‘A’ Level Notes.
Redox potentials refer to reduction potentials (if a reduction
half-equation is written) or oxidation potentials (if an oxidation
half-equation is written).
As long as the equation is given as reversible (ie. equilibrium
double half arrow notation used), it means you can reduce one
species (eg. Fe3+) to a reduced state (eg. Fe2+), and you can also
oxidize the reduced state species (eg Fe2+) to the oxidized state
species (eg. Fe3+).
Hence, for such reversible equations, you can reverse the half
equations and the standard potentials.
When written as a reduction half-equation, the redox potential
is called the (standard) reduction potential (if standard
conditions). When written as an oxidation half-equation, the redox
potential is called the (standard) oxidation potential (if standard
conditions).
The positive/negative sign of the redox potential is reversed
when re-writing a reduction half-equation as an oxidation
half-equation, or vice-versa.
Whichever way (reduction or oxidation) you choose to write it,
the more positive the standard redox potential, the more the
position of equilibrium lies to the right. The more negative the
standard redox potential, the more the position lies to the
left.
You can Google/Wikipedia the technical physics definition of
redox/reduction/oxidation potentials (which is not required for ‘A’
level Chemistry). Technical physics definition aside, it is helpful
for you to think of the meaning of “potential” as “tendency” or
“inclination” or “propensity”.
Hence, the Reduction Potential numerical value (including
positive/negative sign) describes the potential or
tendency/inclination/propensity for a species to be reduced. The
more positive the value, the more the position of equilibrium lies
to the right (ie. the reactant species eg. Ag+ prefers to be
reduced).
(Note : due to formatting problems, the word equation is given
instead)
Eg. Silver cation + electron = (ie. left-to-right arrow) = Silver
atom ( + 0.80V )
( + 0.80V means Silver cation likes to be reduced to Silver
atom)
which can be also be written as :
Silver atom = (ie. left-to-right arrow) = Silver cation + electron
( – 0.80V )
(Indeed, – 0.80V means Silver atom doesn’t like to be oxidized to
Silver cation)
Likewise, the Oxidation Potential numerical value (including
positive/negative sign) describes the potential or
tendency/inclination/propensity for a species to be oxidized. The
more positive the value, the more the position of equilibrium lies
to the right. (ie. the reactant species prefers to be
oxidized).
(Note : due to formatting problems, the word equation is given
instead)
Eg. Potassium atom = (ie. left-to-right arrow) = Potassium cation +
electron ( + 2.92V )
( + 2.92V means Potassium atom loves to be oxidized to Potassium
cation)
which can be also be written as :
Potassium cation + electron = (ie. left-to-right arrow) = Potassium
atom ( – 2.92V )
( – 2.92V means Potassium cation hates to be reduced to Potassium
atom)
Standard cell potential (which must be positive to be feasible,
ie. for the current of a Galvanic/Voltaic cell to flow) is
calculated as :
Oxidation potential at Anode + Reduction potential at
Cathode
or
Reduction potential at Cathode – Reduction potential at
Anode
As long as standard cell potential is positive, the reaction is
feasible, and the current in the Galvanic/Voltaic cell will
flow.
