CHM152Chapter 17

Chemistry (MasteringChemistry Series)

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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’sa 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.
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2 2 Redox Reactions ± Oxidation: loss of electrons ± Reduction: gain of electrons ± 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.
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3 3 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
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4 4 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. Batteries
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CHM152Chapter 17 - Chapter 17 Electrochemistry Redox...

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