What is the standard emf of a galvanic cell made of a Cd electrode in a 1.0 M #Cd(NO_3)_2# solution and a Cr electrode in a 1.0 M #Cr(NO_3)_3# solution at 25°C?

1 Answer
Ernest Z.
Apr 3, 2017

#E_text(cell)^°= "0.34 V"#

Explanation:

You first look up the standard reduction potential for each half-reaction.

#bb("Standard Reduction Potentials"color(white)(m)E°//"V"#
#"Cd"^"2+""(aq)" + "2e"^"-" → "Cd(s)" color(white)(mmmmmml)"-0.403"#
#"Cr"^"3+""(aq)" + "3e"^"-" → "Cr(s)"color(white)(mmmmmmm)"-0.74"#

Now, you convert one of these into an oxidation half-reaction by reversing the equation and changing the sign of #E°#.

You choose the one that gives you a positive cell potential when you add the two voltages. In this case, you reverse the bottom one.

#bb(color(white)(mmmmmmmmmmmmmmmmmmmll)E°//"V")#
#3×["Cd"^"2+""(aq)" + "2e"^"-" → "Cd(s)"] color(white)(mmmmmml)"-0.403"#
#2×["Cr(s)" → "Cr"^"3+""(aq)" + "3e"^"-"]color(white)(mmmmmmml)0.74#
#stackrel(————————————————————)(3"Cd"^"2+""(aq)" + "2Cr(s)" → "3Cd(s)" + "2Cr"^"3+""(aq)")color(white)(ll)stackrelcolor(blue)(——)(bb0.34)#

#E_text(cell)^°= "0.34 V"#

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