The main purpose of the barrier is to keep Cu2+
ions away from direct contact with the Zn electrode. The positive
Cu2+ ions have no tendency
to migrate by electrostatic forces
from right to left, but they could be brought up to the zinc
metal by stirring or agitation. This is what the barrier prevents.
The electrode at which oxidation
takes place always is called the anode,
and the reducing
electrode, the cathode.
In the Zn-Cu cell, the zinc electrode is the anode, and the
copper is the cathode.
Negative ions are called anions
because they flow toward the anode
in an electrochemical cell, and positive ions are cations
because they migrate toward the cathode.
Negatively charged sulfate anions,
for example, migrate from the copper sulfate compartment,
through the porous barrier, and into the zinc sulfate compartment
where the anode is found.
The logic in naming the anode arises because it is the electrode
from which electrons flow up and out of the cell (Greek: ana,
meaning up), and the cathode is the pole at which electrons
flow back into the cell (Greek: cata, meaning down). This
is as hard to remember as the terms themselves.
The best memory device is to recall that Anode and Oxidation
begin with vowels, Cathode and Reduction with consonants,
or that Anode precedes Cathode in the alphabet, and Oxidation
precedes Reduction. (It may not be elegant, but it works.)