Wednesday 2 December 2015

Electrochemical cells, Difference between electrolytic and galvanic cell.

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.
Sources :

No comments:

Post a Comment

Almonds and Archimedes Principle

Why  some almonds float on surface of water while some settle at bottom? You might have observed, when almonds are soaked in water some o...