Electrochemical Reactions

Transcription

1 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.)

2 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

3 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.

4 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 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?

5 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?