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Presentation On Electrolysis

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Published in: Chemistry | Science
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This PPT is suitable for the IGCSE and O Level Chemistry students. It only provides theoretical background of the topic. The practice questions, worksheets and topical questions will be discussed during the tutoring sessions

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  1. Electrolysis Chemistry (IGCSE / O-Level)
  2. What will you learn in this lesson? • About electrolysis, electrolytes and non-electrolytes, electrolytic cell • Electrolysis of Molten Ionic Compounds • Electrolysis of Salt Solutions • Electroplating
  3. Electrolysis The breaking down of an ionic compound in molten or in aqueous solution by the use of electricity Ionic compounds conduct electricity in molten or aqueous solution. E.g., molten lead bromide, sodium chloride solution and copper (Il) sulfate solution etc. Why ionic compounds do not conduct electricity in solid form? graB*e md e battery e heat if necessary graphite rod liquid under test Covalently bounded compounds do not conduct electricity in molten or aqueous solution E.g., ethanol, petrol (gasoline), sugar syrup and pure water etc. Apparatus mr testng the condudivity of liquids (an electrolytic cell).
  4. Electrolytes and Non-electrolytes • Electrolytes: An ionic compound that will conduct electricity when it is molten or dissolved in water. They do not conduct electricity when they are solid • Non-electrolytes: Liquids or solution that do not take part in electrolysis. They do not contain ions sulfuric acid molten lead bromide sodum chloride solution hydrochloric acid copper(ll) chloride solution sodum I-yd roxide solution distilled water ethanol petrol paraffin molten sulfur sugar solution
  5. Electrolytic cell bulb The points where the electric current enters or leaves the battery are called electrodes graphite rod The negative electrode is cathode Electrolyte battery graphite rod The positive electrode is anode liqui d under test heat if necessary Apparatus f'r testing the conductivity of liquids (an eEectrolytic cell).
  6. Electrolysis of Molten Ionic Compound switch battery or power pack graphite anode The electrical energy from the cell has caused a chemical change (decomposition of molten lead bromide into Pb2• and Bri-) graphite cathode molten chloride (PbBr.) 4-0 heat Movement of ions in of a molten salty lead(ll) bromide. Red-brown vapors of bromine formed at anode A bead of liquid lead collects at cathode
  7. Electrolysis of Molten Ionic Compound (contd.) Products of Molten, Binary Ionic Compounds The metal is always formed at the cathode (the negative electrode) The non-metal is always formed at the anode (the positive electrode) ectrolyte (compound o lysed) lead bromide, PbBr2 sodium chloride, NaCl potassium iodide, KI copper(ll) bromide, CuBr2 zinc chloride, ZnC12 aluminium oxide (A1203) Product at cathode (negative Product at anode (positive electrode) lead (Pb) sodium (Na) potassium (K) copper (Cu) zinc (Zn) aluminium (Al) electrode) bromine (Br2) chlorine (C12) iodine (12) bromine (Br2) chlorine (C12) oxygen (02) Table 6.3: Some examples of the electrolysis of molten salts.
  8. Electrolysis of Molten Ionic Compound (contd.) The movement of ions to different electrodes decompose the ionic compound. At anode, each Bri- ion donates an electron to become a bromine atom Then, two bromine atoms combine to form a bromine molecule: + Br2 Each Pb2+ ion accepts 2 electron to become a lead (Pb) atom + 2e— pb
  9. Electrolysis of Molten Ionic Compound (contd.) • During electrolysis, the flow of electrons continues through circuit. • The electrons received at anode flow through the circuit towards the cathode At cathode, metal ions (cations) gain electrons and discharged At anode, non-metal ions (anions except hydrogen) lose electrons and discharged
  10. Electrolysis of Molten Ionic Compound (contd.) Half Equations: Ionic equations showing the separate oxidation and reduction steps in redox reactions, including the reactions at the anode (oxidation) and cathode (reduction) in the electrolytic cell Two half equations are added to give the overall reaction in electrolytic cell Anode reaction involves two steps, the gain of electron and forming a molecule ectro yte lead bromide, PbBr2 sodium chloride, NaCl aluminium oxide (A1203) copper(ll) bromide, CuBr2 cat reaction pb2+ + 2e— pb cu2+ + 2e- cu 2cl- 202_ 23r- reaction Br2 + C12 02 + 4e—
  11. How electrons flow during electrolysis? • Electrons move through the wires of the external circuit under the influence of the battery of DC power source • The cations (positive ions) in the electrolyte move to the cathode erg where they gain electrons from the external circuit • The anions (negative ions) move to the anode where they lose electrons, and these electrons complete the flow of charge by moving through the external wiring to the positive pole of the battery 2Br— fom iheother. cathode brorrfråe + Bromine
