BByy Dr Anand Gupta Mr. Mahesh Kapil Dr. Anand Gupta 09356511518 09888711209 [email protected] [email protected] Electrochemistry EElleeccttrroollyyssiiss Electric energy Chemical energy GGaallvvaanniicc cceellll 2 Electrochemistry The study of the interchange of chemical and electrical energy. Chemical Energy Electrical energy I: A science studying the relationship between chemical energy and electrical energy and the rules of conversion of two energies. II: Electrochemistry is the study of solutions of electrolytes and of phenomena occurring at . electrodes immersed in these solutions EEEllleeeccctttrrrooonnn TTTrrraaannnsssfffeeerrr RRReeeaaaccctttiiiooonnnsss (cid:1)Electron transfer reactions are oxidation-reduction or Redox reactions. (cid:1)Results in the generation of an electric current (electricity) or be caused by imposing an electric current. (cid:1)Therefore, this field of chemistry is often called ELECTROCHEMISTRY. 4 Electrochemical processes are oxidation-reduction reactions in which: • the energy released by a spontaneous reaction is converted to electricity or • electrical energy is used to cause a nonspontaneous reaction to occur 0 0 2+ 2- 2Mg (s) + O (g) 2MgO (s) 2 2Mg 2Mg2+ + 4e- Oxidation half-reaction (lose e-) O + 4e- 2O2- Reduction half-reaction (gain e-) 2 5 TTTeeerrrmmmiiinnnooolllooogggyyy fffooorrr RRReeedddoooxxxRRReeeaaaccctttiiiooonnnsss (cid:1)(cid:1)OOXXIIDDAATTIIOONN——lloossss ooff eelleeccttrroonn((ss)) bbyy aa ssppeecciieess;; (cid:1)OXIDATION—loss of electron(s) by a species; iinnccrreeaassee iinn ooxxiiddaattiioonn nnuummbbeerr;; iinnccrreeaassee iinn ooxxyyggeenn.. increase in oxidation number; increase in oxygen. (cid:1)(cid:1)RREEDDUUCCTTIIOONN——ggaaiinn ooff eelleeccttrroonn((ss));; ddeeccrreeaassee iinn (cid:1)REDUCTION—gain of electron(s); decrease in ooxxiiddaattiioonn nnuummbbeerr;; ddeeccrreeaassee iinn ooxxyyggeenn;; iinnccrreeaassee iinn oxidation number; decrease in oxygen; increase in hhyyddrrooggeenn.. hydrogen. (cid:1)(cid:1)OOXXIIDDIIZZIINNGG AAGGEENNTT——eelleeccttrroonn aacccceeppttoorr;; ssppeecciieess iiss (cid:1)OXIDIZING AGENT—electron acceptor; species is rreedduucceedd.. ((aann aaggeenntt ffaacciilliittaatteess ssoommeetthhiinngg;; eexx.. TTrraavveell reduced. (an agent facilitates something; ex. Travel aaggeennttss ddoonn’’tt ttrraavveell,, tthheeyy ffaacciilliittaattee ttrraavveell)) agents don’t travel, they facilitate travel) (cid:1)(cid:1)RREEDDUUCCIINNGG AAGGEENNTT——eelleeccttrroonn ddoonnoorr;; ssppeecciieess iiss (cid:1)REDUCING AGENT—electron donor; species is ooxxiiddiizzeedd.. oxidized. 6 Another way to remember (cid:1) OILRIG ee xx ss oo ss aa dd ii ss ii uu dd ee nn cc aa tt tt ii ii oo oo nn nn 7 Review of Oxidation numbers The charge the atom would have in a molecule (or an ionic compound) if electrons were completely transferred. 1. Free elements (uncombined state) have an oxidation number of zero. Na, Be, K, Pb, H , O , P = 0 2 2 4 2. In monatomic ions, the oxidation number is equal to the charge on the ion. Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2 3. The oxidation number of oxygen is usually –2. In H O 2 2 and O 2- it is –1. 2 8 4. The oxidation number of hydrogen is +1 except when it is bonded to metals in binary compounds. In these cases, its oxidation number is –1. 5. Group IA metals are +1, IIA metals are +2 and fluorine is always –1. 6. The sum of the oxidation numbers of all the atoms in a molecule or ion is equal to the charge on the molecule or ion. HCO - 3 Oxidation numbers of all O = -2 H = +1 the atoms in HCO - ? 3 3x(-2) + 1 + ? = -1 C = +4 9 Galvanic Cells anode cathode oxidation reduction - + spontaneous redox reaction 10