1. Chapter 18 Notes1 3. standard reduction potentials by convention, half-reactions are written as reductions; to get the overall reaction, use right electrode minus left electrode example: Daniell cell oxidation compartment (left) Zn 2+ (aq)+e -  Zn(s) reduction compartment (right) Cu 2+ (aq)+e -  Cu(s) Cu 2+ (aq)+e -  Cu(s) - (Zn 2+ (aq)+e -  Zn(s)) Cu 2+ (aq)+Zn(s)  Zn 2+ (aq)+Cu(s) electric potentials, E°, arise from charge separation in the electrochemical cell

2. Chapter 18 Notes2 “standard” means elements in their standard states, aqueous ions at concentration of 1 M (really 1 m) potentials are intensive properties: doubling the reaction does not increase the charge separation and does not double the potential! if a reaction is reversed, the potential changes sign potentials for half-reactions are determined by assigning the reduction potential for H 1+ (aq)+1e -  1/2H 2 (g) to E°=0 V exactly; all other potentials are compared to this cell

3. Chapter 18 Notes3 Figure 18.3 measured cell potential is 0.76V assign half-cell potential for H 2 /H + electrode as 0.0 V half-cell potential for Zn/Zn 2+ is thus 0.76 V

4. Chapter 18 Notes4 Figure 18.4 measured cell potential is 0.34V assign half-cell potential for H 2 /H + electrode as 0.0 V half-cell potential for Cu/Cu 2+ is thus 0.34 V

5. Chapter 18 Notes5 example: Calculate the cell potential for the Zn(s)  Zn 2+ (aq)   Cu 2+ (aq)  Cu(s) cell. Zn has a higher reduction potential than Cu, so Zn is a stronger reducing agent (Zn will reduce Cu 2+ ) both Cu and Zn have higher reduction potentials that H 2 so both Cu and Zn can reduce H 1+

6. Chapter 18 Notes6