The Electrochemical Corrosion Corrosion of Iron • Corrosion-An oxidization of a metal, and the oxide flaking off. • All Learning. Cengage © Copyright reserved rights Oxidized metal is commonly called rust 3 • Most commonly oxygen will oxidize a Corrosion metal. • Either by • [Metal] + O2 → [Metal]O • Or • [Metal] + H2O → [Metal]O + H2 Resisting corrosion Examples Steel • Most metals resist corrosion by an oxide • Aluminum very readily loses electrons. • Steel corrodes very readily because iron layer forming on the outside that protects • You would expect it to “rust” easily. oxide doesn’t stick to the surface. the metal inside. • However, aluminum is a very useful metal • It instead falls off exposing new metal to be • It protects the inside metal by preventing because it doesn’t corrode like other metals oxidized. the oxygen (or other oxidizing agent) from can. • This makes iron less useful and explains being able to reach it. • An aluminum oxide layer forms on the outside, why ancient people would prefer other stopping further oxidation from occurring. metals. • This oxide gives aluminum a dull color. • However, the abundance and other properties of iron have made it useful. Galvanic corrosion Galvanic corrosion Preventing oxidation • Two different metals placed next to each other • Iron can be protected by painting the with an electrolytic solution connecting will cause surface or coating it with a different an oxidation reduction reaction to occur. material to prevent the corrosion. • Just like the galvanic cell. • Galvanized steel is steel coated with zinc • Electrons will flow from a more active metal to a to prevent oxidation. less active metal. • Zinc actually oxidizes more readily than • One metal will end up oxidizing the other, but in iron. the process will itself become reduced. • This rapidly oxidized or rusts the one metal but prevents the less active metal from oxidizing (rusting) 1 Galvanic corrosion Cathodic Protection High temperature corrosion • You can also see galvanic corrosion on a • An oxidation reaction like any other battery. reaction occurs faster when heated. • Batteries that are hooked up to a circuit All Learning. Cengage © Copyright reserved rights • Metals that are constantly heated tend to for an extended period of time tend to 11 rust more quickly. become rusted. Noble metals Electrolysis Refining metals • There are certain metals that don’t form • Metals are found as metal oxides (ores) in • an oxide. Electrolysis-Forcing a current through to nature commonly. produce a chemical reaction. • Gold and silver are noble metals. • An electrolysis reaction is commonly used • Water can be electrolysized • Silver will oxidize with sulfur, but not with to produce metals from these ores. • → H + O oxygen. H2O 2 2 • Sodium metal can be produced by melting • Gold does not readily oxidize in nature. • This reaction is very important for fuel cell sodium chloride and passing an electric cars. current through the melt. • It uses electricity to create a combustible fuel for an internal combustion engine. Hall-Heroult Process Charles Hall Electrolysis •An electrolytic cell uses electrical • A student in a chemistry course at Oberlin energy to drive process. • Before 1886 aluminum was a very expensive • College in Ohio was told by his professor, The process is called electrolysis, metal. which involves forcing a current that if anyone could a cheap method to • Even though it is very abundant on the Earth’s through a cell to produce a chemical manufacture aluminum from bauxite they surface, it is only found as bauxite, an oxide. reaction for which the cell potential is could make a fortune. negative. • Since aluminum is so reactive no reducing agent • Using crude galvanic cells Charles Hall was •Now the current is doing work on the could easily turn the ore into a metal. able to achieve this using an electrolysis reaction, instead of the reaction • It was so valuable the Napoleon served his reaction. producing a current. honored guests aluminum silverware and gave •It is very similar to a galvanic cell, but • Yes, he did make a fortune with it. the others gold or silver. everything is reversed. 