Electrolytic Cells Lesson 8 Electrolytic Cells

ELECTROLYSIS Electrolysis is a method of using a direct electric current (DC) to drive an otherwise non- spontaneous chemical reaction. Electrolysis is commercially highly important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell

Electrolytic Cells Characteristics … 1.Nonspontaneous redox reaction 2.Produces chemicals from electricity 3.Forces electrolysis to occur An electrolytic cell is a system of two inert (nonreactive) electrodes (C or Pt) and an electrolyte connected to a power supply. It is just ONE cell…

Electrolytic Cell Oxidation always occurs at the anode and reduction at the cathode Electrons flow through the wire and go from anode to cathode Anions (- ions) migrate to the anode and cations (+ions) migrate towards the cathode. The electrode that is connected to the -ve terminal of the power supply will gain electrons and therefore be the site of reduction. Oxidation is connected to +ve terminal.

For electrolysis to work, you need a liquid sample: 1) molten cells Melt the crystals (produce a "molten" sample ) This is the only way to electrolyse insoluble salts, and is the only way to produce, by electrolysis, pure metals. 2) aqueous cells Dissolve the ionic substance in water (this is the most common)

Type 1 electrolytic cell Inert electrodes immersed in a molten ionic compound

1. Draw and completely analyze a molten NaBr electrolytic cell. Pt DC Power Source - + DC Power Source - + Electrons go from anode to cathode. Na + Br - _ reduction cathode 2Na + + 2e - → 2Na (l) v + oxidation anode 2Br - → Br 2(g) + 2e v 2Na + + 2Br - → Br 2(g) + 2Na (l) E 0 = v e-e- e-e- MTV = v The negative is reduction The positive is oxidation The MTV is the minimum theoretical voltage required to start a reaction Molten or (l) means ions but no water cations to cathode anions to anode

Type 1 electrolytic cell Inert electrodes immersed in a molten ionic compound Things are reversed…meaning: The oxidation half reaction is ABOVE the reduction half-reaction! Non-spontaneous…

Type 2 electrolytic cell Inert electrodes immersed in an aqueous ionic compound Things are different now, you have water to consider! Soooo…what do you do if there is water in the cell?

Reduction of water Water will undergo reduction above anything that is below this line. Below this line, they will undergo oxidation Pb 2+ undergoes reduction in water Water will undergo reduction in a K + solution Treat as if it were here

Reduction If you have water present (aq) 1.0 Msolution Consider the overpotential effect Take the higher reaction on the left The strongest oxidizing agent

Oxidation of water Treat as if here Br - (lower) will undergo oxidation before water Water (lower) undergoes oxidation before F -

Oxidation If you have water present (aq) 1.0 Msolution Consider the overpotential effect Take the lower reaction on the right side of the table The strongest reducing agent

1. Draw and completely analyze an aqueous KI electrolytic cell. Pt DC Power Source - + DC Power Source - + K+H2OI-K+H2OI- The negative is reduction The positive is oxidation Cationor water Reduction Cathode Consonants

highest For Reduction take the highest

1. Draw and completely analyze an aqueous KI electrolytic cell. Pt DC Power Source - + DC Power Source - + K+H2OI-K+H2OI- The negative is reduction The positive is oxidation Anionor water Oxidation Anode Vowels

lowest For Oxidation take the lowest

1. Draw and completely analyze an aqeuous KI electrolytic cell. Pt DC Power Source - + DC Power Source - + Electrons go from anode to cathode. K+H2OI-K+H2OI- _ reduction cathode 2H 2 O+2e - → H 2(g) + 2OH v + oxidation anode 2I - → I 2(s) + 2e v 2H 2 O + 2I - → H 2 + I 2(s) + 2OH - E 0 = v e-e- e-e- MTV = v The negative is reduction The positive is oxidation cations to cathode anions to anode

Overpotential: voltage actually required to drive electrolytic cells. Overpotential effect : a higher than normal voltage required for the half reaction. It can be caused by different reasons. In aqeous solutions, it is due to extra voltage required to produce a gas bubble such as (O 2 ) Final comments on electrolytic cells: Just like an electrochemical cell, they depend on the thermodynamic data of the reaction but ALSO…. They depend on the kinetic energy, activation energy and localized concentrations of the reactions involved and different designs of cells.

Practical applications of electrolysis Electrolytic cells are used extensively in mining and other metallurgy-related industries. –Electro-plating –Electro-refining –Electro-winning –Electrolytic production of Aluminum

Electro-winning It’s a metallurgical term for the electrolytic recovery of a metal from a solution containing its ions. The metal ions are reduced at the cathode where they deposit as metal. Places where this is done are called “smelters” In simple words ‘making metals’ !

Electroplating Purpose: to provide a surface property such as wear resistance, corrosion protection or lustre to a surface that lacks these properties. It is basically coating a conductive material, such as a metal, with a thin layer of a different metal. used in the manufacture of electronic and optic components and sensors, also in automobile parts, etc.

Zinc electroplating cost-effective a protective coating to metallic substances such as nuts, bolts, fasteners, automotive parts, and many other hardware items. the use of zinc also improves the overall appearance of the metals, by giving them a variety of colors, a clean look, brightness, and a nice glowing shine.

Electro refining It is the electrolytic purification of a metal. –The metal of an impure anode is oxidized and then the ions migrate to the pure cathode where they are reduced back to the metal. –Impurities are left behind (anode mud or slime)

Aluminum production(Heroult-Hall Process) Aluminium is the most abundant metallic element, but it is rarely found in its elemental state. Its primary commercial source is bauxite, a mixture of hydrated aluminium oxides and compounds of other elements such as iron.bauxite Aluminium was made by heating ore along with elemental Na or K in a vacuum. The method was complicated and expensive After initial purification, Al 2 O 3.3H 2 O is left. Water is driven off by heat. Left with Al 2 O 3 (Aluminum oxide, aka, alumina) Al 2 O 3 very impractical to electrolyze as its melting point is around 2050 C.

These two chemists came to the rescue!

Aluminum production (Heroult-Hall Process) Two 23-year-olds independently discovered that aluminum oxide dissolves in molten cryolite, Na 3 AlF 6, adding this “flux” would lower the temperature to 1000 C (A flux is a substance used to lower M.P of another substance) Molten aluminum is then electrolyzed using carbon electrodes. Aluminum is still very costly to produce as it consumes enormous electrical energy…

(Heroult-Hall Process)

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