Electrchemical Reactins The first chemical prcess t prduce electricity was described in 1800 by the Italian scientist Alessandr Vlta, a frmer high schl teacher. Acting n the hypthesis that tw dissimilar metals culd serve as a surce f electricity, Vlta set up a reactin between tw metals (silver and zinc) with different electrn affinities. Small plates f silver and zinc were separated by a pad f absrbent material saked in saltwater. The saltwater wuld cnduct electricity, and therefre allw charge t transfer frm ne metal t the ther if the circuit was cmpleted (by a direct cnnectin between the tw metal plates, such as a wire). This type f setup is called a galvanic cell. Galvanic cell: a chemical reactin invlving electrn transfer, arranged s that the electrns must transfer thrugh an external wire in rder t cmplete the circuit, thus generating electrical current. (The chemical reactin causes the electric current t flw.) T increase the flw f electricity, Vlta stacked several f these reactins ne n tp f anther, creating a pile f alternating silver and zinc plates (separated by pads f absrbent material saked in saltwater). When Vlta mistened his fingers and tuched the tp and bttm metal plates, he experienced small electric shcks. Vlta s creatin the vltaic pile, as it came t be called was the first battery. Battery: a repeating pile f galvanic cells. Piling galvanic cells (arranging them in series) generates greater vltage, allwing batteries t act as a practical surce f electricity. Within mnths, William Nichlsn and Anthny Carlisle in England attempted t cnfirm the prductin f electric charges n the upper and lwer plates in a vltaic pile using an electrscpe. In rder t cnnect the plates t the electrscpe, Nichlsn and Carlisle added sme water t the uppermst metal plate and inserted a wire t the electrscpe. T their surprise, Nichlsn and Carlisle bserved the frmatin f a gas, which they identified as hydrgen. Nichlsn and Carlisle then filled a small tube with river water and inserted wires frm the vltaic pile int each end f the tube. Tw different gases were generated, ne at each wire. Nichlsn and Carlisle had discvered electrlysis the electrical current caused water mlecules t decmpse int hydrgen and xygen gases. Electrlytic cell: a chemical reactin invlving electrn transfer which nly prceeds if an external surce f electrical current (electricity) is applied. (The electric current causes the reactin t ccur.)
Batteries in Real Life A regular Duracell r Energizer battery always has a label shwing which end is psitive and which is negative s that yu can line it up crrectly in the devices it pwers. These tw ends, called electrdes, wuld be the tp piece f silver and the bttm piece f zinc in the vltaic pile r the tw wires in the water in the Nichlsn/Carlisle electrlysis. In chemistry, we specify these tw electrdes as the ande (where xidatin ccurs in galvanic cells) and the cathde (where reductin ccurs in galvanic cells). Fr a battery (galvanic cell) t run, there must be an electrlyte (a cnductive slutin) cnnecting the tw electrdes. In bth Vlta s pile and the Nichlsn/Carlisle electrlysis, salt water served this purpse. (Remember frm last unit: inic substances are best at disslving in water and cnducting electricity.) Mdern batteries use a variety f chemicals t pwer their reactins. Cmmn battery chemistries include: Primary Cells (nn-rechargeable batteries): Zinc-carbn battery: The zinc-carbn chemistry is cmmn in many inexpensive AAA, AA, C and D dry cell batteries. The ande is zinc, the cathde is manganese dixide, and the electrlyte is ammnium chlride r zinc chlride. Alkaline battery: This chemistry is als cmmn in AA, C and D dry cell batteries. The cathde is cmpsed f a manganese dixide mixture, while the ande is a zinc pwder. It gets its name frm the ptassium hydrxide electrlyte, which is an alkaline substance. Secndary Cells (rechargeable batteries): Lithium-in battery (rechargeable): Lithium In is the mst cmmn rechargeable battery type fr high-perfrmance devices, such as cell phnes, digital cameras and even electric cars. A variety f substances are used in lithium batteries, but a cmmn cmbinatin is a lithium cbalt xide cathde and a carbn ande. (Nickel-metal hydride (NiMH) and nickel-cadmium (NiCd) batteries were als nce very prevalent, but have becme less cmmn as LiOn batteries have becme easier t prduce.) Lead-acid battery (rechargeable): This is the chemistry used in a typical car battery. The electrdes are usually made f lead dixide and metallic lead, while the electrlyte is a sulfuric acid slutin. Nn-rechargeable batteries, r primary cells, and rechargeable batteries, r secndary cells, prduce current exactly the same way: thrugh an electrchemical reactin invlving an ande, cathde and electrlyte. In a rechargeable battery, hwever, the reactin is reversible. When electrical energy frm an utside surce is applied t a secndary cell, the negative-t-psitive electrn flw that ccurs during discharge is reversed, and the cell's charge is restred. (While attached t the charger, the reactin is electrlytic instead f galvanic.) When it cmes t rechargeable batteries, nt all batteries are created equal. NiCd batteries were amng the first widely available secndary cells, but they suffered frm an incnvenient prblem knwn as
