Unit 1 Chapter 10 Electrolysis PDF

CHAPTER 10 ELECTROLYSIS  1 Chapter10 Electrolysis Electrolysis ❖ Electrolysis is a chemical change ca...

CHAPTER 10 ELECTROLYSIS  1 Chapter10 Electrolysis Electrolysis ❖ Electrolysis is a chemical change caused by passing an electric current through a compound which is either molten or in solution. Electrolyte ❖ An electrolyte is a liquid or solution that undergoes electrolysis. Electrolytes all contain ions. Electrodes ❖ usually made of unreactive metals such as platinum carbon (graphite) which are said to be inert electrodes because which does not participate in a chemical reaction. Two type of electrodes ▪ cathode - the negative electrode ▪ anode - the positive electrode Two type of ions ionic compounds are made up of positive ions and negative ions: ▪ cation - positively charged ions ▪ anions - negatively charged ions Note, ▪ cation moves to cathode - Electron flows in the external ▪ anion moves to anode circuit Electrolytic cell diagram - ions flow in the electrolyte e- - the anode connected to the positive terminal of the power source - the cathode connected to the negative terminal of the power source Edexcel International Chemistry GCSE (9- 1) Note by Dr.Zin  2 I. Electrolysis of molten / fused ionic compound Electrolysis of molten lead (II) bromide (PbBr2) using inert electrodes Predicting the products of electrolysis PbBr2 (aq) Pb2+(aq) + Br-(aq) Pb Br Half reaction 2 1 cathode reaction: Pb2+(aq) + 2e Pb(s) anode reaction: 2Br- (aq) Br2(g) + 2e cathode product – lead deposited anode product – bromine gas evolved Note ▪ Cathode = electron gain ▪ anode = electron lost ▪ When the electrolyte decomposes, - metal is formed at the cathode (negative electrode) - non - metal is formed at the anode (positive electrode) Extra Question. Predict the following products formed at the cathode and anode when the following molten compounds are electrolyzed by using platinum electrodes. (i) aluminum oxide (ii) copper bromide (iii) zinc chloride (iv) sodium chloride (i) aluminum oxide (Al2O3) Al2O3(aq) Al3+(aq) + O2-(aq) Al O cathode reaction: Al3+(aq) + 3e Al (s) 3 2 anode reaction: 2 O2- (aq) O2(g) + 4e cathode product – aluminum, anode product- oxygen evolved Edexcel International Chemistry GCSE (9- 1) Note by Dr.Zin  3 (ii) copper bromide (CuBr) CuBr Cu++ Br− Cu Br + − Cathode reaction: Cu + e Cu 1 1 Anode reaction: 2Br− B r2 + 2e− (iii) zinc chloride (ZnCl2) Zn Cl ZnCl2 Zn2+ + Cl- 2 1 cathode reaction: Zn2+ + 2e Zn anode reaction: 2 Cl- Cl2 + 2e (iv) sodium chloride Na Cl NaCl Na+ + Cl- 1 1 cathode reaction: Na+ + 1e Na anode reaction: 2 Cl- Cl2 + 2e Electrolysis and redox reaction Reduction ❖ Reduction occurs when something gains electrons. cathode reaction: Pb2+(aq) + 2e Pb The lead (II) ions gain electrons at the cathode: Gain of electrons is reduction. Oxidation ❖ Oxidation occurs when something loses electrons. anode reaction: 2Br- Br2 + 2e- The bromide ions lose electrons at the anode: Loss of electrons is oxidation. Bromide ions are oxidised to bromine molecules. Redox reactions, ❖ oxidation and reduction always occur at the same time. Note Cathode = electron added (reduction process) AN OX anode = electron lost (oxidation process) RED CAT When the electrolyte decomposes, - metal is formed at the cathode (negative electrode) - non - metal is formed at the anode (positive electrode) Edexcel International Chemistry GCSE (9- 1) Note by Dr.Zin  4 Reactivity series If the metal is above hydrogen in the reactivity series, hydrogen produced at the cathode instead of the metal. If the metal is below hydrogen in the reactivity series, obtain the metal at the cathode. If the solutions of halides (chlorides, bromides or iodides), obtain the halogen (chlorine, bromine or iodine) at the anode. With other common negative ions (sulfate, nitrate, hydroxide), obtain oxygen at the anode Potassium sodium lithium Decreasing reactivity calcium magnesium aluminium (carbon) zinc iron (hydrogen) copper silver gold Alternatively Discharged Series For positive ions Na+, Mg2+, Al3+, H+, Cu2+ More likely to be