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Predicting Spontaneous Reactions Chapter 19 Pages 768-773 • An electrochemical (voltaic) cell consists of an oxidation reaction and a reduction reaction to produce a voltage for the cell. Example: Batteries • Need to know: which electrode is the cathode, which is the anode, and whether the chemical reaction is spontaneous. •Anode Anode : negative electrode where oxidation occurs •Cathode Cathode :positive electrode where reduction occurs Electrons travel from the anode to the cathode, along a conductive wire . This is the voltage = electricity. Anode Cathode When choosing your anode and cathode follow this rule: • Look at the reduction potential for the ions, the cathode will be the more positive reduction potential (p.11 of data booklet) • Reason: Reason higher reduction potential means more easily reduced, therefore reduction takes place there Table of Standard Reduction Potentials • Shows the relative ability for a substance to be reduced (gain electrons). • It expresses this potential to gain electrons by assigning a voltage for each reduction half reaction in the table. Found on pages 10 & 11 in your data booklet Eonet= Eooxidation + Eoreduction • Eonet = total net voltage of the cell (electric potential energy of the cell) • Must be greater than zero for the reaction to be spontaneous • If less than zero, then non-spontaneous reaction and no electricity produced. • Eooxidation = oxidation half reaction taking place in the cell • Oxidation half reactions are the reverse reaction from the table on p.11, so their sign changes! changes Example: If reduction potential of Ag+ = 0.80V, then the oxidation potential of Ag(s) = - 0.80V • Eoreduction = reduction half reaction taking place in the cell • The larger the reduction potential (more positive), the greater the tendency for the reaction to occur. • The smaller the reduction potential (more negative), the least likely for a reaction to occur. Predicting Spontaneity • Looking at the table on pg 10/11, anything on the left of the arrow will react with anything below it to the right. • Will the following reactions occur? (a) 2 Ag (b) Mg 2+ ( aq ) + Cr( s ) → Mg ( s ) + Cr 2+ + ( aq ) + Cu( s ) → 2 Ag ( s ) + Cu 2+ Shorthand Notation • The shorthand notation for redox rxns start with the oxidation first, (LR from the half rxn) then the reduction (LR from the half rxn) separated by a double line. Ex: Zn | Zn 2+ + || Ag | Ag Oxidation || Reduction Sample Problem 1: 1 • Design an electrochemical cell that uses the reactions of the metals Zn and Ag, and solutions of their ions, Zn2+ and Ag+. a) Identify the anode and cathode b) Write the oxidation and reduction half reactions c) Calculate the cell voltage Solution: Zn+2 + 2e- Zn(s) -0.76V Ag+1 +1e- Ag(s) +0.80V • Ag+ more easily reduced, therefore Ag(s) is the cathode, so oxidation takes place at the anode (Zn) and the oxidation potential half reaction is: Zn(s) Zn+2 + 2e+0.76V a) and b) c) Eonet = Eooxidation + Eoreduction = (+0.76V) + ( +0.80V) = +1.56V Sample Problem #2: An electrochemical cell was designed using the metals Mg and Al and solutions containing Mg2+ and Al3+. a) According to the table, which ion/metal will be reduced? Oxidized? b) Identify the anode and cathode c) Write the oxidation and reduction half reactions d) Calculate the cell voltage e) Write the shorthand notation