the memry effect. Basically, if these batteries weren't fully discharged every time they were used, they wuld quickly lse capacity. NiCd batteries were largely phased ut in favr f NiMH batteries. These secndary cells bast a higher capacity and are nly minimally affected by the memry effect, but they dn't have a very gd shelf life. Like NiMH batteries, LiOn batteries have a lng life, but they hld a charge better, perate at higher vltages, and cme in a much smaller and lighter package. Essentially all high-quality prtable technlgy manufactured these days takes advantage f this technlgy. Hwever, LiOn batteries are nt currently available in standard sizes such as AAA, AA, C r D, and they're cnsiderably mre expensive than their lder cunterparts. With NiCd and NiMH batteries, charging can be tricky. Yu must be careful nt t vercharge them, as this culd lead t decreased capacity. T prevent this frm happening, sme chargers switch t a trickle charge r simply shut ff when charging is cmplete. NiCd and NiMH batteries als must be recnditined, meaning yu shuld cmpletely discharge and recharge them again every nce in a while t minimize any lss in capacity due t the memry effect. LiOn batteries, n the ther hand, have sphisticated chargers that prevent vercharging and never need t be recnditined. Even rechargeable batteries will eventually die, thugh it may take hundreds f charges befre that happens. When they finally d give ut, be sure t dispse f them at a recycling facility, as they cntain substances that can be dangerus if put in a landfill with regular garbage. Based n this reading and the infrmatin with the questins n the reverse, write definitins fr: Electrlytic Cell, Galvanic Cell, Battery, Electrlyte, Oxidatin, and Reductin in yur ntebk fr future reference.
Name: Perid: Intr t Batteries and Electrchemistry 1. Name the scientist(s) wh used a chemical reactin t generate an electrical current. 2. Name the scientist(s) wh used an electrical current t generate a chemical reactin. 3. What tw elements were used in Vlta s reactin (the vltaic pile)? 4. What is piled in a vltaic pile? (Use the scientific term.) 5. Why d yu think Vlta did nt feel a shck frm his reactin until he created a pile? 6. What tw elements are prduced in the Nichlsn/Carlisle reactin? 7. Write a balanced chemical reactin fr the electrlysis reactin perfrmed by Nichlsn & Carlisle. (Hint: the nly reactant is H 2O; there are nly tw prducts bth are elements in their diatmic states because they are prduced as gases.) 8. In a water mlecule, what is the charge f a hydrgen atm? 9. In a water mlecule, what is the charge f an xygen atm? 10. In diatmic hydrgen mlecules (H 2), the hydrgen atms have a charge f zer. (Elements always have a charge f zer in their elemental state.) In the reactin belw, determine the number f electrns that must be prduced t accunt fr the charge difference between the tw types f hydrgen. 2 H 1+ + e - _ 1 _ H 2 0 11. Electrchemical reactins always invlve bth an xidatin cmpnent in which an element s charge increases and a reductin cmpnent in which a (different) element s charge decreases. Is the reactin in the previus questin an xidatin r a reductin?
12. Which type f electrchemical reactin requires an applied current r vltage t prceed? 13. Which type f electrchemical reactin prduces a current r vltage when it ccurs? 14. Use #10 t determine the answer t circle in each statement: a. Reductin causes charge t increase/decrease. Electrns are left/right f the arrw. b. Oxidatin causes charge t: increase/decrease. Electrns are left/right f the arrw. 15. Is a battery a galvanic cell r an electrlytic cell? 16. Given that a battery is a type f chemical reactin, why d batteries eventually die? 17. What is the difference between a primary cell and a secndary cell? 18. What is necessary (in terms f the chemical reactins invlved) in rder fr a battery t be rechargeable? 19. What type f battery is used in autmbiles? 20. In an alkaline battery: a. What substance is the cathde? b. What substance is the ande? c. What is the electrlyte? 21. What substance served as the electrlyte in Vlta s vltaic pile? 22. What is the memry effect? 23. Which types f battery is mst affected by the memry effect? 24. Which rechargeable battery is least affected by the memry effect?