discharged For negative ions SO2− − - - - - 4 , NO3 , OH , Cl , Br , I More likely to be discharged II. The electrolysis of some other solutions using inert electrodes Electrolysis of dilute sodium hydroxide using platinum electrodes NaOH(aq) Na+(aq) + OH-(aq) H2O(l) H+(aq) + OH-(aq) cathode reaction: 2H+(aq) + 2e H2(g) anode reaction: 4OH-(aq) 2H2O(l) + O2 (g) + 4e Edexcel International Chemistry GCSE (9- 1) Note by Dr.Zin  5 cathode product - hydrogen gas liberated anode product - oxygen gas liberated Electrolysis of copper (II) sulphate solution using inert electrodes CuSO4 (aq) Cu2(aq) + + 𝑆𝑂42− (aq) H2O(l) H+(aq) + OH-(aq) The ions in solution are Cu2+, 𝑆𝑂42− , H+ and OH- cathode reaction: Cu2+ (aq) + 2e Cu (s) (reduction) anode reaction: 4OH-(aq) 2H2O(l) + O2(g) + 4e (oxidation) Net result: Cu is deposited at cathode and oxygen gas is liberated at anode. The electrolyte becomes acidic properties. Electrolysis of dilute sulfuric acid solution using inert electrodes H2SO4 H+ + 𝑆𝑂42− H2 O H+ + OH- The ions in solution are 𝑆𝑂42− , H+ and OH- cathode reaction: H+ + 2e H2 anode reaction: 4OH- 2H2O + O2 + 4e Net result: hydrogen gas and oxygen are liberated at cathode and anode. Figure; Apparatus for electrolysing dilute sulfuric acid and measuring the volume of gases produced. In this experiment, the amount of hydrogen is more than twice as much as the oxygen. This is because oxygen is more soluble in water than hydrogen. Edexcel International Chemistry GCSE (9- 1) Note by Dr.Zin  6 Electrolysis of sodium chloride (NaCl) solution The ions in solution are Na+, Cl-, H+ and OH- Half reaction cathode reaction: 2H+(aq) + 2e H2 (g) anode reaction: 2Cl- (aq) Cl2(g) + 2e cathode product - hydrogen gas liberated anode product - chlorine gas liberated When the hydrogen and chlorine bubble off, the Na+ and OH- ions are left behind. So, sodium hydroxide solution is formed. The electrolyte become alkaline solution. Note, The experiment should be done in a fume cupboard (fume hood) or well-ventilated room because chlorine gas is poisonous. Non - electrolytes do not conduct electricity. Why? They do not readily dissociate into ions in their molten or in solution state. Can pure water and sugar conduct electricity molten state? Pure water and sugar solution do not contain ions. Sugar molecules do not break down into ions when dissolved. Therefore, these solutions cannot carry an electric current. Question Page 110. Predict the following products formed at the cathode and anode when the following solution are electrolyzed by using platinum electrodes. (i) KI (ii) MgBr2 (i) Electrolysis of potassium iodide (KI) solution The ion in solution are K+, I-, H+ and OH- Half reaction cathode reaction: 2H+(aq) + 2e H2 (g) anode reaction: 2I- (aq) I2(g) + 2e cathode product - hydrogen gas liberated anode product - iodine gas (ii) Electrolysis of sodium chloride (MgBr2) solution The ion in solution are Mg2+, Br-, H+ and OH- Half reaction cathode reaction: 2H+(aq) + 2e H2 (g) Edexcel International Chemistry GCSE (9- 1) Note by Dr.Zin  7 anode reaction: 2Br- (aq) Br2(g) + 2e cathode product - hydrogen gas liberated anode product - bromine gas liberated Changing the electrodes Electrolysis of Aqueous Solution Copper (II) Sulphate Solution using Inert electrode The ions present in the solution are: H+(aq) , OH−(aq) , Cu2+(aq) and SO42−(aq) Cathode reaction: Cu2+(aq) + 2e Cu (s) Anode reaction: 4OH-(aq) 2H2O(l) + O2 (g) + 4e The net results - The blue colour of the solution fades as the copper ions are discharged. - Because of the copper ions in solution are turning to copper atoms at the cathode but are not being replaced in the solution from the anode. Electrolysis of Aqueous Solution Copper (II) Sulphate Solution using Copper electrode Cathode reaction: Cu2+(aq) + 2e Cu (s) Anode reaction: Cu (s) Cu2+(aq) + 2e Net results Copper ions removed from the solution are replaced by copper ions formed at the anode, resulting in the same deep blue color of the electrolyte. Chapter Questions Page112 No: 1,2,4 Edexcel International Chemistry GCSE (9- 1) Note by Dr.Zin