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Unformatted text preview: 1 1 Chapter 17 Electrochemistry Redox Reactions ¡ Electrochemistry is the study of batteries and interconversion of chemical and electrical energy. ¡ Based on redox (oxidation-reduction) reactions in which one element gains electrons and another loses electrons. ¡ These two processes MUST happen together. ¡ Have to assign oxidation numbers to determine (see Chapter 4). Oxidation Numbers ¡ Elements in their natural states are 0. ¡ Elements in binary ionic compounds are the same as their charges. ¡ H: usually +1 except with alkali metals it’s a ¡ H: usually +1, except with alkali metals it s a -1(LiH, NaH, etc.) ¡ O: usually -2, except in peroxides it’s a -1 (H 2 O 2 , K 2 O 2 ) ¡ The non-oxygen element in a polyatomic ion has to be determined from the other oxidation numbers. Redox Reactions ¡ Oxidation: loss of electrons ¡ Reduction: gain of electrons ¡ LEO the lion goes GER or OIL RIG ¡ LEO the lion goes GER or OIL RIG ¡ The substance oxidized is also called the reducing agent (it caused the other substance to be reduced). And vice versa. ¡ Cu(s) + AgNO 3 Æ Ag(s) + Cu(NO 3 ) 2 Cu(s) + AgNO 3 (aq) Redox Reactions ¡ CH 4 (g) + O 2 (g) Æ CO 2 (g) + H 2 O(g) ¡ Na(s) + HCl(aq) Æ NaCl(aq) + H 2 (g) ¡ For each reaction: ¡ What is oxidized, reduced? ¡ What is the oxidizing agent? Reducing agent? ¡ Combustion, decomposition, combustion, and single-replacement reactions are all redox. 2 2 Balancing Redox Reactions ¡ Cr 3+ (aq) + Be(s) Æ Cr(s) + Be 2+ (aq) ¡ To balance (notice the charges), we break it up into two half reactions , the oxidation half and reduction half. ¡ We have to balance electrons. ¡ Oxidation: (Be(s) Æ Be 2+ (aq) + 2e- ) x 3 ¡ Reduction: (Cr 3+ (aq) + 3e- Æ Cr(s)) x 2 ¡ 2Cr 3+ (aq) + 3Be(s) Æ 2Cr(s) + 3Be 2+ (aq) Group Quiz #14 ¡ Write the ½ reactions for each of the following. Also designate which element is oxidized and which one is reduced. ¡ Zn (s) + Fe(NO 3 ) 2(aq) Æ Fe (s) + Zn(NO 3 ) 2(aq) ¡ Mg (s) + AgNO 3(aq) Æ Ag (s) + Mg(NO 3 ) 2(aq) Galvanic Cells Galvanic Cells ¡ Oxidation reaction: occurs at the anode (mass decreases over time) ¡ Reduction reaction: occurs at the cathode (mass increases over time (mass increases over time) ¡ Electrons are transferred across a wire (through a voltmeter) ¡ Voltage is the cell potential, E (or Electromotive Force, EMF) ¡ Salt bridge - soluble salt solution used to neutralize the ions that build up in each solution....
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