  12. Electrolysis of Solutions The products from the electrolysis of solution of salts may be different from the electrolysis of molten salts. This is because water itself produces ions. most molecules intact only a very few molecules split into ions Not enough ions are produced to make water a good conductor of electricity
  13. Electrolysis of Dilute Sulfuric Acid • To make water a good conductor of electricity, few drops of sulfuric acid are added • Hofmann voltameter keeps the two gases oxygen and hydrogen separate • Oxygen and hydrogen produced in the ratio of 1:2 2H0(g) + 02(g) the catHe at the anode H+ ions move towards cathode and discharged. While OH- and S042-, move towards anode but only OH- gets discharged to give oxygen gas At the + 2e- Ha dilutesülfuricacid platinum electrodes Hofrnam voltameter for the electrolysis of elute sWfuric add. At the anode 40H— + 02 +
  14. Electrolysis of Salt Solutions • In salt solution, hydrogen ions will compete with metals ions to get discharged at the cathode • In concentrated salt solutions, halogens get discharged at anode Electrolysis of Concentrated Sodium Chloride Solution: • The four ions formed in concentrated NaCl solution will be: Na+, • Hydrogen is produces at cathode. Chlorine, a green gas is collected at anode instead of oxygen because Cl- ions are discharged
  15. Electrolysis of Salt Solutions (contd.) Chi. One CC At cathode, only H+ ions get discharged, not Na+ ions because sodium is more reactive than hydrogen and tends to stay in ionic form Overall reaction at cathode would be,
  16. Electrolysis of Salt Solutions (contd.) At anode, Cl- ions discharge more readily than OH- ions: Overall reaction at anode would be, 2CI- — C12 + 2e- Nat and OH' ions are left behind in the solution that creates an alkaline solution of NaOH. Overall reaction that takes place in electrolytic cell would be, 2NaC1(aq) + 2H20(l) — 2NaOH(aq) + H2(g) + C12(g) Similar results are obtained of the concentrated solution of other halides are electrolyzed.
  17. Electrolysis of Salt Solutions (contd.) The product at anode depends on the concentration of solution. If concentration is high chlorine or other halogen is formed at anode. If concentration of halide is low, oxygen is formed at the anode. OHI- ions will be discharged at anode in case of low concentration. concentrated NaCl(aq) concentrated KBr(aq) dilute NaCl(aq) dilute KBr(aq) 40H- *Z--hO+ 4e- Product at the cathode hydrogen hydrogen hydrogen hydrogen Product at the anode chlorine bromine oxygen
  18. Electrolysis of Salt Solutions (contd.) Electrolysis of Copper (Il) Sulfate Solution (with inert carbon/graphite electrodes): At cathode, Cu2 is discharged at cathode (Copper is less reactive than hydrogen) At anode, oxygen gas is produces (OH- ions are discharged rather than S042- ions) Attheanode Overall reaction would be the sum of half equations at anode and cathode, Copper is deposited at cathode while the solution becomes more acidic due to discharge of hydroxide ion
  19. Electrolysis of Salt Solutions (contd.) ConsideraSon M the resuks elearolysis experiments on salt solutions laads to some rules for ion disdlarge at the aectrod es: the cathode: • The more reactive a metil, more it tendsto stay as i0Nand not be discharged. The ionswil] acß«octrons instead. Hydrogen molecetes will be formed, leaving the ions of the reactive metal (e.g. Na— ions) in solution. • In con&ast.theionsoflessreactive metals (e.g. Cuze long) accept electrons readily and form metal atoms. In this cue, the metal will be discharged, leaving the ions in solution (Figure 6.11). NSW Mt2• A' • Far pwtive ions: more to R the anode: • If of a hilogen er— or E) are present in a high enough concentration, they give up electrons more readily than OH— ions van. Mücules of chlorine, bromine or iodine are formed. The OH— ions remain in solution. If no halogen lams are present, or the hA]lde solution is too dill*e, the OH— Ions will gtve up electrons. When OH— tons are disdharaed. •y.en is formed. Sulfate and nitrate ions are not discharged in preference to OH - ions. For negative ions:
  20. Electrolysis of Salt Solutions (contd.) (Il) Sulfate Electrolysis of Copper Solution (with copper electrodes): The cathode gains mass as copper is deposited on the electrode: At the cathode + 2er The anode, however loses mass as copper dissolves from the electrode: Atthe cu(s) + The idea of dissolving anodes is useful in electroplating and in the purification of copper •aihode• soju tion anode@ The color of electrolyte doesn't change
  21. Electroplating A process of electrolysis in which a metal object is coated (plated with a layer of another metal). It is done to prevent rusting, resists scratching and wear, and used to polish to give attractive look The basic rules for electroplating an object with the metal anodé+) The object to be plated must be made the cathode The electrolyte must be a solution of salt of metal to be plated on the object The anode is made of the strip of metal to be plated on the object metaTspmn ethOdé Solution (electrolyte)

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