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A voltage or potential difference is generated between the metal and the solution.
The electrode potential of a metal, measured in volts, and is caused by the charge
built up on the metal rod due to the dynamic equilibrium established between the
metal atoms and the metal ions in solution.
M+(aq) + e
 M(s)
The position of the equilibrium, the charge and size of the electrode potential,
depends on:
 The nature or reactivity of the metal – the electrode potential is more negative
for more reactive metals as they have a greater tendency to lose electrons to
form stable ions.

The ionic concentration [M+] – increasing the concentration of [M+] will alter
the equilibrium in favour of the formation of M(s) , so the electrode potential
becomes more positive.

The temperature – changes in temperature affect exothermic and
endothermic reactions differently.
It is impossible to measure the true potential difference of a metal in a solution of its
ions because there is no complete circuit, so electrodes or half-cells must be
connected to a second (reference) electrode to give a comparison of energy
difference (voltage).
One half-cell will tend to lose electrons and the other half-cell will tend to gain these
electrons: this allows the relative reduction potential of each half-cell to be
calculated.
THE STANDARD HYDROGEN ELECTRODE
The hydrogen electrode is used as a standard or reference electrode to measure the
electrode potential of any half-cell. It is given the reference potential of 0.00 V.
2H+(aq) + 2e - 
H2(g)
Eo = 0.00 V
Eo is measured under standard conditions:
 All solutions are 1 mol L-1
 all gas pressures are 1 atm or 100 kPa
 temperatures are 25oC (298 K)
An inert electrode is used to carry the electrons to and from the hydrogen half-cell,
as neither hydrogen gas nor ions can carry electrons. Platinum is the metal used
because
 it is inert
 it catalyses the reaction that produces hydrogen
+
-2H (aq) + 2e
H2(g)
Once the standard hydrogen electrode is set up, and because it has a reference
potential of 0.00 V , it can be used to determine the electrode potential of all other
half-cells.