4.4 CHAPTER 4. ELECTROCHEMICAL REACTIONS
DEFINITION: Standard emf (E◦cell)
Standard emf is the emfof a voltaic cell operating under standard con-
ditions (i.e. 100 kPa, concentration = 1 mol.dm−^3 and temperature =
298 K). The symbol◦denotes standard conditions.When we want to represent this cell, it is shownas follows:
Zn|Zn2+(1mol· dm−^3 )||Cu2+(1mol· dm−^3 )|CuThe anode half cell (where oxidation takes place) is alwayswritten on the left. The
cathode half cell (where reduction takes place) is always written on the right.
It is important to note that the potential difference across a cell is relatedto the extent
to which the spontaneous cell reaction has reached equilibrium. In otherwords, as the
reaction proceeds and the concentration of reactants decreases and the concentration of
products increases, the reaction approaches equilibrium. When equilibrium is reached,
the emf of the cell is zero and the cell is said tobe ’flat’. There is no longer a potential
difference between thetwo half cells, and therefore no more current will flow.
Uses of standard electrode potential ESCBY
Standard electrode potentials have a number ofdifferent uses.
Calculating the emf of an electrochemical cell
To calculate the emf of acell, you can use any one of the following equations:
E◦(cell)= E◦(right) - E◦(left) (’right’ refers to theelectrode that is writtenon the right
in standard cell notation. ’Left’ refers to the half-reaction written onthe left in this
notation)
E◦(cell)= E◦(reduction half reaction)- E◦(oxidation half reaction)
E◦(cell)= E◦(oxidising agent) - E◦(reducing agent)
E◦(cell)= E◦(cathode) - E◦(anode)
So, for the Zn-Cu cell,
E◦(cell)= 0. 34 − (− 0 .76)