Chapter 18 Notes 1

3. standard reduction potentials

•by convention, half-reactions are written as reductions; to get the overall reaction, use

right electrode minus left electrodeexample: Daniell celloxidation 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

Figure 18.3 •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

Chapter 18 Notes 4

Figure 18.4 •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

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