# 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?

Dec 9, 2015

${E}_{c e l l}^{\circ} = + 0.47 \text{V}$

#### Explanation:

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

" "E^@("V")
$\stackrel{\textcolor{w h i t e}{\times \times \times \times \times \times \times \times \times}}{\textcolor{red}{\leftarrow}}$

$S {n}_{\left(a q\right)}^{2 +} + 2 e r i g h t \le f t h a r p \infty n s S {n}_{\left(s\right)} \text{ } - 0.13$

$C {u}_{\left(a q\right)}^{2 +} + 2 e r i g h t \le f t h a r p \infty n s C {u}_{\left(s\right)} \text{ } + 0.34$
$\stackrel{\textcolor{w h i t e}{\times \times \times \times \times \times \times \times \times}}{\textcolor{b l u e}{\rightarrow}}$

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 $\Rightarrow$

${E}_{c e l l}^{\circ} = + 0.34 - \left(- 0.13\right) = + 0.47 \text{V}$

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

I understand other conventions would reverse the sign of the $S {n}^{2 +} \text{/} S n$ 1/2 cell then add.

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