higher than that of metallic elements They will be more likely to gain electrons than other metals. This is why they tend to be at the bottom of the electrochemical series for cations 23. Reduction potential - anions An overview of the factors affecting the ability of anions to be reduced. 24. For polyatomic anions The larger the anion, the less strongly the electrons are held by the nucleus and the more likely electrons will be easily lost. The ionic radii for polyatomic anions is usually larger than that of other anions. Therefore polyatomic anion tend to not bond strongly with their cation counterparts The bond energy would be higher than that of monatomic anions They will be more likely to lose electrons than other anions. This is why they tend to be at the top of the electrochemical series for anions 25. For halide anions The larger the anion, the less strongly the electrons are held by the nucleus and the more likely electrons will be easily lost. Ionic radii for Group 7 anions increases as you go down the group. Therefore elements lower down in the group tend to not bond strongly with their cation counterparts The bond energy would be increasingly higher as you go down the group Those elements lower down the group will be more likely to lose electrons than other anions. That is,in order of oxidizing power, Cl- > Br-> I- This is why chloride ions (Cl-) is higher in the series than bromide ions (Br-) which is higher than iodide ions (I-) 26. Displacement How an ions position in the electrochemical series affects its displacement from solution 27. For cations, The lower the ion is in the electrochemical series, the more likely it will want to gain electrons (that is be reduced) The cathode supplies electrons to cations in solution Ions lower in the series would therefore be more strongly attracted to the cathode than other cations in solution. CATIONS LOWER IN THE SERIES WILL BE DISPLACED MORE READILY THAN OTHER CATIONS AT THE CATHODE 28. For cations, The lower the anion is in the electrochemical series, the more likely it will want to lose electrons (that is be oxidised) The anode accepts electrons from anions in solution Ions lower in the series would therefore be more strongly attracted to the anode than other anions in solution. ANIONS LOWER IN THE SERIES WILL BE DISPLACED MORE READILY THAN OTHER ANIONS AT THE ANODE 29. Electrolysis of copper (II) sulphate solution At the cathode, pink copper metal is deposited. At the anode, oxygen gas is produced. In solution, the cations are Cu2+ and H+. Since Cu2+ is lower in the series than H+, it would preferentially be displaced at the cathode In solution, the anions are So42- and OH-. Since OH- is lower in the electrochemical series than SO42-, it will be preferentially displaced At the cathode, Cu2+ ions then gain 2 electrons to form solid copper: Cu2+ + 2e- -> Cu(s) At the anode, hydroxide ions lose electrons to form water and oxygen gas: 2OH- - 4e- -> 2H2O (l) + O2 (g) 30. Solution concentration How a solutions molarity can affect the products of electrolysis 31. Concentration defined In chemistry, concentration deals with the amount of solute in a given volume of solvent Higher concentration means that the given solute: solvent ratio is high, that is, there is a high level of solute to a given volume of solvent Low concentration means that the given solute: solvent ratio is low, that is, there is a low level of solute to a given volume of solvent 32. What happens during electrolysis of concentrated solutions? Although the electrochemical series can still predict which ions will be displaced, in concentrated solution, sheer numbers means that other ions slightly higher in the series can be preferentially displaced However, if the ions are very far apart in the electrochemical series, then the lower ion will be discharged preferentially 33. Electrolysis of brine Brine is concentrated sodium chloride solution. At the end of electrolysis, hydrogen is produced at the cathode while chlorine is produced at the anode. The cations in solution are H+ and Na+. Although the concentration of Na+ ions is high, Na+ is very high on the electrochemical series as compared to H+. This great difference between the reduction potentials of the two cations means that it requires a great deal less energy to displace H+ ions compared to Na+ ions. This is why H+ ions are preferentially displaced at the cathode The anions in solution are OH- and Cl-. Since the concentration of Cl- ions is high and there is Cl- is not much higher on the electrochemical series as compared to OH-, it requires a almost the same amount of energy to displace Cl- ions compared to OH- ions. This is why Cl- ions are preferentially displaced at the anode Reduction half-equation at cathode: 2H+ + 2e- -> H2(g) Oxidation half-equation at anode: 2Cl- - 2e- -> Cl2(g) 34. Types of electrodes How inert and active electrodes affect the products of electrolysis 35. Types of electrodes Inert Active Inert electrodes do not actually participate in electrolysis but just provide electrical current Active electrodes actually participate in electrolysis while providing electrical current 36. Example of inert electrodes Graphite Platinum Mercury 37. How inert electrodes work The word inert means unreactive This means that inert electrodes do not actively participate in the reaction They just behave as electrodes should behave accepting and providing electrons to the ions in the electrolyte 38. Active electrodes Usually made of the metal that corresponds to the metallic ion in the electrolyte Actively donates electrons and ions to the electrolytic circuit 39. How active electrodes work The cathode is negatively charged in order to attract cations. With an inert cathode, the electrons are given to the cations, reducing the cation to the corresponding metal. With active electrodes, it really depends on how close the ions in solution are to the metal in the electrode in the electrochemical series If the metal of the electrode is very close or the same that of the ions in solution, it is easier to lose or gain electrons from the electrode itself compared to waiting for electron loss and/or gain from ions in solution. It is really like getting an ice-cream cone from across the street versus one right on the corner from where you are. It is easier to use the store closest to you, just like it is easier to use the region (in this case the electrode) closest to the circuit. 40. Lets compare electrolysis of Copper (II) sulphate using copper electrodes Copper (II) sulphate solution using graphite electrode 41. Electrolysis using inert electrodes Copper(II)sulphatesolutionusing graphiteelectrode Once the power supply is turned on, the Cu2+ ions gravitate towards the cathode as Cu2+ ions are lower in the electrochemical series than H+ ions. The OH- ions go towards the anode as OH- is much lower in the electrochemical series than SO4 2-. At the cathode, the Cu2+ ions gain 2e- to form solid Cu metal: Cu2+ (aq) + 2e- -> Cu(s) At the anode, the OH- ions lose 4e- to form oxygen gas and water: 2OH- (aq) - 4e- -> 2H2O(l) + O2(g) 42. Electrolysis using active electrodes Copper(II)sulphatesolutionusing copperelectrode Once the power supply is turned on, the Cu2+ ions gravitate towards the cathode as Cu2+ ions are lower in the electrochemical series than H+ ions. The OH- ions go towards the anode as OH- is much lower in the electrochemical series than SO4 2-. At the cathode, the Cu2+ ions gain 2e- to form solid Cu metal: Cu2+ (aq) + 2e- -> Cu(s) At the anode, the OH- ions lose 4e- to form oxygen gas and water: 2OH- (aq) - 4e- -> 2H2O(l) + O2(g)