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The positive signs for
free
energy of the zinc and nickel reactions,
and the negative signs
of their cell potentials,
indicate that the reactions are not
spontaneous in the direction written.
Zinc ions will not be reduced spontaneously by hydrogen
gas. Quite the contrary, if metallic zinc is dropped
into acid, in which H+ ions are plentiful, zinc will
be dissolved into Zn2+ ions, and hydrogen gas will be
evolved:
Similarly, Ni2+ ions will not oxidize H2
to H+ spontaneously, but metallic nickel
will dissolve in acid with the release of hydrogen gas,
although not as readily as does zinc.
Copper is not attacked by acid as zinc and nickel are,
because the following reaction is spontaneous:
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