Question

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?

Answer 1

`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"`