3. standard reduction potentials by convention, half-reactions are written as reductions;

1. 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) Zn2+(aq)+e-Zn(s) • reduction compartment (right) Cu2+(aq)+e- Cu(s) • Cu2+(aq)+e- Cu(s) • -(Zn2+(aq)+e-Zn(s)) • Cu2+(aq)+Zn(s) Zn2+(aq)+Cu(s) • electric potentials, E°, arise from charge separation in the electrochemical cell Chapter 18 Notes

2. “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 • H1+(aq)+1e-1/2H2(g) • to E°=0 V exactly; all other potentials are compared to this cell Chapter 18 Notes

3. measured cell potential is 0.76V • assign half-cell potential for H2/H+electrode as 0.0 V • half-cell potential for Zn/Zn2+ is thus 0.76 V Figure 18.3 Chapter 18 Notes

4. measured cell potential is 0.34V • assign half-cell potential for H2/H+electrode as 0.0 V • half-cell potential for Cu/Cu2+ is thus 0.34 V Figure 18.4 Chapter 18 Notes

5. example: • Calculate the cell potential for the • Zn(s)Zn2+(aq)  Cu2+(aq) Cu(s) • cell. • Zn has a higher reduction potential than Cu, so Zn is a stronger reducing agent (Zn will reduce Cu2+) • both Cu and Zn have higher reduction potentials that H2 so both Cu and Zn can reduce H1+ Chapter 18 Notes

6. Chapter 18 Notes