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ELECTROLYSIS

ELECTROLYSIS. Compare and contrast voltaic (galvanic) and electrolytic cells Explain the operation of an electrolytic cell at the visual, particulate and symbolic levels Include: molten and aqueous electrolytic cells. Additional KEY Terms. Electrolysis: “electro” – electricity

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ELECTROLYSIS

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  1. ELECTROLYSIS

  2. Compare and contrast voltaic (galvanic) and electrolytic cells • Explain the operation of an electrolytic cell at the visual, particulate and symbolic levels • Include: molten and aqueous electrolytic cells Additional KEY Terms

  3. Electrolysis: “electro” – electricity “lysis” – break Electrolytic cells • Process of using electricity to forceanonspontaneousreaction Different: it is nonspontaneous so the opposite metals are going to be reduced and oxidized from a voltaic cell • A direct current source (battery) is attached • to inert electrodes – C or Pt Different: electrodes donot take part in reaction – “plate out” or “lose mass”

  4. Inert electrodes must be chargedto attract theappropriate ions to the appropriate side Different: electrodes oppositely charged Same: e- still flow from anodeto cathode Same: anodeoxidized, cathodereduced • Anions migrate to (+) anode and are oxidized • Cations migrate toward (-) cathode to be reduced

  5. 1. Molten Electrolytic Cells • most metals occur naturally as compounds • NaCl , Cu2O, TiO2 ,CuF2 Unwanted elements of a compound are called “impurities” • forcingthe non-spontaneous redox is a common process for obtainingpuremetals Need veryhigh temperatures to make salts molten - “melt”

  6. Down's Cell: Na+(l) + Cl–(l) → Na(s) + Cl2(g) 2 Cl-(l) → Cl2(g) + 2e- 2 Na+(l) + 2e-→ 2 Na(s) Notice: stronger element – Cl is oxidized Notice: weaker element – Na is reduced E°R = -2.71 V E°O = -1.36 V Eocell = - 4.07 V A battery must supply 4.07 V of power to make this electrolytic cell work

  7. Electroplating: • current used to platelayer of metalonto another surface by reducing its ions • objectto be plated – cathode • desiredmetal – anode • immersed in a solution of same metal ions Only one element – the metal is oxidized and then reduces itself in second location

  8. Corrosion: • reaction of metalswithoxygen- spontaneous • metal – oxidized oxygen – reduced Most metals have reduction potentials below oxygen – except gold • Sacrificial anode – second “weaker” elementthat sacrifices its electrons and oxidizes to save metal

  9. Electrolysis of water: • Water can be bothreducedand oxidized Oxidation: 2 H2O(l) → 4 H+(aq) + O2(g)+ 4e– Reduction: 4 H2O(l)+ 4e–→ 4 OH–(aq) + 2 H2(g) Net: 6 H2O(l) → 2 H2(g)+ O2(g) + 4 H+(aq) + 4 OH–(aq) 2 These ions recombine to form 4 H2O Overall: 2 H2O(l) → 2 H2(g)+ O2(g)

  10. 2. Aqueous Electrolytic Cells: • 3 possible reactantsthat will compete for • electrons - cations, anions and water Because water is present – it can oxidize and reduce – you might not get what you expected in the reaction • must predictwhichsubstances will be most likely • oxidized and which reduced This is done by comparing the oxidation and reduction potentials of all possible reactants

  11. What are the products formed at each electrode during the electrolysis of aqueous KI? possible reactants: K+ I– H2O K+ cannot lose more electrons - only I – and H2O can be oxidized Oxidization: 2 I–(aq) → I2(s) + 2e– E°o = – 0.54V H2O(l) → 2 H+(aq) + ½ O2(g) + 2e– E°o = –1.23V • I- has most positive oxidation potential • solid iodine is formed at the anode

  12. possible reactants: K+ I– H2O I-cannot gain more electrons - only K+ and H2O could be reduced Reduction: K+(aq) + 1e– → K(s)E°R = – 2.93V 2 H2O(l) + 2e– → 2 OH-(aq) + H2(g)E°R = – 0.83V • H2Ohas most positive reduction potential • hydrogen gas is formed at the cathode Overall: 2 H2O(l) + 2 I–(aq) → H2(g) + I2(s) Spectator ions created in the reaction would be K+and OH-

  13. CAN YOU / HAVE YOU? • Compareand contrast voltaic (galvanic) and electrolytic cells • Explain the operation of an electrolytic cell at the visual, particulate and symbolic levels • Include: molten and aqueous electrolytic cells Additional KEY Terms

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