Page 328 - Physical chemistry understanding our chemical world

INTRODUCTION TO CELLS: TERMINOLOGY AND BACKGROUND 295

Potential/V Current drawn/A

Figure 7.4 Schematic diagram showing how a cell’s potential decreases with current. We call the
cell potential the emf only when the current is zero

resistance between the two electrodes is zero, which is clearly the case if they should
touch, then the cell potential is zero – we say the cell has ‘shorted’.
Ohm’s law, Equation (7.16), describes the difference between the emf and a voltage
under load:
V = IR (7.16)
where I is the current flowing, R is the resistance of the load and V is the decrease
in the voltage of the cell. When a current is drawn, the potential of the cell decreases
by the amount V in Equation (7.16). We will call this new (smaller) voltage E (load) ,
and its magnitude is given by

E (load) = emf − IR (7.17)
In summary, we say the voltage of a cell is the same as a cell’s
emf if determined at zero current. From Faraday’s laws of elec- The emf can only ever
trolysis, this criterion implies that none of the compositions within be determined at zero
the cell can change. In other words, a cell emf is an equilib- current.
rium quantity.
For this reason, it is not wise to speak of terms such as ‘anode’
of ‘cathode’ for a cell at equilibrium, because these terms relate Why a battery’s emf
to electrodes that give or receive charge during current flow; and
decreases permanently
our definition of equilibrium implies that no current does flows.
after a current has
We therefore adopt the convention: the terms ‘anode’ or ‘cath- flowed is explained on
ode’ will no longer be employed in our treatment of equilibrium p. 328.
electrochemistry.

What is a ‘standard cell’?
The thermodynamics of cells

A standard cell produces a precise voltage and, before the advent of reliable volt-
meters, was needed to calibrate medical and laboratory equipment. It is generally
agreed that the first standard cell was the Clark cell (see p. 299), but the most popular
was the Weston saturated cadmium cell, patented in 1893.