2 Stoichiometry of Electroplating Problem Problem by electrolysis •Faraday’s Law of Electrolysis: the amount • Using a current of 4.75 A, how many • Using a current of 3.75 A, how many of a substance produced at each electrode minutes does it take to plate 1.50 g minutes does it take to plate 1.50 g is directly proportional to the amount of Cu onto a sculpture from a CuSO4 Cr onto a bar from a Cr(NO3)3 electric charge flowing through the cell. solution? solution? • The SI Unit of current is the ampere (A). •1 ampere = 1 coulomb/second or 1 A = 1 C/s •Applying the Relationship Among Current, Time, and Amount of a Substance. Problem Shortcut plating equation Batteries history • What mass of gold will plate onto a • This is not on the equation sheet • Battery- combination on 2 or more electrochemical cells that convert chemical piece of jewelry from Au(NO3)3 M I t • m = or t = energy into electrical energy. solution when a 6.4 A current is run for 2.5 minutes? • m – mass • Luigi Galvini and Allesandro Volta are credited with the invention of the first batteries. • M – molar mass • Galvini came up with the galvanic cell. Volta • I – current (A) connected them together in a series. • t = time (s) • The name battery was coined by Benjamin • n – moles of electrons transferred Franklin, because the batteries at the time were a series of connected jars which • F= Faraday’s constant reminded him of a battery of cannons. Types of batteries The electrolyte Why not HCl • Two major types are: • This is the salt bridge discussed earlier. • HCl would be a very poor choice because • Wet Cell batteries- use a liquid electrolyte • It allows ions to flow freely while the of the redox reaction to allow the ions to freely exchange during electrons travel across our load, the thing • 2 HCl → H2 + Cl2 the redox reaction. you are trying to power. • Hydrogen typically gets reduced • Car batteries or batteries with a liquid • The electrolyte normally needs to be acidic + - • 2 H +2e → H2 inside. or basic to make the redox reaction occur. • But chlorine getting oxidized is very • Dry Cell battery- use a paste that • Sulfuric acid is commonly used, it is dangerous immobilizes the electrolyte. commonly called battery acid. - - • 2 Cl → 2e + Cl2 • AA, AAA, C, D, 9V etc. • Because of the poisonous gas produced. 3 Wet Cell Batteries Lead-Acid • Car batteries are wet cell batteries. • The standard battery used in a car was invented in One of the Six • The obvious problem with these batteries 1859 by Gaston Planté. Cells in a 12–V is the need to be keep them upright or the • It uses a Lead plate and a Lead Dioxide plate in a All Learning. Cengage © Copyright reserved rights sulfuric acid solution. Lead Storage 30 electrolyte, sulfuric acid, will leak out. • Here is the unbalanced redox reaction Battery • However the power they produce is quite • Pb + PbO + H SO ⇌ PbSO substantial. 2 2 4 4 • Reduction half • PbO2 + H2SO4 ⇌ PbSO4 • Oxidation half • Pb +H2SO4 ⇌ PbSO4 Rechargeable Alkaline Batteries A Common Dry Cell Battery • The nice thing about this battery is it is • Normal AA AAA C and D batteries are alkaline. easily rechargeable. • These are dry cell batteries • PbSO will readily form Pb and PbO if All Learning. Cengage © Copyright reserved rights 4 2 • electric current is added back to the cell. The reaction is 33 • This happened completely by chance since • Zn + MnO2 →ZnO + Mn2O3 there was no practical way to recharge the • This occurs in a paste of KOH. battery when it was invented. • http://www.energizer.com/learning-center/Pages/how-batteries-work.aspx • Later the generator would be invented and • This reaction is not reversible! from that a car’s alternator and easily recharge the battery while you drive. These may leak if you try to Strangely enough Lithium Ion Batteries recharge them. • A single AA, AAA, C or D “battery” is not a • Commonly used in cell phones, laptops battery by definition. and other portable electronic devices. • They are all single cells. • Not to be confused with Lithium single use • They are not a battery until you connect them batteries (like energizer e2). together, like you have to in most devices. • These batteries are rechargeable. • A 9 V battery is a battery because it has 6 cells linked together in the rectangular case. • There use a lithium compound as the • Car batteries also have 6 cells linked together. cathode and variety of possibilities for the anode material. 4 Li-Ion Lithium Ion Batteries Other batteries • Zinc-carbon battery - Also known as a standard carbon battery, zinc-carbon chemistry is used in • These batteries are very light for the all inexpensive AA, C and D dry-cell batteries. power the produce The electrodes are zinc and carbon, with an • acidic paste between them that serves as the They can be built to a variety of shapes to electrolyte. fit their device. • Nickel-cadmium battery (NiCd)- The electrodes • Over time, the battery will not be able to are nickel-hydroxide and cadmium, with hold as much of a charge so it will need to potassium-hydroxide as the electrolyte (rechargeable). be recharged more often. • Nickel-metal hydride battery (NiMh)- This • It will take less time to recharge when this battery is rapidly replacing nickel-cadmium occurs. because it does not suffer from the memory effect that nickel-cadmiums do (rechargeable). Other batteries A Mercury Battery • Lithium-iodide battery - Lithium-iodide chemistry is used in pacemakers and hearing aides because of their long life.