Reactivity And Electrochemical Series - Metals
The reactivity series can be used to compare
the relative reactivities of different metals. It lists metals in order of general chemical
reactivity. Metals generally react by losing electrons to form positive ions.
The more readily a metal loses electrons, the more reactive it is – and the
greater its strength as a reductant. Metals higher up in the series can reduce
the ions of those lower down.
The standard electrode potential of a
metal also indicates its strength as a reductant. The more negative the value
of the standard electrode potential of a metal, the greater is its strength as
a reductant. Hence, you might expect the metal reactivity series and standard
electrode potentials to list metals in the same order. However, you must remember that the metal
reactivity series is based on observing a range of reactions, such as displacement
reactions between solid metals and solid metal oxides. Standard electrode
potentials refer specifically to
reactions taking place in aqueous solution.
Below is a comparison of the reactivity
series with the electrochemical series,
which ranks metals according to their standard electrode potentials.
The
obvious discrepancy is the relative positions of sodium and calcium. Calcium is
a stronger reductant than sodium according to Eo (standard electrode potential) values,
but the metal reactivity series suggests that calcium is less
reactive than sodium. This discrepancy arises because calcium reacts at a much slower rate, in
displacement reactions for example, which in turn happens because two
electrons must be removed, not one as for sodium.
Note also that aluminium reacts readily
with oxygen in the air, forming a layer of stable aluminium oxide on its
surface. This impervious oxide coat often causes aluminium to exhibit lower
reactivity than its position in the metal reactivity series indicates.
