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

1 Answer
Dec 9, 2015

Answer:

#E_(cell)^@=+0.47"V"#

Explanation:

You need to look up standard electrode potentials and list them -ve to +ve:

#" "E^@("V")#
#stackrel(color(white)(xxxxxxxxxxxxxxxxxx))(color(red)(larr))#

#Sn_((aq))^(2+)+2erightleftharpoonsSn_((s))" "-0.13#

#Cu_((aq))^(2+)+2erightleftharpoonsCu_((s))" "+0.34#
#stackrel(color(white)(xxxxxxxxxxxxxxxxxx))(color(blue)(rarr))#

You can see that the more +ve 1/2 cell will take in the electrons so the 1/2 cell reactions proceed in the direction indicated by the arrows.

Emf is an experimentally measured quantity and must always have a +ve value.

If you try to measure the emf of a cell and get a -ve reading, it means you have connected the voltmeter to the wrong terminals.

So to get the emf of the cell, always subtract the least positive potential from the most positive potential #rArr#

#E_(cell)^@=+0.34-(-0.13)=+0.47"V"#

I have adopted the convention which is used in the UK.

I understand other conventions would reverse the sign of the #Sn^(2+)"/"Sn# 1/2 cell then add.

Other conventions I have seen write the 1/2 cells in the other direction and reverse the voltage.