Review metal/metal ion couples in a cell

 
For the cell shown, the reaction that would occur if the voltmeter (a big resistance) were removed from the circuit and current were allowed to flow can be deduced from the polarities (signs) of the electrodes,

The positive electrode has the more positive potential.  Electrons would be consumed in reaction at this electrode.
Therefore in the tin-copper cell at copper:
Cu²⁺ + 2e^– Cu

The negative electrode has the more negative potential. Electrons would be generated in reaction at this electrode.
In the tin-copper cell at tin:
Sn Sn²⁺ + 2e^–

If the voltmeter were removed, electrons would flow from tin to copper, and anions would flow in the opposite direction.

The overall reaction that would occur is the sum of the electrode reactions.
One of these may be multiplied by a factor so that electrons lost equals electrons gained.

In the tin-copper cell:
Cu²⁺ + Sn Sn²⁺ + Cu 
 

Oxidant/Reductant	Eo/V
Cu2+(aq)/Cu	+0.32
Sn2+(aq)/Sn	-0.14

The tendency for this reaction to occur is given by the cell potential which is the difference in potentials of the positive electrode and the negative electrode.
In the tin-copper cell:
0.32 V - (-0.14 V) = 0.46 V