For a reaction to happen, current (electrons)
must flow through a complete circuit. Here, you
have two separate reactions. One is the production
of hydrogen, which requires additional energy to
occur (we call this non-spontaneous). The other
reaction is the reaction of zinc and copper, which
happens without added any energy (we call this a
spontaneous reaction). So there are actually two
reactions happening at the same time. One is the
reaction of zinc and copper, which liberates
energy that is used to power the reaction to
create hydrogen gas. In the same way, you could
put a battery into water and create hydrogen gas.
The role of the zinc and copper is to provide
electric energy to make the hydrogen gas.
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