[CHEM] Chapter 4 - Electrolysis

1 DARRELL ER (COPYRIGHTED) © TOPIC 4: ELECTROLYSIS DARRELL ER (COPYRIGHTED) ©

2 CHAPTER ANALYSIS THE ABOUT TIME EXAM WEIGHTAGE • Very difficult chapter • 4 electrolytic cell set-up • 1 simple electric cell set-up • Usually tested in both MCQ & Paper 2 • Will need prior knowledge from other chapters: Oxidation & Reduction, Chemical Equations • Medium overall weightage • Constitute to 4% of marks for past 5 year papers DARRELL ER (COPYRIGHTED) ©

3 Chapter Overview MOLTEN ELECTROLYSIS & DILUTE ELECTRIC CELL SIMPLE ELECTRIC CELL This is probably one of the most difficult chapter in ‘O’ Level Chemistry. So let’s break down the chapter first before proceeding. CONCENTRATED - Only 2 ions present - Both ions will definitely be discharged - 4 ions present - Selectively discharge based on ease of discharge - 4 ions present - Cation follows normal ease of discharge - Anion follows ‘concentrated’ ease of discharge USING REACTIVE ELECTRODE - 4 ions present - Selectively discharged - Electroplating can occur if object is placed at cathode as metal cations would deposit itself onto the object - Use chemical reaction to produce electricity - Require 2 metals of differing reactivity - Does not require electric source DARRELL ER (COPYRIGHTED) ©

4 ELECTROLYTIC CELL MOLTEN IONIC COMPOUND AQUEOUS IONIC COMPOUND CONCENTRATED IONIC COMPOUND ELECTROPLATING All of the 4 scenarios falls under electrolytic cell. Let’s aim to get our basics right! KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

5 ELECTROLYTIC CELL ELECTROLYTIC CELL An electrolytic cell is the set-up used for electrolysis. Key components: Electrolyte is the ionic compound, either in its molten or aqueous state, that would dissociate into ions when a current is passed through it. Electrodes are made of a conductive material. For inert electrodes, it is usually made of lead or platinum. Anode is the positively-charged electrode that is connected to the positive terminal of the electrical source. (Anode is positively charged & attracts anions, hence its called ‘anode’.) Cathode is the negatively-charged electrode that is connected to the negative terminal of the electrical source. (Cathode is negatively charged & attracts cations, hence its called ‘cathode’.) Battery/power source causes electrons to always flow from the anode to the cathode. (For those who take physics, under ‘electricity’, recall that electron flow is from the negative terminal to the positive terminal of the battery.) DARRELL ER (COPYRIGHTED) ©

6 ELECTROLYTIC CELL ELECTROLYTIC CELL Anode is the positively-charged electrode, as it is connected to the positive terminal of the battery. During electrolysis, anions are attracted to the positively charged anode. Anions are negatively charged and are looking to lose electrons to become neutral. Hence, as anions lose electrons at the anode, they undergo oxidation. Cathode is the negatively-charged electrode, as it is connected to the negative terminal of the battery. During electrolysis, cations are attracted to the negatively charged anode. Cations are positively charged and are looking to gain electrons to become neutral. Hence, as cations gain electrons at the cathode, they undergo reduction. AOCR ANODE OXIDATION CATHODE REDUCTION I advise all students to try and understand this concept rather than memorising. Try and understand the logic. Opposite charges attract, so ions move to the respective electrodes that attract them. At the electrodes, they either gain or lose electrons, in order to become neutral again. It is a logical process that once you understand the ‘why’, you no longer need to memorise. DARRELL ER (COPYRIGHTED) ©

7 3 variations MOLTEN DILUTE CONCENTRATED All 3 variations of electrolytic cell follows the same concept you have learnt in the previous 2 slides. Let’s run through how the concept applies and what’s the difference between the 3 set-ups. DARRELL ER (COPYRIGHTED) ©

8 Electrolysis of molten ionic compound ELECTROLYSIS OF MOLTEN SODIUM CHLORIDE This is the most simple set up. There are only 2 ions present: Na+ and Cl-. Cl- goes to anode and gets oxidised to Cl2 (g). Na+ goes to cathode and gets reduced to Na (l). AOCR DARRELL ER (COPYRIGHTED) © Component Explanation Ions present Na+ , Cl- Observation Tiny globules of molten sodium, a silvery liquid, would begin to form at the cathode and float to the surface. Yellow-green chlorine gas is produced at the anode which would turn moist blue litmus paper red and bleaches it. At the anode (positive terminal) 2Cl- (l)  Cl2 (g) + 2e- Chloride ions are attracted to the anode and are oxidised to chlorine gas which will turn damp blue litmus paper red, and bleach it white. At the cathode (negative terminal) Na+(l) + e-  Na (l) Sodium ions are attracted to the cathode and are reduced to globules of liquid sodium metal. Overall change 2NaCl (l)  2Na (l) + Cl2 (g) (redox reaction)

9 EASE OF DISCHARGE Before we move on to the electrolysis of dilute aqueous solution, we need to understand ease of discharge. As there are usually the presence of 2 cations and 2 anions in a dilute aqueous solution, ease of discharge helps us identify which cation & anion gets selectively discharged. For cations, the less reactive is it on the reactivity series, the more easily it can get discharged. For anions, OH- is the easiest to discharge, followed by halogens, then the common anions. (Group VII, reactivity decreases down the group. The less reactive, the more easily it will get discharged.) However, when it is a concentrated solution/the solution becomes concentrated, concentration effect will result in the halogens to be preferentially discharged over OH- ions. EASE OF DISCHARGE *Quick tip: For a normal aqueous ionic compound (excluding copper), H+ & OH- is usually the ones who get discharged. DARRELL ER (COPYRIGHTED) © Cations (goes to cathode & undergo reduction) K+ Na+ Ca2+ Mg2+ Al3+ Zn2+ Fe2+ Pb2+ H+ Cu2+ Ag+ Au+ Anions (goes to anode & undergo oxidation) DILUTE CONCENTRATED SO4 2- SO4 2- NO3 - NO3 - F- OH- Cl- F- Br- Cl- l- Br- OH- l- Ease of discharge increases down the series

10 Electrolysis of dilute ionic compound ELECTROLYSIS OF DILUTE SODIUM CHLORIDE This is the most common set up. There are 4 ions present: Na+ H+, Cl- , OH- Therefore, we have to use the ease of discharge table to decide which ions get preferentially discharged. AOCR still applies. Anode Oxidise, Cathode Reduce. AOCR DARRELL ER (COPYRIGHTED) © Component Explanation Ions present Na+, Cl- , H+, OH- Observation When current is switched on, colourless hydrogen gas bubbles and is formed at the cathode while colourless oxygen gas bubbles and is formed at the anode. At the anode (positive terminal) 4OH- (aq)  O2 (g) + 2H2O (l) + 4e- Both Cl- and OH- ions are attracted to the anode, but OH- is preferentially discharged and is oxidised. Oxygen gas is formed, which will relight a glowing splint. At the cathode (negative terminal) 2H+ (aq) + 2e-  H2 (g) Both Na+ and H+ ions are attracted to the cathode, but H+ is preferentially discharged and is reduced. Hydrogen gas is formed, which will extinguish a lighted splint with a ‘pop’ sound. Overall change 2H2O (l)  O2 (g) + 2H2 (g) (redox reaction) Since only H+ and OH- ions are discharged and the pH of the solution remains the same. However, the solution becomes more concentrated.