How to identify anode and cathode in a cell?
Electrode at which
oxidation occurs is anode and at which reduction occurs is cathode or we can
say that "electrons move from anode to
cathode no
matter what charge each electrode have."
Electrodes
& Charge
The anode of an electrolytic cell is positive (cathode is
negative), since the anode attracts anions from the solution. However, the
anode of a galvanic cell is negatively charged, since the spontaneous oxidation
at the anode is the source of
the cell's electrons or negative charge. The cathode of a galvanic cell is its
positive terminal. In both galvanic and electrolytic cells, oxidation takes
place at the anode and electrons flow from the anode to the cathode.
Galvanic cell
Now, in galvanic cell the reaction proceeds without an
external potential helping it along. Since at the anode you have the oxidation
reaction which produces electrons you get a build-up of negative charge in the
course of the reaction until electrochemical equilibrium is reached. Thus the
anode is negative.
At the cathode on the other hand you have the
reduction reaction which consumes electrons (leaving behind positive (metal)
ions at the electrode) and thus leads to a build-up of positive charge in the
course of the reaction until electrochemical equilibrium is reached. Thus the
cathode is positive.
Electrolytic cell
In an electrolytic cell you apply an
external potential to enforce the reaction to go in the opposite direction. Now
the reasoning is reversed. Electrode
connected to the negative end of the battery will have negative charge or
excessive electrons .These excessive electrons will cause reduction in this
electrode. So the negative electrode will be the one where the reduction reaction will take place and thus it's
the cathode.
At the positive electrode where you have
produced a low electron potential via an external voltage source i.e. positive
pole of battery, will cause oxidation. So the positive electrode will be the
one where the oxidation reaction will take place and thus it's the anode. Refer to the figure:
Differences
between Galvanic cell and Electrolytic cell
Electrochemical cell (Galvanic Cell)
|
Electrolytic cell
|
A
Galvanic cell converts chemical energy into electrical energy.
|
An
electrolytic cell converts electrical energy into chemical energy.
|
Here,
the redox reaction is spontaneous and is responsible for the production of
electrical energy.
|
The
redox reaction is not spontaneous and electrical energy has to be supplied to
initiate the reaction.
|
The
two half-cells are set up in different containers, being connected through
the salt bridge or porous partition.
|
Both
the electrodes are placed in a same container in the solution of molten
electrolyte.
|
Here
the anode is negative and cathode is the positive electrode. The reaction at
the anode is oxidation and that at the cathode is reduction.
|
Here,
the anode is positive and cathode is the negative electrode. The reaction at
the anode is oxidation and that at the cathode is reduction.
|
The
electrons are supplied by the species getting oxidized. They move from anode
to the cathode in the external circuit.
|
The
external battery supplies the electrons. They enter through the cathode and
come out through the anode.
|
Examples:
In the following examples, the
anode is positive in a device that consumes power, and the anode is negative in
a device that provides power:
- In a discharging battery or galvanic cell (diagram at right), the anode is the negative terminal because it is where the current flows into "the device" (i.e. the battery cell). This inward current is carried externally by electrons moving outwards, negative charge moving one way constituting positive current flowing the other way.
- In a recharging battery, or an electrolytic cell, the anode is the positive terminal, which receives current from an external generator. The current through a recharging battery is opposite to the direction of current during discharge; in other words, the electrode which was the cathode during battery discharge becomes the anode while the battery is recharging.
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